Earth-Pages Archive News, Planetary

Since the original analyses of lunar rock samples brought back by the Apollo astronauts is has been widely accepted that they are almost totally anhydrous. Some even contain pristine metallic iron with not a trace of rust after more than 4 billion years. So, therefore, the entire Moon should be bone dry, except for possible rimes of ice preserved in deeply shadowed polar craters. This lack of water is one line of evidence used to support the Moon's origin in a stupendous collision between the early Earth and a smaller companion planet shortly after their accretion. The event may have depleted volatile elements and compounds in the incandescent vaporised rock from which the Moon is believed to have condensed. There are traces of water in glass spherules from lunar dust, but that might have come from the impactors that blasted them from craters. But at this year's Lunar and Planetary Science Conference - the fortieth since the first Apollo landing - evidence for water in lunar minerals was presented (Hand, E. 2010. Old rocks drown dry Moon theory. Nature, v. 464, p. 150-151). The water is in apatite grains that occur as crystals in lunar maria basalts, so must have come from the Moon's mantle through partial melting. Modelling suggests tens of thousand time more water in the lunar interior than believed previously, albeit still much less than in the Earth. Equally surprising is the water's isotopic composition: it has a much greater proportion of deuterium (²H) relative to hydrogen (¹H) than does water in terrestrial igneous rocks. The giant impact hypothesis suggests that the proportions should be the same in both bodies. One possibility is that a fortuitous comet delivered water to a dried-out hot moon soon after it has coalesced from and orbiting incandescent cloud. Hopefully a full publication will appear soon.

Late formation of Earth’s atmosphere

Because the Earth’s mantle is rich in volatiles which escape from magmas that reach the surface, it has long been assumed that our planet’s atmosphere was self-produced by exhalation. But it turns out that noble gases in such exhalations do not match those in the atmosphere isotopically (Holland, G. et al. 2009. Meteorite Kr in Earth’s mantle suggests a late accretionary source for the atmosphere. Science, v. 326, p. 1522-1525). Greg Holland and colleagues from the Universities of Manchester and Houston measured krypton and xenon isotopes in volcanic CO₂ emissions from New Mexico, and found that their proportions matched those in carbonaceous chondrites as does the Kr/Xe ratio. Those in the atmosphere are significantly different, resembling the values in the Sun. Comets may have delivered these gases after the original accretion of the Earth and the catastrophic formation of the Moon.

'Follow the water'

Long, long ago an anonymous Roman wrote, 'The first provision of any civilised society, after a code of law, is a reliable source of clean water'. Personally, I think the phrase 'legalised bureaucracy' in Latin was mistranslated to 'code of law'. Whichever, planetary and life scientists might well like the adage for themselves: the sentiment applies nicely to active planetary tectonics and to the origin and survival of all conceivable life forms. The Earth has plenty of water at the surface and deep in the mantle. Without the second, the main mantle mineral olivine would be too stiff for the mantle to convect. Heat would build up within until magma formed in great abundance and emerged with a dreadful growl, as it did on Venus about 750 million years ago to repave the entire planet. It simply isn't possible to think of answering the questions, 'When did plate tectonics begin and life emerge?' - let alone 'How?' - without first addressing where the Earth's water came from and when our home world become so richly endowed.

In a very practical sense, these are the most important issues in geochemistry. Francis Albarède, of the Ècole normale supérieure de Lyon, President of the European Association for Geochemistry and the first geochemist to deploy a multicollector, inductively coupled, plasma-source mass spectrometer, is a fitting person to review where the subdiscipline stands on them. (An MC-ICPMS is a tool for which many still yearn hopelessly.) His views appeared as a 'Progress' (a rare kind of Nature article) in the 29 October 2009 issue of Nature (Albarède, F. 2009. Volatile accretion history of the terrestrial planets and dynamic implications. Nature, v. 461, p. 1227-1233). The article casts doubt on the long-held views that when the Moon formed after a giant impact on the Earth, both bodies lost huge masses of volatiles, including water, and that Earth's water-rich nature stemmed from repeated bombardment by volatile-rich comets up to about 3800 Ma.

