Category Archives: Planetary, extraterrestrial geology, and meteoritics

Mercury: sometimes a moist, organic-rich world

Posted on January 25, 2013 by | Leave a comment
Full-color image of from first MESSENGER flyby

Full-colour image of Mercury from MESSENGER  (credit: NASA via Wikipedia)

Astronomers welcomed in 2013 by suggesting from Kepler spacecraft data that the Milky Way galaxy alone probably hosts at least a hundred billion extrasolar planets and that a potentially habitable world the size of Earth probably lies within 20 light years of ours (go.nature.com/pxgbbt). OK, so there are at least 10-15 planets out there for every person likely to be alive by the mid-21 century when the technology becomes available to judge whether or not any of them hold a shred of interest for a population facing worsening living conditions right here.

Mercury is closer and currently being peered at in considerable detail by NASA’s MESSENGER mission to the Sun’s closest planet. The venture seems to have justified itself – and probably JAXA/ESA’s forthcoming BepiColumbo to be launched in 2015, arriving in 2022 – by showing that the long suspected ‘cold traps’ at Mercury’s poles have indeed trapped something: ice and abundant organic debris (Neuman, G.A . and 10 others 2013. Bright and dark polar deposits on Mercury: evidence for surface volatiles. Science, v. 339, p. 296-300).

The planet is exceeding rough, having been hit by objects of all sizes yet possessing insufficient internal energy to repave itself. Its axis of rotation is at a right angles to Mercury’s orbital plane, much like that of the Moon, so its polar regions are perpetually short of solar radiation. Deeply shadows places have been measured by infrared radiometry to be as cold as 25 degrees above absolute zero. Any volatile materials that might have landed in them or condensed there from earlier atmospheres might seem likely to stay there indefinitely. Not quite so, for the most likely compound, water ice, can sublimate away (shift directly from the solid to vapour state). Nevertheless, remote sensing shows the north pole region to be somewhat mottled dark and light on shadowed poleward-facing surfaces. The properties of backscattered radar beams and detection of emitted neutrons are consistent with the bright areas being water ice (Lawrence, D.J. and 12 others 2013. Evidence for water ice near Mercury’s north pole from MESSENGER neutron spectrometer measurements. Science, v. 339, p. 292-296). First estimates give a total ice volume of around 10 to 1000 km3 compared with almost 3 million km3 in the Greenland ice cap.

It’s the dark stuff that sets Mercury apart from, say, the Martian or lunar poles, the idea being that comets or icy asteroids impacting Mercury would have delivered complex organic compounds as well as water ice. This would temporarily give otherwise airless Mercury an atmosphere of volatiles parts of which might condense in the perpetually shaded parts of the polar region. Sublimation of exposed ice would have left a residue rich in those organic compounds that eventually protected deeper ice from fading away with time.

Now, imagine how supremely excited exo-planet hunters would be if they picked up such signals from a truly far-off world.

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On-line global geological maps

Posted on January 14, 2013 by | Leave a comment
Global geological map (credit: Commission for the Geological Map of the World

Global geological map (credit: Commission for the Geological Map of the World

Getting hold of geological maps on-line has been a hit or miss affair until recently, and those made available for free are at a variety of scales (generally less than 1:10 million) and vary in reliability and information content.  Scanned versions of paper sheets rendered with JPEG compression can leave a lot to be desired. If you are able to pay, then the situation improves as there are on-line vendors of printed geological  maps. But, all told, browsing the world’s geological features is a slow and generally frustrating task. The best bet might seem to be the Commission for the Geological Map of the World (http://ccgm.free.fr/) . They do, as you might expect, sell global maps, but at 1:50 million detail is sparse, although there is an alternative 3-sheet set (Old World, Americas and Polar regions) at 1:25 million, and it is possible to purchase digital versions and a variety of geophysical sheets. Maps at 1:5 million are available for Europe, Africa (6 sheets), the Middle East and South America plus various tectonic maps. However, to explore full planetary-scale geology at the modestly informative scale of 1:5 million demands visiting a lot of on-line vendors, as there is no one-stop shop for geologists

Small-scale extract from the OneGeology portal with 1:2 million maps for Ethiopia, Kenya, Tanzania and Uganda, and at 1:10 million covering surrounding areas (credit:OneGeology portal)

