Category: Planetary, extraterrestrial geology, and meteoritics

The Martian ‘sexy beast’

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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

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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

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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

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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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Galactic controls

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English: Artist's conception of the Milky Way ...
Artists impression of the Milky Way viewed along its axis. Image via Wikipedia

Palaeoclimatologists are quite content that an important element in controlling the vagaries of climate is due to gravitational forces that cyclically perturb Earth’s orbit, it axial tilt and the way the axis of rotation wobbles in a similar manner to that of a gyroscope. The predictions about this by James Croll in the late 19th century, which were quantified by Milutin Milankovich during his incarceration during World War I, triumphed when the predicted periods of change were found in deep-sea floor sediment records in 1972. Authors of ideas that link Earth system changes  to the progress of the Solar system through the Milky Way galaxy haven’t had the same accolades. One of the first to suggest a galactic link was Joe Steiner (Steiner, J. 1967. The sequence of geological events and the dynamics of the Milky Way Galaxy. Journal of the  Geological Society of  Australia, v.  14, p. 99–132.) but his work is rarely credited.

There has been an upsurge of interest in the last decade or so. In a recent issue of New Scientist Stephen Battersby reviews what galactic ‘forcings’ may have accomplished during the 4.5 billion-year history of our world (Battersby, S. 2011. Earth odyssey. New Scientist, v. 212 (3 December issue), p. 42-45). Having formed probably much closer to the galactic centre than its current position the Solar System has drifted, perhaps even ‘surfed’ gravitationally, outwards to reach its present ‘suburban’ position in one of the spiral arms. There are regularities to the now stabilised orbital movements: once every 200 million years the Solar System completes a full orbit; this orbit wobbles across the hypothetical plane of the galactic disc by as much as 200 light years, moving with and against the Milky Way’s cosmic motion. It has proved impossible so far to detect any sign of the orbital 200 Ma periodicity in events on the Earth, and most attention has centred on the wobble.

Steiner suggested that this motion may have crossed different polarities of the galactic magnetic field, perhaps triggering the periodicity of geomagnetic  changes in polarity, but this now seems unlikely. However, his suggestion that glacial epochs, such as those in the Palaeo- and Neoproterozoic, at the end of the Palaeozoic Era and at present, may have resulted from the Solar System’s passage through dust and gas banding in the Milky Way continues to have its attractions (e.g. Pavlov, A.A. et al. 2005. Passing through a giant molecular cloud: “Snowball” glaciations produced by interstellar dust, Geophysical Research Letters, v. 32, p. L03705). The direction of motion relative to the Milky Way’s cosmic drift governs the exposure to cosmic rays that result from a kind of ‘bow-shock’ ahead of the galaxy

Stellar motion through the Milky Way is semi-independent so that from time to time the Solar System may have been sufficiently close to regions of dense dust and gas that nurture the formation of super-massive stars. These huge objects quickly evolve to end in supernovae, proximity to which would have exposed life to ‘hard’ X- and  γ-rays and would be trigger for mass extinction, for instance by accompanying cosmic rays in destroying the ozone protection from UV radiation from the Sun.

The dynamism of the Earth and the resulting complexity of its surface processes makes it a poor place to look for physical signs of galactic influences. No so the Moon: for almost 4.5 billion years it has been a passive receptor for virtually anything that the cosmos could fling at it, and so geologically inert that its surface layers may well preserve a complete ‘stratigraphic’ record of all kinds of process. Should lunar landings with geological capabilities once more prove economically possible, or politically useful, that hidden history could be read.

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Mercury: anything new?

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Full color image of from first MESSENGER flyby
Mercury from an earlier MESSENGER fly-by. Image via Wikipedia

The Sun’s nearest planet, Mercury, seems odd in some ways; for instance, it has a proportionately larger metallic core than any other planet. That feature has led some to suggest that somehow most of any original silicate mantle was lost. One possibility is that its proximity to the Sun resulted in Mercury’s surface being ablated. Another looks to a huge collision with another body that tore off much of the mantle; similar to the event that the chemical commonality of the Earth and Moon suggests early in Earth history. Both processes should have left a distinct geochemical signature on the surface of Mercury: some kind of residue of solar ablation or evidence of fractional crystallisation of a magma ocean, such as the feldspar-rich lunar highlands that are probably formed of crystals that floated as such a planetary silicate melt cooled and evolved. The seeming strangeness of Mercury helped underpin a well-equipped un-crewed mission, going by the acronym MESSENGER, that finally settled into Mercury orbit in March 2011 after a planned ‘yo-yoing’ path since launch in August 2004 that took it back and forth between Earth, Mercury and Venus in its early stages. Early analysis of results from the now permanent orbit appeared in the 30 September 2011 issue of Science.

