Category: Planetary, extraterrestrial geology, and meteoritics

“Everyone knows” about the huge valley systems on Mars, which through their relationships to other aspects of the planet’s features are thought to have formed catastrophically early in its history.  The high-resolution Mars Global Surveyor images and altimetry bring a new perspective to fluvial features (Hynek, B.M. & Phillips, R.J. 2003.  New data reveal mature, integrated drainage systems on Mars indicative of past precipitation.  Geology, v. 31, p. 757-760).  The authors, from Washington University in St Louis USA, show depressions extracted from the altimetry data by simulation of the paths likely to be taken by rain water falling on the surface.  In some areas, the depressions link up in dendritic networks very like those that occur on the Earth’s surface.  Previous data only picked up disconnected valleys.  The newly outlined valleys are V-shaped, unlike the U-shaped systems that developed on Mars probably by sapping as groundwater emerged, either slowly or catastrophically.  Such profiles are good evidence for surface run-off, and that can only indicate precipitation, either of rain, or as a result of melting snow.  Only 11000 kilometres of valley segments can be identified, and are probably relics of a larger ancient system that later events have masked.  Some however, reach to the rims of large craters and seem to post date them.  Probably, the events that carved these systems occurred in Mars’ early history.

The world has been agog these last few years as evidence has mounted to suggest that Mars still has abundant water buried beneath its dusty surface, in the form of permafrost.  Early in its history there are many signs of vast floods that carved huge meandering canyons and may have filled basins with moderately long-lived seas.  Yet Mars has probably always been pretty cold, as it is now, and the most likely form that surface water would have taken is in glaciers; that is, if there was ever sufficient atmospheric water to precipitate snow.  As on Earth, the likeliest places to look are in mountainous regions, and Mars is not lacking in very high places. By far the largest, and indeed they are the highest mountains in the Solar System, are the shield volcanoes of the Tharsis Rise, topping out around 18 km above the Martian version of the geoid.  The volcanoes have gnarled surfaces, which until recently have been regarded by most as the result of volcano-related processes.  Imaging of the Martian surface has stepped up several notches in resolution in recent years, and details of the small-scale features of the volcanoes are very clear.  Above all else, they resemble aspects of the nearest analogue to Martian conditions on Earth – the Dry Valleys of Antarctica.  Although the Dry Valleys are now largely free of ice sheets, they show many features of former glaciation, perhaps extending back 30 Ma to the Oligocene.  Their frigidity has ensured that any glaciers there were frozen to the surface, rather than having zones of incipient melting at their bases.  Such cold-based glaciers move sluggishly, and produce peculiar features.  Among these are moraines produced by sublimation rather than melting of the ice – they evidence no reworking by melt water – and rock glaciers that are also products of sublimation and sometimes rest on relics of former glaciers.  Probable examples of both occur on the flanks of the Tharsis volcanoes, together with weird track-like assemblies of concentric ridges, that are likely to have formed on the flanks of ablating glaciers as they reached a standstill and then retreated. (Head, J.W. & Marchant, D.R. 2003.  Cold-based mountain glaciers on Mars: Western Arsia Mons.  Geology, v. 31, p. 641-644).  Interestingly, the relationship of the glacial features to impact craters suggests that glaciation took place during the period since about 1.8 billion years ago (the Amazonian phase of Mars’ history) when bombardment had slackened to almost terrestrial rates and liquid water was unable to form on the red planet.  Of course, glaciers do not have to be made of water ice, and there is still a possibility that at such immense altitudes any glaciers might have been made of solid carbon dioxide.  Head and Marchant speculate that some of the features might still sit upon relics of the glaciers.  It could be a bit of a disappointment if future explorers of Mars landed there expecting a water supply.

