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).

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.

One of the annoying features of the Earth as a planet is that it engages in a kind of Saint Vitus’ dance.  The best known of its wandering are those involving variations in the eccentricity of its orbit, and the tilt and precession of its axis of rotation.  These follow from the gravitational influences of massive planets elsewhere in the Solar System, and are implicated in the modulation of climatic change through the last 2.5 Ma.  Rather less well-known, and even more aggravating are far more rapid, but geometrically quite small deviations from good behaviour.  One of these is the habit of the spin axis to wander around the geographic poles within a circle roughly 3 to 6 metres across.  It does this every 14 months.  It takes a certain degree of dedication to chart such a tiny planetary tic.  Chandler Wobble is the single claim to fame of its eponymous discoverer.  Seth Carlo Chandler Jr, an American businessman and amateur astronomer, discovered the quirk in 1891 by observing stars with a degree of single-mindedness that might have put a lesser mortal on the couch.  He set out to verify the famous Swiss mathematician Leonhard Euler’s prediction that the Earth ought to wobble every year, and he did.

So minuscule is Chandler Wobble, that keeping it going is something of a vexing problem, for a single jostle’s effect ought to fade away in a few years.  There are innumerable ways of nudging the Earth, and deciding which is sufficiently regular and just right to maintain the wobble is no easy task.  Following in the great tradition of Seth Chandler, Richard Gross of the Jet Propulsion Laboratory compared Wobbling between 1985 and 1996 with the continual but inconstant motions of atmosphere and oceans, as simulated by super-computer modelling of climate.  The forces of winds and currents are simply insufficient to induce the Wobble, but variations in atmospheric and deep-water pressure, together with their positional shifts are, in the manner of Goldilocks and the little bear’s porridge, just about right.  Because changes in water depth are wind-driven (as for instance with the wandering hump in the Pacific’s surface, linked with El Niño), ‘weather’ is the ultimate driving force for Chandler Wobble.

Why devote time to this picayune curiosity?  The answer is to chart more accurately the position of distant spacecraft; not easy when the measuring platform is behaving like a Womble.

Source:  Richard A. Kerr, 2000.  Atmosphere drives earth’s tipsiness.  Science, v. 289, p. 710.

Up until 10 years back, I was under the impression that as individuals we run little risk of being struck by objects falling on us from between the orbits of Mars and Jupiter.  A slim chance, but one tempered by a recollection of my father’s news clip of a small meteorite landing in the sidecar of a 1930’s biker on his way from Hull to Hornsea.  The biker finished his journey.  These days aliens seem to be falling thick and fast.

Late last year, the sleepy hamlet of Guyra, Australia, about 400 kilometres north of Sydney had a heavenly visitor, or so it seemed. On December 7, an object the size of a cricket ball slammed into the town water supply. In recent months, town officials have been pondering how to exploit their near misfortune.

In early July, a local businessman pledged AU$3,000 to dredge the rock out of the reservoir’s bed so it could be put on display, given to a local university or donated to the Australian Museum in Sydney.  Intrepid snorklers discovered that the object had drilled a 1 metre hole in the mud, after penetrating the reservoir itself.  Because such a small meteorite should have slowed to terminal velocity on entering the atmosphere from space, it is highly unlikely that it would have had enough remaining energy after ploughing through water to have buried itself that deep.  Experts have cautioned the amateur meteorite collectors to leave the object well alone, pending more cautious examination.

If Mars is ever to visited by astronauts, and for there to be any chance of finding living things there, water close to the surface is vital.  Not surprisingly, the search for Martian water, albeit not in a network of canals, is becoming a thriving cottage industry. The last week of June 2000 saw a leaked report from research using images from the Mars Global Surveyor spacecraft, publicised in New Scientist and Science for that week..  Some of these showed systems of V-shaped gullies on steep sides of valleys and craters, which are extremely sharp.  Several workers claim that they were cut by running water in the recent past.  That they are young features is clear, because they are not blurred by dust blown across the Martian surface by it nightmarish winds, and none are cut by craters.    How water might have flowed freely a short time ago is not too clear.  The Martian surface is well below freezing point for most of the time (average temperature -50°C).

The explanation given by the researchers is that a layer of frozen pore water a few hundred metres below the surface can melt because of  built up of pressure.  Where the layer meet the surface in valleys cut through it, the pore water remains frozen, and acts as a dam.  When this becomes breached, water simply squirts out to form the peculiar runnels seen at more then 150 sites.  Several of the gullies lie below signs of collapse on the slopes above, suggesting that water release has removed support for debris on the steep slopes.

