Category: Planetary, extraterrestrial geology, and meteoritics

A year on from the landings of US Mars Rovers, Science devotes much of its early December 2004 issue to findings from the more revealing of the two missions, Opportunity (multi-authored 2004. Opportunity runneth over.  Science, v. 306, p. 1697-1756).  The articles are highly detailed accounts of the main finding from the various instruments aboard Opportunity, including the evidence for the activity of acid waters on the ancient Martian surface.  Equally interesting and considerably more graphic are important findings about volcanic and glacial activity in much more recent times, that come from the European Space Agency’s Mars Express Orbiter and the High Resolution Stereo Camera carried by it (Neukum, G and 42 others 2004.  Recent and episodic volcanic and glacial activity on Mars revealed by the High Resolution Stereo Camera.  Nature, v. 432, p. 971-979). Recently, excitement about evidence for living organisms on Mars rose with the discovery of significant amounts of methane in the Martian atmosphere.  Methane is likely to have a short life span (around 300 years) in the atmospheres of rocky planets.  There are two possible sources: methane-generating bacteria or release from volcanoes.  The High Resolution Stereo Camera shows conclusively that volcanoes were active on Mars until at least 5 Ma, when previously the planet was thought to be magmatically dead.  If fumarole activity continues, that could explain the traces of methane.

Analysis of historic, global seismograph records using sophisticated software allows far more than the detection of various discontinuities in the deep mantle and core that figure in most textbooks.  Essentially, it maps parts of the mantle where P and S waves travel faster or slower than expected from the depth.  Up to now, most results have been interpreted in simple terms of cold (fast) and hot (slow) patches, which have been linked to gross tectonic features such as signs of descending slabs far below the earthquake belts associated with subduction, and possible zones of rising mantle that might (or might not) be plumes.  That leaves a lot unsaid about the mantle, for rising and falling of material is linked to density, and that can be due to temperature anomalies, and also to compositional variations involving either bulk chemistry or different assemblages of minerals in mantle rock.  A difference in seismic wave speed can be an ambiguous indicator of possible motion.  Making the connections between wave speed, temperature and composition is an order of magnitude or more computationally taxing than the tomography itself, but it has been shown to be possible, given supercomputer power and plenty of free time (Trampert, J. et al. 2004.  Probabilistic tomographic maps chemical heterogeneities throughout the lower mantle.  Science, v. 306, p. 853-856).  Trampert and colleagues from the Netherlands and the US factored in mineral physics and temperature data, and were able to calculate the probabilities of tomographic features having a thermal or compositional origin.  Their results will worry some of the earlier workers on seismic tomography who used a simplistic connection with temperature and thus slow = hot = low density and rising, while fast = cool = high density and sinking.  Some zones of low wave speed can as well be connected with high-density mantle as with hot, buoyant material.  That plays havoc with concepts of plumes rising from the core-mantle boundary, that have been all the rage since moderately well resolving tomograms appeared.  Trampert et al’r results, which superficially look just the same as other tomographic renderings of the same seismic data, include statistical evaluations of the likelihoods of wave-speed shifts being either thermal or compositional in origin.  They reveal that many of the slow zones are probably chemical and mineralogical heterogeneities, especially in the deepest mantle levels.  One of the largest slow zones known rises obliquely from the core-mantle boundary around southern Africa towards the surface in NE Africa.  It was leapt on as a reputed superplume, perhaps connected to the last outpouring of flood basalts in Ethiopia and the Yemen around 30 Ma ago, and still active beneath the Afar Depression.  Chances are, from the new work, that it is denser than average and not especially hot.  Mantle geochemists will probably be gleeful at the new look at deep mantle, because they have long been wrangling ideas about gross lateral variations in the source chemistry of basaltic magmas.  Some enthusiastic geotectonic speculators might remain very silent, in the hope that the Dutch-US team’s work is not duplicated, and fades away…

See also:  van der Hilst, R.D. 2004.  Changing views on Earth’s deep mantle.  Science, v. 306, p. 817-818

