Water on Mars?
Having been slightly obsessed by Mars ever since I read Kim Stanley Robinson's sublime Mars trilogy, this counts as big news. If it's water that cuts those gullies, then life is a possibility too. Liquid CO2 is a very good candidate for causing the erosion however, especially given the pressures and temperatures on the Martian surface.
Time for another robot!
BBC NEWS | Science/Nature | Water flowed 'recently' on Mars