Category: Planetary, extraterrestrial geology, and meteoritics

The vast majority of meteorites represent bodies in the Solar System that never became parts of planets; they are fragments of planetesimals.  Of the 20,000 collected meteorites, only about 50 have been suggested from their geochemistry to hail from existing planetary bodies.  They travelled to Earth as fragments that violent impacts on these bodies ejected from their surfaces.  Since most meteorites have been recovered either from glacial ice or the surface of deserts, such suspected planetary fragments arrived recently in geological time, but had probably been travelling for immense periods of time since an impact dislodged them.  Oddly, there are few if any meteorites with Earthly compositions, and only the Moon and Mars seem to be represented in collections.  Suspected planetary meteorites have basaltic compositions, but so too do some likely to have originated from planetesimals.  One of the keys to sorting them is analysis of their oxygen isotopes, as well as conventional element analyses and noble-gas composition.  It was the resemblance of noble gases in the notorious Antarctic meteorite ALH84001, and others like it, to the very imprecise measurements made by the Viking lander in the 1970s that encouraged the view that it was from Mars.  Their odd oxygen-isotope composition has also been said to indicate a Martian origin, mainly because they don’t fit with other specimens most likely to have originated from planetesimals.

In these uncertain times for manned and unmanned space missions, basaltic meteorites are probably as close as planetary scientists will ever get to the objects of their longing, perhaps for several generations. It is hardly surprising that collectors seize on petrogenetically evolved meteorites with glee.  Such a desirable chunk from a desert surface in NW Africa has been analysed comprehensively by scientists from Japan and the USA (Yamaguchi, A. et al. 2002. A new source of basaltic meteorites inferred from Northwest Africa 011.  Science, v.  296, p. 334-336).  Its chemistry fits with no planetesimal or suspected planetary meteorite class, although for the most part it does resemble the eucrites, considered to originate from the large asteroid Vesta.  Rare-earth elements, siderophile metals and oxygen isotopes put it in a class of its own.  Although the authors are content to conclude that it probably evidences a range of planetesimals that underwent differentiation to produce basaltic magmas, some have been tempted to speculate on a planetary origin, perhaps on Mercury (Palme, H. 2002.  A new Solar System basalt.  Science, v, 296, p. 271-273).  I am left wondering why the supposed Martian meteorite class, with all the kudos that such a suggested origin brings, has not been tempered by the likelihood of origin in a large planetesimal; but I am no specialist.

Part of the argument used to support the notion that life may have arisen on Mars early in its history depends on the presence of carbonates in the notorious meteorite ALH84001 found in Antarctica.  Supposedly having been ejected by an impact on the Martian surface (based on its oxygen isotope composition and the blend of noble gases trapped within it), ALH84001 also contains the minute structures that were prematurely announced in a blaze of publicity as fossilized alien life forms by US and British meteorite specialists in 1996.  The discoverers claimed that carbonate minerals within it clearly evidenced the rotting of silicates by liquid water containing dissolved CO₂; so they do in terrestrial rocks. However, carbonates also occur in meteorites that by no shred of the imagination can have formed within sizeable planets.  Many probably accreted in a near vacuum from dusts that occur in clouds within our galaxy, while the solar system was forming.

Using infrared spectra to assess the mineral composition of dust clouds surrounding stars, a team of European and American cosmochemists has found that in two cases such dust contains calcite and perhaps dolomite (Kemper, F. et al. 2002.  Detection of carbonates in dust shells around evolved stars.  Nature, v. 415, p. 295-297).  Because liquid water cannot exist in a near vacuum, production of these carbonates cannot have taken place by the familiar silicate-rotting process.  More likely, they formed on the surfaces of silicate dust or ice grains by reactions between calcium and magnesium ions and those in which carbon and oxygen were combined.

The origin of life on Earth would have been greatly accelerated had some of the compounds used in constructing complex bio-molecules simply rained onto the young planet from outer space.  Carbonaceous chondrite meteorites are known to contain a tremendous blend of many possible precursors, ranging from amino acids to the ampiphile molecules, whose curling-up in the presence of liquid water is seen by many cosmo-biologists as a route to the formation of cell walls.  The latest addition to possible ingredients are sugars and related compounds in the two most important such meteorites, Murchison and Murray (Cooper et al. 2001.  Carbonaceous meteorites as a source for sugar-related compounds for the early Earth.  Nature, v. 414, p. 879-883).  Detection of simpler carbon-based molecules in the spectra of interstellar molecular clouds, from which the Solar System probably accreted, suggests that a complex chain of photochemical reactions followed by thermochemistry as the pre-solar nebula became denser was the route to seeding the vicinity of the Earth with biological potential.  However, the next steps ending in chemical self-replication and its RNA/DNA control remain a great deal more mysterious than detection of suitable reagents.  For one thing, all life-molecules rotate polarized light in only one direction (anti-clockwise), whereas those of abiogenic origin, such as the compounds found in meteorites, rotate it both ways in roughly equal proportions.

