Elysium Planitia
© NASA/JPLElysium Planitia
Stone circles on Mars are prompting a rethink about the planet's ancient climate.

Using cameras on NASA's Mars Reconnaissance Orbiter, Matt Balme of the Open University in Milton Keynes, UK, and his colleagues mapped the Elysium Planitia, a region near the equator. They saw rings up to 23 metres across made up of stones sorted by size into concentric bands.

On Earth, similar structures form via repeated freezing and thawing of ice, but with the stones sorted into layers. Water in soil under stones freezes faster than in surrounding soil, and the expanding ice pushes the stones upwards. Larger stones rise faster, and so layers sorted by size form.

What sorts the material concentrically is a mystery, but if a freeze-thaw mechanism was responsible, there must have been liquid water near the surface recently. This would mean that the climate was once 40 to 60 °C warmer than conventional estimates suggest.

Peter Grindrod from University College London thinks that the circles "would be an interesting target to look for evidence of past water on Mars".