Lots of space has been devoted in science journals to results from NASA’s robot rovers on Mars. Well, haven’t they been exciting? Iron-oxide “blueberries, a cliff with bedded sediments and some iron-aluminium sulphate in a combined traverse of a kilometre at most: imagine a geologist coming back from a terrestrial field trip costing a year’s GDP of a small poor country and writing a report for the funding agency! That is a bit cruel, for in planetary exploration the themes are context, context and context, but we did know that Mars is red and orange, which is enough for most of us to feel happy with a lot of iron coloration. At the same time as the rovers were deployed, the European Space Agency’s Mars Express was going into orbit (so named because it was assembled in something of a hurry). That bristles with the geoscientist’s other modern tools: those aimed at sensing materials from their electromagnetic spectra. There is the High-Resolution Stereo Camera that produces images to rival high-altitude aerial photos of the Earth, and with stereoscopic overlap from which accurate models of Mars’ topographic elevation can be calculated, of which more in the next item. The principal mineral and rock mapping tool is the Observatoire pour la Minéralogie, l’Eau, les Glaces, at l’Activité (OMEGA), that builds on the spectral mapping by NASA’s Thermal Emission Spectrometer deployed by the earlier Mars Global Surveyor and a similar instrument aboard Mars Odyssey. 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. Research geologists don’t get much of that quality of data from Earth, mainly because it is commercially successful in mineral exploration, and very expensive (for much of the Earth, such hyperspectral data is not very useful, because vegetation masks most mineral signatuires). But data are free from Mars Express (or will be when the main investigators have had a reasonable time to satisfy their curiosity) and has a terrestrially useful resolution down to 100m. They also cover an awful lot of the planet’s surface and should eventually give 100% coverage.. The 11 March 2005 issue of Science devotes 24 pages (p. 1574-1597) to summarising OMEGA results. Various papers reveal variations in the composition of pyroxenes in the predominantly mafic Martian surface rocks, those minerals, such as the sulphates gypsum and jarosite, which contain water and signs of weathering by water, and an awful lot about water and CO2 ices around the poles. But this is not the geology in full of course, but driven by the search for potential habitability. Common rocks are not made of sulphates and ice, but silicates, which can be assessed by multispectral thermal emission data that prove very useful on Earth. The lack of information about such fundamental divisions of Martian igneous rocks as ultramafic, mafic, intermediate and felsic is a great disappointment, but perhaps the thermal instrument aboard Mars Odyssey will eventually come up with those more mundane goodies. Oddly, the planetary treasures of Mars are not being revealed by such sophisticated instruments, but by what is still the work horse for a great deal of geological image interpretation, black and white stereo images.
The triumph of the old on Mars
Except perhaps for some of the current generation of geologists, who are immersed in their remote sensing training by false colour images of spectrally revealing multispectral image data, a great many professionals who engage in mapping cut their teeth on what is known simply as photogeology. And it is simple. Provided images are taken of an area from different angles, with the simplest of instruments most people’s innate stereoscopic vision enables them to see startling illusions in three dimensions. Stereoscopy has been to geologists of the mid to late 20th and early 21st centuries what the binoculars were to those earlier scientist who discovered the great nappes of the Alps and thrust belts of the Rockies. A stereoscope of some kind is the latter-day analogue of that “Swiss Hammer”. Two stereo images reveal a great deal more than twice the information of one flat image, no matter how detailed. Using complex software, which converts the parallax differences that enable us to see 3-D to the differences in topographic elevation that cause relative shifts in the position of features on overlapping images creates accurate models of the elevation itself. That enables quantitative measure of many features related to topography, and allows the images to be viewed in perspective, as if they were indeed captured by binoculars from a high view point. Results from the Mars Express High-Resolution Stereo Camera (HRSC) have proved able to revolutionise our understanding of the Martian surface. The 17 March 2005 issue of Nature reports three important new results that stem from HRSC data. For several years the possibility of glaciers having carved some features on Mars have been suspected from lower resolution elevation data. Now it is certain from exquisite perspective views of debris aprons that record the flow of smashed rock from large mountains, almost certainly because the debris was once extremely dirty glacial ice (Head, J.W. et al. 2005. Tropical to mid-latitude snow and ice accumulation, flow and glaciation on Mars. Nature, v. 434, p. 346-351). The flows are reminiscent of rock-rich glaciers in the hyper-arid Dry Valleys of Antarctica. These authors present evidence that suggests that the flows are as young as 130 Ma, and may yet contain water ice. A second paper also reveals the influence of near-surface ice on Mars (Hauber, E. et al. 2005. Discovery of a flank caldera and very young glacial activity at Hhecates Tholus, Mars. Nature, v. 434, p. 356-361). In its case it seems to have been mobilised by an explosive volcanic eruption, possibly as young as 20 Ma, to produce debris flows and also very well preserved drainage channels at a much smaller scale than those known from Mars’ earliest history. The drainages might have resulted from subsurface ice melting by high heat flow and emergence of the “groundwater” to carve the meandering channels. There is an important caution: any dating on Mars depends on assuming a timescale based on counting impact craters and noting their relations to each other and different kinds of surface. The third paper observes something very different (Murray, J.B. et al. 2005. Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars’ equator. Nature, v. 434, p. 352-356). HRSC images reveal an area about the same size as the North Sea that is not only completely flat, but shows features very like those associated with pack ice in the Arctic and around Antarctica. They are plates whose edges can be fitted together, and in some cases islands have resulted in pressure ridges very like those seen where terrestrial pack ice meets land. There are even examples of impact craters that have been flooded. Murray and colleagues attribute all this to a large volume of subsurface water released by very recent volcanism along fissures close to the Martian equator. Basalt floods had been identified in the region before, but not evidence for a possible sea-sized, frozen lake. Similar, but not so revealing features elsewhere on Mars have been interpreted as lava rafts that once floated on flood basalts. Naturally, Mars scientists are very excited about the possibility of a large ice sheet at the equatorial surface, which may be as much as 45 metres deep. Unfortunately, the observations are from an area not yet covered by spectral data that would resolve whether the surface is ice-rich or more mundane lavas.