See also:  Sephton, M.A. 2001.  Life’s sweet beginnngs.  Nature, v. 414, p. 857-858.

Although the press made a great fuss in 1999 about the supposed discovery of signs of life in a meteorite reckoned to have been blasted off Mars by a giant impact, meteorites in general are the only direct means of developing ideas about how the Earth and the rest of the planets formed.  The market in meteorites is beginning to resemble the London Metal Exchange in its frenzied bullishness, but being collectibles it is rare types that command the highest prices, rather than their significance.   An excellent review of current ideas among meteorite specialists appeared in the 6 July issue of Science (Alexander, C.M.O’D., Boss, A.P. and Carlson, R.W. 2001.  The early evolution of the Solar Syetem: a meteoritic perspective.  Science, v. 293, p. 64-68).

Since development of theoretical ideas about the generation of the elements in stellar processes, it has become almost a cliché to ponder about the ultimate dependence of every aspect of the natural world on supernovae and their “seeding” of the galaxy with the chemical mix that is so familiar.  Even the nuclear processes involved are easily grasped.  Not so the means whereby star stuff assembled into planetary systems and laid the potential for life, plate tectonics and virtually everything else.  Alexander and colleagues from the Carnegie Institute of Washington span the interactions between physical conditions around young and rapidly evolving stars, derived theoretically, and the kinds of compounds that they can generate.  Meteorite chemistry and mineralogy, which are very diverse, put flesh on the bones of these ideas.  The tangible properties of different meteorite classes, together with their radiometric ages, are analogous to fossils in piecing together both planetary evolution and the various kinds of environments in the early Solar System.

One conclusion in the review that surprised me concerns the oldest materials known to us – calcium-aluminium-rich inclusions found in some chondrites, such as the famous Allende meteorite that fell in Mexico.  The pale inclusions contain evidence for the former presence of short-lived isotopes, such as ²⁶Al.  So short are their half-lives that the delay between their nucleosynthesis and the assembly of the pale inclusions can have been a few hundred thousand years at most.  There are two possibilities: either such isotopes were generated by energetic particles emitted by the growing early Sun, or they had their source in supernova events.  Theoretical work on local genesis has so far failed to match the relative abundance of all such short-lived isotopes, derived from the amounts of their decay products found in pale inclusions.  It seems highly likely that collapse of a pre-solar cloud of matter to form the nebula out of which Sun, planets and the parent bodies of meteorites emerged was set in motion by shock waves from a nearby supernova.  They would have taken the form of a high-speed interstellar “wind” of gas.  Observed differences in oxygen-isotope proportions in meteorites were once ascribed to heterogeneous mixing of this explosive introduction of exotic matter.  However, the oxygen heterogeneities do not show up in the isotopes of other elements.  That mismatch has led to ideas of chemical fractionation during Solar System evolution, akin to that so familiar from the different behaviours of “light” and “heavy” oxygen during evaporation of water and its uptake in skeletons of living things exposed to different climates.  Differences in oxygen isotopes now form a strand in assigning different meteorites to sources at different distances from the evolving Sun, and in deducing that some rare meteorites did indeed come from Mars.

Clearly behind the hype surrounding promotion of staffed and unstaffed missions to Mars and the increasingly shady world of the meteorite trade, exciting research is being done.

Ice and prebiotic chemistry

The problem with ice on Earth is that it will not support living chemistry.  The process of crystallization excludes impurities from its structure, so that reactions between organic compounds cannot go on.  Comets are mainly ice, and frozen water is a common occurrence in the infrared spectra of interstellar clouds, along with a host of complex CHON compounds (over 100 discovered to date).  How organic molecules form in cold molecular clouds is a difficult problem, or at least it was believed to be until recently. 

