Category: Planetary, extraterrestrial geology, and meteoritics

The use of seismic signals from many receiving stations to probe physical properties of the Earth tomographically is producing increasingly sharp results from the deep mantle. In a fascinating review of the state of that art, combined with results of high-pressure experiments that throw light on deep mantle changes in mineralogy and density, Edward Garnero and Allen McNamara of Arizona State University present some stunning graphics (Garnero, E.J. & McNamara, A.K. 2008. Structure and dynamics of Earth’s lower mantle. Science, v.  320, p. 626-627). Their scope is global, and dominated by thermochemical upwelling plumes and superplumes, zones towards which whole-mantle convection has swept dense material, and some indication of a connection between the two huge phenomena. It seems there are also pockets of magma close to the core-mantle boundary, which are hinted at by abnormally low shear-wave velocities.

Global wildfires at the K-T boundary debunked

Among the minuscule treasures of the K-T boundary deposits across the world are abundant amounts of what researchers have generally called soot. Interpreted literally, these seem to point to massive combustion of living vegetation at the time of the Chicxulub impact. That presupposes two things: that oxygen levels in the late Cretaceous were sufficiently high (~30%) to support combustion of green vegetation and heating from the entry flash of the Chicxulub projectile. The first is possible, but not the second, for not all the planet would have been bathed in the flash caused by compressive heating of the atmosphere ahead of the inbound planetesimal. Nonetheless, global forest fires were the accepted wisdom. A closer look at the ‘soots’ from eight K-T boundary exposures reveals that they are not made of charcoal, which vegetation burning would produce (Harvey, M.C. et al. 2008. Combustion of fossil organic matter at the Cretaceous-Paleogene (K-P) boundary. Geology, v. 36, p. 355-358). Instead the resemble carbonaceous nanospheres that result from incomplete combustion of pulverised coal or oil aerosols in power stations. By chance, the Chicxulub impact was next to what is now one of the most productive oilfields on Earth; the Canterell field in Mexico.

Astonishing stratigraphy of the north pole of Mars

Since, so far as we know, not a single sentient being has set foot on the Martian surface the title of this item might seem strange; but it is true. One of the features of microwave radiation is that it is capable of penetrating through solid surfaces and imaging the subsurface, given the right conditions. This phenomenon is best exploited by ice, and ground-penetrating radar is routinely used for sounding Earths glaciers and ice caps. To a lesser extent sedimentary layers can be penetrated, provided they are very dry. Radar is also an extremely useful remote-sensing tool with which to examine surfaces, and no planetary mission would be complete without some kind of radar instrument. The US Mars Reconnaissance Orbiter carries a radar system targeted at just such penetration – the Shallow Radar or SHARAD.

SHARAD is operated along traverses and provides cross sections of the subsurface that look very like seismic sections, with structure picked out by reflecting surfaces. Crossing the north polar ice cap of Mars, SHARAD reveals a simple layered sequence (Phillips, R.J. and 26 others 2008. Mars north polar deposits: stratigraphy, age and geodynamical response. Science, v. 320, p. 1182-1185). Nonetheless the layering is interesting as it reveals what appear to be cyclical processes involved in the ice cap’s evolution; perhaps by ~million-year periodicity in Mars’s obliquity or orbital eccentricity. The radar transparency of the north polar region is probably down to almost pure ice, around 1 km thick. Therein lie clues to another Martian feature: its lithosphere is very strong and thick. That conclusion stems from the lack of any significant annular topographic bulge around  the ice cap. Kilometre thick ice on Earth would result in a measurable feature of that kind, due to displacement of the underlying asthenosphere. The post-glacial relaxation of such a bulge that once lay to the south of the British ice cap is responsible for the drowning of valleys in SW England especially, and measurable subsidence of southern Britain today.

See also: Kerr, R. 2008. Layers within layers hint at a wobbly Martian climate. Science, v. 320, p. 867.

Other Martian oddities

A wonderfully written and illustrated summary of some of the strange recent findings about Mars appeared in the 24 May 2008 issue of New Scientist (Clark, S. 2008. Fire & ice. New Scientist, v. 198 24 May 2008 issue, p. 35-39). It emphasises the role of water and the chaotic orbital and spin behaviour of the ‘Red Planet’ in shaping its surface. Clark draws a picture of mystery and weirdness that will surely appeal to all Mars buffs.

How to spot impact sites that others have missed

The Earth’s surface is not peppered with obvious impact craters, as are the surfaces of other planetary bodies, because our planet is active tectonically and in terms of weathering, erosion and sedimentary deposition. Craters here get ‘ironed-out’ or buried quickly. Yet there is no way that the Earth could have escaped the episodic rain of objects large and small that results from gravitational perturbation of asteroids and comets by the complex motions of the giant planets. Finding signs of past impacts adds to knowledge of their effects on life, for example, as well as on the processes that accompany ‘mountains that fall from the sky’: it is a damn sight cheaper than doing the field work on the Moon or Mars. Astonishingly, a large impact site straddling a major highway in New Mexico escaped detection until recently (Fackelman, S.P. et al. 2008. Shatter cone and microscopic shock-alteration evidence for a post-Paleoproterozoic terrestrial impact structure near Santa Fe, New Mexico, USA. Earth and Planetary Science Letters, v. 270, p. 290-299). The clue that something swift and terrible had occurred in New Mexico during the late Precambrian were strange structures in road cuttings that looked like cartoons of Christmas trees. They consist of multiple cone-shaped features nested together in masses up to 2 m long and 0.5 m across. Other processes can form these strange structures, but finds of shocked minerals and signs of melting in the rocks affected by the cones confirmed a suspicion of a nearby impact structure. Shatter cones can easily be overlooked by geologists who have never seen such features before. The fact that those in New Mexico occur in recent road cuttings helped the authors spot them. At known impact sites shatter cones occur exclusively within the zone of uplift at the centre of complex craters. Those in New Mexico occur over an area about 3 km across, suggesting a minimum size for the now vanished crater of 6-13 km across.

Back to about 200 Ma ago, charting the motions of plates is relatively simple using the striped patterns of magnetic field strength above the ocean floor, which reflect periodic reversals of polarity of the geomagnetic field. Post-Triassic plate motions can also be assessed in an absolute reference frame with the use of hot spot tracks. Since no ocean floor is older than 200 Ma, the method cannot be used before then. Instead, the inclination and direction of remanent magnetism in continental rocks, suitably corrected for any tilting by deformation, take on the role of tracking motions. The direction is taken as being towards the magnetic poles at the time a rock formed, whereas the inclination supposedly varies in a simple fashion with latitude as it does today; vertical at the poles and horizontal at the ancient Equator. The post-Triassic break-up of Pangaea allows the palaeomagnetic method to be tested, and for that period it holds up extremely well. The models that chart how continental masses separated from a late-Precambrian supercontinent, drifted and then clanged together in the Devonian to early Permian to form Pangaea use the assumption of a consistently dipolar magnetic field that was lined up with the Earth’s axis of rotation: about as uniformitarian as one can get. They are models that delight tectonicians and students alike. There is however, a period in Earth’s history, from about 750 to 600 Ma, when palaeomagnetic positioning gives worrying results. Evidence of glaciation occurs at nearly equatorial palaeolatitudes at least three times.

Taken at face value, these results form the basis for the ‘Snowball Earth’ hypothesis, and the 750 to 600 Ma period has been dubbed the Cryogenian. But there are two other ways of explaining what is about as far from uniformitarian as can be. Maybe there were long periods when the geomagnetic field was neither dipolar nor lined-up with the rotational axis, in which case palaeolatitudes for those periods would be totally meaningless. The other possibility, which is alarmingly odd, is that before about 600 Ma the angle between the Earth’s axis of rotation and the plane in which it orbits the Sun was not about 23.5°, but more than 58°. At a high obliquity, Earth’s rotation would then ensure that high latitudes were warmer than low ones, which would neatly explain away much of the evidence for ‘Snowball Earth’ conditions. It is a worrying idea, simply because some considerable force, i.e. a stupendous impact, would be needed to change the axial tilt from >58° to what it is now and probably has been throughout the Phanerozoic. Settling the matter once and for all seems now to have been achieved by David Evans of Yale University, using a simple yet ingenious approach (Evans, D.A.D 2006. Proterozoic low orbital obliquity and axial-dipolar geomagnetic field from evaporite palaeolatitudes. Nature, v. 444, p. 51-55).

Evans based his study on the uniformitarian assumption that conditions are just right for strong evaporation of shallow, enclosed seas between 15 to 35° of latitude either side of the Equator, which is where evaporite deposits are forming now. If true, and if the geomagnetic field has been much the same as it is now, except during reversals, then all evaporites should give palaeolatitude results with this narrow range. There are lots of them, going back to 2.3 Ga ago, and being quite soft it is easy to drill cores from them. Furthermore they contain wind-blown dust, the magnetic component of which would line up nicely with the geomagnetic field while salts crystallised. The results from 54 world-wide sample are quite a triumph, for no evaporite palaeolatitudes are further than 40° from the Equator, and their means fall within the modern latitude range of an excess of evaporation over precipitation. There are differences between different time periods – before Pangaea existed evaporites formed slightly closer to the Equator than in later times. The fact that they cluster also shows that the dominant component of the geomagnetic field has been consistently been a dipole. However, even though the fundamental assumptions on which palaeomagnetic measurements are based seem sound, there are still problems for the Snowball hypothesis. Are the magnetic measurements up to scratch and do the stratigraphic and radiometric ages of samples refer to the evidence for glaciation?

Bad news for lunar base

Whether or not the Moon becomes once again a target for exploration by astronauts, and for use as a launch pad for Mars, depend on whether there is any water there. There has been considerable optimism that perpetual shadows in some of the deep craters close to the south lunar pole might contain ice that has not been exposed to solar heating. There is a way of telling using radar imaging, and reconnaissance results from orbiting probes had suggested that ice was indeed there, hence the excited men in suits of various kinds. A check using far more revealing radar data produced using the Areceibo radio telescope – it has also produced images of Venus at far greater distances – show that both sunlit and shadowed areas on the Moon can give a signal that is theoretically that from ice (Campbell, D.B. et al. 2006. No evidence for thick deposits of ice at the lunar south pole. Nature, v. 443, p. 835-837). Since ice could never survive in full sunlight, the similar results cast great doubt on ice being anywhere else on the Moon. There also seems to be a correlation in degree of belief with degree of involvement with future lunar exploration preparation.

