Space: seeing things clearly on Mars

The rock outcrop in this scene near the Martian equator is no bigger than the central tower of Christ Church cathedral in Dublin, illustrating the extraordinary seeing-power of America's scientific “spy-satellite” called the Mars Reconnaissance Orbiter. The mound lies in a region of layered rock near the centre of Becquerel Crater, one of the best examples of layered sedimentary rocks on Mars. Sedimentary rocks are often deposited (as sediments) in water, although windblown sedimentary rocks are also common. This location has been suggested as a possible target for the next US rover mission, the Mars Science Laboratory (MSL) in 2009.

Astronauts who landed on the moon during the Apollo programme were relying on photographs that showed nothing smaller than Volkswagens. When the next robotic mission lands on Mars, scientists will already know the location of every boulder and crater in the area. Last week, just eight months away from the launch of the Phoenix lander, flight controllers announced that they'd abandoned the original landing site because the Mars Reconnaissance Orbiter had spotted unexpectedly large boulders in the area. They are now frantically searching for a safer alternative site near the Martian North Pole. Phoenix is a static lander, based on a design that failed to land safely in December, 1999 (see Village, 21 December, 2006).

In this scene near the equator, the majesty of the terrain is matched by tantalising hints that this might be a good place to begin the search for life on Mars. Two criteria are important in selecting a rover landing site: safety and scientific interest. To be safe for the Mars Science Laboratory to land, a site must first be flat and not too windy.

Once the rover lands, it is important that the terrain is suitable for driving: A site covered with lots of rocks may have boulders big enough to impede the rover's driving ability – a rock half a metre high is the biggest the rover can be expected to straddle. Also, experience with the spectacularly successful Spirit and Opportunity rovers over the past three years has shown that areas with a lot of sand or dust can snare a rover's wheels. It is immediately obvious that this particular area of Becquerel Crater would not be suitable, as it is hemmed in by a vast, picturesque but treacherous dune field.

Nonetheless, there is considerable excitement about information gleaned by Europe's Mars Express orbiter about the nature of some of the layered terrain on Mars. Dr Jean-Pierre Bibring of the University of Paris, South, has identified regions that are rich in clay compounds – phyllosilicates. These clays can only have formed in a period of Martian history when the planet was much wetter and warmer than it is today. Clays have also been suggested by other researchers as a likely environment where the chemistry of life began on Earth.