MESSENGER carries several remote sensing instruments: a stereo imaging device to map landforms, and topography; a laser altimeter to back the stereo imager; a visible to short-wave infrared spectrometer to map variations in surface spectra and minerals; gamma-ray spectrometry to map distributions of naturally radioactive isotopes and emissions from other elements triggered by high-energy cosmic ray bombardment; using the Sun as a source of gamma- and X-rays to cause a variety of elements to emit lower energy X-rays – a variant of X-ray fluorescence spectrometry that is a workhorse of lab geochemistry.

The earlier Mercury fly-bys and previous missions clearly showed that its surface is heavily cratered but possesses areas resurfaced by lavas that obliterate older cratering. A little like the lunar maria in age and appearance, these smooth terrains show evidence of accumulations up to a kilometre thick formed by repeated lava flows (Head, J.W. and 25 others, 2011. Flood volcanism in the northern high latitudes of Mercury revealed by MESSENGER. Science, v. 333, p. 1853-1855). As regards the age of these major volcanic features, all that can be said is that they post-date the largest impacts, such as the huge Caloris Basin, and are more sparsely peppered with younger craters. Intriguingly, floors of some of the craters show clusters of small depressions and pits surrounded by light-coloured material of some kind, suggested to be solids condensed from gases that emerged from below (Blewett, D.T. and 17 others 2011. Hollows on Mercury: MESSENGER evidence for geologically recent volatile-related activity. Science, v. 333, p. 1856-1859). While it is only possible to assign youth of these features relative to the craters in which they occur, they indicate an underlying source of volatiles; a factor weighing against previous accounts of Mercury’s evolution by either solar ablation or giant impact.

Considerably more interesting – at least to me – are the results from the geochemically oriented instruments. Calcium, magnesium, aluminium and silicon estimates by the XRF-like instrument present not the slightest evidence for a feldspar-rich component of the early crust akin to the lunar highlands; another blow for the giant-impact and magma-ocean hypotheses. Mercury’s surface seems to be similar in composition to the most ancient terrestrial lavas: Mg-rich mafic to ultramafic komatiites, compared with the more iron-rich tholeiites of the lunar maria (Nittler, L.R. and 14 others. The major-element composition of Mercury’s surface from Messenger X-ray spectrometry. Science, v. 333, p. 1847-1850). They are ten-times more enriched in sulfur than surface rocks on the Earth or Moon, though iron content seems too low to accommodate it in minerals such as pyrite (FeS2). High sulfur content could point to an origin for Mercury from accretion of highly reduced material in the solar nebula, the Earth-Moon system being broadly more oxidised. Gamma-ray spectrometry to analyse the abundances of potassium, uranium and thorium (Peplowski, P.N. and 16 others. Radioactive elements on Mercury’s surface from MESSENGER: implications for the planet’s formation and evolution. Science, v. 333, p. 1850-1852) doesn’t serve previous ideas about the planet’s history either. Potassium, which is moderately volatile, is too high relative to more refractory uranium and thorium to support any notion of solar ablation of the surface, but the U, Th and K proportions are roughly like those of the Earth’s oceanic crust. One of the plots shows K-Th relationships for supposed meteorites from Mars and the extensive gamma-ray data from Mars itself, in which few of the meteorites fall in the K-Th ‘cloud’ for the Martian surface: now there’s a thing….

It must be emphasised that the geochemical results are but a fraction of what should eventually emerge from these powerful instruments. However, these early data place Mercury in much the same envelope as the other rock worlds of the Inner Solar System (Kerr, R.A. 2011. Mercury looking less exotic, more a member of the family. Science, v. 333, p. 1812).