Geochemical data are now available from a comet (Hyakutaki) and it contains twice the amount of deuterium relative to hydrogen that is in terrestrial seawater. The D/H ratio of carbonaceous chondrite meteorites is more Earth-like, and these primitive objects seem a more likely water source than comets. But did cataclysmic formation of the Earth-Moon system dehydrate both bodies and drive off other volatile matter? Planets and smaller bodies formed by gravitational accretion of solids that condensed from the initially hot gas or nebula that dominated the proto-solar system. Experiments show that condensation of the elements occurs in three discrete temperature ranges, separated by ranges in which few elements condense. Above around 1300 K the most refractory elements condensed, including oxides of some elements (Ca, Fe, Mg, Si) that now make silicate minerals, including the dominant mantle mineral olivine. Between 900-1200 K the alkali metals and some of the elements (chalcophile) that readily combine with sulfur emerged in solid form. In the third step from 500-800 K the more volatile chalcophile elements, including lead, and halogens condense, leaving four (Hg, O, N, C) that can take on solid form only below about 300 K. Interestingly, the proportions of volatile elements relative to refractory ones in the Earth, Moon and Martian meteorites are very low compared with those in carbonaceous chondrites. It is likely that volatile elements only accreted to the Inner Planets in small amounts before being swept to the outer reaches by an intense solar wind as the Sun was powering up, i.e. before nebular temperatures had fallen below about 1000 K. From that stems the inescapable conclusion that none of these planets were endowed with much water in their earliest forms.

Proportions of the lead isotopes ²⁰⁶Pb and ²⁰⁴Pb from terrestrial sulfide mineral deposits define a near-perfect linear relationship with the ages of mineralisation, from which an age can be estimated for the time the element lead appeared on Earth. That age is 4400 Ma; about 110 Ma younger than the actual age of the planet, and matches apparent ages derived from I-Xe and Pu-Xe decay schemes; iodine and xenon are volatile elements. This strongly supports the idea that 500-800 K condensates arrived late, and other evidence indicates that they and water ice were delivered by carbonaceous chondrite material falling towards the Sun from far beyond the orbits of the giant planets, once the early solar wind had lessened. That is, the Earth's oceans formed very early in its history, and the mantle gained its water from them once hydrated lithosphere could founder deep into the evolving mantle by subduction. Albarède also summarises fascinating new ideas about the different course followed by Venus and Mars from essentially the same starting point. His 'Progress' is not difficult to read, and by marking the start of a new consensus in planetary evolution is of vital interest to all Earth scientists

Extraterrestrial water is also the subject of a Great Quest by NASA and other space agencies, though sadly an attempt on 9 October to prove that there is ice on the lunar surface, by hurling a US$79 million spacecraft at an obscure polar crater, produced no sensible results. Ironically, a couple of weeks later, three papers appeared in Science that document passive remote sensing evidence that the Moon contains a lot more water than long assumed (the most revealing is: Pieters, C.M. and 28 others 2009. Character and spatial distribution of OH/H2O on the surface of the Moon seen by M3 on Chandrayaan-1. Science, v. 326, p. 568-572). The Apollo samples astonished geologists when they proved to be almost completely anhydrous, any signs of minor hydration being ascribed to contamination after collection. The Moon Mineralogy Mapper (M3) aboard India's first lunar mission Chandrayaan is a hyperspectral imaging device that operates in the visible to SWIR range of EM wavelengths (0.4 - 3.0 μm). That range includes SWIR wavelengths beyond 2.4 μm where OH-, water and water ice have large absorption features that are masked in terrestrial remote sensing by the high moisture content of Earth's atmosphere. Pieters et al. attempted to model hydroxyl and water content in the lunar surface, and discovered significant amounts (a few tenths of a percent) in the polar regions. That they got results when the Moon was fully illuminated by the Sun suggests that this is not due to ice hidden from heating in shadows, but to minerals that contain molecularly bound water and hydroxyl ions. That begs the question of how the water got there. One possibility is the late arrival of volatile condensates as above, another that it is due to hydrogen (protons) from the solar wind reducing iron in silicate minerals to metallic iron and combination with the oxygen released. Expect loud hurrahs from devotees of Star Trek and NASA because one prerequisite of civilised society seems to be there on the Moon. But judging from the bureaucracies involved in space, getting the funds to use it will not be easy.

