If Mars is ever to visited by astronauts, and for there to be any chance of finding living things there, water close to the surface is vital. Not surprisingly, the search for Martian water, albeit not in a network of canals, is becoming a thriving cottage industry. The last week of June 2000 saw a leaked report from research using images from the Mars Global Surveyor spacecraft, publicised in New Scientist and Science for that week.. Some of these showed systems of V-shaped gullies on steep sides of valleys and craters, which are extremely sharp. Several workers claim that they were cut by running water in the recent past. That they are young features is clear, because they are not blurred by dust blown across the Martian surface by it nightmarish winds, and none are cut by craters. How water might have flowed freely a short time ago is not too clear. The Martian surface is well below freezing point for most of the time (average temperature -50°C).
The explanation given by the researchers is that a layer of frozen pore water a few hundred metres below the surface can melt because of built up of pressure. Where the layer meet the surface in valleys cut through it, the pore water remains frozen, and acts as a dam. When this becomes breached, water simply squirts out to form the peculiar runnels seen at more then 150 sites. Several of the gullies lie below signs of collapse on the slopes above, suggesting that water release has removed support for debris on the steep slopes.
There a number of reasons to take these accounts with a pinch of salt. Sure, increased pressure depresses the melting point of water, but at -50°C it would have to be pretty high. In permafrost areas on Earth, waterlogged soil freezes from the top down in winter, thereby trapping the last dregs of water. This becomes pressurised, to remain liquid in a supercooled state. If it breaks out it does not flow, but forms ice almost instantly. As well as forming the famous pingoes (ice cored mounds) of Arctic alluvial plains, this phenomenon almost caused a bizarre disaster during one of the Yukon gold rushes. High-pressure water jetted into a public bath house – the warmth of the building had created a trough of melt water directly beneath – and filled the entire edifice with ice. Fortunately, this happened at night and no prospector was encased. Much the same would probably happen to any such water escape on Mars, unless it was preternaturally warm. Such was the case for the truly huge and unmistakable water-cut valleys on Mars. But they formed far back in Martian history, perhaps as a result of energy introduced by large impacts.
It is tempting to look to other explanations for the gullies. Very dry sand flows down the lee slopes of dunes, often to form runnels with collapse features above them. Perhaps some attention to the physics of dry sand – Mars is a sandy and silty place – under near-airless conditions and suitably reduced gravity, might offer an alternative explanation.
Even more optimistic is the notion that Mars once has seas, based on the discovery of various salts in an Egyptian meteorite that approximate the blend of dissolved ions in Earthly seawater (New Scientist, 1 July 2000, In Brief). The evidence that the class to which this meteorite belongs comes from Mars rests on comparison of its noble-gas content with the extremely imprecise measurements or Mars’ air by the Viking mission in the 1970s. Why the chemistry of Martian ‘seas’, or any of its water for that matter should bear comparison with that for waters derived from a planet with both weather and highly evolved continents seems to demand an explanation. Oh well, no doubt we will get answers when astronauts do get there – it is not inconceivable that all the papers suggesting it is important to go have some relation to NASA’s decades long fight for funds to do that.