Recent data from Europe's Mars Express spacecraft confirms that Mars was once a wet world, perhaps covered with large, Earth-like lakes. An infrared scanner aboard the probe detected scattered deposits of water-bearing minerals. One of these, phyllosilicate, forms as liquid water interacts with volcanic rock over thousands of years. "Water has pooled in a lot of places, in the canyons and local lows," says Michael Carr, a geologist with the U.S. Geological Survey.
Meanwhile, readings from Mars Express's radar experiment suggest that some of that water may still be around—it just went underground. The instrument beamed radar waves deep into the planet's crust. The way the waves bounced back hints at the presence of a thick 100-mile-wide puddle of ice in a hidden crater, detectable only via radar, that is buried one mile beneath a region where liquid water seems once to have flowed. Combined with other discoveries of recent volcanic activity on Mars, those results strengthen the case that Mars could have warm, wet havens hidden beneath an icy crust.