And now; salt domes on Mars

The front cover of the August 2009 issue of Geology could be mistaken for an exaggerated oblique aerial view of part of Iran’s Zagros Mountains, well known for their dissected salt domes. It is, however, a simulation of an aerial oblique using digital elevation data from the Valles Marineris area on Mars (Adams et al. 2009. Salt tectonics and collapse of Hebes Chasma, Valles Marineris, Mars. Geology, v. 37, p. 691-694). Hebes Chasma is a roughly oval, steep-sided depression the margins of which show clear signs of some kind of erosion. However, the depression has no outlet, so looks quite bizarre by terrestrial standards: and it is not the only such feature. At its core is a pericline of material that was formerly buried deeper than the flanks of the chasma, which are pretty much horizontal. Unlike the larger, nearby Valles Marineris, Hebes Chasma cannot have formed by erosion of the surface by a huge mass of flowing water, yet 100 thousand cubic kilometres of rock has simply disappeared. Explaining such a gigantic, weird feature taxes the imagination, but the authors do come up with a hypothesis. They reckon that the 10⁵ km³ of material became some kind of thin, briny slurry during an early Martian heating event, which drained downwards into a vast aquifer. For that to happen demands a thick, subsurface layer of dirty ice that melted, and an extremely porous substrate able to channel away the escaping muddy brine. How the pericline formed is not explained, except that it appears in a lab model made of sand, glass beads and ductile silicone polymer, when the silicone drained out through slots in the model’s base. There is plenty of evidence that the surroundings of the chasma collapsed spectacularly, and if the pericline formed by the rising of low-density material dominated by ductile salts (or ice) then it is a likely story. But where did the 100 thousand km³ of gloopy brine go? My guess it followed a secret passage to emerge into the far larger Valles Marineris... Even if there is a crewed mission to Mars, to land anywhere near Valles Marineris would be suicidal, it is so precipitous. So, this is yet another Martian mystery that will linger in a febrile kind of way.

Is there a giant impact basin beneath the Antarctic ice?

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...

Possible effects of mid-Ordovician bombardment

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.

So, when did the core form?

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.

Two Archaean birds with one stone

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.

Mercury in the news

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.

Complexities of the deep mantle

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.

Astronomical connection for the K-T event

That an asteroid about 10 km across hit the Earth at the time of the K-T mass extinction is now generally accepted as part of the cause for the mass killings. But what set it on its collision course? Today, there are a number of bodies that orbit the Sun outside the roughly circular paths of those that make up the Asteroid Belt between Mars and Jupiter, and which might some day whack into the Earth. Yet it seems there are none that would pack anywhere near the punch of the K-T body. These near-Earth objects appear to represent random gravitational processes that fling small objects out of the Asteroid Belt. There have, however, been periods during the Phanerozoic when destructively large asteroids have landed in clusters and others when unusually large amounts of tiny meteoritic particles have dusted the Earth. The late Eocene witnessed an unusually frequent flux both of interplanetary dust and large impactors, two creating the largest known craters of the Cenozoic: the 100 km Popigai structure in Siberia and one 85 km across on the sea floor off Chesapeake Bay, Maryland. There are also yet-to-be-dated, possible ‘sisters’ of the Mexican Chicxulub crater connected with the K-T event. It seems that from time to time larger magnitude events set asteroidal material on collision courses with the Inner Planets.

Czech and US planetary astronomers have analysed the orbital parameters and size of a group of asteroids ranging from about 2 to 40 km in diameter, whose spectral properties suggest that they are all related to the largest, called Baptistina (Bottke, W.F. et al. 2007. An asteroid breakup 160 Myr ago as the probable source of the K/T impactor. Nature, v. 449, p. 48-53). The analysis is complex, but suggests that the whole group formed from a major collision in the Asteroid Belt that smashed an asteroid around 170 km across. Estimated to have occurred 160 Ma ago, the debris flung off by the break-up slowly responded to gravitational and other forces to be delivered into Earth-crossing orbits. There is a 90% likelihood that one of the larger fragments became the Chicxulub impactor, and another produced the large, rayed crater Tycho on the Moon.

See also: Claeys, P. & Goderis, S. 2007. Lethal billiards. Nature, v. 449, p. 30-31.

Hadean diamonds

Proportionate to their total mass (< 1 mg), a great deal more technical effort and imagination have been expended on the pre-4 Ga zircons from Western Australia than on any other Earth materials. The latest find is that some of them contain even tinier grains of diamond (Menneken, M. et al. 2007. Hadean diamonds in zircon from Jack Hills, Western Australia. Nature, v. 448, p. 917-920). This stemmed from a search by German-Australian geochemists for inclusion of other minerals in a thousand zircons separated from the host conglomerate, which ranged in age from 3.1 to 4.3 Ga. About 5% contain diamonds, detected by Raman spectroscopy. Although most are of Archaean age, some date back to 4.25 Ga.
Inclusions in zircons are often taken to represent the mineralogy of the source of the magma from which the zircons crystallised. Some, such as diamond, may also reflect the pressures to which the zircons had been exposed. Assuming the crystallisation of zircons at 680°C (from a felsic magma), the diamonds suggest pressures at depths around 100 km. That more or less rules out melting from subducted continental crust, as previously believed by some. But perhaps the diamonds were not incorporated during any melting process. They could have begun as graphite inclusions that were subsequently metamorphosed by transport to mantle depths; a means of introducing graphite could have been reduction of CO2 in fluid inclusions. In fact, there are other possibilities too. The diamonds could be leftovers from Earth accretion or products of extreme high-pressure events, such as impacts during the Hadean, and they may have been recycled – like the zircons – several times. The authors admit difficulties in explaining their presence; importantly, the absence of any other high-pressure minerals included in the zircons. Although their opinion is one of ultra-high pressure metamorphism and therefore the existence of thick lithosphere capable of transfer to depth, as with other inferences from the Earth’s oldest materials, the jury may be out for a long time.