Christianity had a hard time in its first four centuries as a faith, especially at the centre of the Roman Empire.  Persecution of Christians ended abruptly with the conversion of Emperor Constantine in 312 AD.  Legend has it that, while faced with the double problem of northern barbarian hordes at the gates of Rome and dissident Christians within, Constantine saw a vision in the sky while preparing to take on the invaders.  Immediately converting to Christianity, he saw off the hordes, albeit temporarily, and the rest, as they say, is history.  One version of the legend, from the Sirente region of Central Italy, tells of a new star that came nearer and nearer to disappear behind the mountains, with a blaze of light from horizon to horizon and ground shaking.  Unsurprisingly, impact theorists latched onto this because of its similarity to what probably happens when a substantial meteorite strikes the Earth.  Geologists from Sweden have discovered a small crater field in the Sirente area, that consists of a 125 m wide, circular lake with a raised and deformed lip, and several lesser craters dotted around it.  Preliminary dating gives an age of 412+­ 40 years.  Although this date is a century later than Constantine’s conversion, contamination with later material might have reduced the actual age.  If the link does prove to be substantial, the Sirente impact will rank with other catastrophes that literally made history, such as the filling of the Black Sea which has been argued to be the inspiration for the Biblical Flood and the Epic of Gilgamesh, and the explosive volcanism of Santorini that wiped out Minoan civilisation on Crete and may well be recorded apocryphally in the Old Testament.

Source:  Chandler, D.L. 2003.  Crater find backs falling star legend.  New Scientist, 21 June 2003, p. 13.

Middle Devonian extinction and impactite layer

Around 380 Ma there was a major extinction event (~40% of marine animals) that is recorded world-wide, along with negative shifts in ¹³C.  As with other extinctions since the discovery that the Chixculub crater was exactly the same age as the famous K/T extinction, there has been a quest to link this Middle Devonian event to an extraterrestrial cause.  Now there seems to be a positive result (Ellwood, B.B. and 4 others 2003.  Impact eject layer from the mid-Devonian: possible connection to global mass extinctions.  Science, v.  300, p. 1734-1737).  A Devonian section in Morocco contains a thin layer rich in shocked quartz, microspherules of devitrified glass, and metals, that also has low d¹³C.  The carbon-isotope shift could have resulted from either of two possible consequences: collapse of the marine ecosystem; or massive release of methane from gas hydrates destabilised by the impact.  Only one crater coincides wit the date of the layer and the extinction, Kaluga in Russia, but it is only 15 km wide, so cannot have had any dramatic biological effect.  However, the very presence of a moderate crater at exactly the right age might signify other impacts, because it is becoming increasing clear that impacts come in clusters, perhaps because large, approaching bodies break up before they hit the Earth.

As mentioned several times in Earth Pages News, geologists have been slow to accept that the Earth’s evolution has been substantially affected by impacts of extraterrestrial bodies.  In hindsight, this stubborn scepticism seems perverse.  The discovery of impact-induced melt spherules in the Late Triassic sediments of SW England (see Britain’s own impact in EPN, December 2002) went almost unnoticed.  However, there is still an entrenched view that nothing really big has happened.  When similar spherule beds were reported from the Early Archaean greenstone belts in Australia and South Africa in 1986, and deduced to have formed by an impact, the authors were pounced on by those who thought they could plausibly explain the very odd rocks by unremarkable, Earthly processes. How satisfied Donald Lowe and Gary Byerly, of Stanford and Louisiana State Universities must be to find their view now proven beyond doubt, and to share in publishing the evidence.  The proof comes from isotopic studies of three spherule beds in the 3200 Ma-old Barberton greenstone belt in South Africa (Kyte, F.T. et al. 2003.  Early Archean spherule beds: Chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type.  Geology, v. 31, p. 283-286).  Chromium isotopes in the rocks are so unearthly, that explaining them requires that they contain up to 60% of extraterrestrial material, probably from carbonaceous chondrite impactors.  Compared with the global spherule-bearing and iridium-rich K/T boundary layer (3 mm thick on average), that is the ejecta from the Chicxulub impact, the Barberton beds are much thicker (10-20 cm).  The authors estimate that, if the Barberton layers are globally representative, the impactor responsible for their formation could have been 50 to 300 times more massive than that which terminated the Mesozoic Era.  Besides that, three such layers formed within 20 Ma, and that suggests bombardment flux more than ten times that late in Earth evolution.

