Category: Planetary, extraterrestrial geology, and meteoritics

At present there are only two reliable means of surveying variations in the Earth’s gravitational field: at the surface using gravimeters and from space, by processing measurements the height of the ocean surface from radar measurements or by accurately measuring the variation in distance between two satellite travelling in tandem over the Earth’s surface. The last is used by the Gravity Recovery and Climate Experiment (GRACE) designed by NASA and the German Space Agency. It is the only realistic means of usefully precise gravity surveys over Antarctica. A truly multinational team (von Frese, R.R.B. et al. 2009. GRACE gravity evidence for an impact basin in Wilkes Land, Antarctica. Geochemistry,Geophysics, Geosystems, v. 10, Q02014, doi:10.1029/2008GC002149 – on-line journal) has discovered a prominent positive free-air gravity anomaly over a roughly 500-km diameter subglacial basin in Wilkes Land. A basin filled with low-density ice would normally give a negative gravitational ‘signature’, so the positive anomaly suggests either unusually dense crustal rocks beneath it, or that the mantle is unusually close to the surface; i.e. the crust is thin. The authors suggest that the central anomaly is surrounded by roughly concentric circular features, and that it is a hitherto unsuspected impact structure, three time larger than the Chicxulub structure (also mapped by gravity data off the Yucatan Peninsula of Mexico) that caused an upward bulge of the mantle. To my eye, the hypothesis only becomes convincing when concentric circles are drawn around the undoubted major anomaly, and the evidence for them is scant compared with the similarly detected structures of Mars and the Moon. What intrigues the authors is the position of the anomaly on a Permian continental reconstruction, It is at the antipode of the Siberian Traps flood basalt province, implicated strongly in the end-Permian mass extinction: the most devastating known. This harks back to speculation that the undoubted Chicxulub structure and caused the mantle to melt beneath its antipode to form the Deccan Traps…

Limestones dated at around 470 Ma in Sweden contain highly altered chondritic meteorites, ranging in mass up to 3.4 kg and up to 20 cm across, along with chromite grains and high iridium. There are so many that investigators have estimated a flux of extraterrestrial debris that was a hundred times greater than at present. The remarkable repository is matched in age by sediments rich in chromites in central China. The Darriwilian Stage (460-470 Ma) of the Ordovician is also notable for evidence of powerful downslope sediment movement in many continental margin sequences. John Parnell of Aberdeen University reviews the many megabreccias or olistostromes of this geologically short time span (Parnell, J. 2009. Global mass wasting at continental margins during Ordovician high meteorite influx. Nature Geoscience, v. 2, p. 57-61). Most seem to be associated with continental margins of the mid-Ordovician Southern Hemisphere. While some occur at what were probably seismically unstable volcanic arcs, most are associated with stable carbonate platforms. Together with the link in time to evidence for enhanced meteorite flux, this association suggests slope failure associated with large impacts. However, the megabreccias are so widespread that they are unlikely to have been formed by a single tsunami resulting from one giant impact. Indeed there is no evidence for a catastrophic event, either as a large crater or evidence for mass extinction: the mid Ordovician was a time of rising faunal diversity (see The Great Ordovician Diversification in September 2008 issue of EPN). Parnell calculates that there may have been as many as 10 Chicxulub-sized impactors per million years during the Darriwilian, but the lack of catastrophic consequences suggests that the megabreccias may have resulted from a great many smaller events, probably of bodies less than 300 m across. That would also explain the lack of global evidence traditional sought to identify impacts, such as iridium, glass spherules and shocked mineral grains. If he is correct, then other olistostromes of different ages in aseismic settings could point to extraterrestrial causes.

Experiments on formation of organic compounds by impacts

Many mechanisms have been speculatively proposed for the origin of complex organic chemicals from which life may have originated on Earth. The best known of these is the 1929 Oparin-Haldane hypothesis that life began with simple organic compounds formed from methane and ammonia in the early atmosphere, followed by more complex compounds formed in the seas through a variety of reactions. This was tested by Miller and Urey in the 1950s, using electrical discharges through a simulation of such a reducing atmosphere, but current views are that the early atmosphere was rich in CO₂ and nitrogen rather than reduced methane and ammonia. Another possibility is synthesis of organic compounds as a result of impact energy; very abundant early in Earth’s history. This idea has been tested experimentally using a propellant gun to create high-velocity impacts into a mixture of solid carbon, iron, nickel, water and nitrogen: a highly simplified scenario of ordinary chondrites bombarding atmosphere and ocean (Furukawa,Y. et al. 2009. Biomolecule formation by oceanic impacts on early Earth. Nature Geoscience, v. 2, p. 62-66). The experiments were performed under conditions that excluded possible contamination. Yet they yielded a wealth of organic molecules, including fatty acids, amines and an amino acid (glycene) found in DNA. Scaling up the experimental yields to the mass of meteoritic material accreted to the Earth during the Hadean Eon (of the order of 10 ²⁴ g), the authors estimate that at least 10¹⁷ g of organic material would have been present in the surface environment by the time life eventually emerged. Furukawa et al. rule out the delivery of ready-made organics by carbonaceous chondrites, in which a great variety has been found. As well as their decomposition by the heat of entry, the lack of metallic iron in carbonaceous chondrites would promote oxidation rather than reduction of organic compounds preformed in early evolution of the Solar System.