One theological mode of discourse is casuistry, best known for disputing the number of angels who can sit on a pinhead. Amongst astronomers, at least those who meet every three years at the General Assembly of the International Astronomical Union (IAU), this form of sophism crops up from time to time.  It does too among geologists, and probably more often, as they have a many things to argue about. At 13.32 GMT on the 24^th of August the 26^th GA of the IAU in Prague upset a great many people by casting Pluto, formerly known as Planet Pluto, into the indignity of dwarf-planet status. NASA may be well-miffed, as their New Horizon probe has been on its way there since mid-January 2006.

The issue of Pluto’s status popped up after a larger Sun-orbiting object was announced in 2005 (2003 UB₃₁₃), which, like Pluto is beyond the orbit of Neptune. That new body is the largest known in the dim and distant Kuiper Belt, and Pluto may well be a stray from that region, having a very odd orbit. IAU decided, somewhat late in its existence, to define ‘planet’. Committees were appointed. The primary criterion decided by the final committee to report to IAU was that planets need to orbit the Sun, not another bigger planet. Second, they have to have sufficient mass for their gravitational force to make them nice and round. Sadly, it seems that the committee made quite a gaffe. In order to distinguish trans-Neptunian planets that take more than 200 years to orbit, they suggested the term ‘pluton’ (oh dear). Whatever, that would give the Solar System 12 planets: trans-Neptunian Pluto, Charon (in binary orbit with Pluto) and 2003 UB₃₁₃; and Ceres, formerly just the largest asteroid known. But the Kuiper Belt might easily have lots of other massive and round objects in it, awaiting discovery. So, has the old Jesuitical mind-expanding exercise been ‘larged-up’? Probably not, in a strictly scientific sense, because additional criterion for planetary status, added by the 26^th GA of the IAU, is that one should be massive enough either to have ‘swept’ its orbit clear of minor bodies early on, or to have flung them far away. Since Pluto and Ceres have done neither, they are officially to be considered ‘of diminished stature’. Some worry that traumatised children, fond of Pluto, will be driven from an interest in science. Who knows? But if IAU persists in the name ‘pluton’ as a sop to public opinion, there will be trouble…

Painstaking work on meteorites and their re-evaluation has only a small, non-specialist readership, but now and again developments in the science and its bearing on how the Solar System and its planets formed need a review. The latest of these (Wood, B.J. et al. 2006. Accretion of the Earth and segregation of its core. Nature, v. 441, p. 825-833) doesn’t deviate much from generally accepted ideas, except in detail. For a long while it has seemed inescapable that gravitational potential energy accumulated from accretion of mass, together with energy released by decaying short-lived isotopes formed by a supernova near the dust cloud that gave birth to the Solar System would have led to hot protoplanets. So core formation by segregation of dense immiscible metal and sulphide melts was likely to have been sooner rather than later – such melts form at lower temperatures than do those made of silicates.

The daughter isotope (¹⁸²W) of one short-lived isotope (¹⁸²Hf) is especially revealing in both meteorites and the Earth. Hafnium favours entry into silicates while tungsten has an affinity for metallic iron; they are siderophile. So, when metallic melts form in a silicate body the Hf/W ratio increases in the silicates. If that segregation occurs before most ¹⁸²Hf has decayed – within about 45 Ma – then the silicate part will express an excess of ¹⁸²W while metals have a deficiency. In the case of metallic meteorites, ¹⁸²W is so low as to indicate segregation of the metal from silicate within less than 5 Ma of the ultimate origin of the Solar System. Inevitably, Earth would have incorporated some of these early-formed metallic parts during its accretion. Tungsten isotopes from terrestrial rocks, however, suggest that core formation lasted about ten times longer, and imply that this early metal re-mixed with silicate in the mantle during accretion, and formation of the core was a secondary product of heating of the growing planet. The mantle has an excess of siderophile elements, which poses a problem. There are three possibilities: core formation was never completed, some of these elements remaining locked in silicate; it took place while overall chemical conditions changed from reducing to oxidizing, so that the most siderophile ended up in the core during the reduced phase then less siderophile elements progressively favoured silicate entry as conditions became oxidising; as the Earth grew the pressure under which segregation of core materials increased. The third scenario invokes a deep ‘ocean’ of magma through which droplets of metal fell, equilibrating with silicate melt and then forming a pond on the ‘ocean’ floor, ultimately to descend as large masses.

Wood et al. examine these three scenarios in the light of recent data and planetary modelling, suggesting that the second was the most likely by a process of ‘self-oxidation’ as its size increased, perhaps linked with the formation of perovskite in the deep mantle once a limiting radius had been achieved. Such a heterogeneous accretion and core segregation would explain the disparity between estimates of the timing of the core from tungsten and lead isotopes (~12 and ~28 Ma respectively)   They also revisit the oddly low density of the liquid outer core – about 8% less than expected of an iron-nickel alloy, ascribing it to a mixture of the low-atomic weight elements, silicon, sulphur, carbon and hydrogen, with an unknown proportion of oxygen.

Titan, where Kurt Vonnegut’s Sirens sang, is, as we all know, a foggy world shrouded in hydrocarbons. The Huygens probe that sank to its surface revealed a tantalising glimpse of its strangeness, with possible erosion by liquid methane rivers and sediments of icy substances. But Huygens didn’t really tell us much, like the probe that lasted a few minutes on equally obscure Venus. To map a foggy world you need orbital radar. The Cassini mission, the mother ship for Huygens, carried a high-resolution radar imaging system, and the results are astonishing; Titan has monster sand dunes (Lorenz, R.D. and 39 others 2006. The sand seas of Titan: Cassini RADAR observations of longitudinal dunes. Science, v. 312, p. 724-727). They dwarf all but the largest terrestrial dunes in Namibia, rising to 200 m. They are linear dunes, spaced at around 4 km, and trend parallel to Titan’s Equator, where there must be a wind belt. So far only a few images have been returned, so the extent of the dune systems is unknown.  However, they correlate with optically dark material that is extensive in the equatorial region, so Titan may be dominated by dunes. For dunes to form presupposes an abundant supply of particles small enough to be picked up and transported by winds. The images from different latitudes suggest that transport is equatorwards. What those particles are made of is impossible to tell from radar returns, but most likely they are either organic solids or ice. Notions of Titan being bathed in hydrocarbon oceans now fall flat, as the areas that are not dunes seem to be topographic highs.

To most geologists minerals are a means to an end. Identifying them and working out their relative proportions in a rock provides a quick means of assessing its rough chemical composition. Textural relations between minerals help work out the sequence of processes that were involved in its evolution, and in the case of metamorphic minerals what pressure and temperatures were involved. In the case of the Earth’s mantle, however, mineralogy comprises only one or two abundant minerals – olivine and pyroxene at shallow depths, and the mineral perovskite (MgSiO₃) at depths greater than about 670 km – and dominates the mantle’s physical properties and bulk behaviour. There are distinct, narrow zones or discontinuities that separate different seismic properties and these have long been considered to represent changes in mineralogy of the more or less uniform bulk composition of the mantle. The most likely phase transition is from olivine + pyroxene to perovskite, in response to increasing pressure, thought to occur at about 670 km down. That transition was confirmed by high-pressure experiments, but whether that simple mineralogy persists down to the outer core has remained a mystery. Using tiny diamond anvils in a laser-heated furnace to create the enormous pressures at depths up to 2700 km is fraught with technical difficulties, but Kei Hirose and Shigeaki Ono of the Japan Marine Science and Technology Centre have finally achieved them (see Cyranoski, D. 2006. Magical mantle tour. Nature, v. 440, p. 1108-1110).

Hirose and Ono discovered that perovskite itself collapses to produce another, more tightly-packed molecular structure – post-perovskite with a sheet-like structure. This phase transition occurred experimentally under conditions that characterise the thin D” layer just above the core-mantle boundary. Seismic tomography has suggested that a number of weird things happen there. For instance, seismic S waves near the CMB have different speeds according to their direction of travel, and even accelerate in some parts. The platy structure of post-perovskite, unlike the more regular perovskite, is likely to create such physical anisotropy, especially if grains are aligned. The mineral, when iron enters its structure, may also help to explain thin (5-40 mm) zones in the D” layer in which seismic wave speeds fall by 5 to 30% compared with expected values (Mao, W.L. et al. 2006. Iron-rich post-perovskite and the origin of ultralow-velocity zones. Science, v. 312, p. 564-565). When first detected by seismic tomography, these zones had been assumed to involve regions in which partial melting occurred. It also seems that the phase transition is temperature- as well as pressure-dependent, so that post-perovskite could form at shallower depths in cooler regions. Being denser than its parent, that could result in sinking: like slab-pull at shallow depths, such a gravitational force would contribute to whole mantle convection by displacing hotter D” material. That in turn would ‘flip’ through the phase transition in the reverse fashion to become less dense, perhaps encouraging the initiation of rising plumes.

Sure enough, what might seem to be a boring bit of exotic mineralogy promises to exert some control over speculation on what happens at the bottom of the mantle. But it is too early to say how seminal the discovery might be – the errors in the experiments correspond to a depth range of about 350 km. On top of that, other experiments need to be conducted under these extremely difficult conditions, such as finding out if post-perovskite can chemically interact with the iron-rich outer core, and if its electrical properties are in some way different from those of better-understood perovskite.