Triggering core formation at the microscopic level

Since Birch’s discovery in the 1950’s that the Earth’s excessive density compared with exposed rocks could be explained by a metallic, iron rich core, whose presence was detected by studies of seismic waves, there have been many explanations for core formation.  Some regarded the process as a slow accumulation of iron-rich melt as it sank from the mantle, others that it formed during Earth’s initial accretion from the iron-rich parents of metallic meteorites.  Lead and tungsten isotope studies indicate clearly that the core formed very early in Earth’s evolution, taking as little as 30 Ma.  However, for such a vast mass to have quickly segregated from the rest of the Earth poses awesome mechanical problems.  Alloys of iron, nickel and sulphur do have much lower melting temperatures than silicate minerals, and planetary accretion releases gravitational potential energy.  That serves to heat up a growing planet, but core-forming materials would melt long before the dominant silicates that envelop them, if indeed mantle materials did melt substantially.  So, at the centimetre scale of rocks, a melt fraction, however dense, would have to migrate and accumulate in globules with sufficient gravitational potential to sink through the viscous early mantle.  The boundaries of pores in which melts form are critical.  If the angles between silicate facets and melt-filled pores are large, tiny amounts of molten metal cannot become interconnected and migrate, unless the silicates begin to melt too or are actively deformed.  Since coexisting silicate and metal melts are not supported by geochemical evidence and deep planetary interiors are probably static, the fact that the interfacial angles of crystalline minerals are high poses quite a problem.  Geochemists at the University of Yokohama in Japan have performed complex experiments at high pressure and temperatures to simulate likely conditions during planetary accretion (Yoshino, T. et al. 2003.  Core formation in planetesimals triggered by permeable flow.  Nature, v. 422, p. 154-157).  They discovered that if metallic melts account for more than 5% by volume of the accreting body, then this melt can percolate through the solid rock, because the angles separating melt and solid fall below the critical value of 60º.

The implication is that even quite small planetesimals (>30 km radius) can quickly develop metallic cores, using energy released by the decay of short-lived isotopes that were plentiful early in Solar System history.  This is borne out by studies of metallic meteorites  Of course, the immense gravitational energy released by accretion of larger planetary bodies would result in the same differentiation, but if they formed by accumulation of smaller differentiated bodies there is no need to postulate within-planet processes on the microscopic scale.  The core would be “pre-manufactured”, only requiring blending of many smaller cores of accreting planetesimals

See also: Minarik, B. 2003.  The core of planet formation.  Nature, v.  422, p. 126-127.

Despite the evidence from the neutron detector on Mars Odyssey for the possible existence of subsurface water on Mars (Water on Mars, August 2002 Earth Pages News) not everyone accepts that minor rills and channels on its surface are due to periodic melting of buried water ice (Water on Mars, July 2000Earth Pages News).  Two small pieces in New Scientist contest that view.  In a letter, Wytse Sikkema of Shell likens them to features carved by turbidity flows (suspensions of solid particles in a fluid, such as avalanches, ash flows and submarine turbidity currents) which they resemble more than stream channels (Sikkema, W. 2003.  Rivers of Dust.  New Scientist, 18 January 2003, p. 24).  Sikkema suggests that the supposed ocean-like basins on the Red Planet are filled with dusts carried by such flows.  Support for such a mechanism emerges from observations of gullying in progress during Mars’ late spring near the poles, when temperatures were too low for liquid water to exist.  Nick Hoffman of the University of Melbourne, suggests that the active gullying that he observed  on successive Mars Global Surveyor images involves rapid vaporisation of CO₂ snow and ice to lubricate dust avalanches (Nowack, R. 2003.  Ravines hint at gas avalanches on Mars.  New Scientist, 18 January 2003, p. 14-15).  Hoffman also considers that massive release of gas by boiling of buried CO₂ liquid could have carved the much larger valley systems on Mars by massive flows of dust-gas mixtures.  If he is correct, there is no reason to consider Mars either as a haven for early life or one for intrepid astronauts.  Britain’s Beagle 2 probe and two unnamed NASA Mars rovers, due for launch this year, should resolve the issue, but if water is not confirmed, there will be huge disappointment for both teams involved with those missions.

One of the most fascinating spin-offs of detailed palaeontology is that the growth layers in corals and the carbonate shells of other organisms can record how many days there once were in a year.  Records of shell growth can even chart variations in the lunar cycle, backed up by subtle features in cyclical sediments.  Such data infer that the speed of the Earth’s rotation has changed (Ravilious, K.  2002.  Wind up.  New Scientist, 23 November 2002, p. 30-33).  As well as the general slowing through the Phanerozoic, from a rate that gave 420, 21-hour days in a Cambrian year, there have also been times when the rate has strangely speeded up again.  Such curious events occurred at 400 Ma and again around 180 Ma.

