"At the American Geophysical Union (AGU) meeting last week in San Francisco, ESA scientists announced Mars Express' OMEGA instrument has discovered evidence of past water on Mars' surface.
Mars Express' visible and infrared mineralogical mapping spectrometer, called OMEGA, found deposits of clay minerals and other phyllosilicates. This is the first time an orbiting spacecraft has detected deposits created by exposure to large and long-lasting non-acidic liquid water.
OMEGA had previously found patches and outcrops of hydrated sulfate salts speckled throughout Mars' surface and exposed by either erosion or meteor impacts. These features support the Mars Exploration Rovers' findings that some places on ancient Mars had liquid water on or immediately beneath the surface. But this water seems to have been mixed with sulfuric acid — which greatly lowers water's freezing point."
http://www.astronomy.com/asy/default.aspx?c=a&id=3782(may require registration)
The clay suugests that water may have existed long enough for some life to have evolved on Mars during its first 800 million years.