Since George Bush announced that US manned planetary missions are back on the agenda, albeit in an uncertain future for NASA, barely a month goes by without some kind of scientific justification for a return to the ‘good old days’. The latest as regards future lunar missions was in the 1 April 2006 of New Scientist, as a special report ‘It’s time to go back’.  It seems there are unique opportunities that the Moon presents for a range of scientific work (Chandler, D.L. 2006. The ultimate lab. New Scientist,1 April 2006 issue, p. 33-37). The lunar far side, being shielded from radio noise from Earth, is well suited to deploying an array of miniature radio telescopes. Half a dozen 1 m dishes spread over 20 km could simulate an enormous dish. The lack of an atmosphere suggests ideal stable conditions for optical telescopes, although being on a body with a large gravitational attraction would expose instruments to meteor flux. The lunar south pole is said to look good for science. For a start, there is a 5 km peak always lit by the Sun for continuous solar power, as well as data relay back to Earth. Nearby is the deep Shackleton crater that is never lit, and is immensely cold; ideal for an infrared telescope, and maybe harbouring water ice to support a manned lunar base.

The Apollo missions returned sufficient rock and soil samples to whet planetary scientists’ appetites.  They answered a lot of questions, and did revolutionise issues of planetary origins, evolution and bombardment history, yet they raised other interesting questions. Answering geological questions from the rocks of other worlds depends a great deal on luck, and the few small sites visited by the Apollo astronauts undoubtedly left out a great deal. What is needed, it seems is a ‘Serendipity Base’. The best one would be a deep crater with steep, rocky sides, and there is one that seems just right. The Aitken basin is 12 km deep and exposes a layered structure in its walls.

Perhaps the greatest attraction is the fact that anything that falls on the Moon remains in its pristine state for all time, provided it is not buried by accumulated meteoritic dust and impact ejecta. The Moon could be a really happy hunting ground for meteorite specialists, although finding interesting ones on the dull, grey surface might pose problems – you can tell a meteorite on Earth, if you search ice sheets, deserts and saline flats, by their contrast with the background.  There is a very odd notion, however, that well-preserved ejecta from impacts on the Earth and other planets that found their way to the lunar surface might hold the keys to the origin of life (Ward, P. 2006. House of flying fossils. New Scientist, 1 April 2006 issue, p. 38-41). The reasoning goes like this: like the Moon, all planets in the Solar System have for 4.55 Ga been whacked by impacts, which must have flung debris outside their gravitational attraction. Having a strong gravitational field itself, the Moon must have swept up a sizeable representative sample of all such debris hurtling around the Solar System.  Some of the biggest impacts – again as revealed by the lunar surface – were early in planetary evolution. Debris from them would therefore be samples of materials before they had been affected by later geological processes on their parent planets. Analyses of particles in the Apollo samples indicate that perhaps 3 kg of the third of a tonne of material is non-lunar, of which a few grams might be from Earth.

Terrestrial geology effectively stops once we go back to about 4 Ga, besides which very old rocks on Earth have been subject to all manner of chemical, erosive, tectonic and metamorphic influences. That is the reason why incontrovertible fossils and geochemical evidence for life have yet to be found before 3 Ga at the earliest. There are whiffs of earlier life, which people choose to believe or otherwise, but the potential for dispute fuels continual debate. But escaped ejecta from Hadean impacts on the Earth wouldn’t have been altered so much. They could be dated, and thereby tell geoscientists about the earliest crust, now vanished apart from a few minute grains of pre-4 Ga zircons. Most attractive is the possibility that they could harbour well-preserved organic materials that are traces of the very earliest life forms or their complex precursor chemicals. But would they survive the impacts that produced them? Although impacts from objects as small as 100 m could fling debris beyond the Earth’s pull without heating it too much, Hadean impacts would have had awesome energy because the colliders were huge, as witness the mare basins on the Moon that are over 100 km across. Much of the debris from those lunar big hits is in the form of once melted glasses, and the holes that they left filled with magma generated by the huge energies involved. Some meteorites do preserve their original magnetization, which suggests they never reached temperatures above the Curie points of the minerals responsible for it. Ward cites this evidence in support of once living materials being able to survive in ancient terrestrial ejecta that almost certainly will lie on the lunar surface. But he uses it to say that meteorite internal temperatures must have stayed below 100°C: the Curie point for common magnetic minerals is around 600°C. Given the date of publication, might we be reading of a pudding with too much egg? Whatever, the origin, if not the meaning of life exerts more pull on science purse strings than the prospect of gold nuggets hiding in shadowed craters…

Yet another weird world

Saturn is well-endowed with moons: 35 with names and a whole lot of moonlets.  The Saturnian System is astonishing in its diversity, and part of the Cassini probe’s mission is to examine in detail as many moons as possible– 20 flown by in the last year. Enceladus is by no means the largest (504 km in diameter), yet it is very odd indeed. One of its singular features is its ability to jet vast amounts of water from warm spots, and the fact that it seems to snow there.  The 10 March 2006 issue of Science magazine devotes 40 pages to articles on the oddities of Enceladus. To jet water ice and vapour to more than twice its diameter – in fact to drench much of the planetary system and replenish parts of the famed ring system – there must be a powerful heat source.  Just what that is has yet to be worked out: it could be bound up with internal radioactive decay or with vast tidal sources from Saturn itself, and maybe something else entirely. Its south pole is curiously its most active part, with sufficient heat energy beneath to create a major positive anomaly in long-wave infrared images. This is where much of Enceladus’s resurfacing by snow takes place. Saturn’s tidal forces have rucked up the surface to create hilly ridges, perhaps assisted by a kind of icy volcanism. Tidal or internal forces have also opened up great cracks in the surface, which false-colour images that use UV, green and short-wave infrared reveal to be compositionally different from the water-ice bulk of the surface. That may have resulted from hydrocarbon deposits leaking from deeper layers. It is the moon’s interior that causes most excitement.  In order for it to spray off watery jets, there must be a deep source of liquid water, either a liquid shell on which an ice ‘lithosphere’ floats or produced as internal plumes by melting at an interface with a rocky core.  That there are hydrocarbons suggests that some of the watery solids include gas-hydrates (ices that incorporate both water and gases).

Dating of detrital zircon grains found in moderately old Archaean sediments from Western Australia first pushed known geological time beyond the previously impenetrable 4 Ga barrier. The record now goes back to around 4.4 Ga, within 95% of the date when the Earth and the Solar System came into being (4.55 Ga).  There has been much written about the oxygen isotopes in this tiny number of resistant minerals regarding whether or not they originated in a crust permeated by liquid water.  Because zircon is a mineral most usually associated with rocks of granitic composition, the very presence of extremely old ones seems to suggest that some degree of fractionation of primitive basaltic magmas must have taken place in the Hadean to form highly evolved magmas.  But did actual continental material arise so early? Processes in island arcs can generate evolved magmas in which zirconium is moderately enriched.  If such a host for the pre-4 Ga zircons was small in volume, it may have been easily recycled back to mantle depths, yet would enough zircons have been eroded from it to yield those preserved in sediments a billion years younger? It is possible to probe the processes involved in zircon formation by using the extremely sluggish radioactive decay of an isotope of the rare-earth element lutetium. The half-life of the ¹⁷⁶Lu to ¹⁷⁶Hf decay scheme (~37 Ga) is far longer than the time since the Big Bang, so detecting changes in the proportion of ¹⁷⁶Hf to other hafnium isotopes is a tough nut to crack, the more so as ¹⁷⁶Lu is very rare indeed.

A consortium of geochemists from Australia, the US, France and the UK have used the famous Jack Hills zircons to test the widely believed hypothesis that substantial continental crust has only emerged since 4 Ga ago (Harrison, T.M. et al. 2005. Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 Ga. Science, v. 310, p. 1947-1950). They found that deviations of ¹⁷⁶Hf/¹⁷⁷Hf from those assumed to characterise the bulk Earth (in fact the proxy of chondritic meteorites) show large variations in the zircons. Some of the deviations are negative, which is consistent with the very early formation of continental crust – perhaps from very soon after the Earth formed. On the other hand, some zircons show positive deviations, a sign that the mantle was depleted, also pointing to crust forming events. The authors boldly suggest that such anomalies refer to a very early geochemical upheaval in the Earth, that likely produced continental material. But the 4 Ga barrier for whole rocks seems clearly to suggest that none remains: either it was all subducted away, or was only a tiny fraction from which the Jack Hills zircons miraculously emerged on their long journey to a final resting place.

Commenting on the paper, Yuri Amelin of the Canadian Geological Survey, points out that no one agrees on the true composition of the bulk Earth (Amelin, Y. 2005. A tale of early Earth told in zircons. Science, v. 310, p. 1914-1915). Other isotopic evidence raises the spectre of our planet having accreted from a mixture of geochemically different meteorite types, and has never mixed thoroughly. Moreover, zircons are notorious for being compositionally zoned, as a result of being able to survive engulfment in later magmas from which new layers of zircon grow. The measurement of ¹⁷⁶Hf/¹⁷⁷Hf ratios is so difficult that only whole zircons give useful results, but those data hide the variations among the zones. Finally, he points out that studies of the ¹⁷⁶Hf/¹⁷⁷Hf in post 4 Ga basalts – and therefore the mantle from which they were derived – show that there is a clear divergence from chondritic meteorites that began around 4 Ga, the start of the record of existing continental rocks. In the kindest way, Amelin casts doubt on the sense in studies of such tiny relics of the Earth’s distant past.

In the last ten years the new technology of seismic tomography that produces ghostly images of high and low density mantle has convinced many geoscientists that two major dynamic features extend to almost to the core mantle boundary (CMB). Dense, high-velocity zones descend from subduction zones, suggesting that the slabs continue to fall through the entire mantle below the ~700 km maximum depth of the earthquakes that Bennioff and Wadati used to define subduction.  Some hotspots seem to be above diffuse zones of low seismic velocity that are supposed to signify hot, low density plumes that rise from the CMB. An inkling of a grand theory of mantle convection might then be that the descending slabs ruck up the deepest and hottest mantle layers to set them rising as narrow diapirs. Yet, other tomographic features appear to be restricted to the uppermost mantle, less than the 660 km depth of a major discontinuity long considered to be due to a mineral phase change at high pressure. A whole-mantle theory of convective heat transfer should transfer some geochemical trace of an exchange between core and silicate mantle. Osmium isotopes from plume-related magmatism suggest that there might be an exchange, but those of tungsten do not (see: Mantle and core do not mix, February 2004 issue of EPN).  The oldest and perhaps most convincing evidence against whole-mantle convection comes from study of helium in volcanic rocks, neatly reviewed by Francis Albarède (Albarède, F., 2005. Helium feels the heat in Earth’s mantle. Science, v. 310, p. 1777-1778).