Bedout end-Permian “impact” hammered

The claim that a large circular feature beneath the sea bed between Australia and New Guinea is linked to the end-Permian mass extinction (Becker, L. et al. 2004. Bedout: A possible end-Permian impact crater offshore of northwestern Australia.  Science Express 14 May 2004 – www.sciencexpress.org)  (See Crater linked to end-Permian extinction, June 2004 EPN) has met with a flurry of sceptical comment in letters to the editor of Science(2004, v. 306, p. 609-613).  Becker and colleagues have published several articles on the P-Tr boundary, including data on noble gases from the boundary in China, which are alleged to be consistent with an extraterrestrial influence, a meteorite from Antarctica which they consider to be a fragment of the impacting body and this year the claim for shocked minerals and impact glass in sedimentary core over the Bedout structure.  There have been unsuccessful attempts to duplicate the results on the noble gas analyses, the Antarctic meteorite is regarded as being insufficiently altered to be as old as 250 Ma, and as regards the Bedout material, the authors of the letters to Science consider none of the evidence to stand up to proper scrutiny.  One letter from specialists in the US, Russia, South Africa, Austria and the UK (Renne. P.R. and 7 others 2004.  Is Bedout an impact crater?  Take 2.  Science, v. 306, p. 610-611) also claims that the 250 Ma argon-isotope age for Bedout samples is misconceived and without objective basis.  One of the authors, Jay Melosh of the University of Arizona, is reported to have said that the Becker group, “..have deeply muddied the waters about what is going on at the Permian/Triassic boundary”.  These and material in the other letters are tough words indeed.  Becker’s group is funded by NASA, and when the flurry of letters hit home earlier in October, NASA sent a team of three scientists, including Becker, to resample the Chinese P-Tr boundary section.  Ten geochemistry laboratories will receive splits of the material to settle the issue of noble-gas evidence for an end-Permian impact.  But it looks very much as if a major scandal may break when the multi-lab analyses are published next year.  That is not to imply that there are no other skeletons lurking in cupboards along with impact-related materials.  A few years ago, editors of a major journal were asked to withdraw or refute a paper that used analyses of impact-related materials that had found there way to several laboratories without the permission of their originators or their names being mentioned.  The kudos associated with publishing on extraterrestrial influences on biological extinction patterns seems hard to resist…..

See also:  Dalton, R 2004.  Comet impact theory faces repeat analysis.  Nature, v. 431, p. 1027.

So, you are a geoscientist and you are interested in Mars.  Excellent!  Now read pages 793 to 845 of the 6 August 2004 issue of Science v. 305.  There is much to learn from 11 papers about the less revealing of the two Mars Exploration Rovers, Spirit.  Rover Opportunity has been getting the headlines, with its discoveries that relate to the influence of surface and subsurface water on superficial Martian minerals, such as the now well-publicised “blueberries” made of hematite, and the presence of sulphates.  A more informative digest of the mineralogy of Mars appears in the same issues’ News Focus (Kerr, R.A.  2004, Rainbow of Martian minerals paints picture of degradation.  Science, v. 305, p. 770-771).  Kerr makes clear that the really revolutionising instrument is orbiting Mars; the Visible and Infrared Mineralogical Mapping Spectrometer or OMEGA.  That is part of the payload of the ESA Mars Express, and measures radiant energy from the Martian surface with such spectral and spatial resolution, that the results can be compared with standard spectra of terrestrial minerals to see what the Martian surface is made of.  Hopefully, OMEGA will produce a hyperspectral database for the entire planet.  The on-surface readings from the various instruments on the NASA Rovers play much the same role as a field geologist would, by providing “ground truth” to validate the broader scope of the OMEGA instrument.  The hematite that dominates the overall red colour of Mars, has been confirmed by the Rovers, but to nobody’s great surprise.  The exciting find is just how much is owed to sulphate minerals, such as orange iron potassium sulphate, or jarosite.  The sulphate-rich veneer could well point to the influence of sulphuric acid, let alone water in Mars’ early surface environment, probably emitted as sulphur dioxide during intense volcanic activity.  Interestingly, the incompatibility of highly acid surface water with the preservation of carbonates could have thwarted drawdown of CO₂ from the Martian atmosphere (Fairén, A.G. et al. 2004.  Inhibition of carbonate synthesis in acidic oceasn on early Mars. Nature, v. 431, p. 423-426).  Formation and preservation of soil carbonate minerals would have collapsed the “greenhouse” warming mechanism demanded by the now proven influence of flowing water early in Martian history.  So long as sulphurous volcanic emissions overwhelmed carbonate formation, Mars might have stayed wet and warm.  The key is the duration of massive volcanism, which could be tied down by seeing how lavas have been affected by impacts in the minute detail possible from another Mars Express imaging instrument, the High Resolution Stereo Camera.  Planetary volcanic specialists reckon massive volcanism lasted for a considerable time

