One of the key questions in planetary science is whether the early climate of Mars was warm enough for liquid water - and hence extraterrestrial life - to exist. The layered rocks, thought to be around 4000 million years old, are deposited horizontally and appear to consist of fine-grained material. Sedimentary rocks can be created in several ways - by wind, water, volcanoes, or even meteorite impact - but many of the newly discovered outcrops are in ravines or basins in the planet's crust, suggesting that the Martian surface was once dotted with lakes. "Some of the images show hundreds of identically thick layers, which is almost impossible to have without water", says Malin. The researchers have not established the origin of the sedimentary matter and believe that clues to where the matter came from have been eroded over time.

But Malin and Edgett have still not ruled out an alternative theory. It is possible that dramatic changes in atmospheric pressure - altering the composition of the atmosphere and therefore its ability to carry dust - might have caused rock layers to form.

The photographs were taken by the Mars Orbiter Camera on board the Mars Global Surveyor, which was commissioned to follow up Mariner and Viking data collected in the 1960s.