Small-scale extract from the OneGeology portal with 1:2 million maps for Ethiopia, Kenya, Tanzania and Uganda, and at 1:10 million covering surrounding areas (credit:OneGeology portal)

Such frustration is set to change, because in the last few years there have been moves to compile digital geology in a manner akin to Google Earth, now available at the OneGeology portal (http://portal.onegeology.org/). As soon as you enter the portal, the reason why the Commission for the Geological Map of the World is so irritating immediately becomes clear: the CGMW world map is what shows at the global scale and it doesn’t show much. Progressive zooming-in removes the 1:50 million map, to be replaced by a compilation of regional maps at scales ranging from 1:2 million to 1:12.5 million scales that does cover the entire Earth’s continental surface. A mouth-watering prospect until you start to look for legends! In fact, the associated tool box provides a means of pointing to individual stratigraphic units on the maps to get information (metadata), but whether and how it works depends on the source of the maps and the scale of viewing. For instance, the 1:10 million map of Africa gives no information, while the 1:5 million map of Europe gives quite a lot.

With a zoom to better than 1:10 million display, lots more detail appears in the form of country maps, but coverage is not comprehensive. In East Africa country maps are available for Ethiopia, Kenya, Rwanda and Tanzania – ranking with the current offerings from the USA. Moving to Europe, the range of scales improves on a country-by-country basis, generally 1:1 million to 1:250 thousand, but the UK truly grabs attention by providing digital geology at up to 1:50 thousand scale. The British Geological Survey has systematically rendered all its bedrock map data digitally to this scale, and is to be congratulated at making the ‘Full Monty’ available on the OneGeology portal. Full BGS metadata shows for all the visible stratigraphic and lithological units, together with faults and superficial deposits.

British Geological Survey bedrock mapping in Cumbria at 1:50 thousand scale. (credit: OneGeology portal)

British Geological Survey bedrock mapping in Cumbria at 1:50 thousand scale. (credit: OneGeology portal)

It soon becomes clear that OneGeology is a work in progress, but what a work it will be! If I have a criticism it is that geology is not linked to topography and cartographic features. The ever-present base data is the NASA Blue Marble mosaic of natural colour MODIS imagery. Unfortunately, outside of areas bare of vegetation this does not have any useful lithological connection, and is presented at such a large pixel size that only the coarsest topography shows up. At scales better than 1:2 million it is an irritating patchwork of square pixels. Far better would be shaded relief based on the ubiquitous ASTER GDEM data at up to 30 m resolution, especially as it is possible to vary the opacity of the geological maps to show the link with surface morphology. Maybe that is on its way and possibly oblique perspective 3-D viewing: one has to bear in mind that Google Earth wasn’t built in a day and geoscientific data are not yet standardised – a hugely costly endeavour, as that would involve not only digitising all maps but lengthy negotiations.

Most geologists are likely to be interested in maps that show rock units with stratigraphic age, but Jens Hartmann and Nils Moosdorf of the University of Hamburg, German have mined regional geological maps to assemble a global, purely lithological database (Hartmann, J. & Moosdorf, N. 2012. The new global lithological map database GLiM: A representation of rock properties at the Earth surface. Geochemistry, Geophysics, Geosystems, v. 13, doi:10.1029/2012GC004370) in cooperation with CGMW. Their Global Lithological Map (GLiM) consists of over 1.25 million digital polygons (ESRI shape or *.shp format), classified lithologically in three levels to give a total of 42 rock-type classes, 16 used in previous global lithological maps and two more lithologically specific sets of 12 and 14 subclasses . Though the database is said to be presentable at up 1:3.75 million scale, the version of GLiM that the reader can download is not in vector format but as a series of cells numerically coded according to class in a georeferenced grid. Since that is 360 rows x 720 columns, i.e. 0.5 degrees of latitude by 0.5 degrees of longitude, that version is useful only for rough statistics, such as the percentage of North America that is covered by evaporates, for instance. Perhaps the most useful aspect of the GLiM paper is the comprehensive referencing of the source maps. GLiM, apparently, is not an on-line resource, but no doubt the authors can provide interested parties with the *.shp files (contact jens.hartmann@zmaw.de or nils.moosdorf@zmaw.de)