The 1800 Ma old Sudbury complex in eastern Canada is one of the largest repositories of nickel ores and contains commercial platinum deposits.  It has also been ascribed to a major impact that produced a crater over 200 km across.  The evidence is the common presence of shocked minerals and a sheet of very homogeneous, once molten rock, whose andesitic major-element composition suggests that it represents melting of the local upper crust.  However, the trace elements, including platinum group metals, have all the hallmarks of the lower crust (Mungall, J.E. et al. 2004.  Geochemical evidence from the Sudbury structure for crustal redistribution by large bolide impacts.  Nature, v. 429, p. 546-548.).  The melt sheet is mixed with upper crustal rocks, including sediments that formed in a shallow marine basin into which the meteorite plunged.  This suggests that impact not only affected the whole crust, but excavated it as well, so that a 30 km deep crater formed at the instant of collision.  The bulk of the homogenised crustal melt remained molten for long enough for complex fractional crystallisation to take place, thereby forming the classic layered Sudbury Igneous complex, in which the nickel ore bodies are located.  They may well represent relics of the impactor itself, that mixed with molten crust.

In mid May news spread fast that a nearly circular feature that shows up in gravity data over the north-western continental margin of Australia could be a crater, about 220 km across, which formed at the end of the Permian (Becker, L. et al. 2004. Bedout: A possible end-Permian impact crater offshore of northwestern Australia.  Science Express 14 May 2004 – www.sciencexpress.org). Australian and US scientists have examined drill cuttings from exploratory oil wells that penetrate to the level of the hidden feature.  They describe breccias and associated melt rock. A plagioclase separate from the exploration well has an Ar/Ar age of 250.1 ± 4.5 Ma, that is within error of the age (251 Ma) of the largest Phanerozoic mass extinction.  Unfortunately, they have not discovered the easily recognised signs of shock damage to minerals – distinctive banded lamellae in quartz – nor any meteoritic chemical signature.  Nevertheless, the structure is huge and looks very like the gravitational expression of the Chixculub crater off the Yucatan Peninsula of Mexico, drill core from which shows all the signs of having formed by an impact at the end of the Cretaceous.  Evidence is accumulating from the Permian-Triassic boundary sequence that some event did produce all the signs usually attributed to a major impact in a global ejecta blanket (see Permian-Triassic boundary and an impact?, December 2003 EPN).  Despite glass being included in the breccias, many experts on impact processes and products are sceptical that the Bedout structure was produced by an impact.  But probably the only way in which such melts might have formed is by some kind of seismic shock, although that could have occurred during volcanism..  The structure is so huge that if it does have an origin by internal processes it ranks among the biggest to be found – could this ironically be a product of a Verneshot event (see Mass extinctions and internal catastrophes, above)?!

Two lines of evidence from the current robotic explorations of Mars add to less tenuous ones that the planet is really wet – icy to be precise.  One is mineralogical.  Spectroscopy of the surface being slowly trundled across by a NASA rover, shows abundant signs of the hydrated, iron-potassium sulphate jarosite, which probably can only form under wet conditions.  When it was precipitated is not known with certainty, but it occurs in layered sediments that contain structures that clearly point to transport in and deposition from surface water.  The time when liquid water could exist at the surface probably goes back to the earliest events on Mars, tied to the famous canyons and more recently discovered dendritic drainage patterns.  The other evidence stems from even more remote sensing, that captures short-wavelength infrared radiation emitted by the Sun and reflected from the Martian surface.  Ices of water and carbon dioxide have distinct and unique reflected spectra, because of the different ways in which they absorb a small proportion of solar radiation.  Results from the OMEGA instrument aboard the European Space Agency’s Mars Express satellite show that the south polar region contains as much as 15% water ice mixed with solid CO₂ (Bibring, J-P et al. 2004.  Perennial water ice identified in the south polar cap of Mars.  Nature, v. 428, p. 627-630).