Helium is generated by the decay of radioactive uranium and thorium isotopes as alpha particles (⁴He), which generates much of the Earth’s geothermal heat flow. There should be a close correlation between helium and helium, but at mid-ocean ridges the amount of ⁴He is only 5% of that expected from the associated heat flow. One explanation for this is that somewhere in the mantle there is a barrier to upward movement of helium, yet is allows heat to pass through: a thermally conductive layer that bars convective mass transfer. Albarède cites recent work that uses the flow of heat and helium through groundwater in an aquifer (Castro, M.C. et al., 2005. 2-D numerical simulations of groundwater flow, heat transfer and ⁴He transport — implications for the He terrestrial budget and the mantle helium–heat imbalance. Earth and Planetary Science Letters, v. 237, p. 893-910) as analogy of mantle processes. There too helium is less than might be expected, the reason being that the aquifer is recharged by rainwater, low in He.  Likewise, ocean-floor basalts are probably affected in the same way by hydrothermal circulation of seawater, thereby diluting the flux of helium from the mantle and perhaps helping to account for anomalously low helium flux. Another widely accepted view that the high ³He/⁴He ratios of hotspot basalts is evidence for their source in primitive mantle – ³He is probably a product of nucleosynthesis and therefore primordial as far as the Earth is concerned – is challenged by a recent paper that shows that helium is dissolved in mantle minerals (Parman, S.W. et al., 2005. Helium solubility in olivine and implications for high ³He/⁴He in ocean island basalts. Nature, v. 437, p. 1140-1143).  Parman et al.’s measurements suggest that the high ³He might result from residues of earlier melting in the mantle, rather than coming from parts that have remain in the state they were when the Earth accreted.

Vanished Martian sea or not?

The Mars Rover data from the Opportunity site that showed up masses of sulfate minerals in the large depression that it has roamed for 2 years prompted the notion that they formed as a sizeable body of surface water evaporated. The Rover Opportunity scientists have also speculated on Mars once having had highly acidic ‘weather’, in the form of sulfuric acid rain from SO₂ emitted by volcanoes. The sediments at the Opportunity site also show signs of fluid transport in the form of bedding and cross stratification, ascribed to moving water. Most independent-minded scientists confronted by a united front of vast teams of highly focused scientists sometimes feel that there is more than one way of skinning a cat.  Such is the case of Paul Knauth and Donald Burt of Arizona State University and Kenneth Wohletz of the Los Alamos National Laboratory in New Mexico. The visualise the dramatic evidence from Opportunity in an altogether more mundane scenario (Knauth, L.P. et al., 2005.  Impact origin of sediments at the Opportunity landing site on Mars. Nature, v. 438, p. 1123-1128). Their main point of departure is quite simple; acidic water full of hydrogen ions is a powerful means of weathering and the production of clay minerals. Clays are very uncommon on Mars, particularly at the Opportunity site, and have only shown up rarely on hyperspectral remote sensing images.

Layered sediments are evidence for fluid deposition, but not only water produces them. As well as wind transport and deposition, they are also formed by gas-rich base surges from explosive volcanism and meteorite impacts – and also during surface nuclear explosions that mimic impacts, hence the Los Alamos connection. Knauth et al. explain the Opportunity deposits as debris originally made of rock, sulphides brines and ice flung from a massive impact. They explain the sulfates as products of interaction between melted ices and sulfides. The extension of the Opportunity team’s hypothesis of evaporating surface water is that it would have been long-lived, perhaps sufficiently so for the emergence of acid-loving organisms, similar to those that infest groundwater in terrestrial massive sulfide deposits. Should the deposit prove to have formed during an extremely rapid event, such as an impact, the idea of it having hosted primitive life forms becomes extremely unlikely. Gleefully, Knauth et al. almost exactly match the Opportunity image mosaic of layered sediments with a photograph of a New Mexico layered, volcanic surge deposit. Surges from large impacts, and Mars was intensely bombarded in its early history, can extend hundreds of kilometres from the crater rim. Many other examples of layered sequences are being revealed by high-resolution orbital images of Mars, and interpreters regularly ascribe them to wind, flowing water or volcanic processes. Ockham’s Razor demands the most likely and simplest explanation for phenomena, and impacts could have formed the lot. The earliest detection of features that only flowing water could have carved – the sinuous canyons on Mars, originally prompted such a simple explanation, that water was released en masse by early massive impacts. Perhaps there is a much wider link between many Martian features and the most common geological agent in the Solar System.

Scottish Gaelic mythology includes the ‘Dread Coruisk’, the largest of the each uisge, or water horses.  “ ‘Tis a thing of which we dinnae care tae speak”, say locals of the Isle of Skye, whose shores it nightly stalks. The same could be said of one of the most daring, and amusing, hypotheses of modern geosciences: that of the ‘Verneshot’ (see Mass extinctions and internal catastrophes in June 2004 issue of EPN).  Phipps Morgan, Reston and Ranero explored the possible consequences of a build-up of volatiles in plume-related magmas at the base of thick continental lithosphere beneath cratons, prior to the eruption of continental flood basalts. The suggested that pressure would eventually result in an explosive release at a lithospheric weak point, followed by collapse above the plume head that would propagate upwards, at hypersonic speeds. Modelling the forces involved, the authors of the novel idea considered that they would be sufficient to fling huge rock masses into orbit.  The notion neatly might explain the circumstances around mass extinctions: coincidence of CFB events; large impact structures, most likely at the antipode of the event; global debris layers containing shocked rock, melt spherules; unusual element suites and compounds (including fullerenes); and enough toxic gas to cause biological devastation.  As with the ‘Dread Coruisk’, little has been said, neither in support nor in dispute over the last year.  My comment at the time was, “As with all departures from “accepted wisdom”, the Geomar group’s ideas will come in for a lot of stick, quite possibly from the fans of giant impacts, who not so long ago were themselves dismissed as “whizz-bang kids” by many geoscientists.

It is good to be proved perceptive once in a while. One of the original butts of adverse opinion in the early days of impact hypotheses, Andrew Glikson of the Australian National University, has been the sole commentator (Glikson, A.Y. 2005. Asteroid/comet impact clusters, flood basalts and mass extinctions: Significance of isotopic age overlaps. Earth and Planetary Science Letters, v. 236, p. 933– 937).  He points out that Phipps Morgan et al. overlooked 6 overlaps of impact clusters and CFBs, three of which were associated with mass extinctions. Rather than adding grist to their mill, he goes on to say that it is the geochemical blend associated with impactite layers that points unerringly to an extraterrestrial source for the mass involved in creating large impact craters, rather than any known terrestrial rocks. Moreover, the extreme shock-metamorphism that is the hallmark of impactites has never been observed near any gas-rich volcanic structure formed by explosive venting.   He returns to the view that impacts of alien origin have sufficient energy to induce large-scale partial melting of the mantle, and thereby generate large igneous provinces.

Unsurprisingly, the original authors were onto Glikson’s comment, in leopard-like manner (Phipps Morgan, J., Reston, T.J & Ranero, C.R. 2005. Reply to A. Glikson’s comment on ‘Contemporaneous mass extinctions, continental flood basalts, and ‘impact signals’: Are mantle plume-induced lithospheric gas explosions the causal link?’. Earth and Planetary Science Letters, v. 236, p. 938– 941).  First they emphasise that their concept of the tremendous power of a ‘Verneshot’ is not based on the explosive release of volatiles, but on the shock pressures associated with the collapse of ~80 km tall pipes due to gas venting, in a very short period of time. As regards the geochemical blend in impactite-related layers, dominated by iridium yet a dearth of other platinum-group metals, they cite evidence that very similar element proportions are released in the carbon- and sulfur-rich gas phases of plume-related volcanoes, as in Hawaii and Reunion. They are not crustal, but of mantle origin, carried by escaping volatiles, and fall in the field normally said to be meteoritic. Phipps Morgan et al. also dispute the likelihood of extraterrestrial-impact induced magmatism from its statistical unlikelihood – the chances of a one in 100 Ma bolide coinciding with 1 in 30 Ma CFB events is, on their count, 1 in 3000 Ma – and from the standpoint of the powers and work involved.  They agree that indeed there are extraterrestrial impact structures.

Surely, their well-argued idea is worth bearing in mind and considering as evidence continues to emerge – they do list a plausible set of characteristics that a ‘Verneshot’ would probably produce. There is some essential philosophy that has a good track record in the history of the geosciences, that of plate tectonics for one: the absence of evidence is not evidence of absense.

 

In Joseph Heller’s Catch 22, Hungry Joe is noted for ‘…snorting, stamping and pawing the air in salivating lust and grovelling need’. That is a close metaphor for reactions among some scientists (and astronauts) to observations that seem to support the notion that indeed, there is life on Mars. Remember the meteorite ALH84001? In 2004, a spectrometer carried by ESA’s Mars Express probe detected methane in the Martian atmosphere above areas that probably carry sub-surface water ice. Many exobiologists attributed this to exhalations by methanogen bacteria perhaps living in the ice, which seemed plausible. Sadly, it seems that hydrous alteration of the mineral olivine, which is widespread at the Martian surface, to serpentine is even more likely. The reaction can yield hydrogen, which generates methane by reducing carbon dioxide. Exobiologists are keeping their options open…. Meanwhile, it is not implausible that hydrogen from this simple reaction might be used to resolve global warming: olivine is the most abundant mineral in the rocky planets. Incidentally, it is serpentinisation of ultramafic rocks that best explains methane exhalation from the deep ocean floor and from crystalline basement, which Thomas Gold thought had a deep-mantle origin and was responsible for all hydrocarbon deposits.

Source: Schilling, G. Martian methane: rocky birth then gone with the wind? Science, v. 309, p. 1984.

Where do impactors come from?