Researchers at the NASA Ames Research Center in California have probed the structure of solid water under all manner of physical conditions.  Below a temperature of 200 K (about that of liquid nitrogen)  the hexagonal symmetry of ice, familiar from snowflakes, changes to the simpler cubic form.  At yet cooler temperatures 10 to 125 K), ice has no crystalline structure.  Like flint, it is cryptocrystalline or amorphous.  Curiously, even only a few degrees above absolute zero it can flow like a viscous medium, in the manner of glass, when irradiated with ultraviolet radiation.  The breaking and reforming of hydrogen bonds, as in liquid water, but slower, creates the conditions for retaining impurities and their chemical combination.  This odd behaviour at precisely the temperatures of molecular clouds explains their richness in organic molecules.  Quite probably comets form by accretion of such interstellar icy material.  The experiments revealed that warming of amorphous ice above 125 K does not result in a complete transition to cubic ice, that would exclude impurities.  Instead, around two thirds retains its odd properties.  The discovery strongly hints that much of the basic work of producing precursors to life’s chemistry is not only feasible in interstellar space, but that they can be delivered to planets as they collide with comets giving a kick start to the origin of life.

Source:  Blake, D.F. and Jenniskens, P.  2001.  The ice of life.  Scientific American, August 2001, p. 36-41.

Mars is the only planet in the Solar System that has landscapes that bear any resemblance to those we see on Earth.  The one factor common to both planets is that surfaces have been shaped by flowing water.  On Mars, that was a one-off event early in its history, and thereafter shaping the planet has been through continual movement of dust in its thin, but energetic atmosphere, the formation of impact craters and volcanism.  Evidence for fluvial processes occurs in the highland regions, which were built mainly by volcanic activity., and stems from careful examination of high-resolution photography from orbiting probes.  Whether the various kinds of valleys formed by catastrophic, short-lived floods of melt water released by impacts into deep frozen ground, through steady release of groundwater or actually by precipitation  are the ground for speculation and controversy.  A means of assessing the possibilities is using accurate data on topographic elevation.  Digital elevation models for the Earth, even at coarse resolution (GTOPO30 data at 1 km resolution), map out the intricacy of surface drainage of the continents.  A DEM produced by the laser altimeter aboard Mars Orbiter allows not only the various models to be assessed, but enables quantitative work on the amount and rate of water erosion and deposition of sediment when combined with evidence for the age and duration of Mars’ fluvial event (Hynek, B.M. and Phillips, R.J.  2001.  Evidence for extensive denudation of the Martian highlands.  Geology, v. 29, p. 407-410).

Hynek and Phillips show that the event was long lived, lasting 350 to 500 Ma around 4 billion years ago.  Their study was of an area the size of Europe.  Scaled up, their findings suggest that of the order of 5 million cubic kilometres of sediment was transported, equivalent to deposition of a 120 metre thick sediment layer in the flat plains of Mars’ northern hemisphere.  The average rate of erosion during the event compares closely with that typical of temperate maritime areas of mountains on Earth.  It is difficult to see how such prolonged erosion could have taken place without runoff fed by precipitation on the surface, and that implies a much warmer climate and thicker atmosphere than on modern Mars, albeit only for a very early episode in its evolution.

Seismic waves generated by large earthquakes arrive at different times at seismographs arranged in a world-wide network.  When they arrive depends on the relative positions of epicentres and receivers, but most importantly on variations in physical properties within the Earth that affect the speed at which they travel.  Given enough high-quality seismic records and powerful computing, such data allow geophysicists to map how wave speeds change with depth in the mantle and produce 3-D models.  In other words, seismic energy can produce geophysical homologues of medical CAT scans.  The second important means of visualizing the unseeable comes from the geochemistry of basaltic lavas formed by partial melting of the mantle in different tectonic settings.  Results from such studies reveal that the composition of the mantle is not homogeneous.  Combining information from both sources, in the light of motions of the lithosphere, provides a powerful input to modelling how the Earth behaves as a whole  (see Earth Pages, July 2000, Geodynamics).

Seismic tomography’s most important derivative stems from the manner in which wave speed depends on variations in the mechanical properties of the mantle.  For P-waves, speed varies with the mantle’s differing resistance to compression, and S-wave speed is directly proportional to the rigidity of the mantle.  Unusually high mantle temperatures cause decreases in compression resistance and rigidity, and therefore drops in the speeds of both kinds of body wave.  The cooler the temperature, the higher both speeds.  So, velocity variations in seismic tomographs are proxies for changing mantle temperature, and in turn for regions of different density – the hotter a material is, the lower is its density.  The implications are quite simple; high-speed anomalies signify cool, potentially sinking regions in the mantle, whereas low speeds suggest that matter is able to rise.  In practice, modelling the fundamental dynamics of the Earth’s mantle using seismic tomography is computationally difficult, often ambiguous and blurred because of the lack of suitable data.

