THE past few days have brought good news and bad for those who hope to find Martians of some description or other. The good news is that on May 25th Phoenix, America's latest mission to Mars, landed successfully where many others have failed. The bad is that an analysis of the results of previous successful missions suggests it is unlikely that life ever got going there in the first place, let alone survived the transition to the harsh Martian conditions prevailing today. The balmy, watery past conjured up in the minds of the eternal optimists who inhabit the exobiology departments of the Earth's universities has been replaced by something more akin to a planet-sized version of the Dead Sea.
To start with the positive, though, Phoenix's landing seems to have been picture-perfect. Literally. The photograph above, which was taken from a spacecraft called Mars Reconnaissance Orbiter during Phoenix's descent, shows how it first deployed a parachute to slow itself. It then, in classic sci-fi fashion, fired retro-rockets to make a controlled touch-down on three spindly legs, rather than swaddling itself wimpishly in balloons and bouncing around for a bit before settling, as some previous missions have done.
More pleasingly still, the first images it sent back could have come out of a geography textbook's chapter on arctic terrain. They show a surface broken into polygonal slabs by repeated freezing and thawing of the sort that happens above permafrost. For, unlike any previous successful mission to Mars, Phoenix has landed not in the planet's tropics but near one of its poles. And the reason for going there is the same as the reason for the polygonal slabs. It is that the Martian poles are repositories of ice, and the hunt for water on Mars has now become almost obsessive.
Ere the winter storms begin
Phoenix got its name from its precarious beginnings. In 1999 NASA, America's space agency, suffered two failed Mars missions on the trot. One, Mars Climate Orbiter, had a particularly humiliating crash, because the fault lay with a group of engineers who, in defiance of common sense, had continued to use imperial units of measurement in their calculations when all around had adopted the metric system. The crash of the other, Mars Polar Lander (intended to touch down near the south Martian pole), was not caused by that sort of carelessness; landing things on Mars is inherently hard and mission failures are inevitable. Nevertheless, rather than risk a third consecutive loss, NASA put things on hold. One result was that an unlaunched craft similar to Mars Polar Lander was tucked away in a clean room while the agency had a rethink.
The outcome of that rethink was a bid by the University of Arizona to take the mothballed lander, refit it and have another go at visiting a Martian pole (this time the northern one). The idea of naming the craft after a mythological bird that self-immolates and then rises anew from its own ashes was thus irresistible. It was also poignant, because Phoenix's active life is expected to be a mere three months. It will be killed beyond reasonable hope of resurrection not by heat but by the encroaching cold of winter.
Watching that winter arrive will be spectacular. Vastitas Borealis, the area that Phoenix has landed in, will be covered in a metre-deep layer of frozen carbon dioxide. Phoenix's main camera will have an impressive view of this happening. The instrument everyone is really concentrating on, however, is not the camera but the digging arm. Measurements from orbit suggest that the Martian permafrost is within half a metre of the surface. If that is correct, the arm, which is fitted with a scoop and a second camera, should be able to dig into the permafrost, photograph its details and bring samples back into the main body of the probe for analysis.
The hope—the one that propels all missions to Mars—is to find a set of conditions that would allow living creatures to survive. Since cold-tolerant microbes do just that in terrestrial permafrost, it is quite possible that appropriate conditions will, indeed, be found. That does not, of course, mean that anything will be living in them. Nor is Phoenix equipped to detect micro-organisms, though it will be able to sniff out organic molecules if these are around, and that will be a tantalising clue. It will also examine the general chemistry of the sub-surface layers, to see if it is within the limits tolerable by living things on Earth.
The shore of a poison sea
The importance of those limits is emphasised by the second piece of news, a paper analysing Mars's rocks that was published a few days after Phoenix landed. The message seen written in those rocks when they were first examined was of abundant water. Lots of them are layered in ways that indicate they formed at the bottoms of lakes or seas. However, Nicholas Tosca of Harvard University and his colleagues have been looking more closely and, as they report in Science, the second glance is not as promising as the first.
Mars's geology (if that is not an oxymoron) has been studied spectroscopically by several satellites, and has also been prodded and probed at a local level by ground-based rovers. The upshot is that the layered rocks do, indeed, seem to have formed underwater. They appear to be sandstones, but with a lot of minerals such as magnesium, calcium and iron sulphates that form when brine evaporates. It is these minerals that concern Dr Tosca.
His calculations suggest that the waters they formed from would have been highly acidic. That is bad enough for those who imagine them brim-full of bacteria. But they would also have been highly osmotic. The dissolved chemicals within them would have served to lock up the water so that it could not take part in biochemical reactions.
The degree to which water is locked up by such solutes is measured by a number called the “water activity” of a solution. Pure water has an activity of 1.0. Seawater's activity is 0.98. The water of primeval Mars, Dr Tosca calculates, was probably between 0.78 and 0.86, and may sometimes have been as low as 0.5. That is far lower than is tolerable by any living thing known on Earth, even from places like the Dead Sea.
This result will probably not dampen the ardour of exobiologists. Living organisms, they will argue, are amazingly adaptable, so why should they not be born in conditions even more extreme than those found on Earth? Like a phoenix, perhaps. But then the phoenix is mythological.
This article appeared in the Science & technology section of the print edition under the headline "The Phoenix has landed"