Inverting Robert Oppenheimer’s memory of the line in the Bhagavad Gita, “I am become Death, the destroyers of worlds”, during his Road-to-Damascus moment when the first atomic weapon was tested, may seem an odd headline for an article on geochemistry.  But geochemists sometimes do give the air of being on the verge of solving the “Big Question”.  Alex Halliday of ETH in Zurich is one of them (Halliday, A.N. 2004,  Mixing, volatile loss and compositional change during impact-driven accretion of the Earth.  Nature, v. 427, p. 505-509). It is now well accepted that Earth’s early evolution was one of repeated big impacts during planetary accretion.  It probably culminated in a collision with a Mars-sized planet that not only created the Moon from the debris splattered from both bodies, but set the Earth’s chemistry for all subsequent time; a sort of geochemists’ Year Zero.  When that happened and what ensued has all manner of connotations (see Geoscience consensus challenged in EPN for January 2004).  Halliday reviews evidence from several isotopic systems (Pb, Xe, Sr, W) that are reckoned to be appropriate “fingerprints” for the environments in which planets accreted.  His treatment takes the data as a whole, rather than separated into one or another isotopic system. He begins with the assumption in most accretion models that metallic cores form continuously and in equilibrium with the silicate outer mantle of rocky planets.  That is important in using W isotopes to model the “when”, since tungsten is likely to enter iron-rich metal rather than silicates (see Mantle and core do not mix in EPN February 2004).  In fact estimates for the time taken for the Earth to gather 2/3 of its mass based on W isotopes (~11 Ma) are a lot faster than those based on other isotopes (between 15 to 40Ma).  Halliday’s explanation is the seemingly sound one that when big things form from smaller ones (whatever contributed to core and mantle), the chances of them mixing and reaching equilibrium, before they definitively separate into the inner and outer Earth, are not good.  Reviewing the somewhat bewildering permissiveness of isotopic data from Earth and Moon that bear on “Year Zero” he concludes that the massive loss of xenon (and other “volatile” elements) that characterises Earth, by comparison with what is known about the Solar System’s pre-planetary composition, was 50 to 80 Ma after the “start of the Solar System”.  The Moon has provided insufficient data for its age of formation to be tied down isotopically.  Although its Hf-W age might be >44 Ma relative to the Earth’s beginning, there again, perhaps >54 Ma, and it may have formed even later.  Eventually we reach modelling (read “speculation”?) that takes us to the putative composition of the culprit for Year Zero, “Theia” (a Titan and the product of incestuous liaison between Uranus and his mother Gaia).

What seems odd to me is that some of the parent isotopes for those used in fingerprinting (e.g. ¹⁸²Hf for ¹⁸²W, and plutonium for a Xe isotope) can only form in supernovae events, and are so short-lived that the balance between their formation and their influence on partitioning of their daughters in planets is pretty delicate in terms of timing.  Indeed all radioactive isotopes, and every element with greater atomic mass than iron, in the Solar System have this origin, because it is impossible for a star the size of the Sun to form them.  Massive stars that become supernovas are common enough, and when they “go off” and what blend of heavy elements they produce depend on how big they were and when they formed.  Interstellar material is surely a mix of debris from a number of such events of different ages, and new stars and planetary systems form from that.  Maybe they are triggered by nearby supernovas, but that also contributes to the isotopic mix that has evolved since a galaxy formed.  Just suppose that the mix for the Solar System was heterogeneous, with differently aged uranium, thorium, rubidium, hafnium and other elements heavier than can be formed inside small stars like the Sun, and must have formed in big ones that eventually blasted their products into interstellar space.  If the Earth accreted as an open, non-equilibrated system, then what of the Solar System itself?  Bit early to say, really….