Seismic tomography gave the first clues to the idea that subducted slabs penetrate all the way down to the core mantle boundary, and that at least some of the plumes suspected to underpin hot spots have their source at such depths.  Together, these findings support whole-mantle convection.  As well as improving the amount of high-quality seismic data and the software to analyse them, combining physical parameters with sketchy knowledge of variations in mantle chemistry and mineralogy is the next step in “sharpening” the focus of mantle models.  That seems to have been taken by Alessandro Forte and Jerry Mitrovica of the Universities of Western Ontario and Toronto (Forte, A.M. and Mitrovica, J.X. 2001.  Deep-mantle high-viscosity flow and thermochemical structure inferred from seismic and geodynamic data.  Nature, v. 410, p. 1049-1056).  Their work confirms the concept of whole-mantle convection resulting from thermal anomalies, but has an added bite.  They show evidence for vary large variations in deep-mantle composition – to megaplumes they have added “mega-blobs”.  Although the results of their analyses are limited by data availability and reliability, and by simplifying assumptions, they imply that such blobs can respond to temperature changes by rising and sinking periodically.  That is, the mantle may move as vast domes and downwellings as well as in the more tightly constrained plumes and sinking slabs.  One intriguing possibility is that such blobs may be primitive and retain high concentrations of elements that evolution of other parts of the mantle has transferred to the continental crust.  Such primitive signatures are passed on to the geochemistry of basalts forming from plumes beneath ocean islands.  However, there is a long way to go before a blob-plume-ocean island connection can be made.  If it proves to be plausible, then such ancient blobs would have to be very viscous to have resisted mixing over time with more evolved mantle.  Another possibility is that the blobs are themselves highly evolved, through the progressive accumulation of subducted slab material.

(See also:  Manga, M.  2001.  Shaken, not stirred.  Nature, v. 410, p. 1041-1042)

Atmospheric oxygen: yet more

Following last month’s Earth Pages briefing (Mantle overturn and oxygenation of the atmosphere)  Nature (19 April 2001) ran a news feature on the competing theories for when oxygen began to accumulate in Earth’s atmosphere (Copley, J.  2001.  The story of O.  Nature, v. 410, p. 862-864).  The paradox between evidence for oxygen production by photosynthetic cyanobacteria since 3.5 Ga and that supporting the first major influence of oxygen in redbeds at 2.2 Ga may be resolved by the ideas of Hiroshi Ohmoto of Pennsylvania State University.

Redbeds – terrestrial sediments containing abundant ferric hydroxides – form when iron enters its Fe-3 state, and are insoluble.  That results in weathering processes being unable to leach soils of their iron content, unless the waters involved have been rendered reducing by bacterial activity.  The most dramatic expression of this is laterite that blankets ancient erosion surfaces of most of the Gondwanan continents, much of which formed in Palaeocene times.  Palaeosols older than 2.2 Ga do not show the characteristic laterite ferricrete cap, implying that iron existed consistently in its soluble Fe-2 form and could be leached away.  Most geochemists regard that as evidence for a reducing atmosphere, lacking oxygen except as a trace.  Ohmoto suggests that organic acids formed by terrestrial cyanobacteria might also create the reducing conditions necessary for iron leaching..  He sees such “blue-greens” as having had a dual role, fixing iron in soils through oxidation and then releasing it to solution by formation of organic acids.  Ohmoto and Antonio Lasaga are developing a geochemical model for the iron, oxygen, carbon and sulphur cycles during the Archaean.  Early runs suggest that only 30 Ma after the appearance of cyanobacteria at 3.5 Ga their release of oxygen would have built up high levels in the early atmosphere.

That bucks the evidence for low oxygen provided by detrital sulphides and uranium oxide grains in Archaean high-energy sediments, such as the conglomerates of the Witwatersrand basin in South Africa – in the presence of oxygen, both should break down quickly in water.  Archaean banded iron formations, thought to form by reaction between Fe-2 ions in ocean water and oxygen produced locally by shallow-water cyanobacteria, have a dual significance – abundant oceanic Fe-2 suggests global lack of oxygen, and BIF deposition of ferric oxide would have formed a sink for any oxygen in the environment.  Ohmoto cites the re-appearance of BIFs at several times in the Proterozoic Eon as a sign that BIF formation was possible when atmospheric oxygen was abundant.

The debate seems destined to run, for two reasons.  Studies of sulphur isotopes – Ohmoto’s speciality – give evidence for fractionation through the influence of ultraviolet radiation.  Once oxygen rose in the air, its formation of ozone gas would have blocked UV and ended this kind of selective take-up of sulphur isotopes.  James Farquhar of the University of California in San Diego has found its effects common in Archaean rocks, but no sign in later rocks.  That favours an oxygen-poor early atmosphere.  Ohmoto counters with abundant evidence in the Archaean for the activity of bacteria that reduce sulphate ions to sulphide – in an oxygen-poor world, sulphate formation would have been suppressed.