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The useful geoneutrino

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The Sudbury Neutrino Detector
A neutrino detector in Canada similar to KamLAND. Image via Wikipedia

While the wires were hot with news of neutrinos possibly having exceeded light speed as they were fired through the Alps by the Large Hadron Collider, steady research has been seeking answers rather than perhaps transmuting physicists’ hubris into a death wish. (Note:  it has to be said that  British theoretical physicist Jim Al-Khalili has sufficient confidence that the speeding ticket issued to the neutrinos will be rescinded that he promises to eat his underpants if it is upheld.) The more tangible work concerns antineutrinos that the Earth emits, dubbed ‘geoneutrinos’ to distinguish them from extremely exotic ones from deep space which, worryingly for some, pass from one side of the Earth to the other and through us as well.  When unstable isotopes, such as those of uranium, thorium and potassium that help heat the Earth, decay they emit antineutrinos as well as electrons, helium nuclei and gamma-rays. Notoriously elusive, neutrinos and antineutrinos can now be detected with sufficient precision to make useful observations, as well as produce results that have many theoretical physicists quivering in cellars from which they emerge, from time to time, covered with chalk dust from their desperate exertions to explain a material speed faster than ‘little c’. To geoscientists, the results of an experiment using geoneutrinos at the Japanese Kamioka Liquid-Scintillator Antineutrino Detector (KamLAND), which involved 66 individuals from 15 Japanese, US and Dutch institutions, are much more interesting: they help resolve a long-standing puzzle about the source of geothermal heat that flows from the Earth’s surface at a rate of about 44 TW (The KamLAND Collaboration 2011. Partial radiogenic heat model for Earth revealed by geoneutrino measurements. Nature Geoscience, v. 4, p. 647-651).

A model of the Earth that assumes it accreted from chondritic meteorites with well-known abundances and proportions of heat-producing U, Th and K isotopes, supported by some measurements of peridotites from the mantle, suggests that less than half the geothermal flux is radiogenic, implying that a great deal is heat originally trapped in the Earth when it formed. This view depends on several assumptions: that the Earth’s mantle is indeed chondritic below the 200 km or so from which samples have been brought by volcanism; that the core doesn’t produce any heat by radioactive decay; and that a geophysical model of a well-mixed mantle is correct. Not surprisingly, geophysical and geochemical evidence is so flimsy that many different views have had their champions: that the core contains potassium; that there is a deep, barely tapped inner-mantle layer of high heat production formed from now-rare meteoritic material, and so on. Geoneutrinos, if distinguishable from those from elsewhere in the cosmos and indeed measurable, could help home-in on one or other hypothesis. Based on a spherical balloon containing 1000 t of hydrocarbon liquids in a deep mine shaft that floats in an 18 m metal sphere filled with buoyant oil, KamLAND relies on detecting the light emitted by very rare interactions of neutrinos with protons. That is hard enough, but the site is surrounded by Japan’s 53 neutrino-emitting nuclear reactors, so a great deal of cunning operating conditions and data processing is needed to sort the ‘wheat from the chaff’; at present errors are large, but now sufficiently constrained to throw light on the great heat-flux issue. The KamLAND Collaboration reports that between 16 and 68% of heat flow is due to decay of the most productive isotopes 232Th and 238U – there is insufficient 235U and 40K in the Earth for geoneutrinos generated by their decay to be meaningfully estimated. Fuzzy as the results are, they are sufficient to support the view that Earth’s ‘primordial’ heat of formation is still a major source of geothermal energy, thus narrowing down the geochemical aspects open for disputation.

  • See also: Korenaga, J. 2011. Clairvoyant geoneutrinos. Nature Geoscience, v. 4, p. 581-582

Asteroid dust said to resolve a conundrum

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In September 2005 a Japanese space probe, Hayabusa, twice landed lightly on the small (700 m long) asteroid Itokawa that habitually crosses the orbit of Mars. The plan was to scoop up a substantial amount of its rubbly surface and return it for lab analysis. In the event the main sampling device malfunctioned. The dismayed Hayabusa team were mollified to some extent by the second landing impact fortuitously directing dust particles up to 0.2 mm across into the sampler. After Hayabusa landed safely in Australia on 13 June 2010, the team thankfully recovered 1574 tiny grains. Most were made of single minerals: olivine, pyroxene, feldspar (including 14 alkali feldspar grains), sulfides, chromite, Ca phosphate and iron-nickel alloy. About 450 were silicate mixtures some containing K-bearing halite (NaCl) (Nakamura, T. and 21 others. Itokawa dust particles: a direct link between S-type asteroids and ordinary chondrites. Science, v. 333, p. 1113-1116  – followed by 5 other papers from the Hayabusa team in the same issue). The sample analyses clearly show that Itokawa chemically and mineralogically resembles ordinary LL chondrites that make up most meteorites found on Earth.