There a number of reasons to take these accounts with a pinch of salt.  Sure, increased pressure depresses the melting point of water, but at -50°C it would have to be pretty high.  In permafrost areas on Earth, waterlogged soil freezes from the top down in winter, thereby trapping the last dregs of water.  This becomes pressurised, to remain liquid in a supercooled state.  If it breaks out it does not flow, but forms ice almost instantly.  As well as forming the famous pingoes (ice cored mounds) of Arctic alluvial plains, this phenomenon almost caused a bizarre disaster during one of the Yukon gold rushes.  High-pressure water jetted into a public bath house – the warmth of the building had created a trough of melt water directly beneath – and filled the entire edifice with ice.  Fortunately, this happened at night and no prospector was encased.  Much the same would probably happen to any such water escape on Mars, unless it was preternaturally warm.  Such was the case for the truly huge and unmistakable water-cut valleys on Mars.  But they formed far back in Martian history, perhaps as a result of energy introduced by large impacts.

It is tempting to look to other explanations for the gullies.  Very dry sand flows down the lee slopes of dunes, often to form runnels with collapse features above them.  Perhaps some attention to the physics of dry sand – Mars is a sandy and silty place – under near-airless conditions and suitably reduced gravity, might offer an alternative explanation.

Even more optimistic is the notion that Mars once has seas, based on the discovery of various salts in an Egyptian meteorite that approximate the blend of dissolved ions in Earthly seawater (New Scientist, 1 July 2000, In Brief).  The evidence that the class to which this meteorite belongs comes from Mars rests on comparison of its noble-gas content with the extremely imprecise measurements or Mars’ air by the Viking mission in the 1970s.  Why the chemistry of Martian ‘seas’, or any of its water for that matter should bear comparison with that for waters derived from a planet with both weather and highly evolved continents seems to demand an explanation.  Oh well, no doubt we will get answers when astronauts do get there – it is not inconceivable that all the papers suggesting it is important to go have some relation to NASA’s decades long fight for funds to do that.

Not only geologists are waking up to the influence that stray asteroids and comets have had on geological and biological evolution, but so too are politicians.  Despite the minuscule chances of a sizeable body hitting the Earth within our lifetime, the devastation would be awesome.  Insurance actuaries have calculated the risk from such rare events, taking into account the number of likely deaths in the same way as for airline disasters.  You or I are more likely to perish in the aftermath of an asteroid or comet strike than from botulism or a fireworks accident, and the risk is comparable with that of intercontinental flying.  Governments are beginning to find money to support systematic mapping of bodies that may pose a threat; not a lot, but sufficient to spot bad news and refine the risks.

On June 22, a French-US team released a first assessment of the near-Earth objects (NEOs) that pose the biggest threat; those more than 1 kilometre in diameter (Bottke, W.F. et al., 2000.  Understanding the distribution of near-Earth asteroids.  Science, 288, p. 2190-2194).  They estimate about 900 big asteroids in orbits that will pass eventually within a few moon distances of us. “Sometime in the future, one of these objects could conceivably run into the Earth,” warns astronomy researcher William Bottke at Cornell University. “One kilometer (about .6 of a mile) in size is thought to be a magic number, because it has been estimated that these asteroids are capable of wreaking global devastation if they hit the Earth.”  Much smaller objects caused the celebrated Meteor Crater in Arizona (20 000 years ago) and the Tunguska explosion (1905), and seem to pose the greatest hazard, being undetectable at present.

The Cambridge-Conference Network (CCNet) freely provides a regular electronic newsletter about research into short-lived catastrophic events, including climate change, the effects of supervolcanoes, and impacts, both in the geological record and possible in future from NEOs.  To subscribe, contact the moderator Benny J Peiser at b.j.peiser@livjm.ac.uk .

The K-T event is back for the death of the dinosaurs

Just when those palaeontologists who don’t like ‘whizz-bang’ theories for the fossil record had begun once more to feel comfortable, the geological record has bitten back.

One of the main planks against an impact cause for the extinction of all the dinosaurs at the end of the Cretaceous Period was the raraity of their remains in the top 3 metres of the Hell Creek Formation in the Great Plains of North America.  The Hell Creek Formation is noted for clear signs of the Chixculub bolide strike very close to its top, as well as for a rich dinosaur fauna.  Previous workers stated that a rarity of dinosaur signs just below this signified that they were under considerable evolutionary stress before any catastrophe; support for a gradualist notion of mass extinction.  A team of geologists and biologists from the US have just published the results of a painstaking survey of the Hell Creek (15 thousand hours of field survey of 11 million square meters of its outcrops in North Dakota and Montana) (Sheehan, P.M. et al., 2000.  Dinosaur abundance was not declining in a “3 m gap” at the top of the Hell Creek Formation, Montana and North Dakota.  Geology, 28, p. 523-526).  Their work finds that the top 3 metres are just as rich in dinosaur signs as any of the strata below it, right up to the layer immediately beneath the signal of Chixculub.  They do not report any findings from above the impactite, though dinosaur teeth are reported to be present by earlier workers.

As journalists say, this will run and run!