When an embattled US president, who as a Texan never visited the Johnson Space Flight Center in Houston, unveils plans for staffed missions to set up a lunar base and land on Mars, 10 years at the earliest after he becomes an ex-president, anyone become suspicious of an election stunt.  Former Democratic Vice-president Gore made the following observation that seems to stand above the tedium of US politics, “[It is]… an unimaginative and retread effort to make a tiny portion of the moon habitable for a handful of people”.  Much the same could be said of a Martian mission, when billions of Earthbound people find their homelands barely habitable.  The word “hubris” (insolent pride) springs to mind, for scientists who support such pies in the sky, as well as for politicians in an election year.  During the Apollo lunar missions the justification for sending people was that they could use their eyes, ingenuity and knowledge to collect samples.  The fact is that planetary scientists on terra firma specified the landing sites and told the astronauts what to collect, and of course all the sample analyses were made on Earth.  They did indeed revolutionise our understanding of how the Earth began its evolution and its record of bombardment by interplanetary debris.  Human hands were needed then, because robotics (servo-mechanisms, machine vision and remote control) were too primitive to collect material efficiently.  Within a month since Christmas Day 2003 three robotic laboratories and collecting systems have landed on the Red Planet.  One, a marvel of miniature sophistication (Beagle-2) seems to have died on touchdown.  The other two are NASA vehicles able to roam under close control and send back detailed close ups and make some analyses.  At the same time, imaging systems in orbit are providing more detail about Martian surface geology and landforms than exists for our home world, despite the efforts of geologists over the last two centuries.  Given 10 years or so of further robotic development, surface rock samples and cores of soils could be returned.  Look at it this way; a staffed mission has to send and return say 2 or 3 humans weighing upwards of 150 kg, along with all their requirements for a long mission, plus various weighty safety shields.  Given the same spacecraft without passengers, we are looking at more than half a ton of samples that could be returned for a fraction of the cost, if 2 or 3 humans forewent the massive privilege of standing on a not too welcoming planetary surface for a couple of days.

What issues remain to be addressed scientifically on the lunar and Martian surfaces?  For the Moon, the far side remains little known, but on which no human mission is likely to be landed, because it would be devoid of constant communication.  More samples of rock from the side that faces Earth would always be welcome, but robotics can grab them and bring them back.  For Mars the question is that of early life, but mainly to see if it did emerge in what increasingly seem likely to have been favourable albeit brief conditions, and if traces remain.  Geological matters are secondary to that, but nonetheless fascinating.  Yet, Mars is a far more complicated place than the Moon, and to properly grasp its evolution and composition, and whether it spawned and supported organisms, needs more than one mission to one site for a few days – all that a staffed mission could realise.  The Bush “vision” already threatens the single most important scientific instrument in orbit – the Hubble telescope.  The cost of developing human expeditions to both Moon and Mars would probably sterilise funds for more ambitious robotic exploration.  Indeed robots could invalidate their entire scientific justification long before the astronauts set off.  In order to check out the health risks of lengthy space missions, the so-far functionless International Space Station is to have life breathed into it, in the manner of a Frankensteinian white elephant.  The ageing and dangerous Shuttle fleet is to be kept alive, solely to service this legacy of Ronald Reagan’s bizarre two terms of office.  But, let’s live in the real world.  Who would stump up the funds necessary for a proper planetary exploration programme, when there will be no-one gazing steely-eyed into the camera saying how awed they are to be on Mars, Mr President?

Evidence from the neutron detector on Mars Odyssey suggested the possible existence of subsurface water on Mars (Water on Mars, August 2002 Earth Pages News).  I reluctantly succumbed to all the hype about what is implied by that, the more so when reports came in of dendritic drainages revealed by high-resolution elevation data (Case for Martian rainfall strengthens in October 2003 issue of EPN).  In planetary exploration, including remote sensing of the Earth’s surface features, progressive improvement in resolution generally reveals novelty.  The Mars Orbiter Camera, deployed by the Mars Global Surveyor mission has a resolution from 15 down to 2 metres.  For the Earth, you can get 15 m images freely from the ASTER programme, but to match the 2 m images would be very costly.  Given a broadband or better connection you can download the lot for Mars (http://pds-imaging.jpl.nasa.gov/atlas/).  It is this resource that scientists from Brown and Boston Universities in the USA and the Kharkov National University of the Ukraine have used to reveal the latest paradigm buster from the Red Planet (Head, J.W. et al. 2003.  Recent ice ages on Mars.  Nature, v. 426, p. 797-802).