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A glimpse of the deep Moon

Posted on December 30, 2012 by | Leave a comment

Charting the variation in gravitational potential across a planet provides a measure of the distribution of mass beneath its surface. That depends on both the planet’s actual shape and on internal variations in rock density. The Earth’s gravity has been mapped with varying degrees of precision, depending on sample spacing, by surface measurements using gravimeters. Doing gravity surveys from space cannot be so direct, however. One ingenious approach for the gravitational field over the oceans is to measure the mean height of the ocean surface using radar beams from a satellite. Since this is affected by variations in the gravitational field, partly due to bathymetry and partly because of varying density beneath the ocean floor, removing the calculable bathymetric effect leaves a gravitational signal from the underling lithosphere and deeper mantle. The first satellite to illuminate the Earth with radar microwaves, Seasat, gradually built up such a gravitational map of the deep Earth over a period of 105 days in 1978, which was followed up by other satellites such as the ERS series and Topex-Poseidon.

GRAIL lunar probes

The GRAIL satellites in lunar orbit (credit: Wikipedia)

It is not so easy to map gravity precisely above a solid planetary surface, but through the GRACE experiment this can be done by measuring very precisely the distance between a pair of satellites that follow the same orbit. As the gravitational field changes so too does the separation between the tandem of satellites; an increase in gravity pulls the satellites closer together and vive versa. GRACE has provided some fascinating data, such as estimates of the withdrawal of groundwater from large sedimentary basins and shrinkage of ice caps. However, GRACE is limited in its resolution of gravitational anomalies by the fact that Earth has an atmosphere above which such tandems must be parked in orbit to avoid burning up. The higher the orbit, the more degraded is the resolution. This effect is much less for Mars and non-existent for the Moon.

Gravity field of the moon as measured by NASA's GRAIL mission. The far side of the moon is at the centre, whereas the nearside (as viewed from Earth) is at either side. (credit: NASA/ARC/MIT)

Gravity field of the moon as measured by NASA’s GRAIL mission. The far side of the moon is at the centre, whereas the nearside (as viewed from Earth) is at either side. (credit: NASA/ARC/MIT)

A sister experiment to GRACE has been orbiting the Moon since September 2011: the Gravity Recovery and Interior Laboratory (GRAIL). First the tandem orbited at 55 km, then 22 and for a brief period 11 km, before running out of thruster fuel on 17 December 2012 and crashing into the lunar surface. Results from the highest orbit resolve lunar gravity to 13 km cells, recently reported on-line in three papers (Zuber, M.T. and 16 others 2012. Gravity field of the Moon from the Gravity Recovery and Interior Laboratory (GRAIL) Mission. Science, doi 10.1126/science.1231507; Wieczorek, M.A. and 15 others 2012. The crust of the Moon as seen by GRAIL. Science, doi 10.1126/science.1231530; Andrews-Hanna, J.C. and 18 others 2012. Ancient igneous intrusions and early expansion of the Moon revealed by GRAIL gravity gradiometry. Science, doi 10.1126/science.1231753). From crater gravitational signatures due to variations in surface topography it seems that the early bombardment of the lunar surface far exceeded previous assumptions. Impact effects dominate the GRAIL data at this resolution, but 2% of the information relates to structures hidden at depth.

500 km linear anomaly in the Moon's far-side gravitational field. (credit: NASA/JPL-Caltech/CSM)

500 km linear anomaly in the Moon’s far-side gravitational field. (credit: NASA/JPL-Caltech/CSM)

There are linear gravity anomalies extending over hundreds of kilometres, which may be huge igneous intrusions in the form of dykes; perhaps reflections of early influences of early extensional tectonics in the Moons lithosphere. Estimates point to this having been due to an up to 5 km increase in the lunar radius, probably as a result of thermal changes. The dominant feature of the lunar surface is not the near-side flat basaltic maria, visually prominent as they are, but the far more rugged lunar highlands which stand far higher because of the lower density of their constituent feldspar-rich anorthosites. GRAIL permitted a bulk estimate of the density of highland crust that turned out to be substantially lower, at 2550 kg m-3 – compared with 2600-2700 for granite and 2800-3000 for basalt – than originally estimated from samples returned by the Apollo mission. This forces a reassessment of the thickness of highland crust from 50-60 km to between 34 and 43 km, with a near-surface layer that has a porosity of around 12%, probably resulting from its awful battering. A thinner highland crust than previously assumed presents a bulk geochemical picture that need not be more enriched in ‘refractory’  elements, such as aluminium and calcium, than is the Earth.