Planetary spin can be set in motion or changed by very large impacts, in the manner of whipping a spinning top.  But there is little sign for such drama at those times.  Another possibility is a change in the Earth’s moment of inertia by a shift of mass relative to the spin axis, in the manner of a skater speeding up a spin by pulling in her arms.  What could induce such an effect at the scale of our planet?  Cold, dense lithosphere continually sinks at subduction zones, but that is normal behaviour in balance with rotation.  One possible trigger for sudden changes in moment of inertia is the breaking away of a substantial chunk of the mantle that lies above the discontinuity 670 km beneath the surface to sink to deeper levels.  This dramatic suggestion stems from modelling by Philippe Machetel and Emilie Thomassot of the University of Montpellier in France (Machetel , P. & Thomassot , E. 2002.  Cretaceous length of day perturbation by mantle avalanche.  Earth and Planetary Science Letters, v. 202, p. 379-386).  Their model focussed on the transition zone between lower and upper mantle around the 670 km discontinuity, and how it might respond to the fluid dynamics of Earth’s convective heat transfer, particularly that involving heat originating in the core.  The transition, they claim, acts as a “lid” to efficient heat transfer between lower and upper mantle.  Their model suggested that additional deep-mantle heat flow might destabilise the transition’s strength, so that it would no longer support the mass of cooler and more rigid mantle above it.  Failure could then allow a massive slab of upper mantle literally to fall to the core-mantle boundary, spreading out to displace material there upwards as the precursor of a superplume. 

The link to day-length comes from Machetel and Thomassot’s search for evidence that such collapses might have occurred, and they concentrated on the 180 Ma change (Mid Jurassic).  Around 170 Ma the current round of continental drift began in earnest.  In the Early Cretaceous (130 Ma) the geomagnetic field became locked into quiescence, remaining with the same polarity for an unprecedented 40 Ma during which the giant Ontong Java oceanic flood volcanism took place.  Their explanation for both is that upper mantle avalanched, eventually to reach the core-mantle boundary.  When the mass “bottomed out” it cooled the outer core, settling it into regular motion, so that the geomagnetic field became constant.  Coincidence?  I am reminded that when skaters wish to stop their spins, they throw out their arms.  The law of conservation of angular momentum also demands that the Earth behaves in the same way.  In fact it applies to the Earth-Moon system, so that the general slowing of Earth’s rotation has been accompanied by the Moon receding into ever more distant orbit, and gaining momentum.

While evidence has been accumulating for the influence of asteroid and comet strikes elsewhere, the British geological community has had a disproportionate share of sceptics; those who thought it was all a matter of “whizz-bang” science.  It is welcome news that we now have our own “piece of the action”, for geoscientists from Aberdeen University and the Open University have a discovered a well-preserved impact horizon in Late Triassic terrestrial sediments that contain both devitrified glass spherules and shocked quartz grains (Walkden, G. et al. 2002.  A Late Triassic Impact Ejecta Layer in Southwestern Britain.  Science Express –www.scienceexpress.org, 15 November 2002).  It is not associated with the Triassic-Jurassic boundary, which witnessed on of the “Big Five” mass extinctions, but is dated at 214±2.5 Ma, within error of the major impact at Manicougan (~100 km diameter; Quebec; 214±1 Ma) the lesser Rochechouart structure (~25 km diameter; France; 214±8 Ma).  The Ar-Ar dating did not use spherule glass, but authigenic potassium feldspar that postdates the spherules, but may have formed from potassium released when they became hydrated.  Given its size and position relative to Britain on a Triassic plate reconstruction, Manicougan is a likely culprit.  However, despite its considerable size, there are no signs of significant faunal changes at the time of the Manicougan impact.  The host Triassic rocks in Somerset rest directly on Carboniferous limestones, and primitive mammal remains are known from infillings of a palaeokarst surface in the Mendip Hills.  Now the deposit has come to light, the search is on for similar materials in Late Triassic marine sediments.

