Geoscientist Online

Geoscientist 17.2 February 2007

Emily Baldwin reports on the latest “spot the difference” exercise being conducted on photgraphs of Mars…

Until late last year, planetary scientists regarded the possibility of water flowing on Mars five million years ago as “recent”. Now they may have discovered evidence that water may have flowed within just the last five years.

By comparing images from the Mars Global Surveyor (MGS) with Mars Orbital Camera (MOC) pictures taken recently, scientists have observed changes in the Martian surface that they think can only be accounted for by fluid flow1. Photos of thousands of gullies first observed by MGS were retaken by the MOC in an attempt to identify evidence for recent water activity. A handful of locations - mostly craters - showed changes between 1999 and 2005.

Image: NASA/JPL/Malin Space Science Systems

In several images, apparently fresh, light coloured deposits in lobes several hundred metres long can be seen, possibly created by water-bearing fluids that might contain deposits of mud, salt or ice.

"The shapes of these deposits are what you would expect to see if the material were carried by flowing water" Michael Malin of Malin Space Science Systems, San Diego and lead author, told reporters. "They have finger-like branches at the downhill end and diverted around small obstacles." Malin is principal investigator for the MOC.

Liquid water cannot remain stable for long on Mars’s surface because low temperature and pressure cause it to freeze or sublimate. However the discovery, as interpreted by Malin et al., offers support for the idea that liquid water, lying close to the Martian surface, can occasionally burst out. Subsurface water has serious implications for life on Mars. Water, together with a stable heat source, provide all the ingredients favourable to life.

Image: NASA/JPL/Malin Space Science Systems

Caption: An enlargement of a portion of another image from August 2005, showing details of the new, light-toned gully deposit. The new material covers the entire gully floor, from the point at which the gully emerged from beneath a mantled slope, to the spot at which the channel meets the crater floor. At this break in slope, the gully material, as it was emplaced, spread out into five or six different fingers. Note 75-metre scale bar.

Double impact

It also seems that the swansong of Mars Global Surveyor has two verses. Image comparisons reveal that Mars suffers more high-velocity impacts than scientists suspected. In fact 20 fresh craters, ranging from two to 148 metres across, are known to have formed in the last seven years. Counting craters is the key to determining the relative ages of a planetary surfaces, so this finding could have important implications for calibrating modern impact rates. This in turn holds out the prospect of increasing the method’s resolution, and hence of reaching more accurate age determinations for younger geological features.

So, while NASA will be encouraged by the evidence for water in the Martian subsurface, it is clear that any visitors will have to watch the skies a little more closely than expected.