Such unanticipated results from the low-resolution mode of the GRAIL experiment have its science team almost salivating at prospects from the sharper ‘pictures’ that will arise from the lower altitude orbits.

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New twist on lunar origin

Posted on October 22, 2012 by | Leave a comment
English: Giant impact - artist impression. Čes...

Artistic impression of the moon-forming giant impact. (credit: Wikipedia)

Although a few would-be space faring countries have ambitions, a post-Apollo crewed mission to the Moon is unlikely for quite a while. Yet moon-struck curiosity goes on: currently there is a surge in re-examining the lunar samples brought back more than 40 years ago. The Lunar Sample Laboratory Facility in Houston holds about a third of a ton of rock and regolith. I suppose part of the reason why lunar rocks are being re-analysed – in fact some for the first time – is because new or improved methods are available, but frustration among  a growing community of planetary geochemists having little more than meteorites to peer at probably plays a role as well. Since Hartman and Davis first suggested it, the giant impact theory for the Moon’s origin has dominated geochemical ideas. Most tangible is that of a magma ocean, floated plagioclase crystals from its fractional crystallisation probably having formed the glaring white lunar highlands composed of anorthosite. More subtle are ideas about what happened to the Mars-sized planet that did the damage to Earth and flung vaporised rock into orbit to accrete into the new Moon, and the effects of the stupendous energy on the geochemistry of all three bodies. Directed at all that is new research on isotopes of zinc (Paniello, R.C. et al. 2012. Zinc isotope evidence for the origin of the Moon. Nature, v. 490, p. 376-379).

The focus on zinc is because it is easily vaporised compared with more refractory materials, such as calcium an titanium, and as well as being ‘volatile’ it has five naturally occurring isotopes with relative atomic masses of 64 (the most abundant), 66, 67, 68 and 70. In general, isotopes of an element behave in slightly different ways during geological and cosmological processes, which changes their proportions in the products; a process known as ‘mass-fractionation’. Paniello and colleagues from Washington University, Missouri and the Scripps Institution of Oceanography, California USA found that Moon rocks are enriched in the heavier isotopes of zinc yet depleted in total zinc compared with terrestrial rocks and meteorites supposed to have come from Mars. Unlike those two planets the Moon’s zinc deviates from its abundance relative to other elements recorded by chondritic meteorites. This zinc depletion tallies with volatile loss from incandescent vapour blurted from the colliding planets. But it doesn’t help with the detailed predictions from the giant-impact model. A variety of scenarios suggest that the Moon should be made from remnants of the inbound impactor’s mantle, yet studies of other elements’ isotopes indicate that the Moon is rather Earth-like. But not those of zinc, so it looks like they have to be explained by a complete rethink of the whole hypothesis (Elliott, T. 2012. Galvanized lunacy. Nature, v. 490, p. 346-7).

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Are Martian clays magmatic in origin?

Posted on September 16, 2012 by | Leave a comment
593496main pia14840 full Curiosity Touching Do...

Artist’s Concept of Curiosity’s touchdown(credit: Wikipedia)

The remote detection of spectral features in the infrared that suggest abundant clay minerals on the surface of Mars is the basis for a widely-held view that Mars may once have had moist climatic conditions that encouraged life to form (see The Martian ‘sexy beast’ in September 2012  EPN). The presence of clays, along with suggestive landforms, has also been used to speculate that Mars once harboured long-lived lakes and perhaps even a huge ocean on its northern hemisphere, between 3.7 to 4.1 Ga. It was the clays that pitched the recently arrived Curiosity (aka Mars Exploration)Rover at the Gale crater and its central Aeolis Mons. The latter, also known as Mount Sharp, preserves about 5 km of layered rocks, the lowest of which are clay-rich and hypothesised to be sediments laid down in a lake that filled the crater. Provided Curiosity operates according to plan, we will know soon enough whether or not the layered rocks of Mount Sharp are indeed sediments, but a soon-to-be-published article suggests another explanation than weathering for the production of abundant clay minerals on Mars (Meunier, A. et al. 2012. Magmatic precipitation as a possible origin of Noachian clays on Mars. Nature Geoscience, published online 9 September 2012; DOI: 10.1038/NGEO1572).

