Author: zooks777

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.