Mercury: sometimes a moist, organic-rich world

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.

Mercury: anything new?

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).