The increasing use of finely-resolving 3-D seismic surveys in offshore exploration for hydrocarbons reveals exquisite detail of structure in strata beneath the sea floor.  So it is no surprise that oil-company geophysicists are able to image features that would otherwise remain hidden to researchers in universities.  If such discoveries are of little interest commercially, their finders are free to publish.  During routine surveys in the southern North Sea, an array of seismic profiles gradually built up a picture of something more reminiscent of the surface of an icy moon of Jupiter than a sequence of basinal sediments (Stewart, S.A. & Allen, P.J.  2002.  A 20-km-diameter multi-ringed impact structure in the North Sea.  Nature, v. 418, p. 520-523).  The circular feature found in strata at the top of the Cretaceous, might have been passed off as the product of deeper rise of salt diapirs from the widespread Permian evaporites of the North Sea basin, but for several features.  The surveys revealed no signs of the low-density Permian salt having bulged upwards below the structure, and disruption stops at depth.

The feature consists of at least 10 concentric rings extending to 20 km diameter, and at its centre is a bowl-shaped depression around a clear peak.  Not only is it an impact structure, but one of a particular class known as multi-ringed basins.  Those known from the Moon, are vastly bigger and are thought to have formed by such immense energy that the lunar surface rippled to fail along large concentric faults.  Lunar and terrestrial craters of the size of the North Sea structure usually have no concentric structure, being circular pits with rims and occasionally a central peak cause by rebound of the crust after impact.  The only similar features known are from moons of the Giant Planets that are made mostly of ice.  It is surprising that the North Sea example closely resembles them.  Modelling of such craters on Callisto suggests that they form when surface materials are underlain at depth by weaker ones; possibly an ice-liquid slush on ice moons.  The North Sea impact was into the Upper Cretaceous Chalk, whose upper strata are more homogeneous than those at deeper stratigraphic levels, which contain layers of mudstone.  Had impact occurred while the strata were not completely lithified, then the clays would have allowed inward movement to fill the crater excavated by impact, the more rigid upper Chalk having fractured during this movement.

Whether or not the impact accompanied the Chicxulub crater, implicated in the end-Cretaceous mass extinction, is not certain, although it does seem to predate Tertiary sedimentation in the North Sea.  There are probably many more impact structures on the sea floor, buried by marine sediments, but only in hydrocarbon-rich basins are they likely to be unmasked by seismic surveys.

Evidence builds for major impacts in Early Archaean

Following the discovery that anomalous tungsten isotope compositions of some Early Archaean rocks suggest a major component of extraterrestrial material in them (See Earth Pages News, August 2002, Tungsten and Archaean heavy bombardment), geochemists from Louisiana State and Stanford universities report evidence of debris from very large impacts in the same period (Byerly, G.R. et al. 2002.  An Archean impact layer from the Pilbara and Kaapvaal cratons.  Science, v. 297, p. 1325-1327).  Their case rests on the occurrence of layers of rock containing spherules of what formed as molten silicate droplets, in Early Archaean greenstone belts of the Barberton and Warrawoona areas of South Africa and Australia.  Zircons from a single layer in both areas yield identical ages of 3470 Ma, suggesting that the layers formed during a single impact event.  The authors speculate that a major unconformity in the Archaean of the Pilbara province in Australia, which is around the same age, may be the result of tsunamis induced by the impact.  It seems as if the responsible impact had a global effect, and may have released 1 to 2 orders of magnitude more energy than that responsible for the K/T event.  Judging by the lunar cratering record, this and previous finds help confirm expectations of similar bombardment on Earth during the Early Archaean.

Very early differentiation of planetary bodies

The radioactive decay of ¹⁸²hafnium to ¹⁸²tungsten seems likely to resolve the influence of impacts on the Earth ‘s evolution (See Earth Pages News, August 2002, Tungsten and Archaean heavy bombardment).  It is even more useful in refining ideas about the evolutionary pace of the parent bodies of meteorites.  The half-life of ¹⁸²Hf is only 9 million years (all of it has decayed away in the Solar System by now), so the amount of radiogenic ¹⁸²W associated with hafnium in a meteorite is a guide to pervasive geochemical processes early in the history of their parent bodies.  Hafnium has an affinity for silicates, whereas tungsten is siderophile and likely to enter planetary cores, should they form.  Because ¹⁸²Hf decays so quickly, it is not easy to work out its original abundance, relative to stable ¹⁸⁰Hf, in the source material for the Solar System.  That is a prerequisite for estimating when the hafnium-tungsten differentiation took place in a planetary body.  Two papers in the final August 2002 issue of Nature agree on this initial ratio (Yin, Q. et al. 2002.  A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites.  Nature, v. 418, p. 949-952.  Kleine, T. et al. 2002.  Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry.  Nature, v. 418, p. 952-955), which has important connotations; it is less than half the previously assumed value.  They determined this initial ratio using Hf-W data from independently dated carbonaceous-chondrite meteorites, whose parent bodies were never fractionated.

