Wet Moon, dry Moon

Regular readers will remember my remarkable though very reluctant conversion to the notion that there may be water on Mars. My stubborn reaction had been against the background that shrouded the hypothesis with a certain desperation; the need of any future crewed mission to Mars for a water supply and thereby one of hydrogen fuel, plus the determination of the whole Mars-oriented community to justify such a mission by hyping ‘xenobiology’ on the ‘Red Planet’. A similar desperation claoked the search for surface water on the Moon, although one more dominated by the ‘Everest’ syndrome: since the boot prints and flags appeared, everyone wants to go. The Moons internal water is an entirely different kettle of fish. The hypothesis of the Moon’s formation by condensation from an incandescent mass flung into orbit after a planet – planet collision involving the Earth has the corollary that the lunar mantle ought to be bone dry: and so it seemed to be from bulk analyses of rocks brought back by the Apollo missions. In fact, there are a number of possibilities to explain vanishingly small amounts of internal water: the Moon is made of impactor that happened to be dry rather than terrestrial material; Earth and Moon are a mix of both and both Earth and impactor started out dry, but the Earth later received its water from comets; low pressure condensation of the Moon ruled out water entering itss silicate minerals and so on. Then water was found in apatite grains from lunar maria basalts (see Moon rocks turn out to be wetter and stranger in May 2010 issue of EPN). Within a couple of months we are back to the dry-as-an-alco’s-throat view (Sharp, Z.D. et al. 2010. The chlorine isotope composition of the Moon and implications for an anhydrous mantle. Science, v. 329, p. 1050-1053). Both terrestrial and meteoritic chlorine isotopes are in remarkably consistent proportions, but lunar rocks show an 25 times greater spread by comparison. To cut a long and complicated discussion short, such a range could only have formed if chlorides of a variety of metals were vaporised from lunar magmas each having its own effect on fractionation of Cl isotopes. In turn, combination of chlorine with metal ions requires virtually no hydrogen ions and therefore vanishingly little water in the moon, otherwise chlorine would have been combined in HCl and not subject to any fractionation when that volatilised on eruption. So that seems settled, then…

Carbonates on Mars

Ancient valley systems, huge water-carved gorges and sedimentary deposits signify with little room for doubt that early in its history Mars was wet; it must therefore have been warm. A thick CO2-rich atmosphere seems obligatory to give the kind of greenhouse warming that prevented Earth from freezing over when the young Sun was weaker than now. The question is, where did the CO2 go so that the planet became chilled? Gravity on Mars is sufficient to have retained the gas, unlike water vapour that dissociates to hydrogen and oxygen, of which hydrogen easily escapes even a much stronger gravitational field. A consensus is developing that it resides in carbonate minerals. The other likely greenhouse gas is sulfur dioxide, for whose drawdown there is ample evidence in the form of sulfates detected from orbit and by surface rovers. Carbonates have a relatively simple, and unique spectrum of reflected solar radiation, with an absorption feature at a wavelength around 2.3 micrometres. Carbonates have been detected on Mars using orbital hyperspectral imaging, but only in patches. The NASA rovers rely on serendipity for any discovery, yet Spirit did stumble on a large carbonate-rich outcrop identified by its on-board Mössbauer spectrometer (Morris, R.V. and 12 others 2010. Identification of carbonate-rich outcrops on Mars by the Spirit rover. Science, v. 329, p. 421-424). It appears to be a Fe-Mg variety in association with olivines, and carbonate makes up to 34 % of part of the outcrop. The texture is granular, yet the area abounds with evidence for hydrothermal activity in the form of sulfates and silica-rich materials, implying that some kind of circulation system deposited the carbonates. The associated olivine is odd, as that mineral is prone to rapid breakdown to serpentines in the presence of water.

The discovery of carbonate rock does help the CO2 early greenhouse theory and the fate of the warming gas, but aside from the fact the identification has been done at vast distance does it rank with geoscience that can be accomplished on Earth? It is a small piece in the jigsaw of Mars’s climatic evolution, but cannot resolve the issue of drawdown of greenhouse gas. The real drama there lay in the finding of abundant signs of water erosion on many scales set against today’s surface hyperaridity; evidence for glaciation and subsurface water ice in apparently large volumes. Earth had to have had a thick CO2-rich atmosphere at the same time as that of Mars, but we are still not sure where all that carbon ended up in the early Precambrian, despite limestones and carbon-rich mudstones dating back to 3.4 Ga: as we cannot quantify that aspect of Earth’s history, neither can we expect an early answer for Mars. Indeed, what is the benefit set against the cost?

See also: Harvey, P. 2010. Carbonates and Martian climate. Science, v. 329, p. 400-401.


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