Moon-forming impact dated

One of the major discoveries that arose from the lunar samples returned by the Apollo astronauts was that the pale-coloured lunar highlands were made almost entirely of calcium-rich plagioclase feldspar: they are made of anorthosite. In the early 1970s Joe Smith of the University of Chicago realised that the only way vast amounts of such single-mineral igneous rocks could have formed was by massive fractional crystallisation. Low-density feldspar must have floated on top of what had been literally a magma ocean. Although Smith did not put forward the idea that a molten moon had formed through a giant collision between the Earth and a passing Mars-sized planet, it was his concept that pointed strongly in that direction. Inevitably, much of the Earth would also have been melted by such a monstrous catastrophe – material that eventually became the Moon had probably been vaporised before condensing to form our satellite.

The Apollo samples are still objects of research, especially as new analytical methods develop. One such new method is the dating of single, tiny zircons; even of their individual zones. Later impacts on the Moon formed a variety of breccias, samples of which are handy as they include fragments of many rock types in one specimen. One of these has helped zero-in on just when the magma ocean began to crystallise (Nemchin, A. et al. 2009. Timing of crystallization of the lunar magma ocean constrained by the oldest zircon. Nature Geosciences, v. 2, p. 133-136). In fact advanced mass spectrometry dated 41 tiny spots in a single half-millimetre zircon grain, revealing a spectrum of ages between <4.35 Ga and a maximum of 4.417 ± 0.006 Ga. The oldest marks the minimum age for the start of crystallisation of the molten Moon and thus for the impact that formed the Moon. For comparison, the earliest material found on Earth – also a zircon but one transported in sediment to become part of a much younger sandstone – is 4.404 Ga old. The authors suggest that the bulk of the lunar highland crust had solidified within 100 Ma of the collision

Sometime early in its history the Earth underwent two gigantic redistributions of its chemistry: a gargantuan collision that formed the Moon; separation of a metal plus sulfide core from a silicate remainder. These ‘set the scene’ for all subsequent geological (and perhaps biological) evolution. The current theory about core formation stems from a marked disparity between Hf-W and U-Pb geochronology of the mantle. The first suggests a metal-secreting event about 30 Ma after formation of the Solar System – tungsten is siderophile and would have become depleted in the mantle following segregation of a metallic core. The second points to lead partitioning into a sulfide mass descent to the core around 20-100 Ma later; assuming that lead is chalcophile. The key to explaining the disparity and validating the dual core formation hypothesis lies in establishing just how chalcophile lead is, relative to other metals that are present in the mantle (Lagos, M. et al. 2008. The Earth’s missing lead may not be in the core. Nature, v. 456, p. 89-92). The German and Russian geochemists set up experiments to determine directly the partition coefficients of lead and the other ‘volatile’ elements cadmium, zinc, selenium and tellurium between metal, sulfide and silicate melts at mantle pressures. They found that Pb and Cd are moderately chalcophile and lithophile, but never siderophile; Zn favours silicate melts, and is exclusively lithophile under mantle conditions; Se and Te are both chalcophile and siderophile, so would enter the core in both molten sulfide and metal.

The measured partition coefficients give a basis for comparing the relative proportions of the volatile elements estimated in the mantle with those predicted by the two-event model of core formation. This elegant approach strongly suggests that sulfide or iron-nickel metal segregation from the mantle to the core can explain neither the mantle abundances of the five ‘volatile’ elements nor the lead-isotope ratios in the mantle. It even questions the existence of terrestrial sulfur in the core. The postulated Moon-forming mega-impact alone could have produced the measured geochemical features of the mantle as a result of vaporisation of ‘volatile’ elements.