Oxygen build-up demands complementary burial of organic matter formed by photosynthesis before it oxidized.  The influence of organic carbon burial  is to take with it ¹²C that biological processes favour over heavier ¹³C, so that carbon-rich rocks show higher ¹²C than carbonates precipitated from the seawater that was left.  Such enrichment in ¹²C shows up most clearly after 2.7 Ga ago, when carbon burial must have been stoked up somehow.  That points to a late build-up of oxygen in the air.  But why?  James Kasting, also of Pennsylvania State University, suggests a change in the Earth’s mantle from reducing to oxidizing conditions.  Before that time volcanic gases would have been dominated by reduced gases that could mop up any free oxygen.  Afterwards, oxidized volcanic gases could have co-existed with free oxygen.

Jupiter’s giant moon Ganymede is an icy world, as are many satellites of the Outer Planets.  But is also one of the few showing signs of some kind of tectonics.  Its surface is made up of dark, cratered material, presumably an ancient mixture of rocky debris and ice, riven by swaths of lighter surface.  The latter, which covers two-thirds, has little cratering and is a later feature of the moon’s surface.  Somehow, Ganymede underwent a resurfacing, perhaps in a similar manner to neighbouring Europa – a simple ice ball – but not so all-consuming.

The event probably stemmed from the coming together of Jupiter’s largest moons into orbital resonance that generated sufficient gravitational energy to cause internal melting.  Precisely how this achieved the intricacies of Ganymede’s surface is something of a mystery.

Images from Voyager and Galileo missions form stereoscopic pairs from which the moon’s topography can be derived with useful precision (Schenk, P.M. et al.  2001.  Flooding of Ganymede’s’ bright terrains by low-viscosity water-ice lavas.  Nature, v. 410, p.57-60).  Using digital elevation data with high-resolution Galileo images, Schenk et al. have been able to subdivide the light swaths into three kinds of surface, reticulate, grooved and smooth at different elevations from highest to lowest.  Large elevation differences of the order of 2 km are involved.  That in itself is evidence that ice at the prevailing temperature behaves more like rock than glacial ice.

The greater surprise is that the lowest, smooth unit shows evidence of having formed by processes akin to volcanism, with calderas and features that engulf earlier structures.  However, even the fine resolution of the latest images does not reveal “lava” flows.  Some rifting mechanism seems to have encouraged emergence of water-ice “magma” to form the low smooth terrains.  All very counter-intuitive for terrestrial volcanologists, because water “magma” must be more dense than the solidified flows forming from it, unlike silicate liquids or those rich in sulphur on Io.  That makes the formation of high volcanoes impossible.

Presumably, the much higher grooved and reticulate terrains started in the same manner, as linear troughs, then to be deformed and thickened by “water tectonics”.

Mars seem quite massive enough to have held a substantial atmosphere, as have Earth and Venus.  That it has barely any is a major puzzle.  One possible reason is that Mars has a tiny magnetic field.  A strong magnetic field on Earth serves to deflect the solar wind, a stream of charged particles emitted by the outer part of the Sun.  Undeflected in this way, the solar wind would gradually strip off an atmosphere.  Currently, Mars has so little atmosphere that photosynthetic life that combines water and carbon dioxide to build carbohydrate is impossible, despite the fact that most of what little air there is  comprises CO₂.

In the great chattering about prospects for Martian life at some time in the planet’s past, a central issue is the timing of atmospheric loss.  It is inconceivable that Mars never had an atmosphere, because it possesses the largest volcanoes in the Solar System which must have vented mantle gases.  If its magnetic field slowly dwindled, that gives ample time for life to have emerged.

Unsurprisingly, one of the tasks of NASA’s Mars Global Surveyor Mission has, for the last two years, been a global survey of the Martian ionosphere.   That is a proxy for regional variation in magnetic field strength.  A recent meeting of the Mars Global Surveyor team revealed the maps and their implications to the public.  The oldest terrains – those showing the greatest density of impact structures, as in the Lunar Highlands – show evidence of remanent magnetism.  Those affected by the youngest major impacts – analogous to the 4 billion-year old lunar maria – do not.  This suggests that Mars lost its magnetic field some time in its first half billion years, and thereby any substantial atmosphere.  One possible reason for this loss is that Mars has long been a geologically sluggish planet.  It is turbulent motion in the Earth’s liquid outer core that generates a magnetic field.  That turbulence is probably kept in motion by convective heat transfer in the mantle – it is a companion of terrestrial plate tectonics or any kind of regular mantle overturn.  Mars’ mantle does not do that, either by tectonics or through plume activity (unlike Venus), so its core may well be devoid of motion.