Focusing the 100-millimeter Mastcam [detail]

Layered rocks on the flanks of Mount Sharp in Gale crater from Curiosity’s Mastcam (NASA Goddard via Flickr)

The French-US team provides evidence from terrestrial lavas that abundant iron- and magnesium-rich clays, known as smectites, may form at a late stage during crystallization of magma. If magma contains water – and most magmas do – as more and more anhydrous silicates crystallise during cooling water builds up in the remaining liquid. Once silicate crystallisation is complete there remains a watery fluid capable of reacting with some of the silicates to form clay minerals; a process often referred to as pneumatolysis. How much clay is formed depends on the initial water content of the magma. Pneumatolysisoperates on hot lava, whereas weathering occurs at ambient temperature provided the climate is able to support liquid water at the surface. Mars is currently far too cold for that, and ideas of a wet surface environment earlier in the planet’s history demand an explanation for a much warmer climate. Clay minerals do not appear to be present in Mars’s younger rocks, so Meunier and colleagues suggest that as the planet’s mantle evolved early water-rich magmas were gradually replaced by ones with less water: interior Mars was gradually de-gassed and its magmas lost the ability to alter minerals that crystallised from them.

Now, clay minerals are extremely resistant to change except through high-temperature metamorphism. Once formed they can be blown around – Mars has probably always been a very windy place – to end up in aeolian sediments that are plentiful on Mars.  Also, if occasionally water flowed on the surface perhaps by subsurface water venting suddenly, fine-grained pneumatolytic clays would easily be picked up, concentrated as flow speed lessened and deposited in waterlain sedimentary layers.  A dilemma that faces the Curiosity science team is what significance to assign to clays in sediment layers, when they no longer provide unequivocal evidence of weathering.  But will the resistant layers on Mount Sharp turn out to be pneumatolytically altered lava flows?
Note added 28 September 2012: The first scientific triumph of the Curiosity Rover is imagery of sediments in what had been suggested to be an alluvial fan washed into Gale crater. They show gravels with rounded pebbles.

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Whence Earth’s water?

Posted on August 25, 2012 by | Leave a comment
English: Carbonaceous chondrite Meteorite. The...

Carbonaceous chondrite meteorite. (credit: Mila Zinkova via Wikipedia)

English: Image of comet C/1996 B2 (Hyakutake),...

Comet Hyakutake. (credit: E. Kolmhofer & H. Raab via Wikipedia)

Because they can be so big, consist mainly of water ice and there are probably a great many lurking in the outer reaches of the solar system impacting comets have long been thought to have delivered the water that makes the Earth so dynamic and, so far as we know, the only place in near-space that hosts complex life. Remote sensing studies of the isotopic composition of water in one comet (Hartley 2) caused great excitement in 2011 by showing that its ratio of deuterium to hydrogen was very similar to that of Earthly ocean water. Other D:H ratios have recently been published from a suite of meteorites gleaned from the surface of Antarctic ice (Alexander, C.M.O’D. et al. 2012. The provenances of asteroids, and their contributions to the volatile inventories of the terrestrial planets. Science, v. 337, p. 721-723). These meteorites are carbonaceous chondrites thought to be the source of much of the solid material in planets of the Inner Solar System. To cut short a long and closely argued argument, it seems that the CI-type chondrites’ water is isotopically quite different from that in analysed comets, knocking another popular hypothesis on the head; that comets and carbonaceous chondrites formed in the same part of the Solar System.

Since hydrocarbons in comets – known from interplanetary dust particles – contain hydrogen with a far richer complement of its heavy isotope deuterium than does cometary water ice, the crashing of entire comets onto planets such as the Earth would not produce the observed terrestrial D:H ratio even though their water ice alone does match it. The US, British and Canadian meteoriticists conclude what seems to be a unifying explanation whereby CI chondritic solids and volatiles alone would have been able to form the Inner Planets and their various complements of water by initial accretion. Comets as a second-stage source, in this account, are relegated to mere curiosities of the Solar System with little role to play other than occasional big impacts that may, or may not, have influenced evolution by the power that they delivered not through their chemistry.

