Mineral mapping and the history of Mars’ rocks and water

The principal mineral and rock mapping tool for Mars is the Observatoire pour la Minéralogie, l’Eau, les Glaces, at l’Activité. OMEGA is every remote sensing geologist’s dream machine, because its coverage of the short-wave end of electromagnetic radiation by 350 narrow bands can match spectra reflected from rocks and soils with those measured under laboratory conditions for several hundred important minerals. For over 18 months it has been steadily building up mineralogical maps of the Martian surface in a series of narrow swathes would round the planet in the manner of wool in a ball (see Mineral maps of Mars in April 2005 issue of EPN for early results). The 90% complete data, combined with dating of surface regions from crater counts and other means of stratigraphic analysis, is beginning to chart the history of the Martian surface in familiar terms of geology and the effects of water (Bibrin, J-P, and a great many others in the OMEGA team 2006. Global mineralogical and aqueous Mars history derived from OMEGA/Mars Express data. Science, v. 312, p. 400-404).

An interesting correlation is emerging. Where Mars’s surface is dominated by large amounts of pyroxene – the stratigraphically older regions of heavily cratered volcanic rocks – there is evidence of hydrated clay minerals (products of non-acid water alteration) and sulfates (formed by acid, hydrous alteration). The younger, brighter regions, which probably formed by surface processes after about 3.5 Ga, are dominated by anhydrous iron(III) oxides that give Mars its overall red colour. Although on Earth this hematite commonly forms by dehydration of iron(III) hydroxide or goethite, there is no sign of relic goethite on Mars. The authors attribute the red-staining hematite to direct oxidation of iron-rich silicates, without the role of water. It seems that in terms of surface processes, water played a role in the very earliest weathering to form clays. For a while conditions became acidic by the oxidative breakdown of igneous sulfides, thereby encouraging the formation of sulfate encrustations and sediments. This ‘wet’ phase may well have involved water vapour emanating from early, huge volcanoes. Once global volcanism became extinguished the supply of water was shut off, and since 3.5 Ga the planet has been hyper-arid. Hydrated minerals above the 5% level are not common on Mars, and if they did in fact encourage some life forms to emerge, the search for them can be finely focused by the OMEGA results.

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