Wet or just slipped off?
H2O is not only found on the blue planet, but also on the red planet - there, however, only in a form that has been frozen for millions of years. But wait a minute: What are those new puddles in the pictures from last winter?
Count through: Two rolling ground explorers, four – no, only three – snapping orbit scouts, makes five actives. In addition, starting with the first success, there are twelve US veterans who have since been retired or have long since disappeared into space, not to mention 13 Russian-Soviet and one Japanese mission. Mars, it seems, has an enormous attraction for terrestrial planetary explorers and their expensive probe toys. And one question has always remained the same when evaluating all the data collected from the Red Planet, ever since image interpreters first bent over extraterrestrial close-ups of the planet on the evening of July 14, 1965: have we found traces of life? Or at least places where there could be niches of life? No one gets bored – although the answer has actually been the same for 41 years: "No."
That sounds more definitive than it is, because with better and better instruments you can search more and more thoroughly. In addition, life as we know it on Earth is extremely imaginative and tough: some terrestrial bacterium could theoretically withstand a lack of oxygen and temperatures similar to those on Mars. There is almost only one exclusion criterion: where no water flows – and never – there is no life. The reverse conclusion, which everyone who has been interested has mastered for decades, is very simple: where water flows, there might be …
In short: In the past decade, in order to keep life fantasies alive, Mars explorers have been looking specifically for liquid water, or at least for traces of water that once or occasionally flowed. And before that can get boring, the current generation of scientists presents the photographically captured, "convincing, best so far" evidence that water must not have bubbled over the otherwise dusty Martian soil at some point, but must have actually continued to do so in the last four years.
The river evidence comes from the estate of the Mars Global Surveyor (MGS), who had just been retired due to a loss of radio contact. Since 1997, its camera had been delivering images of craters, dunes and deserts on our neighboring planet from Mars orbit, making it possible to track the seasonal change over the years. Two such series of images are now alarming the Martian water seekers: On old photos from 2001 and 1999, uninterpretable lighter areas had already been noticed on the otherwise monotonous surface in the sloping gullies of two crater walls. These were still present in the new pictures from 2005 and 2004, but they had suddenly grown: over the past few years, very similar, smaller deposits had appeared out of nowhere right next door. The new gully tracks, a little more than a kilometer long, with their finger-shaped branches at the lower end, actually appear as if they were cast and meandered around boulders lying in the way, describes the responsible image interpreter Michael Malin.
Are these water tracks suddenly emerging from an unknown source and then frozen over? Not so hasty, say critics such as Oded Aharonson from the California Institute of Technology or Allan Treiman from the Lunar and Planetary Institute, and open up a small exchange of opinions: Couldn't dust or debris also have slipped here? And dust and debris are definitely more common on Mars than water.
Right, but: Nowhere else on Mars has a landslide suddenly uncovered lighter ground, in no young crater is the impact base colored so differently, none of the impressive Martian dust tornadoes left behind lighter ground when they pulled away, nor did the tire tracks of the Rover Spirit and Opportunity. Malin and Co point out that dust and scree can also be found there and are easy to recognize, where there are also large amounts of dust, sand and scree - which is obviously not the case in the now suspect craters.
Critics from another quarter, on the other hand, warn that instead of dust or water, flowing carbon dioxide may have left the bright traces. CO2 could have escaped from an underground but ground-level reservoir and – in regions where, according to their calculations, the temperature at ground level is minus 107 degrees Celsius – rushed down the valley as a bright, liquid avalanche. At such surface temperatures, however, liquid water should only occur several kilometers below the ground.
But the average temperatures actually prevailing make this carbon dioxide scenario very unlikely, counters Malin: After all, the temperatures observed in the latitudes observed on Mars are often so high that carbon dioxide does not appear in liquid form, but in the same gaseous form and therefore hardly ever as a bright spot allowed to freeze. No: The bright spots are most likely just frozen puddles of water ice.
Water, according to Malin's stain formation scenario, is trapped in near-surface underground reservoirs that occasionally erupt from the ground. Despite the low temperatures and the low pressure of the Martian atmosphere, the water probably remains liquid at least for a short time - for example because it is saturated with high s alt concentrations - in order to then freeze over, form the typical bright regions and finally gradually sublimate, i.e. without a transition to vaporize into the icy state.
New studies should help to clarify the dispute over the photo interpretation. Of course, an exact temperature measurement on site could give clear indications - but Malin and colleagues are eagerly awaiting the on-site deployment of the youngest Mars scout in orbit, the Mars Reconnaissance Orbiters (MRO), which is just in time to replace the old MGS scout made its debut with impressively detailed images of the surface. The life trace seekers are hoping more for the SHARAD radar on board the probe than for the camera - at least theoretically this device can also identify water veins under the Martian soil.
So if MRO flies over the now conspicuous crater rims, Malin says he might be able to spot the subterranean water source of the bright spots directly. The rest of the latest chapter in the oldest Martian story has already been told: If there's water down there, then maybe there could be… The story, it seems, continues to flow.
