Water on Mars

From time to time Earth Pages News has tried to temper the flood of papers that seek every which way to support the notion that Mars is still well-endowed with water.  That is what NASA seeks in order to fuel its bid for the vast funds needed to launch a staffed mission to the Red Planet.  The evidence in each case was ambiguous.  I have always thought that attention and money would be better directed towards the one sixth of the human population who have no access to safe and abundant water supplies.  That remains my view, but the appearance of 10 pages of Science forces me to accept near proof of Martian water in abundance (Feldman, W.C. and 12 others 2002.  Global distribution of neutrons from Mars:results from Mars Odyssey.  Science, v. 297, p. 75-78.  Mitrofanov, I and 11others 2002.  Maps of subsurface hydrogen from the high energy neutron detector, Mars Odyssey.  Science, v. 297, p. 78-81.  Boynton, W.V. and 24 others 2002.  Distribution of hydrogen in the near surface of mars: evidence for subsurface ice deposits.  Science, v.  297, p. 81-85).

The neutron and gamma-ray detectors aboard Mars Odyssey only needed to operate for a month to reveal the abundance of hydrogen across the surface of Mars.  It varies a great deal, the highest levels showing up at high northern and southern latitudes.  Preliminary modelling suggests that these regions have at least several metres of ice-rich debris, containing between 25-35 % water ice.  Quite possibly the modelled ice-rich layer could reach a kilometre in thickness.  High anomalies at lower latitudes are modelled as being due to hydrated minerals in the Martian soil.

More results at higher precision are to come from Mars Odyssey, and experts emphasize that the reported modelling of neutron fluxes and those of gamma rays emitted by neutron-capture reactions is complex and preliminary.  However it does look like NASA scientists will soon by selecting sites for future landings on Mars.  Even more certain, it will have sent a frisson of excitement through those intent on the glory of finding signs of life there.

Tungsten and Archaean heavy bombardment

One of the major revelations that arose from the Apollo missions to the Moon is that the vast maria basins, filled with basalt, formed when a series of huge impacts wracked the lunar interior.  Surprisingly, they formed between 4 to 3.8 Ga ago, rather than in the earlier evolution of the Moon, and this “late heavy bombardment” (LHB) spans the period when the oldest rocks were forming on the Earth.  Controversy has raged for 3 decades about whether the LHB had a major influence on early Archaean geology.  The problem was that direct evidence has been hard to find, and difficult to get across to critics of such outlandish notions.  A careful investigation by geochemists from the Universities of Queensland and Oxford seems likely to force some critics to eat hat (Schoenberg, R. et al. 2002.  Tungsten isotope evidence from ~3.8-Gyr metamorphosed sediments for early meteorite bombardment of the Earth.  Nature, v.  418, p. 403-405).

Because stable 182W forms by the decay of 182Hf, with a short (9 Ma) half life, virtually none will have formed since the Earth accreted.  The 182W/183W ratio of objects from different parts of the Solar System should show distinct differences, and so they do.  Different classes of meteorites show tungsten isotopes that are significantly different from one another, and from products of mantle melting on Earth.  Ronny Schoenberg and co-workers analysed tungsten from two early-Archaean sources: the dominant grey gneisses, which are probably calc-alkaline igneous rocks formed at mantle depths, and metasediments from the famous Isua area in Greenland and another around the same age (~3.8 Ga) in Labrador.  The gneisses show no difference from later products of mantle processes, but the metasediments deviate significantly from the terrestrial isotopic composition of tungsten, towards that characteristic of meteorites.  They conclude that the metasediments mix debris formed by weathering and erosion of normal early Archaean crustal rocks with that formed in major blankets of ejecta from meteorite-induced impacts.

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