Rovers show there was once abundant water on Mars

• The twin rovers on Mars - now reviving having survived the freezing depths of Martian winter - have found new signs that water was once plentiful on Mars.
• The Mars rover Opportunity, which has nearly reached the bottom of the 160-metre-diameter Endurance crater, has now found clear signs that the area was drenched twice in the past, say NASA scientists.
• Its twin, Spirit, which landed in what was thought to be an ancient lakebed, had failed to find direct signs of water for months as it crossed two kilometres of pure basaltic plains.
• The rovers landed on the Red Planet in January 2004.
• But since Spirit reached Mars' Columbia Hills in July, the rocks have changed completely, showing evidence of abundant water.
• "We have not seen a single fresh volcanic rock since we crossed the line" from the plain to the hills, says rover chief scientist Steven Squyres of Cornell University, US.
• By measuring the differences between the spectra of rocks on the plains and those in the hills - using Spirit's Alpha Proton X-Ray Spectrometer - the team discovered that the hillside rocks were all significantly depleted in potassium, sulphur, bromine, and chlorine.
• These are the most soluble elements and would be the first to go if the rocks got wet.
• While attempting to retreat from the lower levels of the Endurance crater, the rover happened upon a slab of rock - dubbed Escher by the team - which is fractured into polygonal shapes (see image).
• The rock itself is made up of fine layers that run across the direction of the polygonal cracks, explains John Grotzinger, a sedimentologist at MIT and a member of the science team.
• The shapes suggest that the original sedimentary rock formed in a lake or shallow sea - which then dried and solidified - before undergoing a second wet episode and yet another drying out.