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The Martian ‘sexy beast’

Posted on July 27, 2012 by | Leave a comment

Artist’s concept of NASA’s Mars Science Laboratory (Curiosity) near a canyon on Mars. (Credit: NASA-JPL via Wikipedia)

Why is ’Curiosity’ the latest Mars rover aimed to land at Gale Crater? It seems to have been filled with stratified sediments deposited in the crater over perhaps as long as two billion years after it formed by a meteorite impact. The sediments now occur as a relic of later aeolian erosion at the centre of the crater in the form of a large mound that Curiosity is designed to climb and sample. The big attraction is the detection of clays and sulfate minerals in the sediments using multispectral remote sensing. They clearly suggest the influence of water in the formation of the sediments, hence the suggestion that they are lake sediments. On that assumption, Gale Crater is hoped to be a fruitful site for seeking signs of former biological processes: given the technical circumstances of the mission it is deemed the best site there is on Mars for NASA’s Mars Science Laboratory.

Sulfates on Mars have excited geologists enormously, along with their companion clays, because they signify the influence of abundant acid water in the breakdown of Martian primary igneous rocks from which the sediments have undoubtedly been derived. Their formation is undoubtedly the geoscientific ‘sexy beast’ of the last four or five years. Given multi-channel remotely sensed data – and Mars labs are awash with them from several previous missions – sulfates are easy to detect from their distinctive reflectance spectra so there has been abundant pay-back for geologists involved with the Red Planet. But there is water and there is…water. It is hoped to be proved that the depositional medium was standing water or at least abundant subsurface aqueous fluids, which may have lingered for long enough for living organisms to have formed. But there is a possibility that sulfates can form, and so too clays, by superficial weathering processes beneath a humid atmosphere.

English: This oblique, southward-looking view ...

An oblique view of Gale crater showing the landing site and the mound of layered rocks that NASA’s Curiosity rover will investigate. The landing site is outlined in yellow. (Credit: NASA-JPL via Wikipedia)

Erwin Dehouck and  team of French geochemists set out experimentally to recreate conceivable atmospheric and climatic conditions from Mars’s early history to mimic weathering processes (Dehouck, E. et al. 2012. Evaluating the role of sulfide-weathering in the formation of sulfates or carbonates on Mars. Geochimica et Cosmochimica Acta, v. 90, p. 47-63). The experiment involved liquid water and hydrogen peroxide (detected in Mars’s present atmosphere and probably produced photochemically from water vapour) in contact with a CO2 atmosphere.  Martian surface conditions were simulated by evaporation of H2O and H2O2 to mix with dominant CO2, which allowed ‘dew’ to form on the experimental samples. The samples consisted of ground up olivine and pyroxene, important mineral constituents of basalt – feldspar was not used. – mixed with the iron sulfide pyrrhotite, commonly found in terrestrial basalts and meteorites judged to have come from Mars. Samples of each pure mineral and mixtures with the sulfide were left in the apparatus for four years and then analysed in detail.

Even in such a short exposure the silicate-sulfide mixtures reacted to produce sulfate minerals –hexahydrite (MgSO4_6H2O), gypsum (CaSO4_2H2O) and jarosite( KFe3 (OH)6(SO4)2), together with goethite (FeOOH) and hematite (Fe2O3). Without the presence of sulfides, the silicate minerals barely broke down under the simulated Martian conditions but did produce traces of magnesium carbonate. The sulfate bearing assemblages look very like those reported from many locations on Mars. The acid conditions produced by weathering of sulfides to yield sulfate ions are incompatible with preservation of carbonates, as the experiment indicates. However, there are reports of Martian sediments that do contain abundant carbonate minerals.

The researchers’ conclusions are interesting: “These results raise doubts on the need for a global acidic event to produce the sulfate-bearing assemblages, suggest that regional sequestration of sulfate deposits is due to regional differences in sulfide content of the bedrock, and pave the way for reevaluating the likelihood that early sediments preserved biosignatures from the earliest times”. Weathering by dew formation seems quite adequate to match existing observations.

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The oldest impact structure

Posted on July 26, 2012 by | Leave a comment
Ilulisat Isfjord

Ilulisat Grenland (credit: Wikipedia)

Various lines of evidence, such as sedimentary deposits of glass spherules and shocked minerals or signs of unusual isotopic chemistry (see Ejecta from the Sudbury impact and Evidence builds for major impacts in Early Archaean in EPN April 2005 and August 2002) point to the predicted intensity of meteorite or comet bombardment of the early Earth, and evidence is growing for some events that had global effects. Yet no actual impact sites from the Archaean Eon have been found, until recently. That is not entirely unexpected because erosion during the last few billion years will have removed all trace of the characteristic surface craters. But perhaps there is cryptic evidence in Archaean terrains for the deeper influence of impacts: after all, the depth of penetration of large meteoritic ‘missiles’ would have been of a similar order to their diameter where shock structures in minerals would slowly anneal and impact-generated melts would crystallise slowly enough to masquerade as plutonic igneous rocks.