Hardly a surprise, then… Yet it was, for Itokawa is an S-type asteroid – the most common – whose spectra do not match those of ordinary chondrite meteorites despite the logic that commonly found meteorites ought to come from the break-up of commonly seen asteroids. S-type asteroids have annoyed astronomers for decades because of their cryptic appearance, and now they are broadly relieved. Any object floating around the inner Solar System for billions of years inevitably undergoes a process for which terrestrial weathering is a metaphor; it is affected by the stream of charged particles that constitutes the solar wind, by bumping other bodies and attracting debris from such collisions. The Itokawa dust particles turn out to have extremely thin veneers of sulfide and metallic blobs on the scale of a few nanometres that are thought to result from condensation of matter vaporised either by tiny impacts or the solar wind. This veneer gives Itokawa and probably other S-type meteorites their irritatingly uniform reddish colour. It strikes me that there is a problem here: all asteroids, no matter what their mineralogy and chemistry, would be subject to the same kind of process and end up with a similar veneer. Itakawa may well be an ordinary chondrite, but what about all the other S-type asteroids?

See also: Kerr, R.A. 2011. Hayabusa gets to the bottom of deceptive asteroid cloaking. Science, v. 333, p. 1081.

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A big hit in the Moon’s evolution

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South pole Aitken basin highlighted on the moo...
The South Pole - Aitken basin (blue-magenta) and part of the high lunar far side (yellow-red) on an elevation map. Image via Wikipedia

The most significant discovery from the Apollo lunar landings is that the Earth and Moon shared a fiery early history, when a planetary body around the size of Mars slammed into the Earth to fling off vaporised rock that condensed to create the Moon. Such a catastrophic event reset the geochemistry of the Earth, and both it and the Moon likely had an early phase dominated by a deep ocean of magma. The evidence for a magma ocean comes mainly from the lunar highlands which are dominated by almost pure calcium plagioclase feldspar (the rock anorthosite), suggesting that this high-temperature, low-density silicate mineral crystallised and then floated to the surface of the Moon. Yet there is a great deal of evidence about the Moon that did not depend on people setting foot on its surface. For instance, detailed photographic records of the surface and extremely precise measurements of the surface elevation stem from cheaper orbital missions, including coverage of the unvisited far side of the Moon.

The face of the Moon never seen from Earth has long been known to have one of the largest impact basins in the solar system, the South Pole – Aitken basin. Analysis of the far side’s surface elevation data from the Lunar Orbiter Laser Altimeter (LOLA) also shows that it is significantly higher than the near side. It is also far more heavily cratered than the near side. Now there is a plausible explanation for the dichotomy: the Moon received another stupendous blow (Jutzi, M & Asphaug, E. 2011. Forming the lunar farside highlands by accretion of a companion moon. Nature, v. 476, p. 69-72). But how come that didn’t blast the Moon apart or re-melt it and allow it to re-shape to a near perfect sphere? The modelling study suggests that if the culprit slowly collided – around 2-3 km s-1 – it would have wrapped around the early Moon to plaster the surface with debris, nicely shown by the paper’s graphics.  Such a ‘slow’ impact is only possible from a co-orbital companion moon, objects from outside the Earth-Moon system inevitably being accelerated by gravity to at least the equivalent of its escape velocity (about 11-12 km s-1). That exceeds the speed of sound through rock, leading at least to a very large hole, shock metamorphism and, with a massive body, to extensive melting (the energy would be ½ mv2) rather than the observed lunar far-side bulge. Jutzi and Asphaugs’s modelling comes up with a companion moon around 1200 km across, that may have formed from the same massive event that created the Moon itself. It could have accreted from the impact-induced vapour disc at a Trojan point in the lunar orbit, where gravitational forces balance to keep orbital objects apart. The gradual expansion of the lunar orbit in response to tidal forces – large in the early history of the Earth-Moon system – could have destabilised the balance so that the companion moon slowly drifted towards the Moon and eventual collision.

