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A changed Mars

The 800m diameter Victoria crater showing the unusual scalloped walls, and sief dunes on the floor. Photo by NASA. Image taken 3 October 2006 by the High Resolution Imaging Science Experiment (HiRISE).

Joe McCall presents a personal view of the 2006 William Smith Meeting…

The William Smith Meeting (8-9 November 2006) Planetary geosciences provided well-structured coverage of Mars on day one and a mixed bag of presentations on Mars, Venus, the Moon, Titan and the Solar System on day two. I shall here concentrate on the red planet, based on audience participation and the abstract volume1

John Bridges and colleagues led off with Mars’s climatic variations through time, using the measured abundance of carbonate in SNC meteorites and imagery of Mars to calculate the amount of CO2 sequestered in its crust. Studying nakhlites, they used thermodynamic models to constrain the pCO2 during the Amazonian epoch (back to ~670Ma) to 50-100 mbar. The overall picture obtained from the meteorites is consistent with occasional short-lived fluids (days, weeks) within a basaltic crust during the post-Noachian, whereas the picture for the Noachian allows the presence of liquid water for protracted lengths of time.

Lionel Wilson and Peter Mouginis-Mark presented a picture of Martian eruptive volcanic styles, and concluded that the important gaseous sulphur compounds would control water chemistry and allow significant amounts of water, sulphur and carbon dioxide species to be exsolved from magmas as they neared the surface. Various factors suggested to them that basaltic eruption would be very explosive; yet few pyroclastic constructs have been observed around vents, and even vent locations on most Martian volcanoes have been difficult to detect.

The Martian counterparts of Earth’s shield volcanoes probably commenced with eruption from multiple fissure sources. A candidate for such a dyke-fed system has possibly been recognised - in a long fissure/collapse pit/graben structure near Jovis Tholus, associated with possible spatter ramparts. Surface eruption on Mars results in much larger structures than on Earth. Lionel Wilson suggests that the huge magma reservoirs required beneath, say, Olympus Mons2 relate to different mantle-depth on Mars, with increased partial melting. The differences in atmospheric pressure with altitude mean that eruption at the base of a great volcanic pile like Olympus Mons would differ markedly in character from that at the summit2.

Red Mars, blue Mars

After a lucid presentation by Susan Harrison et al. on digital elevation models for processing Mars Orbiter Laser Altitude (MOLA) data, the focus returned to the main thrust of the day’s proceedings: ice and water/ brines and salts on Mars. Martin Hovland et al. noted that photographs of fracture patterns in the eastern Martian Elysium Plains show clear evidence of break-up with horizontal drift, recently interpreted as a frozen lake with a surface of pack ice, which from crater counts had been determined to be only five (plus or minus two ) million years old. Previously the fracture patterns had been interpreted in terms of a rafts of solid lava floating on a surface of large flood basalts (an interpretation which, if true, would raise doubts about the validity of crater-count dating, for surely Mars was never volcanically active so recently?).

It is now known that Mars contains abundant sulphates and water-soluble minerals: drawing on analogy with deposits in the Red Sea, and experience in the hydrocarbon industry, workers now propose that sulphates and salts on Mars form by brine circulation and ‘supercritical outsalting’, rather than as surface-derived evaporites. The brines are probably sourced in deep reservoirs – e.g. fracture zones above magma reservoirs - and migrate through cracks and fissures in the subsurface before venting on the surface. A lake with a smooth, level surface would thus form, and evaporation would leave a crust of anhydrite, gypsum, kieserite and others salts subsequently subjected to episodes of disruption, producing the ’ice-flow landscape’. The salts are thought to arise as a result of endogenic hydrothermal processes related to magmatism.

After interesting presentations by Bridges and Matthew Balme on Becquerel Crater as a possible target for palaeoenvironmental studies, and another by John Parnell et al. on selection of targets for search for evidence of life on Mars, we heard a stimulating presentation by John Murray and Balme on the alternative hypothesis - that the Elysium structures described by Hovland et al. were frozen seas near the equator.

And so we came to the William Smith Lecture, delivered by Steve Squyres - and the ongoing issue of water, sulphur and salts on Mars came to a head. His presentation focused on the work of rovers Spirit and Opportunity, which have achieved 16km of traverse (10 times their design expectation). The highlight of the Gusev crater traverse came in the Columbia Hills, where float and large outcrops of layered bedrock were analysed. Scientists were able to analyse granular breccias and finely laminated deposits in the layered rocks, which are older than the Gusev basalt plain rocks. Geochemical and mineralogical data revealed much more alteration, involving low water/rock levels. These rocks were formed early in Martian history, since when eruptivity in Gusev ended and the environment since remained cold, arid and quiescent.
At Meridium Planum, Opportunity, which on landing was blown into a crater, performed the first outcrop-scale study of ~300m thick sedimentary rocks, ‘disconformable’ on ancient Noachian dissected cratered terrain. The sequence is aeolian, formed by the erosion and redeposition of fine-grained siliciclastics and sulphates ultimately derived by chemical weathering of olivine basalts by acid waters. The stratigraphic section, >7m thick, is dominated by dune and sheet-sand facies, often with spectacular tabular cross-bedding. The upper 0.5m displays trough cross-bedding, interpreted as indicating subaqueous deposition, in a interdune playa-lake setting.

Silicate minerals and sulphates dominate outcrop geochemistry, with haematite and jarosite (the latter indicating precipitation at low pH). The “blueberry” spheroidal haematitic concretions and crystal moulds indicate a complex history of early diagenesis, mediated by ambient groundwaters (which must surely have taken place during very shallow burial, as there would appear to be little or no mechanism for deep burial on Mars, except under thick volcanic rocks). The early environment of deposition and diagenesis was arid, acidic and oxidising - and would have posed significant challenges to life.

The complexity of alteration of all but the youngest rocks on Mars revealed by Opportunity and Spirit was unexpected and suggests that detailed will be a laborious process with so many small scale variations in mineralogy, some isochemical. The early involvement of water is significant, but neither it nor the sediments confirm suggestions that there were once expansive oceans on Mars.

The rovers are still going, but dust will eventually terminate their heroic performances. Opportunity is at present on the rim of Victoria crater (picture), about 800m across with scalloped, collapsing walls, of soft, downhill-moving, altered, layered rock The dunes in the crater floor are remarkable - presumably ‘sand’ is driven by the wind into the crater with sufficient velocity to form these seif dunes. However, dunes of this type on Earth require a consistent wind vector, so such dunes within craters, which are comparatively sheltered, are surprising.

Finally, Steve Squyres mentioned the finding of several iron meteorites by the rovers3,4. Iron meteorites only account for ~4% of those falling to Earth, so their apparent dominance over stones is surprising. They have apparently found at least one stony meteorite, and it may be that this dominance of irons is indeed more apparent than real. Irons will stand out on the Martian plain, whereas stones, (which are also much smaller), will tend to blend in. However, it is also very important to consider the effects of Mars’s thin atmosphere on meteoroid approach3.

References cited:

  1. Geological Society William Smith Meeting 2006 abstract volume.
  2. McCall, G J H, 2006: A caldera volcano of Brobdingnagian scale, Geoscientist 16 (4) 28.
  3. McCall, G J H, 2005. An iron meteorite on Mars. Geoscientist 15 (7); 1
  4. McCall, G J H, Bowden, A J & Howarth, R J 2006. The History of Meteoritics & Key Meteorite Collections. Geological Society of London Special Publication 256: pp 13 & 501-2