See also: Williams, I.S. 2007. Old diamonds and the upper crust. Nature, v. 448, p. 880-881.

Light element in Earth’s core likely to be silicon

The overall density of the Earth can be worked out from its mass, derived from its astronomical behaviour, and its volume. Geophysicists have a good idea what the density of the mantle is from the composition of nodules in lavas and experiments to determine the likely mineral changes as pressure increase with depth. From these two estimates it is possible to work out the density of the core, and it turns out to be significantly less that would be expected from iron alloyed with a bit of nickel; the long accepted model. So there must be a light element in the core. Several candidates have been suggested: sulfur, oxygen and silicon are abundant enough on the cosmic scale of things. So, how to test the hypotheses? It is quite a rigmarole, involving determining the relative abundances of the stable isotopes of silicon from meteorites, the Moon and terrestrial crustal rocks (Georg, R.B. et al. 2007. Silicon in the Earth’s core. Nature, v. 447, p. 1102-1106). In terms of the abundances of ³⁰Si and ²⁹Si relative to ²⁸Si, rocks from the Moon and the Earth are very different from the meteoritic materials from which it is believed that the rocky planets were made. They are also different from the meteorites reckoned to have arrived from Mars after a large impact—those materials are similar in silicon isotopes to other meteorites. Georg and colleagues conclude that the most likely explanation for the Earth-Moon anomaly is that silicon had entered the Earth’s core when it formed, and core formation had happened before Moon matter was flung off by a giant impact early in Earth history. But does that reveal that the low density of the core is due entirely to silicon? Undoubtedly not, and both sulfur and oxygen are still just as likely to be there.

The purpose of Georg et al’s work is, however, a little more sophisticated, as they suggest that any silicon in the core would have needed highly reducing conditions in the mantle from which the core was secreted. Yet the evidence from volcanic rocks of all ages is that the mantle has been much more oxidizing since 4 Ga. Consequently, the mantle somehow evolved (or flipped?) to a more oxidized state after core formation, and that ties in with evidence from isotopes of iron. The mantle contains anomalously high amounts of the heavier isotopes of iron, which can be accounted for by iron’s isotopic fractionation between the mantle and the core under increased oxidizing conditions. Yet that in turn implies that during whatever the oxidizing event or episode was, iron, and presumably other elements, were in continuous flux between core and mantle.

See also: Elliott, T. Silicon-enhanced core. Nature, v. 447, p.1060-1061

Magnetic field present for at least 3.2 billion years

Convection within the Earth’s molten outer core of iron and nickel is the source for the geomagnetic field and the periodic reversals in polarity revealed by igneous rocks and some sediments. Because heating involved in igneous intrusion and metamorphism resets the remanent magnetisation in rocks, if temperatures rise above 250^o C, the record of magnetism is patchy the further back in time attempts are made to measure it. Or, at least, that is the way it seemed to be. Iron-rich minerals that are responsible for the bulk of remanent magnetism are indeed prone to this resetting. However, the finer the grain size of a magnetized mineral, the more strongly it retains its magnetisation, only losing it once above its Curie temperature. Isolating such retentive materials from the effects of larger magnetic minerals is possible, if they occur in otherwise non-magnetic host minerals. Quartz contains iron oxides and so too do alkali feldspars. Such iron-rich traces are responsible for the occasional pink tinge of these otherwise colourless minerals. The challenge is measuring the tiny magnetic field.