James Head and his colleagues focused on the smooth terrains, or mantles, which drape over older deposits above 30º latitude on both Martian hemispheres, especially where water had been indicated by the Mars Odyssey neutron detector.  They were looking for signs of what on Earth would be regarded as periglacial features, formed by the growth and melting of subsurface ice.  They found lots, including signs of flowing ice-bound debris, but they do not show them in the Article, which deals with the implications of their findings.  An important conclusion is that at least some of the mantle may have formed by what could be described as very dirty snow – a mixture of ice and wind blown dust.  Judging the age of the deposits directly depends on the standard stratigraphic method for all planets other than the Earth and Moon, their relationship to signs of impacts.  There are very few fresh craters in the mantle, but many that have been “blurred” by it.  Head et al. suggest that the mantle dates to at most 10 Ma.  They resort to modelling climate shifts on Mars from its orbital and rotational history. Its rotational axis undergoes the greatest obliquity shifts of any planet, from about 15 to 35º over a 124,000-year cycle (unlike Earth’s tilt, which slowly rocks through a range of only 4 degrees thanks to the stabilising tuggings of our large Moon).  At high obliquity, the polar caps probably evaporate. loading the atmosphere with water vapour, so unlike the Earth it is global warming that induces low-latitude ice accumulation.  It is this modelling that encouraged the authors to suggest an ice age between 2 million and 400 thousand years ago.

More than 20 years since the proposal that the end-Cretaceous mass extinction coincided with a major impact, confirmed by the discovery of Chicxulub, nobody has produced convincing evidence for an extraterrestrial culprit for others.  Were geologists implanted with GPS tracking devices as soon as they graduated (no doubt on the cards in new health and safety regulations planned by the Blair government in Britain), then Big Brother would see strong clusters close to a number of boundaries on the geological map of the world.  There would be many at P-T sites.  Electronic tagging would have shown personnel from several US universities (Rochester, Harvard, California) in the Transantarctic Mountains, from time to time in the last few years.  Allegedly, that near-pristine area exposes rocks at the juncture between Permian and Triassic strata over less than a metre.  It is marked by the sudden disappearance of the famous Glossopteris flora, just below a clay breccia, from which this group of scientists have previously extracted evidence for shocked quartz and extraterrestrial fullerenes (football-shaped organic molecules) that contained odd noble-gas isotopes.  Two members of the team have made other finds of fullerenes, at the P-T boundary in China and Japan, the K-T boundary and the ancient Sudbury impact in Canada, whereas other workers have not been so lucky.  In fact, the duo are also the only people to have found fullerenes in meteorites, which is key evidence linking terrestrial finds to possible impact events.  The team has hit the headlines again (Basu, A.R et al. 2003.  Chondritic meteorite fragments associated with the Permian-Triassic boundary in Antarctica.  Science, v. 302, p. 1388-1392).  At first sight their discovery of pristine fragments of forsterite-enstatite rock with probable chondrules at the boundary suggests that indeed a major impact coincided with the biggest of all Phanerozoic mass extinctions.  They even report tiny grains of metallic iron with an astonishing purity, perhaps formed by condensation from the plasma cloud associated with a really big meteorite impact.  What is really odd, however, is that sedimentary rocks a quarter of billion years old should have preserved such highly unstable minerals.  All other finds of fossil meteorite fragments have been highly altered relics, as any geologist would expect.  There is a clamour for the Antarctic samples from other laboratories, so that the results can be confirmed or refuted. 

See also: Kerr, R.A. 2003.  Has an impact done it again?  Science, v. 302, p. 1314-1316, and Oxygen depletion before P-T extinction (above)