One such modelling becomes closer to known reality, i.e. the far-side bulge, it gets more tempting to look for secondary possibilities. One of these the effect of such a ‘slow’ impact on the remaining magma ocean on the Moon. It may have blurted that by then deep molten layer to the side opposite the impact. That, the authors suggest, may be responsible for the geochemical peculiarities of the flood basalts that filled the much later lunar maria on the near side. There are no signs of these KREEP basalt floors to large later craters on the far side, such as the Aitken basin, formed around 4.0 to 3.8 Ga ago at the same time as the near-side maria. A variety of new instruments orbit the Moon and more are planned, so this model presents a nice hypothesis for them to test: what is the betting that a robotic lander might eventually be sent to return samples from the enigmatic far side?

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Explosion of the exoplanets

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The size of Kepler's first five planet discoveries
First five confirmed planets discovered by Kepler mission Image via Wikipedia

There is little doubt that it can be done, but what is so compelling about the search for worlds that orbit other stars?

By the end of the 21st century’s first decade 500 such exoplanets had been discovered, ranging from super gas giants almost 10 thousand times the mass of the Earth to a few that are comparable in size to our home world. At present the records of size and orbital radius are biased by the relative ease

of detecting large bodies over that of Earth-sized objects. Another bias is the greater chance of observing the change in luminosity of a star as one of its planets passes between us and the star – a transit – if the planet’s orbital period is short, being close to the star. The majority of known exoplanets are less than about 8 times the Earth’s orbital radius (1 astronomical unit or AU) away from their star, although some truly huge bodies have been spotted that are up to a thousand times more remote from a star than ours is.

Labeled illustration of the Kepler spacecraft
Kepler spacecraft. Image via Wikipedia

The rate of discovery is set to burgeon now that data from NASA’s Kepler exoplanet-finding mission, launched in 2009, is producing data (Reich, E.S. 2011. Beyond the stars. Nature, v. 470, p. 24-26). The 0.95 m Kepler space telescope gazes continually at a patch of sky containing 150 thousand Milky Way stars, many of which are like the Sun. It uses the transit method, and because it is fixed on only one star field it can potentially pick up the variation of stars’ luminosity due to transiting planets that are about the size of the Earth and larger. The computations are, unsurprisingly, massive and any dips in the light curves for pixels that represent individual stars have to be confirmed by other methods or by Kepler detecting repeats of the fluctuation. One drawback is that the transit method only provides the radius of a planet and its orbital period. Mass is needed to work out an exoplanet’s density and that requires another method using the red-shift of a star due to the gravitational effect of a planet causing it to wobble; a technique fraught with difficulties and best applied to dwarf red stars. The density is important for discriminating silicate-rich exoplanets from gas-liquid bodies. The main aim of planet finders is to find those around the same size and mass as the Earth that orbit a star at a distance where they would be warm enough for liquid water to exist but not so warm that it existed only as a vapour: in the so-called ‘Goldilocks zone’.

There was an initial flurry of excitement in the press in 2010 when a scientist on the Kepler programme was misinterpreted while giving a conference presentation that resulted in headlines that hundreds of distant Earths had already been discovered in the experiment’s first year. So far Kepler has only 15 confirmed planets to its credit that range from 800 times to twice the Earth’s radius all with orbits less than that of the Earth around the Sun. Nonetheless, a couple orbit within their star’s Goldilocks zone. So there is a way to go before real excitement is justified, but Kepler data will undoubtedly be used to seek funds for other planet-dedicated programmes that can fill in the gaps and perhaps confirm the existence of distant worlds that bear some resemblance to ours. Out of Kepler’s 1235 candidate detections since launch, 68 would be Earth-sized if confirmed (Shiga, D. 2011. What’s an alien solar system like? New Scientist, v. 209 (26 February 2011 issue) p. 6-7). For such remote detection to suggest an exoplanet on which life has evolved demands that atmospheric composition can be deduced from spectra of electromagnetic radiation from the body itself: a far more difficult undertaking that finding and weighing. Free atmospheric oxygen, so far unique to the Earth, is an obvious target. However, its absence would not rule out life that did not use photosynthesis to split water molecules in making living matter, and there are plenty of life forms here that do that.

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