Unfolding the radar antennae used by Mars Express proved to be tricky.© ESA

The first experiment to look for water deep below the surface of Mars has revealed hints of a subterranean ice lake, trapped in a buried crater.

The MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument, carried on board the European Space Agency's orbiting Mars Express probe, uses radar to see beneath the surface of the red planet.

Radio waves from its 20-metre antennae penetrate the planet's surface before reflecting off layers of different materials. The strength and time delay of these echoes can help scientists to locate deposits of ice, or even trapped aquifers of liquid water.

The results of its first scan of the planet, made in July this year, show what scientists believe to be a impact crater, up to 250 kilometres across and buried 1.5-2.5 kilometres below the surface.

The basin is at least partly filled with what could be an ice-rich material. This is surprising because it lies in a relatively temperate mid-latitude region called Chryse Planitia, where no ice is seen at the surface, says Jeff Plaut, a member of the MARSIS team from NASA's Jet Propulsion Laboratory in Pasadena, California.

Finding more of these buried craters could help to reveal what Mars looked like billions of years ago, before the holes were covered by dust or lava flows.

Ice magic

"It's really interesting because it allows us to reconstruct the geological history of Mars in much more detail," says John Murray, a planetary scientist at the Open University in Milton Keynes, UK.

"Chryse is at the downslope end of Valles Marineris, the Grand Canyon of Mars," Murray says, "and there have obviously been flooding events there in the past." He thinks that the ice in the crater could once have flowed as water through the enormous channels of Valles Marineris, before pooling in the crater and being covered by sediment.

MARSIS has also looked at the ice sheet at the martian north pole, and found that it is about 1.8 kilometres thick. Plaut says that there was a chance of finding melted water at the base of this layer, but MARSIS has found no evidence of liquid there. The results are presented in this week's edition of Science¹.

Rocky start

The MARSIS experiment was delayed for more than a year, after mission engineers feared that deploying the antennae might destabilize the spacecraft. The arms were finally unfurled in June, and more results are expected to follow soon. "What we have so far is just the first few weeks of data," says Plaut.

He explains that MARSIS can only see through Mars's surface when its closest approach to the planet is during the martian night, because sunlight increases the amount of charged particles in the ionosphere, which can disrupt the radio waves bouncing between MARSIS and the planet. And the orbit of Mars Express has kept its closest approach in the sunlight for the past few months.

"But this month, we return to the night side," enthuses Plaut. 

• References

  1. Picardi G., et al. Science, published online doi:10.1126/science.1122165 (2005).