Enlarge / The highly reflective signals projected on top of a graphic showing Mars Express scanning the planet. (credit: ESA/Davide Coero Borga)

Today, a team of Italian scientists announced evidence that Mars has lakes of liquid water under its polar ice caps. While the evidence is a bit indirect and requires a number of assumptions, the alternatives that the researchers have considered fit the data worse. The announcement is likely to get scientists thinking about how to use existing instruments orbiting Mars to give this region a more careful examination.

Under the poles

Most parts of Mars spend much of the time at temperatures too low to support liquid water at the surface. Adding salt—and we’ve found lots of evidence of salt on Mars—can lower the freezing point of water by turning it into a brine. But that’s not enough to overcome the extremely low pressures of the Martian atmosphere, which would cause any ice to sublimate off into vapor instead of melting.

So the only realistic hope for significant amounts of liquid water is under the surface, at sufficient depth to create enough pressure to allow salt to overcome the low temperatures. Calculations have suggested that these conditions may be met at the Martian poles, where large ice caps composed of both water and frozen carbon dioxide exist. There are obvious parallels to Antarctica, where lakes have been discovered beneath glaciers in several areas of the otherwise frozen continent.