All the rocky bodies in the Solar System (the Moon, Mars, Mercury, Venus, Earth and moons of the giant planets) preserve to some extent the signs of collisions with errant bodies. One period stands out dramatically: the Late Heavy Bombardment or LHB (4.0-3.8 Ga) that produced the lunar maria, and left its signature in Archaean rocks on Earth (see Tungsten and Archaean heavy bombardment, August 2002 EPN). The planet Venus was entirely resurfaced about 500 Ma ago, and its plains record the later flux of impactors in much smaller more widespread craters, as do the lunar maria, parts of Mars and to a very limited degree the Earth. The LHB stopped abruptly, having appeared equally out of the blue. The influence of astronomical collisions on planetary histories may be an established fact, but is still something of a mystery as regards its pace and intensity. High resolution images of large rocky bodies sustain a thriving cottage industry of measuring, counting and dating craters; the latter from stratigraphic evidence of relative age, such as craters that have been cratered, and ejecta mantles that bear signs of impact themselves.

Hidden inside such statistics are clues to the astronomical processes that lead to impacts (Strom, R.G. et al. 2005. The origin of planetary impactors in the Inner Solar System. Science, v. 309, p. 1847-1850). The crater-size distributions for the early events and those after 3.8 Ga are very different. Those of the later generation show features very like the size distribution of objects whose orbits intersect that of the Earth (near-Earth Objects or NEOs) and largely reflect the element of chance in a more or less stable late Solar System. The LHB pattern extends to craters more than an order of magnitude larger than the younger one, and resemble the size distribution of bodies that now orbit quite happily in the Main Belt of asteroids. It seems that during the period between 4.0 and 3.8 Ga, some main belt asteroids were flung out of their orbits to enter the Inner Solar System in large numbers. The analysis by Strom et al. suggests that the gravitational disturbance during that period might have been due to gradual migration of the giant Outer Planets before they took up their present stable orbits.

Slowly, geochemists as well as planetary scientists have been taking up the implications of a likely infernal origin for the Earth-Moon system that resulted from a Mars-size planet colliding with the proto-Earth, shortly after planetary accretion.  The chemistries of both Earth and Moon have sufficient similarities for a common origin to be almost certain.  There is one difference: lunar rocks are more depleted in volatiles than those accessible on the Earth.  Terrestrial rocks were at some stage in their evolution purged of some volatile elements.  The Moon’s early history seems to be extraordinarily simple.  It is recorded in the pale rocks of the lunar highlands that are made dominantly of feldspars.  Their low density and abundance suggest that feldspars floated to the top of completely molten rock, in much the same way as similar anorthosites on Earth seem to have formed in large magma chambers. The difference is that lunar anorthosites probably once formed the entire crust of the early Moon, and formed by simple differentiation of a deep, all-encompassing magma ocean.  The late Dennis Shaw applied this simple notion to the Earth’s earliest evolution during the 1970s, but his vision was largely ignored by his geochemist peers.  A mantle-wide zone of complete melting was resurrected when William Hartmann’s giant impact theory appeared: the energy involved seems to make this an inevitable corollary of his idea.

Indirect analysis of the mantle from the geochemistry of its basaltic products has shown that the mantle is not homogeneous.  Some has been partially stripped of basalt-forming elements, and there are other chemical heterogeneities.  However, examined from the standpoint of isotopes of neodymium (¹⁴²Nd and ¹⁴⁴Nd) more or less every magmatic rock has been considered to have been ultimately derived from material with the same isotopic composition as chondritic meteorites, and by extension, that of the Galaxy in the vicinity of what became the Solar System.  That observation has been a major counter argument to the notion of an early terrestrial magma ocean. Differentiation of such a fundamentally molten Earth would have separated some of the samarium-146 (the source of ^142‑Nd through radioactive decay) from ¹⁴⁴Nd, thereby imparting different growth histories for ¹⁴²Nd/¹⁴⁴Nd ratios to different mantle ‘reservoirs’.  The half-life of ¹⁴⁷Sm is about 100 million years, so that radiogenic ¹⁴²Nd would accumulate most in Earth’s early history, thereafter tending towards a constant proportion of neodymium, unlike the ¹⁴³Nd used in radiometric dating that accumulates much more slowly from decay of ¹⁴⁷Sm (half life about 100 billion years).

There was a flaw in this counter argument.  The similarity of chondritic and terrestrial Nd isotope patterns might have stemmed from isotopic measurements that were insufficiently precise to detect significant differences. Mass spectrometry has undergone a near-quantum leap in precision.  Applied to the chondrite-Earth rock comparison, the neodymium data for chondrites remains as determined earlier, but the ¹⁴²Nd/¹⁴⁴Nd ratios of terrestrial rocks turn out to be 20 parts in a million higher than for chondrites (Boyet, M & Carlson, R.W. 2005. ¹⁴²Nd Evidence for Early (>4.53 Ga) Global Differentiation of the Silicate Earth.  Science, Published online June 16 2005; 10.1126/science.1113634).  That doesn’t seem very much, but quite sufficient to suggest plausibly that indeed the Earth’s mantle did indeed evolve from a magma ocean.  Its upper part was enriched in samarium by its fractionation as a solid that probably crystallised downwards.  Whatever was left of the original liquid would be at the base of the protomantle, and in it many other elements that favoured melt over crystals – so-called ‘incompatible’ elements – would have been enriched.  Boyet and Carson suggest that such a deep, enriched layer may amount to between 5 to 30% of the current mass of the mantle. 

The implications, if the ideas are confirmed, are enormous, because geochemists up to now have taken the bulk of the mantle that supplies basalt magmas – and whose composition is quite well constrained – to represent the whole silicate Earth.  That may satisfy geochemical parameters, but worries geophysicists.  The ‘standard’ Earth has insufficient radioactive uranium, thorium and potassium to account for the heat that flows to the surface. In fact it generates about a half, leaving the rest to speculation. One school looks to supposed gravitational potential energy locked in the core when it formed by inward collapse of iron-nickel alloy and slowly released thereafter.  Another theorises about radioactive potassium-40 combined in sulphides of the core, which also ‘leaks’ out.  The possible existence of the last dregs of an early magma ocean, near the core-mantle boundary (CMB), would not only account for 43% of surface heat flow, but might also drive convection in the liquid outer core as a means of generating Earth’s magnetic field.  Even more important, it might fuel the rise of plumes from the CMB that are increasingly implicated in periodic repaving of the Earth’s surface by flood-basalt volcanism.  Since flood basalts are a popular source for mantle geochemists’ data, why are the signs of such a peculiar source region not clear in their analyses?  Either they are not looking with the requisite precision, or the source itself does not move with plumes, merely setting them in motion.  Eminent geochemists see a bit of a hectic time ahead….. 

See also: Kerr, R.A. 2005. New geochemical benchmark changes everything on Earth.  Science, v. 308, p. 1723-1724.

Sudbury in Ontario, Canada hosts one of the largest nickel and platinum-group metal deposits, and it in turn is associated with the world’s second largest impact structure (260 km diameter), dated at 1850 Ma.  About 650 km to the WNW is another of Canada’s Precambrian treasures, the Gunflint Chert beds that contain the earliest incontrovertible fossil cells.  Those cherts are also roughly the same age as the Sudbury impact structure, so what better place to seek material excavated and ejected by the offending meteorite? No need either to thrash around the bush to collect rocks; the succession has been penetrated by 5 drill cores near Thunder Bay and in northern Minnesota.  Sure enough, all the cores show signs of impact ejecta (Addison, W.D. et al. 2005.  Discovery of distal ejecta from the 1850 Ma Sudbury impact event.  Geology, v. 33, p. 193-196).  The proof takes the form of shocked quartz and feldspar grains and melt spherules, but in a sequence of silicified carbonates above the level of the Gunflint Chert.  Ejecta material is about 0.6 m thick.  Because the carbonates contain no volcanic horizons, establishing the age of the ejecta depends on a thin volcanic ash 5 m above it, which yielded zircon U-Pb ages between 1827 to 1832 Ma.  There are no other known impacts around this time, so Sudbury is the most likely source of the ejecta.  Apart from being the oldest impactite layer known that can be tied to a source, there are a couple of intriguing features.  The ejecta layer occurs almost at the top of the Gunflint Formation famous for its cellular remains, yet the overlying strata contain no sign of fossils.  The authors wonder if this might represent mass extinction, but these slightly younger sediments are clastic rocks in which cell microfossils are unlikely to have been preserved.  However, they do show signs of anoxia, including high organic carbon content and sulfide minerals.  Hopefully carbon isotope data from the section might throw light on how impacts in a world exclusively that of single-celled organisms affected the biota: an interesting comparison with the K-T boundary.  The other puzzle is that the ejecta are in shallow-marine sediments.  Being only a few hundred km from the linked impact structure, some sign of disturbance by tsunamis or water-release by huge seismic shocks might be expected within the sediments.  No signs of such disturbances have been reported.

One of the oddities of the deep Earth is the presence of zones of the order of 1 to 10 km thick close to the boundary between the lower mantle and the outer core that have seismic wave speeds well below those expected at such depths.  Because wave speed is inversely proportional to density, the chances are that they are “ponds” of extremely dense solid materials.  Denser in fact than basalt might become in the form of eclogite, even compressed appropriately to these extreme depths.  The zones have been a puzzle, but that seems to have been resolved by work from University College, London (Dobson, D.P. & Brodholt, J.P. 2005.  Subducted banded iron formations as a source of ultralow-velocity zones at the core-mantle boundary.  Nature, v. 434, p. 371-374).  The densest materials found commonly at crustal levels are iron oxides and hydroxides, but today they are disseminated through much larger volumes or quartz-rich sediments.  Up to about 1.8 billion years ago, they were produced in huge abundance in sedimentary rocks, along with interbedded cherts, to form banded iron formations (BIFs).  That is widely agreed to have been a phenomenon only possible when the ocean was oxygen free so that iron could be dissolved in the oceans, and that they were precipitated when that Fe(II) came into contact with oxygen being produced by photosynthesising blue-green bacteria in shallow water.  Without any shadow of doubt, BIFs are the densest sediment that the Earth has ever produced, with a 50:50 mix of iron oxide and chert having a density of 3900 kg m^-3 at near-surface pressures, compared with 3100 for the upper mantle.  Long ago, Bob Newton of the University of Chicago reckoned that they “didn’t oughta be around still”: Precambrian BIFs are so vast and so dense that they are even more likely to be subducted than oceanic basalt converted to eclogite.  And they would not even need to be metamorphosed to do that.  So, it has taken a long time for someone to cotton on to Newton’s typical prescience.  Quite possibly, BIFs were a tectonic driving force at a time when the basalt-eclogite transformation was thermodynamically unlikely. Dobson and Brodholt observe that BIF density can only get larger (much larger; 6600 kgm^-3 at CMB pressure) if they sink  This is a nice hypothesis, for BIFs fit the bill exactly for the ultra-low velocity zones, and carries some interesting corollaries.  BIFs contain a great deal of oxygen, in fact probably the entire productivity of the early Precambrian biosphere: that would have a biogenic isotope signature.  Could that be added to any plume material emanating from the CMB?  Equally, BIFs contain unusually high concentrations of transition metals, and there is another possibility for deep-mantle geochemists to juggle with. The authors also observe that iron-oxides have high electrical conductivity compared with silicates, and ponder on the electromagnetic consequences of that so close to the core.  One thing seems certain; iron oxides probably would not melt, but, depending on the amount of oxygen in the core, they might dissolved in the molten outer core.