Mantle heat transfer by radiation

After some early speculation about efficient heat transfer in the mantle by radiation, it became generally accepted that convection and conduction dominate at depth in the Earth. Yet the Stefan-Boltzmann law has the radiant energy flux of a body increasing proportionally to the fourth power of its absolute temperature. So at deep mantle temperatures of up to 4300 K radiation ought to be significant unless mantle minerals become opaque at high pressures. Mantle mineralogy is dominated by iron-magnesium silicates that adopt the perovskite structure. High-pressure experiments with perovskites reveal surprisingly high transparency to visible and near-infrared radiation (Keppler, H. et al. 2008. Optical absorption and radiative thermal conductivity of silicate perovskite to 125 gigapascals. Science, v. 322, p. 1529-1532).  It seems that a higher than expected radiative contribution to heat transfer should stabilise large plume structures in the zone above the core-mantle boundary.

There are two major issues concerning the Archaean mantle: was the mantle hotter than it is now; was it in a reduced or oxidised state? The first has implications for Archaean plate tectonics. If loss of the higher radioactive heat produced in the mantle was accomplished by processes similar to those today, i.e. dominantly by mid-ocean volcanism the Archaean geotherm would have been similar to today’s and plate tectonics would have been similar. If this means of heat loss could not cope, then temperatures would increase more rapidly with depth, with implications for the style of plate tectonics, especially subduction. A mantle with reducing conditions would be expected to emit reduced gases, such as methane, as well as carbon dioxide, to produce a reducing atmosphere. If oxidising conditions prevailed, then CO₂ would be a dominant emission to the atmosphere. There have been arguments over these two aspects of the Archaean for decade, but now they may have been resolved (Berry, A.J. et al. 2008. Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle. Nature, v. 455, p. 960-963). One factor alone allowed the arguments to damp down: A 2.7 Ga ultramafic lava flow from Zimbabwe preserved a pristine sample of the original magma in the form of small glass blobs trapped in olivine. Measured proportions of Fe(II) and Fe(III) in a melt indicate those in its source, and hence the redox state of the source, mantle peridotite. The Zimbabwe melt inclusions are similar in this respect to those found in modern mid-ocean ridge basalts; they show a high degree of reduction. In turn that suggests that the melting that formed them was almost anhydrous, otherwise dissociation of water would have added oxygen that would have upped the content of Fe(III) in the melt. Experiments show that the degree of anhydrous partial melting of peridotite needed to form ultramafic magma is compatible only with temperatures around 1700ºC, about 400 degrees hotter than those that form modern basalt magma. Significant volumes of the late Archaean mantle, and by extension that of earlier times, had to have been a great deal hotter than it is today.

It has been more than 3 decades since the Mariner 10 mission took a close look at the surface of the innermost planet Mercury. In January 2008 NASA’s MESSENGER spacecraft flew past and the 4 July issue of Science contained a special section on the early observations (Several reports 2008. Messenger Special Section. Science, v. 321, p. 58-94). These involve images, spectral observations, laser altimetry, estimates of chemistry in Mercury’s surrounding space and measurements of the mercurial magnetic field. The data bear on surface mineralogy, geological structures, regolith formation, cratering – especially the giant Caloris Basin, and evidence for volcanism.

Oh dear; water on the Moon…

The accepted wisdom about the Moon is that it is and always has been supremely dry. That notion stems from analyses of every single solid rock brought back by the Apollo astronauts, and the probability that the Moon formed from incandescent vapour blasted into orbit by a giant collision between the original Earth and an errant planet as big as Mars. Water and indeed most volatile elements and compounds ought to have been driven off the orbiting gas and debris that coalesced to form the Moon around 4.5 Ga ago. Most people believe that more or less everything the astronauts dragged back to Houston has been analysed: not so. There are millions of glass beads that constitute a sizeable fraction of the lunar regolith. Some of these turn out to be volatile rich, and may have been blown out by early lunar volcanism (Saal, A.E. et al. 2008. Volatile content of lunar volcanic glasses and the presence of water in the Moon’s interior. Nature, v. 454, p. 192-195). If the glasses are volcanic in origin, that implies there is water in the Moon’s mantle. So, you might ask, how come the Moon is not a vibrant place rather than being as dead as a doorknob? The Earth is so interesting partly because it is a wet planet. The Moon has very little in the way of heat production, so even if its mantle contained hydrous phases, it cannot reach basalt solidus temperatures unless energy is delivered mightily by impacts. That did happen around 4 Ga, when the lunar maria formed and became floored by gigantic floods of basalt. Yet those basalts are extremely dry, thereby posing a bit of a question for Saal and his colleagues.

See also: Chaussidon, M. 2008. The early Moon was rich in water. Nature, v. 454, p. 170-172.

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.