South African and US geophysicists have designed an approach that is both sensitive and capable of revealing more about the Earth’s field at the time it was ‘captured’ in remanent magnetism (Tarduno, J.A. et al. 2007. Geomagnetic field strength 3.2 billion years ago recorded by single silicate crystals. Nature, v. 446, p. 657-660). They have adopted a means more familiar to anyone who has done Ar-Ar radiometric dating: step-heating the samples. At low temperatures this erases any resetting of the magnetisation by heating long after the host rock first formed. Above 400^o C up to the Curie point (580^oC for magnetite) the magnetism should be that induced by the Earth’s magnetic field at the time the tiny minerals crystallised. Using quartz and feldspar from a 3.2 Ga South African granite Tarduno and colleagues found such constancy in this temperature range that they were able to estimate the geomagnetic field strength as well as its direction in the middle Archaean. It turns out to have been similar to its value today (about 7 x 10²² A m²). So the Earth acquired its magnetism before 3.2 Ga, so more work of the same kind needs to be done to discover when it was first initiated during evolution of the Earth’s core.

The importance extends beyond the geophysical community, because it is planetary magnetism that currently protects life from charged, highly energetic particles, such as the solar wind. This damps down the random genetic mutation on which evolution has partly depended – along with environmental change – and to some extent protects established life forms from very rapid, fatal mutation rates. Yet, the astonishing emergence of life and its self-replicating nucleic acids might have depended on such bombardment, if life and evolution were to get going at all. At present, estimates of when the core had segregated a solid inner core, on which the circulatory dynamics of its outer part depend, arise from far-off information: the gas content of lunar minerals. Nitrogen and argon isotopes in some minerals from the Moon probably arrived as a result of the pre-magnetism solar wind stripping the Earth’s atmosphere into space. Dates for the affected minerals are older than 3.9 Ga, so perhaps the geomagnetic field began at that time. No doubt older rocks will be magnetically analysed in such detail, now that the procedure has been so successfully demonstrated.
See also: Dunlop, D.J. 2007. A more ancient shield. Nature, v. 446, p. 623-625.

Planetary, extraterrestrial geology, and meteoritics

Moon rocks turn out to be wetter and stranger

Late formation of Earth’s atmosphere

'Follow the water'

And now; salt domes on Mars

Is there a giant impact basin beneath the Antarctic ice?

Possible effects of mid-Ordovician bombardment

Experiments on formation of organic compounds by impacts

Moon-forming impact dated

So, when did the core form?

Mantle heat transfer by radiation

Two Archaean birds with one stone

Mercury in the news

Oh dear; water on the Moon…

Complexities of the deep mantle

Global wildfires at the K-T boundary debunked

Astonishing stratigraphy of the north pole of Mars

Other Martian oddities

How to spot impact sites that others have missed

Astronomical connection for the K-T event

Hadean diamonds

Light element in Earth’s core likely to be silicon

Magnetic field present for at least 3.2 billion years

Earth-like planet in Libra?

The tune that Earth hums

Long-term stability of the magnetic poles

Bad news for lunar base

So, farewell planet Pluto…

Accretion and core formation reviewed

Has Dune been discovered?

Mantle behaviour and the influence of minerals

Afar and the African superplume

Crustal spreading from the Tibetan Plateau

Yet another weird world

Puffing up the Moon

Zircons and early continents no longer to be sneezed at

Helium and how the Earth convects

Vanished Martian sea or not?

A dialogue concerning world-shattering events

Where do impactors come from?

Martian methane: a bit of a blow

Ejecta from the Sudbury impact

Curiously low-velocity material at the core-mantle boundary (CMB)

Plotting meteorite falls

Mars, planet of 2004

Mars in Science and Nature

Bedout end-Permian "impact" hammered

Linking seismic tomography to chemical mantle heterogeneity October 2004

Mars issue of Science

The creators of worlds

Perspective on the Moon and Mars

Recent snowfall on Mars

Case for Martian rainfall strengthens

Glaciers of Mars

Divine intervention?

Middle Devonian extinction and impactite layer

Chromium isotopes and Archaean impacts

Triggering core formation at the microscopic level

Carbon dioxide and Martian channels

Mantle avalanches and length of the day?

Britain's own impact

Bizarre impact structure beneath North Sea

Evidence builds for major impacts in Early Archaean

Very early differentiation of planetary bodies

Water on Mars

Tungsten and Archaean heavy bombardment

Early Argentines did not witness a meteorite impact

The mantle’s breath and Earth's early evolution

A basaltic meteorite, but from where?

Interstellar carbonates and "fossils" from Mars

Erosion on Mars

How the Earth works: "mega-blobs" in the mantle

Atmospheric oxygen: yet more

Ganymede's water volcanism

Loss of Martian atmosphere

And now, Martian glaciers

Pushing back the "vestige of a beginning"

Early life survived lunar cataclysm

More evidence for water on early Mars?

Earth’s core