As 2004 was but a few days old, there was much cheering at NASA’s Jet Propulsion Laboratory as the two Mars landers touched down safely and unleashed the two Rovers to deploy their instruments.  Celebrations at ESA were not so universal, as the Beagle-2 miniature geochemistry laboratory vanished without trace.  Beagle could in principle have proved the existence or otherwise of Martian life, had it survived and landed on suitable ground.  Still, ESA’s Mars Express orbiter was safe and promised oodles of highly detailed pictures and other data.  What followed was an embarrassment of riches from both the US and EU missions, more or less throughout the year.  Then ESA had real cause for partying as 2005 opened, as its Huygens probe landed on the largest and most enigmatic moon in the solar system, Saturn’s Titan, but that is a story that will run this year, and it was carried courtesy of NASA’s Cassini mission.  New Scientist featured an excellent summary of the achievements on Mars in its 15^th January 2005 issue (Chandler, D.L. 2005.  Distant shores.  New Scientist 15 January 2005, p. 30-39).  Everything has worked better than expected, Rovers Spirit and Discovery having the benefit of sand blasts that cleared the dust off their solar cells.  They are still functioning, though not exactly prancing – it has taken a year for them to travel just over 5 km between them.  But the treasures they have unfolded have delighted lots of geologists.  There is ample evidence at least for the former influence of liquid water at the surface, which has both weathered the Martian surface to produce iron minerals that witness both water and highly acid conditions and also laid down sediments in layer after layer.  Some hint at the former existence of a large shallow, salty sea where Discovery landed.  Mars Express’s imaging devices have produced high-resolution pictures that confirm the influence of water’s sculpting, seemingly late in its history, and the presence of recent glacial deposits.  The orbiter also carries a deeply penetrating radar device (MARSIS) capable of finding water up to a kilometre beneath the surface, though it has yet to be deployed.  Perhaps the most intriguing find is that Mars’ atmosphere has more methane in it than seems possible, unless something is continually emitting it.  That “something” could be volcanism (2004 also revealed signs of previously unknown, recent eruptions), methane may be leaking from sub-surface gas-hydrates similar to those beneath Earth’s sea floor, it could be emitted by icy material from comet debris, and maybe it signifies some primitive, methanogen life forms that are respiring.  The last needs to be tied down very rigorously before scientists get over excited.  Even if it matches up with signs of emitted water vapour, which it does, that could still be an abiogenic phenomenon.  There can be little doubt that Mars is proving irresistible as a political draw, riding on its kudos to hammer out the old message that “Man Must Go  There!”. But consider this: had today’s robotic technology and analytical miniaturisation been possible 35 years ago we would know vastly more than we do about the evolution of our neighbour the Moon.  Instead of carrying astronauts and their weighty life support systems, the Apollo missions would have brought back an equivalent mass of lunar rock.  The same goes for Mars, surely, on the old basis of getting “more bangs for your buck”.  But that is a scientific outlook, and maybe the bucks can only be raised by the romantic notion of some brave souls treading where Edgar Rice Burrough’s John Carter once rode astride his banth.  But of course, robotic science can also ride on that “vision”, for what could be more catastrophic to whichever US president succeeds in making George W. Bush’s dream come true to find that it is not safe enough out there, and the astronauts do not come back.

Plotting meteorite falls

Museums host collections of thousands of meteorites donated by collectors over more than a century.  Although they are the source of much of our understanding about the timing and processes involved in the origin of the solar system and of the Earth itself, the collections are biased towards those that are most easily spotted on the ground.  Metallic meteorites show up much more readily than do those made of silicate minerals, which resemble ordinary terrestrial rocks in colour and density.  Only when collectors pore over very uniform, light coloured surfaces, such as ice caps, deserts and bare limestone plateaux, can they be assured of a truly representative selection of types.   Also, many meteorite samples are weathered and contaminated with earthly materials, because they have lain around on the ground for a long time.  Improved precision and detection limits of the chemical analytical tools that meteorite specialists use demand fresh material, as do researchers interested in organic materials carried from space – the embarrassment of having an announcement of a fossil bacterium in a meteorite and then finding that it is some common bug from soil is career threatening.  Most important are trying to overcome the compositional bias and to see from which part of the sky different kinds of meteorite come.  Phil Bland of Imperial College, London is trying to solve all problems at a stroke.  His idea is to set up a network of wide-angle sky cameras to record meteor trails, so that computer analysis of the film will triangulate the point of impact and also work out the precise orbit of the offending body.  The ideal place – easy to get to, safe, flat, dry unvegetated and dominated by pale rock – is the infamous Nullarbor (“No Tree”) Plain of SW Australia, which is one of the most featureless places on Earth.  Bland already has one sky camera in place that has sensors that only turn it on if the sky is clear, and an internet connection that e-mails him if something as malfunctioned.  In one year it spotted 12 trails bright enough to have resulted in meteorites falling to the surface.  With three cameras, he hopes that results will be sufficiently accurate to narrow search areas to a square kilometre.  If funded, the extended project will even incorporate e-mail alerts to teams of local collectors, whenever a trail exceeds a certain brightness.  They should then be able to pristine recover material in a few days.

Source:  Muir, H. 2004.  Catch a falling star.  New Scientist, 25 December 2004, p. 45-47.

A year on from the landings of US Mars Rovers, Science devotes much of its early December 2004 issue to findings from the more revealing of the two missions, Opportunity (multi-authored 2004. Opportunity runneth over.  Science, v. 306, p. 1697-1756).  The articles are highly detailed accounts of the main finding from the various instruments aboard Opportunity, including the evidence for the activity of acid waters on the ancient Martian surface.  Equally interesting and considerably more graphic are important findings about volcanic and glacial activity in much more recent times, that come from the European Space Agency’s Mars Express Orbiter and the High Resolution Stereo Camera carried by it (Neukum, G and 42 others 2004.  Recent and episodic volcanic and glacial activity on Mars revealed by the High Resolution Stereo Camera.  Nature, v. 432, p. 971-979). Recently, excitement about evidence for living organisms on Mars rose with the discovery of significant amounts of methane in the Martian atmosphere.  Methane is likely to have a short life span (around 300 years) in the atmospheres of rocky planets.  There are two possible sources: methane-generating bacteria or release from volcanoes.  The High Resolution Stereo Camera shows conclusively that volcanoes were active on Mars until at least 5 Ma, when previously the planet was thought to be magmatically dead.  If fumarole activity continues, that could explain the traces of methane.

Analysis of historic, global seismograph records using sophisticated software allows far more than the detection of various discontinuities in the deep mantle and core that figure in most textbooks.  Essentially, it maps parts of the mantle where P and S waves travel faster or slower than expected from the depth.  Up to now, most results have been interpreted in simple terms of cold (fast) and hot (slow) patches, which have been linked to gross tectonic features such as signs of descending slabs far below the earthquake belts associated with subduction, and possible zones of rising mantle that might (or might not) be plumes.  That leaves a lot unsaid about the mantle, for rising and falling of material is linked to density, and that can be due to temperature anomalies, and also to compositional variations involving either bulk chemistry or different assemblages of minerals in mantle rock.  A difference in seismic wave speed can be an ambiguous indicator of possible motion.  Making the connections between wave speed, temperature and composition is an order of magnitude or more computationally taxing than the tomography itself, but it has been shown to be possible, given supercomputer power and plenty of free time (Trampert, J. et al. 2004.  Probabilistic tomographic maps chemical heterogeneities throughout the lower mantle.  Science, v. 306, p. 853-856).  Trampert and colleagues from the Netherlands and the US factored in mineral physics and temperature data, and were able to calculate the probabilities of tomographic features having a thermal or compositional origin.  Their results will worry some of the earlier workers on seismic tomography who used a simplistic connection with temperature and thus slow = hot = low density and rising, while fast = cool = high density and sinking.  Some zones of low wave speed can as well be connected with high-density mantle as with hot, buoyant material.  That plays havoc with concepts of plumes rising from the core-mantle boundary, that have been all the rage since moderately well resolving tomograms appeared.  Trampert et al’r results, which superficially look just the same as other tomographic renderings of the same seismic data, include statistical evaluations of the likelihoods of wave-speed shifts being either thermal or compositional in origin.  They reveal that many of the slow zones are probably chemical and mineralogical heterogeneities, especially in the deepest mantle levels.  One of the largest slow zones known rises obliquely from the core-mantle boundary around southern Africa towards the surface in NE Africa.  It was leapt on as a reputed superplume, perhaps connected to the last outpouring of flood basalts in Ethiopia and the Yemen around 30 Ma ago, and still active beneath the Afar Depression.  Chances are, from the new work, that it is denser than average and not especially hot.  Mantle geochemists will probably be gleeful at the new look at deep mantle, because they have long been wrangling ideas about gross lateral variations in the source chemistry of basaltic magmas.  Some enthusiastic geotectonic speculators might remain very silent, in the hope that the Dutch-US team’s work is not duplicated, and fades away…

See also:  van der Hilst, R.D. 2004.  Changing views on Earth’s deep mantle.  Science, v. 306, p. 817-818