Exactly when magnetism stopped, with the attendant effect of the solar wind on any atmosphere, is crucial for estimates of how long life might have had to appear and begin evolving.  The results certainly rule out evolution beyond the most primitive life forms.  However, establishing that date must await future Mars landers, either staffed or robotic, on which the most important experiments will aim at detecting signs of former of extant life.  The magnetic data are not encouraging for exobiologists.

(Source:  Samuel, E.  2001.  The day the dynamo died.  New Scientist, 10 February 2001 issue.)

And now, Martian glaciers

Readers will have seen scornful comments in Earth Pages, regarding the desperate search for evidence of liquid water on modern Mars.  That water once was there seemed cut and dried from the giant valleys scoured across the Red Planet’s surface.  It was said that vast volumes of deep-seated ice catastrophically melted to flood from large impact sites.  Like the supposed evidence for active watery emissions in recent time, that for past flooding which cut the large valley systems rested on interpretation of the landforms themselves.  Re-examination of the valleys shows that they almost exactly mimic features revealed by sonar sounding on the sea floor surrounding the Antarctic ice sheet.  The Antarctic features probably formed during increased flow regimes when sea level stood at its lowest during glacial maxima.  Such surges can flow uphill, and sure enough the valley systems on Mars do have uphill tracts.

Baerbel Luchita of the US Geological Survey applied work on structure beneath the Ross Ice Shelf to Mars, suggesting that impact-melted water froze on emergence at the surface to flow in a more or less glacial fashion.  Undoubtedly, ice flow is far more capable of large-scale excavation than an equal volume of water, but to form the 1000 km long systems on Mars implies a considerable head.  Also its branching nature forces the assumption of many coalescing glaciers over a very large area.  That meets problems in imagining a widely distributed source of energy that caused the melting.  Impacts are at points, so perhaps yet another mechanism, such as seismicity, will need to be invoked.

(Source:  Hecht, J.  Sliced by ice.  New Scientist, 27 January 2001 issue)

About 4.5 billion years ago the Moon formed, probably as a result of a stupendous collision between the original Earth and a body about the size of Mars.  That would have left Earth with its outer parts molten in a global magma ocean, and without any atmosphere.  Such a dreadful condition formed the point of departure for all subsequent evolution of our home world; the beginning of geological history.  No matter how many terrestrial rocks geochronologists analyse, it seems pretty clear that they are never going to push back their erstwhile grail of the oldest one beyond 4 billion years.  Among the oldest rocks, those from Akilia in west Greenland contain sedimentary evidence for flowing water and the isotopic signature of established life.  The date 4 billion years before the present seems to be the maximum for every aspect of geological research that might support theory with concrete evidence, which is sad, because both continents and oceans existed, the planet was inhabited, some form of tectonics operated and water moved matter around.  Studying the emergence of such broadly familiar processes is a lost cause, at least on this planet, for a half billion years has simply vanished.

The enduring outer skin of the Earth, continental crust, is made mainly of two minerals, quartz and feldspar.  Feldspar can be dated, but it breaks down to clay and soluble compounds, so the weather removes it as a source of information,.  Quartz offers not a single clue to when it formed, even though its hardness and stable molecule mean that it is durable.  Its abundance of silicon demands several stages of evolution from the silicon-poor mantle.  Quartz is quintessentially continent stuff.  Probably among those quartz grains found on a beach or in a sandstone some date back to the emergence of the first crust, but you would never know.  Even more durable is zirconium silicate, or zircon, tiny amounts of which settle from many sands because it is denser than quartz.  Zircon’s structure is hospitable to several elements rarer still, including radioactive uranium and thorium. Build up of radiogenic lead isotopes inside zircon crystals means that grains carry their own history.  Zirconium finds no easy resting place in minerals that form the bulk of the mantle.  So it tends selectively to enter magma formed there.  Nor are the minerals of oceanic crust particularly accommodating.  Naturally, zirconium becomes concentrated in materials that end up as continental crust, so to form zircons.  A handful of zircons from beach sands continually sorted according to density on the Coromandel Coast contains the entire history of the formation of the Indian continent – they are sold in bottles by urchins at tourist resorts as one of Lord Krishna’s five varieties of “rice”.