The two research groups, from Harvard University and the French Laboratoire des Sciences de la Terre, and the universities of Münster and Köln, Germany, respectively, use the new initial ratio to estimate the age of core formation from a range of meteorites.  Their estimates dramatically shorten the time between original accretion and core formation in a variety of bodies whose Hf-W isotopes have been studied previously.  The parent of the eucrite class of meteorites, probably the asteroid Vesta, differentiated within only 3 to 4 Ma, whereas the cores of the Earth and Mars took a little longer – about 29 and 13 Ma respectively.  In geological terms, accretion and core formation probably accompanied one another.  Of course, such estimates based on isotopic decay systems assume that the initial ratios existed at the time of accretion.  That may not be valid if the pre-Solar nebula took millions of years to evolve to the stage of self-collapse under gravity, which is the prerequisite for the formation of a planetary system.  However, there is evidence from short-lived decay systems involving other radioactive isotopes, such as ²⁶Al, in meteorites, that points to the influence of a nearby supernova that triggered the formation of our Solar System.  Such an event is required to synthesize short-lived isotopes anyway.  Moreover, the shock from a supernova could accelerate collapse to mere few tens of thousand years.

See: Cameron, A.G.W. 2002.  Birth of a Solar System.  Nature, v. 418, p. 924-925.

From time to time Earth Pages News has tried to temper the flood of papers that seek every which way to support the notion that Mars is still well-endowed with water.  That is what NASA seeks in order to fuel its bid for the vast funds needed to launch a staffed mission to the Red Planet.  The evidence in each case was ambiguous.  I have always thought that attention and money would be better directed towards the one sixth of the human population who have no access to safe and abundant water supplies.  That remains my view, but the appearance of 10 pages of Science forces me to accept near proof of Martian water in abundance (Feldman, W.C. and 12 others 2002.  Global distribution of neutrons from Mars:results from Mars Odyssey.  Science, v. 297, p. 75-78.  Mitrofanov, I and 11others 2002.  Maps of subsurface hydrogen from the high energy neutron detector, Mars Odyssey.  Science, v. 297, p. 78-81.  Boynton, W.V. and 24 others 2002.  Distribution of hydrogen in the near surface of mars: evidence for subsurface ice deposits.  Science, v.  297, p. 81-85).

The neutron and gamma-ray detectors aboard Mars Odyssey only needed to operate for a month to reveal the abundance of hydrogen across the surface of Mars.  It varies a great deal, the highest levels showing up at high northern and southern latitudes.  Preliminary modelling suggests that these regions have at least several metres of ice-rich debris, containing between 25-35 % water ice.  Quite possibly the modelled ice-rich layer could reach a kilometre in thickness.  High anomalies at lower latitudes are modelled as being due to hydrated minerals in the Martian soil.

More results at higher precision are to come from Mars Odyssey, and experts emphasize that the reported modelling of neutron fluxes and those of gamma rays emitted by neutron-capture reactions is complex and preliminary.  However it does look like NASA scientists will soon by selecting sites for future landings on Mars.  Even more certain, it will have sent a frisson of excitement through those intent on the glory of finding signs of life there.

Tungsten and Archaean heavy bombardment

One of the major revelations that arose from the Apollo missions to the Moon is that the vast maria basins, filled with basalt, formed when a series of huge impacts wracked the lunar interior.  Surprisingly, they formed between 4 to 3.8 Ga ago, rather than in the earlier evolution of the Moon, and this “late heavy bombardment” (LHB) spans the period when the oldest rocks were forming on the Earth.  Controversy has raged for 3 decades about whether the LHB had a major influence on early Archaean geology.  The problem was that direct evidence has been hard to find, and difficult to get across to critics of such outlandish notions.  A careful investigation by geochemists from the Universities of Queensland and Oxford seems likely to force some critics to eat hat (Schoenberg, R. et al. 2002.  Tungsten isotope evidence from ~3.8-Gyr metamorphosed sediments for early meteorite bombardment of the Earth.  Nature, v.  418, p. 403-405).