Bedout end-Permian “impact” hammered

The claim that a large circular feature beneath the sea bed between Australia and New Guinea is linked to the end-Permian mass extinction (Becker, L. et al. 2004. Bedout: A possible end-Permian impact crater offshore of northwestern Australia.  Science Express 14 May 2004 – www.sciencexpress.org)  (See Crater linked to end-Permian extinction, June 2004 EPN) has met with a flurry of sceptical comment in letters to the editor of Science(2004, v. 306, p. 609-613).  Becker and colleagues have published several articles on the P-Tr boundary, including data on noble gases from the boundary in China, which are alleged to be consistent with an extraterrestrial influence, a meteorite from Antarctica which they consider to be a fragment of the impacting body and this year the claim for shocked minerals and impact glass in sedimentary core over the Bedout structure.  There have been unsuccessful attempts to duplicate the results on the noble gas analyses, the Antarctic meteorite is regarded as being insufficiently altered to be as old as 250 Ma, and as regards the Bedout material, the authors of the letters to Science consider none of the evidence to stand up to proper scrutiny.  One letter from specialists in the US, Russia, South Africa, Austria and the UK (Renne. P.R. and 7 others 2004.  Is Bedout an impact crater?  Take 2.  Science, v. 306, p. 610-611) also claims that the 250 Ma argon-isotope age for Bedout samples is misconceived and without objective basis.  One of the authors, Jay Melosh of the University of Arizona, is reported to have said that the Becker group, “..have deeply muddied the waters about what is going on at the Permian/Triassic boundary”.  These and material in the other letters are tough words indeed.  Becker’s group is funded by NASA, and when the flurry of letters hit home earlier in October, NASA sent a team of three scientists, including Becker, to resample the Chinese P-Tr boundary section.  Ten geochemistry laboratories will receive splits of the material to settle the issue of noble-gas evidence for an end-Permian impact.  But it looks very much as if a major scandal may break when the multi-lab analyses are published next year.  That is not to imply that there are no other skeletons lurking in cupboards along with impact-related materials.  A few years ago, editors of a major journal were asked to withdraw or refute a paper that used analyses of impact-related materials that had found there way to several laboratories without the permission of their originators or their names being mentioned.  The kudos associated with publishing on extraterrestrial influences on biological extinction patterns seems hard to resist…..

See also:  Dalton, R 2004.  Comet impact theory faces repeat analysis.  Nature, v. 431, p. 1027.

So, you are a geoscientist and you are interested in Mars.  Excellent!  Now read pages 793 to 845 of the 6 August 2004 issue of Science v. 305.  There is much to learn from 11 papers about the less revealing of the two Mars Exploration Rovers, Spirit.  Rover Opportunity has been getting the headlines, with its discoveries that relate to the influence of surface and subsurface water on superficial Martian minerals, such as the now well-publicised “blueberries” made of hematite, and the presence of sulphates.  A more informative digest of the mineralogy of Mars appears in the same issues’ News Focus (Kerr, R.A.  2004, Rainbow of Martian minerals paints picture of degradation.  Science, v. 305, p. 770-771).  Kerr makes clear that the really revolutionising instrument is orbiting Mars; the Visible and Infrared Mineralogical Mapping Spectrometer or OMEGA.  That is part of the payload of the ESA Mars Express, and measures radiant energy from the Martian surface with such spectral and spatial resolution, that the results can be compared with standard spectra of terrestrial minerals to see what the Martian surface is made of.  Hopefully, OMEGA will produce a hyperspectral database for the entire planet.  The on-surface readings from the various instruments on the NASA Rovers play much the same role as a field geologist would, by providing “ground truth” to validate the broader scope of the OMEGA instrument.  The hematite that dominates the overall red colour of Mars, has been confirmed by the Rovers, but to nobody’s great surprise.  The exciting find is just how much is owed to sulphate minerals, such as orange iron potassium sulphate, or jarosite.  The sulphate-rich veneer could well point to the influence of sulphuric acid, let alone water in Mars’ early surface environment, probably emitted as sulphur dioxide during intense volcanic activity.  Interestingly, the incompatibility of highly acid surface water with the preservation of carbonates could have thwarted drawdown of CO₂ from the Martian atmosphere (Fairén, A.G. et al. 2004.  Inhibition of carbonate synthesis in acidic oceasn on early Mars. Nature, v. 431, p. 423-426).  Formation and preservation of soil carbonate minerals would have collapsed the “greenhouse” warming mechanism demanded by the now proven influence of flowing water early in Martian history.  So long as sulphurous volcanic emissions overwhelmed carbonate formation, Mars might have stayed wet and warm.  The key is the duration of massive volcanism, which could be tied down by seeing how lavas have been affected by impacts in the minute detail possible from another Mars Express imaging instrument, the High Resolution Stereo Camera.  Planetary volcanic specialists reckon massive volcanism lasted for a considerable time

The 1800 Ma old Sudbury complex in eastern Canada is one of the largest repositories of nickel ores and contains commercial platinum deposits.  It has also been ascribed to a major impact that produced a crater over 200 km across.  The evidence is the common presence of shocked minerals and a sheet of very homogeneous, once molten rock, whose andesitic major-element composition suggests that it represents melting of the local upper crust.  However, the trace elements, including platinum group metals, have all the hallmarks of the lower crust (Mungall, J.E. et al. 2004.  Geochemical evidence from the Sudbury structure for crustal redistribution by large bolide impacts.  Nature, v. 429, p. 546-548.).  The melt sheet is mixed with upper crustal rocks, including sediments that formed in a shallow marine basin into which the meteorite plunged.  This suggests that impact not only affected the whole crust, but excavated it as well, so that a 30 km deep crater formed at the instant of collision.  The bulk of the homogenised crustal melt remained molten for long enough for complex fractional crystallisation to take place, thereby forming the classic layered Sudbury Igneous complex, in which the nickel ore bodies are located.  They may well represent relics of the impactor itself, that mixed with molten crust.

In mid May news spread fast that a nearly circular feature that shows up in gravity data over the north-western continental margin of Australia could be a crater, about 220 km across, which formed at the end of the Permian (Becker, L. et al. 2004. Bedout: A possible end-Permian impact crater offshore of northwestern Australia.  Science Express 14 May 2004 – www.sciencexpress.org). Australian and US scientists have examined drill cuttings from exploratory oil wells that penetrate to the level of the hidden feature.  They describe breccias and associated melt rock. A plagioclase separate from the exploration well has an Ar/Ar age of 250.1 ± 4.5 Ma, that is within error of the age (251 Ma) of the largest Phanerozoic mass extinction.  Unfortunately, they have not discovered the easily recognised signs of shock damage to minerals – distinctive banded lamellae in quartz – nor any meteoritic chemical signature.  Nevertheless, the structure is huge and looks very like the gravitational expression of the Chixculub crater off the Yucatan Peninsula of Mexico, drill core from which shows all the signs of having formed by an impact at the end of the Cretaceous.  Evidence is accumulating from the Permian-Triassic boundary sequence that some event did produce all the signs usually attributed to a major impact in a global ejecta blanket (see Permian-Triassic boundary and an impact?, December 2003 EPN).  Despite glass being included in the breccias, many experts on impact processes and products are sceptical that the Bedout structure was produced by an impact.  But probably the only way in which such melts might have formed is by some kind of seismic shock, although that could have occurred during volcanism..  The structure is so huge that if it does have an origin by internal processes it ranks among the biggest to be found – could this ironically be a product of a Verneshot event (see Mass extinctions and internal catastrophes, above)?!

Two lines of evidence from the current robotic explorations of Mars add to less tenuous ones that the planet is really wet – icy to be precise.  One is mineralogical.  Spectroscopy of the surface being slowly trundled across by a NASA rover, shows abundant signs of the hydrated, iron-potassium sulphate jarosite, which probably can only form under wet conditions.  When it was precipitated is not known with certainty, but it occurs in layered sediments that contain structures that clearly point to transport in and deposition from surface water.  The time when liquid water could exist at the surface probably goes back to the earliest events on Mars, tied to the famous canyons and more recently discovered dendritic drainage patterns.  The other evidence stems from even more remote sensing, that captures short-wavelength infrared radiation emitted by the Sun and reflected from the Martian surface.  Ices of water and carbon dioxide have distinct and unique reflected spectra, because of the different ways in which they absorb a small proportion of solar radiation.  Results from the OMEGA instrument aboard the European Space Agency’s Mars Express satellite show that the south polar region contains as much as 15% water ice mixed with solid CO₂ (Bibring, J-P et al. 2004.  Perennial water ice identified in the south polar cap of Mars.  Nature, v. 428, p. 627-630).

Inverting Robert Oppenheimer’s memory of the line in the Bhagavad Gita, “I am become Death, the destroyers of worlds”, during his Road-to-Damascus moment when the first atomic weapon was tested, may seem an odd headline for an article on geochemistry.  But geochemists sometimes do give the air of being on the verge of solving the “Big Question”.  Alex Halliday of ETH in Zurich is one of them (Halliday, A.N. 2004,  Mixing, volatile loss and compositional change during impact-driven accretion of the Earth.  Nature, v. 427, p. 505-509). It is now well accepted that Earth’s early evolution was one of repeated big impacts during planetary accretion.  It probably culminated in a collision with a Mars-sized planet that not only created the Moon from the debris splattered from both bodies, but set the Earth’s chemistry for all subsequent time; a sort of geochemists’ Year Zero.  When that happened and what ensued has all manner of connotations (see Geoscience consensus challenged in EPN for January 2004).  Halliday reviews evidence from several isotopic systems (Pb, Xe, Sr, W) that are reckoned to be appropriate “fingerprints” for the environments in which planets accreted.  His treatment takes the data as a whole, rather than separated into one or another isotopic system. He begins with the assumption in most accretion models that metallic cores form continuously and in equilibrium with the silicate outer mantle of rocky planets.  That is important in using W isotopes to model the “when”, since tungsten is likely to enter iron-rich metal rather than silicates (see Mantle and core do not mix in EPN February 2004).  In fact estimates for the time taken for the Earth to gather 2/3 of its mass based on W isotopes (~11 Ma) are a lot faster than those based on other isotopes (between 15 to 40Ma).  Halliday’s explanation is the seemingly sound one that when big things form from smaller ones (whatever contributed to core and mantle), the chances of them mixing and reaching equilibrium, before they definitively separate into the inner and outer Earth, are not good.  Reviewing the somewhat bewildering permissiveness of isotopic data from Earth and Moon that bear on “Year Zero” he concludes that the massive loss of xenon (and other “volatile” elements) that characterises Earth, by comparison with what is known about the Solar System’s pre-planetary composition, was 50 to 80 Ma after the “start of the Solar System”.  The Moon has provided insufficient data for its age of formation to be tied down isotopically.  Although its Hf-W age might be >44 Ma relative to the Earth’s beginning, there again, perhaps >54 Ma, and it may have formed even later.  Eventually we reach modelling (read “speculation”?) that takes us to the putative composition of the culprit for Year Zero, “Theia” (a Titan and the product of incestuous liaison between Uranus and his mother Gaia).