The mount Narryer Quartzite of Western Australia is a similarly well sorted, though 3 billion-year old sedimentary repository.  Fourteen years ago, Bill Compston and Bob Pidgeon managed to extract 17 tiny zircons from it that extinguished at a stroke the ambitions of other geochronologists to date the oldest rock in the world.  Their ages, obtained by methods based on the build up of lead isotopes from decayed uranium and thorium reached back to 4.27 billion years.  They had discovered the oldest continent, but one sneeze and they would have lost the lot.  Mount Narryer made the front pages early in January by providing even older zircons that post-date “Year Zero” by a mere hundred million years.  Some continental material was around 4.4 billion years ago (Wilde, S.A. et al.  2001.  Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.  Nature, v. 409, p. 175-178).  Oxygen isotopes in these tiny, aged grains offer another insight.  They have contents of ¹⁸O that are too high to have formed other than in an environment that involved liquid water reacting with the source of the zircon-forming magma (Wilde et al., 2001; Mojzsis, S.J et al. 2001.  Oxygen-isotope evidence from ancient zircons for liquid water at the Earth’s surface 4,300 Myr ago.  Nature, v. 409, p. 178-181).

Evidence for such old liquid water drew attention from many planetary scientists.  Life is impossible without it.  The conclusion drawn is that it could have been around so close to “Year Zero” .  But evidence for early water is no surprise.  Earth’s high content of volatiles ensures that water in one phase or another must always play a role in its internal processes.  Hot as it must have been immediately following Moon formation, convection in its “magma ocean” and radiation from its surface (proportional to the fourth power of surface temperature) would have been so efficient that cooling to permit liquid water at the surface may have taken less than 100 million years.  The maximum temperature of the liquid water that interacted with the zircon-forming magma depended on the pressure of the environment where that happened.  That was not necessarily an ocean or even “some warm little pond”.  Water is liquid, if the pressure is high enough, at temperatures up to 274°C, which is too high for most of life’s molecules.

The Mars Orbiter Camera aboard the Mars Global Surveyor spacecraft is one of those little irritations that irks Earth-oriented remote sensers.  It captures pictures with resolutions as fine (1.5 m) as those from “spies in the sky” of a decade back, and the best commercially available imaging systems in orbit around our home world (they cost between US$16 to 44 per km²).  Nor surprisingly, geologists interpreting features of the Martian surface are having a heyday (there is no damned cloud or atmospheric haze either, and it’s the dry season all the time!)

Nearly every report focuses on water, either that supposed to have flowed after recent (most unlikely) melting of ice in the upper veneer of Martian “soil” (see Earth Pages xx  2000, and the episode of catastrophic melting early in Mars’ history  that cut huge valleys.  The latest shows abundant topographic features that speak plainly of layer-cake sediments (Malin, M.C. and Edgett, K.S.  2000.  Sedimentary rocks of early Mars.  Science, v. 290, p. 1927-1937).  Even unconformities and exhumed channel-like features show up, and some of the deposits partly fill ancient impact craters.  While aeolian and volcanic processes, and those associated with impact ejecta might all form sediments – we can be certain that all these processes have operated on Mars – to conclude that some of the sediments might be waterlain is not so easily assumed.  Thankfully, Malin and Edgett are cautious, for there is no definitive sign that the Martian sediments are waterlain – but some might have been.

Having just returned from a technical meeting with people working for humanitarian relief agencies, and heard of their needs for remote-sensing data that should show up habitations clearly enough to estimate numbers of people affected by disasters, I did not read this paper with any great relish.  NASA’s determination to convince itself that indeed water lies waiting to be tapped on the “Red Planet” by the first staffed mission there sits uneasily with the fact that the best part of a billion people on Earth have neither enough nor much with a safely drinkable quality.  It’s a pity that there isn’t an “Earth Orbiter Camera” that would serve their needs rather than those of a few earnest astronauts and some ambitious bureaucrats.

Early life survived lunar cataclysm

The last real “geology” on the Moon was the formation of the maria and their filling with basaltic magma.  Both resulted from the unimaginable energies released by a storm of impacts on the lunar surface, from which the Earth cannot conceivably have escaped.  This “late, heavy bombardment” occurred between 4.15 and 3.8 billion years ago, and overlapped the ages of Earth’s oldest rocks in West Greenland and Northern Canada (The Akilia supracrustals and the Akasta Gneiss respectively, dated around 4 billion years).  Such was the energy involved in each of the maria-forming impacts – and the Earth would have had more and bigger impacts at that time – that it seems likely that any surface water on our planet would have boiled away.  That poses the issue of whether life emerged several times, only to be literally blown away and having to start over.  Two sets of new data help answer this awful question.

