The different temperatures tell us how the rocks of Mars are responding to the influence of sunlight and night, and from this we can see details of rock structure that are invisible to the naked eye, or hidden under a thin shroud of dust. For example, a thick dusty layer is a good insulator of heat, so it warm up fast in sunlight and cools off quickly at night, while solid bedrock conducts heat much better, and warms up and cools down slowly.

So an IR picture tells us about how rocky or dusty the ground is, and these differences help us discriminate between loose dust, cemented fine sand, and solid rock, which all show different "colours" of IR light. Today at the AGU conference in San Francisco, members of the Themis team reported to fellow scientists on the first 6 months of IR images of Mars, and the audience listened eagerly to the new results.

Phil Christensen (Arizona State University) began with a stunning visual tour of Mars as seen by infra-red light, showing both monochrome and colour-composite images. Detailed colour maps of Valles Marineris - the "grand canyon" on Mars, showed an outcrop of distinctive purplish rock at the base of the cliffs on both sides of the valley.

Diagnosis of the IR spectrum from the same area using the Mars Global surveyor TES spectrometer showed the distinctive signature of olivine-rich basalt. Olivine is a common mineral in some types of primitive basalts on Earth, but it weathers rapidly in a wet environment to rusty iron oxides. The presence of unoxidised iron in this valley casts doubt on the idea that the valleys were carved by vast outrushes of liquid water.

Phil then turned his attention to the Haematite area, where one of the twin MER rovers may be targeted after its launch next year. He showed images of flat layered outcrops with another distinctive colour in IR., picking out the Haematite-bearing unit. Other images showed incredible complexity of buried rock layers, hidden under a sheet of bland dust.

Jim Rice (ASU), followed with more analysis of the fluvial valleys of Mars, where giant floods appear to have emerged from the ground some 3 billion years ago. Evidence of regular outcropping layers showed that the islands within the channels were carved out from pre-existing layers, not deposited in situ by the floods - a distinction that had been hard to make from visible light images.

Furthermore, the lack of bright streaks down the cliffs bordering the islands, when viewed by night IR, shows that the layers are composed of fine grained material with no large rocks, and that they weather easily to fine sand or dust. This ease of erosion may explain how the floods carved so deeply, and why they leave no ridges and banks of boulders, unlike floods on Earth.

Other images from Kasei Valles showed strange irregular blobby formations, rather than the expected regular deltas, so perhaps we don't fully understand the mechanisms of these floods.

All these units showed signs of later stripping and erosion, perhaps by late stage floods or more recent wind erosion, but there are no dust mantles so all the eroded material has been swept or blown away. Meanwhile, in the cratered highlands, there is more evidence of fluvial valleys dating from the earlier Noachian era (around 3.7 billion years old). IR images show more valleys on a finer scale than do visible light images.

Alfred McEwan (University of Arizona) examined the Athabasca Vallis region of Mars where quite recent floods have emerged from fissures in the ground - the Cerberus Fossae. The value of IR data in discriminating lava flows from river sediment was clearly shown.

Several distinctive foci of lava effusion were identified, plus a series of fluvial channels, and the cross-cutting relationships between these pointed to a rich chronology of lava and water activity in this single valley.

Alfred also showed that many of the small craters in the valley are locally-derived secondary craters, emanating from a very young 10 km crater some 400 km away to the southeast. This has two very interesting consequences. Firstly, since over 50% of the local small craters are fall-back ejecta from this single event, the surface age of the Athabasca valley is wrong. Surface ages are based on counting craters and assuming they are the product of the slow random accumulation of individual impacts from space If a whole bunch arrive in a single event, then the assumptions are wrong and the surface even younger than we believed previously.

Another consequence is that the crater and all its secondaries are so fresh looking and unweathered, even when the crater ejecta is fine dusty material, that they must be very young. Alfred raised a laugh among the scientific audience by noted that he had checked the photos from the Viking mission some 25 years ago to be sure it hadn't occurred since then!

Alfred noted that the ejecta velocities were close to those required to launch a meteorite all the way to Earth, after escaping Mars' gravity pull. It is possible that some of the Mars meteorites currently being examined on Earth may have come from this very crater!

Steven Ruff (ASU) showed detailed studies of Syrtis Major - a huge and low-angle shield volcano. He showed that IR images can distinguish two different terrains within this single volcanic caldera. One matches the basaltic signature seen over much of the southern highlands and the other is very close to the "andesitic" signature of the northern plains.

Finding both signatures in a single caldera, and typing both via IR images to solid rock lava flows shows that we need to extend our understanding of magma formation on Mars to show how two such different types can be produced in a single crater, apparently very closely spaced in time.

Anton Ivanov (JPL) looked at the southern polecaps using THEMIS. He showed images of the "Swiss Cheese" terrain, where strange round pits are eroding in the solid CO2 ice. His images clearly showed that the dark walls of the pits were warmer than their floors or the surrounding polecap, and warmer than solid CO2 can survive.

Either the walls are covered in dust which insulates the underlying CO2 ice from the warm surface, or they are not made of pure CO2 ice, but some other substance with a higher melting point, like water ice, or CO2 clathrate - the joint ice of water and CO2. Anton emphasized the value of THEMIS IR in integrating different datasets on Mars, at different scales.

Patrick Russell (Brown University) presented an expanded study of large impact craters in the northern plains, all dating to the last 500 million years or so. His study examined whether any of these craters were large enough to punch through the frozen permafrost and release floods of water from the presumed deep aquifers that are believed to underlie Mars.

In his study of over a dozen craters, up to 250 km in diameter, he showed that there was no sign of any water escape, even though many of the craters should have punched right through the permafrost. The conclusion is that either our modelling of the craters is wrong somehow, or a powerful aquifer no longer exists in the northern plains.

Justin Ferris (USGS) and Norbert Schorghofer (Caltech), and their respective co-workers both studied the enigmatic dark slope streaks of Mars. Both teams have compiled a detailed image catalogue and presented fascinating examples of these streaks.

Both discussed the potential role of liquid water in forming these features, perhaps as slow trickles of liquid water down the slopes, or dry avalanches triggered by melting ice. Neither team had a final answer, but both are homing in on the evidence required to identify the nature of the flows.

Dr. Nick Hoffman is a lecturer of Geography at the University of Melbourne

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