Close to the Arctic Circle in SW Greenland Archaean gneisses are associated with a roughly 200 km wide geomagnetic anomaly and regionally curvilinear features that suggest a series of concentric closed structures over a 100 km diameter area (Garde, A.A. et al. 2012. Searching for giant, ancient impact structures on Earth: The Mesoarchaean Maniitsoq structure, West Greenland. Earth and Planetary Science Letters, v.  337, p. 197-210). Adam Garde and colleagues from the Greenland Geological Survey, Cardiff University UK and Lund University Sweden focused on the central part of these anomalies where gneisses are extensively brecciated with signs of annealed shock-induced lamellae in quartz, feldspar melting and fluidization of highly comminuted mylonites. They ascribe this assemblage of features on a variety of scales to the effects of a major meteorite impact on 25 km deep continental crust. The metamorphic complex contains the famous Amitsoq Gneisses that once had the status of the world’s oldest rocks at around 3.8 Ga, but is dominated by migmatites formed around 3.1 Ga that are akin to the Nuuk Gneisses from further south.

The possible signs of a deeply penetrating impact are cut through by small ultramafic intrusions, zircons from which yield 207Pb/206Pb ages between 3.01 and 2.98 Ma, confirming the structures’ Mesoarchaean age. An interesting and unanswered question concerns the origin of these magmas together with marginally younger, voluminous granites. Were the ultramafic magmas generated by high degrees of partial melting of mantle as a result of the immense energy of impact?  Having temperatures well above those of basaltic melts, could the ultramafic intrusions in turn have induced crustal melting within the depths of a large impact basin?

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A mighty sag or a big wrench for Mars

Posted on July 10, 2012 by | Leave a comment
MOLA colorized relief map of the western hemis...

Colour-coded relief map of the Thatsis bulge on Mars, with Valles Marineris at left centre (Credit: Goddard Space Flight Center, NASA, via Wikipedia)

In the Solar System topographic features don’t come larger than Valles Marineris on Mars. At between 5 to 10 kilometres deep and extending along a fifth of the planet’s circumference, it makes the Grand Canyon and The Gorge of the Nile look puny.

The base and margins of this stupendous valley contains all manner of evidence for erosion, huge landslips and signs of collapse into voids in Mars’s crust. Much of the erosion on Mars seems to have stemmed from catastrophic floods several billion years ago, though whether they were all of water or if some were volcanic in origin is being debated (Leverington, D.W. 2011. A volcanic origin for the outflow channels of Mars: Key evidence and major implications. Geomorphology, v. 132, p. 51-75 http://www.webpages.ttu.edu/dleverin/leverington_mars_outflow_channels_geomorphology_2011.pdf  , but see http://www.universetoday.com/94367/did-water-or-lava-carve-the-outflow-channels-on-mars/)

It is difficult to imagine anything other than some kind of fault control over the almost straight, roughly east-west trend of Vales Marineris, but the scale suggests, again, an unmatched scale of tectonics. It has long been thought that the massive canyon resulted from extensional rifting that created a major weakness etched out by later erosion and/or collapse into huge subsurface voids in the crust. Yet there is little sign of commensurately large faults, through there are some. But the structure is an integral part of yet another superlative. It is on the eastern flank of the mighty Tharsis bulge on which several humongous volcanoes, including Mons Olympus, developed: perhaps there is a causal link between the two dominating features.

Jeffrey Andrews-Hanna of the Colorado School of Mines in the US has tried to model the bulge-chasm pair, coming to the conclusion that there is little sign of major extension. The finale of his study zeroes-in on the possibility of dominant subsidence producing the structure (Andrews-Hanna, J.C. 2012. The formation of Valles Marineris:  3. Trough formation through super-isostasy, stress, sedimentation, and subsidence.  Journal of Geophysical Research, v. 117, E06002, doi:10.1029/2012JE004059).

In this model, the Tharsis bulge and its associated volcanic province rose so high that on the scale of the planet it must have created a large positive gravitational anomaly. This remains for the most part, but in the Valles Marineris region the crust is now either in isostatic balance or has large negative gravity anomalies, complicated by the fact that the very carving of the canyon system must have resulted in some uplift through unloading. For a while the whole bulge was supported in this gravitationally unstable state by the strength of the Martian lithosphere, and most of it is still in a state of disequilibrium.