Because stable ¹⁸²W forms by the decay of ¹⁸²Hf, with a short (9 Ma) half life, virtually none will have formed since the Earth accreted.  The ¹⁸²W/¹⁸³W ratio of objects from different parts of the Solar System should show distinct differences, and so they do.  Different classes of meteorites show tungsten isotopes that are significantly different from one another, and from products of mantle melting on Earth.  Ronny Schoenberg and co-workers analysed tungsten from two early-Archaean sources: the dominant grey gneisses, which are probably calc-alkaline igneous rocks formed at mantle depths, and metasediments from the famous Isua area in Greenland and another around the same age (~3.8 Ga) in Labrador.  The gneisses show no difference from later products of mantle processes, but the metasediments deviate significantly from the terrestrial isotopic composition of tungsten, towards that characteristic of meteorites.  They conclude that the metasediments mix debris formed by weathering and erosion of normal early Archaean crustal rocks with that formed in major blankets of ejecta from meteorite-induced impacts.

Ten years ago, planetary scientist Peter Schultz and Argentine pilot Ruben Lianza observed several depressions shaped like tear drops while flying over the Pampa. Because they also found meteorites and tektite glass when they examined the structures on the ground, it seemed certain that the depressions had formed by the impact of bodies travelling almost parallel to the Earth’s surface.  The structures were clearly no more than a few thousand years old, and the discovery encouraged lurid artistic impressions of terrified native South Americans cowering from an extraterrestrial firestorm.  The Rio Cuarto structures were a godsend for those who fear social and economic disaster from Earth-bound NEOs (near-Earth objects), and have been lobbying for a sky watch for impending doom.

In reality, the Pampas of northern Argentina has hundreds of similar structures over an area of more than 50 thousand square kilometres, and their long axes parallel the prevailing wind direction (Bland, P.A. and 10 others 2002.  A possible tektite strewn field in the Argentinian Pampa. Science, v. 296, p, 1109-1111).  They are “blow-outs” developed in the fine loess soils of the Pampa, and much the same structures affect most loess plains.  Being formed of wind-blown silica and clay dust, loess is not well known for its content of objects above a millimetre in size, so any larger objects found on wind-deposited plains stand a high chance of having arrived by some extraterrestrial process.  Meteorites and tektites are rare, but ablation concentrates them in wind-blown depressions as they are too heavy to be blown away.  That is the likely origin of the objects that Schultz and Lianza used in support of their hypothesis of impact devastation wrought on early South Americans.  Phil Bland of the Open University, and his colleagues from Brazil, the USA, Australia, Russia, Argentine and Britain, were able to date organic matter in the Rio Cuarto structures using the C-14 method at 4000 years.  Yet Ar-Ar ages of the meteorites range from 52 to 36 thousand years, so the two are unconnected.  The glassy tektite fragments provided yet another age of 57 thousand years.  Along with similar glasses at a couple of other sites in Argentina, these support melting of the homogeneous loess by an impact around that time, although no crater from which they might have been ejected is known.  The search is on for the source of a hitherto unknown field of strewn tektites, although it seems strange that in the featureless plains of southern South America one hasn’t shown up long before now.

The mantle’s breath and Earth’s early evolution

Many lavas contain bubbles, which form when gases dissolved under pressure in magma froth out at low pressures.  For the most part the gas is water vapour, carbon dioxide and sulphur dioxide.  It comes from mantle peridotite, and represents the volatile fraction of the deep Earth.  But there are traces of other gases, the most revealing of which are the noble gases helium, neon, argon, krypton and xenon, because some of their isotopes originate from radioactive decay of other elements (mainly potassium, uranium and thorium.  Noble gases in basalts offer important insights into how the mantle has evolved since the origin of the Earth.  Chris Ballentine of the University of Manchester, reviews how such trace-gas isotopes in basalts help resolve some otherwise intangible challenges (Ballentine, C.J. 2002.  Tiny tracers tell tall tales.  Science, v. 296, p. 1247-1248).