Though they have been sitting in Houston for a generation, the Apollo lunar samples still provide useful information.  In the early 1990s precise dating of glass spherules in lunar soil samples found evidence for 12 impacts, but they clustered around 3.9 billion years.  It was this find that supported the cataclysm  proposed on stratigraphic grounds from photo interpretation of the maria.  When planets form, they undoubtedly do so by accreting debris from the vicinity of their orbits.  However, their growing gravitational attraction intuitively suggests that the big chunks are swept up early in planet formation.  On those grounds it can be predicted that additions tail off in mass and impact energy over time.  So there should be a spread of ages from about 4.5 billion years onwards of a dwindling number of big events.  The lunar glasses buck that trend severely, as do the ages of the voluminous maria lavas, for there are few ages between 4.5 and 4.0 billion years.  One objection has been that later events obliterate signs of earlier ones.  Another centred on how a clutch of whopping impactors might survive in Earth’s orbit without having been swept up early on, or how a maria-forming storm of many such bodies might have appeared in the Earth-Moon vicinity almost simultaneously from elsewhere in the Solar System.

The monster events are mainly on the Moon’s near-side, which is where the Apollo samples come from.  Consequently, the objection to the “late, heavy bombardment” seems valid – the data could be biased.  Meteorites found on the Earth, which have geochemistries signifying a lunar origin, potentially offer a check, because they could have formed by late impacts anywhere on the lunar surface, including the unanalysed far-side.  Barbara Cohen, Timothy Swindle and David Kring of the University of Arizon, Tucson, report ages of glasses from four such meteorites (Cohen, B.A. et al., 2000.  Support for the lunar cataclysm hypothesis from lunar meteorite impact melt ages.  Science, v. 290, p. 1754-1755).  All the glasses show evidence of having originated from the ancient, anorthositic lunar highlands, which dominate the far-side.  The results show seven distinct events, and none are older than 3.9 billion years.  Although the work began as a way of perhaps disproving the cataclysm, it turns out to support it even more strongly.  It still poses the question of how and where the bulky culprits appeared.  One possibility lies in the idea that the outermost giant planets, Uranus and Neptune entered their present orbits far later than expected.  Harold Levinson (in press, Icarus) of the Southwest Research Institute of Boulder , Colorado, has suggested that the two planets’ materials accreted between Jupiter and Saturn, but eventually became orbitally unstable, and zoomed off into the outer limits.  The gravitational perturbations by such a theorized event would have been immense, sufficient to set the asteroid belt and the much more distant source of comets juddering.  [See also:  Kerr, R.A.  2000.  Beating up on a young Earth, and possibly life.  Science, v. 290, p. 1677].

Whatever the debate about the “late, heavy bombardment’s” possible tight time span, at the time the Moon did experience awesome delivery of impact energy, and so must have the Earth.  Hence the deep interest in its effect on living processes.  The Akilia sedimentary rocks of West Greenland formed at least 3.85 billion years ago.  Carbon isotopes trapped in minerals that are resistant to metamorphic effects show beyond any reasonable doubt that living things, probably primitive bacteria, dwelt in the waters that laid down the Akilia sediments.  If the cataclysmic bombardment still going on at that time had been continually thwarting lifes puny efforts at survival, then the Akilia rocks should contain a lot of elements concentrated in asteroidal material.  They should be rich in iridium, the ubiquitous signalling element of the Chicxulub impact that terminated the Mesozoic.  Curiously, they are not unusual in that respect.  In a paper soon to be published in the Journal of Geophysics Research (Planets), Ariel Anbar and Gail Arnold of the University of Rochester in New York will report a distinct lack of success in finding iridium spike in the Akilia sedimentary rocks (Source:  Hecht, J.  2000.  It’s a bug’s life.  New Scientist,1 December 200 issue, p. 11). 

Other searches for iridium spikes in early Archaean rocks have also proved fruitless, although impact-generated glass spherules have been found in the sediments of the Barberton greenstones of Swaziland.  That rules out a continuous bombardment by giant impactors.  Quite possibly big impacts came only every 10 to 100 Ma.  Also, the discovery of primitive bacteria living today in cracks in hot, deep rocks as well as around ocean-floor hydrothermal vents, suggests a high chance that such hyper-thermophilic life might well have survived anything the Solar System might have flung at it.  Molecular phylogenies of bacteria seem to point strongly to all life having arisen ultimately from heat-loving ancestors.  Quite possibly, the “late, heavy bombardment” shaped the molecular basis for all later biological evolution.  Certainly, many bio-molecules in all modern cells are but a short chemical step away from the heat-shock proteins possessed by modern hyper-thermophiles.