Andrews-Hanna’s novel view is that a small amount of extension allowed residual magma to rise in linear zone along the eventual length of Valles Marineris as dykes. The magmas and their heating effect reduced the strength of the lithosphere, locally removing support for the huge load, which subsided. By creating greater slope on the surface of Tharsis the subsidence would have become a focus for both erosion and sedimentation, the increased sedimentary load adding to the subsidence to give the present stupendous depth of the canyons and chasms.

Polski: NASA World Wind - Mars (MOLA Shaded el...

Simulated oblique view of the topography of Valles Marineris looking westwards (Credit: Goddard Space Flight Center, NASA, via Wikipedia)

But this isn’t the only model for the canyon system (Yin, A. Structural analysis of the Valles Marineris fault zone: Possible evidence for large-scale strike-slip faulting on Mars. Lithosphere, v. 4 doi:10.1130/L192.1). An Yin of the University of California used a combination of remote sensing data from Mars Reconnaissance Orbiter and Mars Odyssey to perform detailed lithological and structural mapping  along Valles Marineris. What emerged were several  fault zones up to 2000 km long. Instead of an expected extensional sense of movement they are strike-slip faults, with displacements of the order of 100 km in a left-lateral sense. Yin’s model is that the canyon system bean as a zone of transtensional  deformation: very different from that of Andrews-Hanna. It also begs the question of the underlying tectonic processes, because strike-slip zone on Earth are usually associated with distributed stress from plate tectonics.

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Two smoking barrels on the Moon

Posted on March 21, 2012 by | 1 Comment
This image is an elevation map of the South Po...

Elevation map of the South Pole-Aitken basin on the Moon, from the NASA/SDIO probe Clementine mission. magenta and blue show the lowest elevation rising through a rainbow spectrum to red, the highest elevations

The South Pole and the farside of the Moon contain, at 2500 km across and 13 km deep, the largest impact structure in the Solar System: the South Pole-Aitken (SPA) basin. Being partly camouflaged by many later craters up to several 100 km across, typical of the lunar far side and the lunar highlands in general, the SPA basin formed early in the Moon’s cratering history, and is unlike the mare basins of the near side that are filled with basalt lavas. The light colour of the lunar highlands into which the SPA basin was excavated signifies that they are dominated by almost pure feldspar in the form of anorthosite rock. These anorthosites are prime evidence for the former melting of much if not all of the Moon at the time of its formation: low-density feldspar with a very high melting point could only have accumulated with the degree of purity of anorthosite if early-formed crystals floated to the top of the magma ocean.

Total magnetic field strength at the surface o...

Total magnetic field strength at the surface of the Moon from the NASA Lunar Prospector mission

The other feature of feldspars is that they are among the least magnetic of minerals, so it came as a surprise that the northern rim of the SPA basin is studded with positive magnetic anomalies (Wieczorek, M.A. et al. 2012. An impactor origin for lunar magnetic anomalies. Science, v. 335, p. 1212-1215). Lunar samples returned by the Apollo Programme are consistently lacking in all but the weakest remanent magnetism, suggesting that the Moon either never had a magnetic field or if it did the field was extremely weak. Even if it did once have a magnetic field, the anomaly patterns are small with high amplitude and reminiscent of a target hit by a shotgun blast. Similar anomalies are scattered on the near side.

The SPA basin is elliptical, suggesting that the projectile responsible for it struck at an oblique angle. The far=side magnetic anomalies cluster exactly where impact modelling would suggest for debris displaced by impact from a northward travelling body. The interpretation arrived at by Mark Wieczorek of the Parisian Institut de Physique du Globe and colleagues from MIT and Harvard University in the US is that the anomalies mark landing sites for large fragments of an easily magnetised,  iron-rich asteroid that excavated the basin. Moreover, the same impact might explain magnetic anomalies much further from the basin, on the lunar near side. The remaining mystery is how fragments of the impactor came to be magnetised. The impact would have ensured their being heated well above the temperature of the Curie point at which even the most magnetically susceptible materials lose their magnetisation. The most likely possibility is that the fragments attained their magnetised state at a time when the moon did have a core-generated magnetic field, albeit weak.

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