The Long Search for Water on Mars

A Seen through a telescope, Mars is a rusty, reddish disc, and the spacecraft we have sent to it confirm the first impression: a cold, dry and dusty desert, swept by thin winds and bathed in radiation. Its air is so thin that liquid water on the open surface would boil away or freeze almost at once. And yet, for more than a century, no other planet has so gripped the imagination of scientists, precisely because there are strong hints that Mars was once a far warmer and wetter world. The search for water, past and present, has become the guiding thread of almost every mission sent there, shaping the design of the rovers and the choice of the places they are sent to explore.

B The most striking evidence is written in the shape of the land itself. Cameras aboard orbiting spacecraft have photographed features that look unmistakably like the work of water: long winding channels that resemble dried riverbeds, networks of branching valleys of the kind that rivers carve, and flat basins ringed by what appear to be ancient shorelines. On Earth such landforms are made by flowing water acting over very long stretches of time, and their presence on Mars strongly suggests that rivers and lakes, and perhaps even shallow seas, once lay upon its surface in the distant past.

C Shape alone, however, can mislead, and so scientists have looked for chemical evidence as well. Robotic rovers creeping across the Martian ground have examined the rocks directly and found minerals, among them certain clays and salts, that on Earth form only in the presence of liquid water. These minerals act as a kind of chemical memory, preserving in their very structure the record of the wet conditions under which they were made. Taken together with the shape of the land, they have largely settled the question in the minds of most researchers: liquid water was once present on Mars, and in considerable quantity.

D If Mars was once wet, the obvious question is where all the water went. The answer seems to lie in the planet's small size. Mars is far smaller than the Earth, and over the ages it has lost most of the thick atmosphere it is thought once to have possessed. As that blanket of air thinned, the pressure and warmth at the surface fell, until liquid water could no longer survive there; what did not freeze into the ground escaped as vapour into space. The loss of the atmosphere was made worse by the fading of the planet's magnetic field, which had once shielded it from the stream of particles flowing out from the Sun.

E Not all of the water, however, has been lost. A great deal of it survives on Mars today, though in frozen form. The planet's poles are capped with ice, and beneath the surface across wide regions there lies a hidden store of frozen water, buried much like the ground-ice of the Earth's coldest lands. There are even signs, still debated among scientists, that on rare occasions small amounts of extremely salty water may briefly trickle across the surface before vanishing again. Mars, in short, is not entirely dry; it is a world whose water has mostly been locked away out of sight.

F The reason all this matters so much is that water and life are closely bound together. Every living thing we know of needs liquid water, and so a Mars that once had rivers and lakes is a Mars that might once have supported life, at least of the simplest kind. Much of the present effort is devoted to searching the ancient rocks for chemical traces that living things might have left behind. There is a practical side too: the buried ice could one day supply drinking water, breathable oxygen and even fuel for human explorers, turning a hostile desert into a place where visitors from Earth might one day survive.