Water on Mars? The most recent images of Mars' surface have come from the Mars Orbital Camera (MOC), on board the Mars Global Surveyor. Many features imaged by Mariner 9 and Viking have been photographed by the MOC at much higher optical resolution, showing channel and valley networks in greater detail. "Rivers"? What has emerged from these images has been a re-interpretation of these fluvial (river-like) features. When satellite images are taken of river valleys on the Earth and as the resolution (or magnification) of the images is increased, then an increasingly fine structure is observed, with tributaries joining the main flow of the river. The same effect does not seem to be observed on Mars: higher resolution images of channels show that very few 'tributaries' flow into the main pathways, see the picture. These images have been interpreted as indicating that either the channels were formed by ice, rather than liquid water, or that the flow was below the surface, rather than across the surface of the planet. Whichever interpretation is correct, the implication is still that water was relatively abundant in surface or sub-surface locations in Mars' past. How distant was that past is also a matter for debate. Again, images from the MOC have been used to suggest that liquid water might have percolated to the martian surface in much more recent times than had previously been believed, perhaps as recently as 1 million years ago, and indeed might even still be present in sub-surface locations today. Water and life For water to have been present at Mars' surface, the martian atmosphere must have been much thicker, and surface temperatures much warmer than they are today. A thicker atmosphere engenders greater protection from solar radiation; in a previous epoch, Mars would have had a warmer and wetter climate and provided all the conditions suitable for the emergence of life. Given this framework, it is not surprising that so much interest has been focussed on the potential for life on Mars. Who knows, maybe the Earth's sister planet has also harboured life at some time?

This picture shows the equatorial region of Mars. In the bottom right there are valley-like formations present. This picture suggests that there are layers of sediment which could have been deposited by water flowing through this valley. This image was obtained by Mars Orbital Camera on the Mars Global Surveyor.