Category: Planetary, extraterrestrial geology, and meteoritics

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.