What seems odd to me is that some of the parent isotopes for those used in fingerprinting (e.g. ¹⁸²Hf for ¹⁸²W, and plutonium for a Xe isotope) can only form in supernovae events, and are so short-lived that the balance between their formation and their influence on partitioning of their daughters in planets is pretty delicate in terms of timing.  Indeed all radioactive isotopes, and every element with greater atomic mass than iron, in the Solar System have this origin, because it is impossible for a star the size of the Sun to form them.  Massive stars that become supernovas are common enough, and when they “go off” and what blend of heavy elements they produce depend on how big they were and when they formed.  Interstellar material is surely a mix of debris from a number of such events of different ages, and new stars and planetary systems form from that.  Maybe they are triggered by nearby supernovas, but that also contributes to the isotopic mix that has evolved since a galaxy formed.  Just suppose that the mix for the Solar System was heterogeneous, with differently aged uranium, thorium, rubidium, hafnium and other elements heavier than can be formed inside small stars like the Sun, and must have formed in big ones that eventually blasted their products into interstellar space.  If the Earth accreted as an open, non-equilibrated system, then what of the Solar System itself?  Bit early to say, really….

When an embattled US president, who as a Texan never visited the Johnson Space Flight Center in Houston, unveils plans for staffed missions to set up a lunar base and land on Mars, 10 years at the earliest after he becomes an ex-president, anyone become suspicious of an election stunt.  Former Democratic Vice-president Gore made the following observation that seems to stand above the tedium of US politics, “[It is]… an unimaginative and retread effort to make a tiny portion of the moon habitable for a handful of people”.  Much the same could be said of a Martian mission, when billions of Earthbound people find their homelands barely habitable.  The word “hubris” (insolent pride) springs to mind, for scientists who support such pies in the sky, as well as for politicians in an election year.  During the Apollo lunar missions the justification for sending people was that they could use their eyes, ingenuity and knowledge to collect samples.  The fact is that planetary scientists on terra firma specified the landing sites and told the astronauts what to collect, and of course all the sample analyses were made on Earth.  They did indeed revolutionise our understanding of how the Earth began its evolution and its record of bombardment by interplanetary debris.  Human hands were needed then, because robotics (servo-mechanisms, machine vision and remote control) were too primitive to collect material efficiently.  Within a month since Christmas Day 2003 three robotic laboratories and collecting systems have landed on the Red Planet.  One, a marvel of miniature sophistication (Beagle-2) seems to have died on touchdown.  The other two are NASA vehicles able to roam under close control and send back detailed close ups and make some analyses.  At the same time, imaging systems in orbit are providing more detail about Martian surface geology and landforms than exists for our home world, despite the efforts of geologists over the last two centuries.  Given 10 years or so of further robotic development, surface rock samples and cores of soils could be returned.  Look at it this way; a staffed mission has to send and return say 2 or 3 humans weighing upwards of 150 kg, along with all their requirements for a long mission, plus various weighty safety shields.  Given the same spacecraft without passengers, we are looking at more than half a ton of samples that could be returned for a fraction of the cost, if 2 or 3 humans forewent the massive privilege of standing on a not too welcoming planetary surface for a couple of days.

What issues remain to be addressed scientifically on the lunar and Martian surfaces?  For the Moon, the far side remains little known, but on which no human mission is likely to be landed, because it would be devoid of constant communication.  More samples of rock from the side that faces Earth would always be welcome, but robotics can grab them and bring them back.  For Mars the question is that of early life, but mainly to see if it did emerge in what increasingly seem likely to have been favourable albeit brief conditions, and if traces remain.  Geological matters are secondary to that, but nonetheless fascinating.  Yet, Mars is a far more complicated place than the Moon, and to properly grasp its evolution and composition, and whether it spawned and supported organisms, needs more than one mission to one site for a few days – all that a staffed mission could realise.  The Bush “vision” already threatens the single most important scientific instrument in orbit – the Hubble telescope.  The cost of developing human expeditions to both Moon and Mars would probably sterilise funds for more ambitious robotic exploration.  Indeed robots could invalidate their entire scientific justification long before the astronauts set off.  In order to check out the health risks of lengthy space missions, the so-far functionless International Space Station is to have life breathed into it, in the manner of a Frankensteinian white elephant.  The ageing and dangerous Shuttle fleet is to be kept alive, solely to service this legacy of Ronald Reagan’s bizarre two terms of office.  But, let’s live in the real world.  Who would stump up the funds necessary for a proper planetary exploration programme, when there will be no-one gazing steely-eyed into the camera saying how awed they are to be on Mars, Mr President?

Evidence from the neutron detector on Mars Odyssey suggested the possible existence of subsurface water on Mars (Water on Mars, August 2002 Earth Pages News).  I reluctantly succumbed to all the hype about what is implied by that, the more so when reports came in of dendritic drainages revealed by high-resolution elevation data (Case for Martian rainfall strengthens in October 2003 issue of EPN).  In planetary exploration, including remote sensing of the Earth’s surface features, progressive improvement in resolution generally reveals novelty.  The Mars Orbiter Camera, deployed by the Mars Global Surveyor mission has a resolution from 15 down to 2 metres.  For the Earth, you can get 15 m images freely from the ASTER programme, but to match the 2 m images would be very costly.  Given a broadband or better connection you can download the lot for Mars (http://pds-imaging.jpl.nasa.gov/atlas/).  It is this resource that scientists from Brown and Boston Universities in the USA and the Kharkov National University of the Ukraine have used to reveal the latest paradigm buster from the Red Planet (Head, J.W. et al. 2003.  Recent ice ages on Mars.  Nature, v. 426, p. 797-802).

James Head and his colleagues focused on the smooth terrains, or mantles, which drape over older deposits above 30º latitude on both Martian hemispheres, especially where water had been indicated by the Mars Odyssey neutron detector.  They were looking for signs of what on Earth would be regarded as periglacial features, formed by the growth and melting of subsurface ice.  They found lots, including signs of flowing ice-bound debris, but they do not show them in the Article, which deals with the implications of their findings.  An important conclusion is that at least some of the mantle may have formed by what could be described as very dirty snow – a mixture of ice and wind blown dust.  Judging the age of the deposits directly depends on the standard stratigraphic method for all planets other than the Earth and Moon, their relationship to signs of impacts.  There are very few fresh craters in the mantle, but many that have been “blurred” by it.  Head et al. suggest that the mantle dates to at most 10 Ma.  They resort to modelling climate shifts on Mars from its orbital and rotational history. Its rotational axis undergoes the greatest obliquity shifts of any planet, from about 15 to 35º over a 124,000-year cycle (unlike Earth’s tilt, which slowly rocks through a range of only 4 degrees thanks to the stabilising tuggings of our large Moon).  At high obliquity, the polar caps probably evaporate. loading the atmosphere with water vapour, so unlike the Earth it is global warming that induces low-latitude ice accumulation.  It is this modelling that encouraged the authors to suggest an ice age between 2 million and 400 thousand years ago.

More than 20 years since the proposal that the end-Cretaceous mass extinction coincided with a major impact, confirmed by the discovery of Chicxulub, nobody has produced convincing evidence for an extraterrestrial culprit for others.  Were geologists implanted with GPS tracking devices as soon as they graduated (no doubt on the cards in new health and safety regulations planned by the Blair government in Britain), then Big Brother would see strong clusters close to a number of boundaries on the geological map of the world.  There would be many at P-T sites.  Electronic tagging would have shown personnel from several US universities (Rochester, Harvard, California) in the Transantarctic Mountains, from time to time in the last few years.  Allegedly, that near-pristine area exposes rocks at the juncture between Permian and Triassic strata over less than a metre.  It is marked by the sudden disappearance of the famous Glossopteris flora, just below a clay breccia, from which this group of scientists have previously extracted evidence for shocked quartz and extraterrestrial fullerenes (football-shaped organic molecules) that contained odd noble-gas isotopes.  Two members of the team have made other finds of fullerenes, at the P-T boundary in China and Japan, the K-T boundary and the ancient Sudbury impact in Canada, whereas other workers have not been so lucky.  In fact, the duo are also the only people to have found fullerenes in meteorites, which is key evidence linking terrestrial finds to possible impact events.  The team has hit the headlines again (Basu, A.R et al. 2003.  Chondritic meteorite fragments associated with the Permian-Triassic boundary in Antarctica.  Science, v. 302, p. 1388-1392).  At first sight their discovery of pristine fragments of forsterite-enstatite rock with probable chondrules at the boundary suggests that indeed a major impact coincided with the biggest of all Phanerozoic mass extinctions.  They even report tiny grains of metallic iron with an astonishing purity, perhaps formed by condensation from the plasma cloud associated with a really big meteorite impact.  What is really odd, however, is that sedimentary rocks a quarter of billion years old should have preserved such highly unstable minerals.  All other finds of fossil meteorite fragments have been highly altered relics, as any geologist would expect.  There is a clamour for the Antarctic samples from other laboratories, so that the results can be confirmed or refuted. 

See also: Kerr, R.A. 2003.  Has an impact done it again?  Science, v. 302, p. 1314-1316, and Oxygen depletion before P-T extinction (above)