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THE first few weeks of data from Europe鈥檚 Mars Express orbiter leave little doubt that Mars was once wet, and also hint at how the planet may have lost its water. The mission鈥檚 team is most excited about detecting water ice directly for the first time on the southern polar cap, while colour images reveal further evidence of a planet shaped by flowing water.

An infrared camera called OMEGA detected the water ice by studying the sunlight reflected from the planet鈥檚 surface, the European Space Agency announced last week at mission control in Darmstadt, Germany. Water ice has previously been measured on the northern polar cap, which loses its covering of carbon dioxide ice during the northern summer. And hydrogen atoms detected in the soil suggest there is subsurface water over large areas of the planet. But the southern polar cap is covered year-round by CO₂ ice, and until now has yielded only indirect hints, including results from NASA鈥檚 Odyssey orbiter last year revealing temperatures expected for water ice in exposed areas.

鈥淭hese are very important measurements, as they get at the abundance and distribution of the two ices 鈥� water and carbon dioxide,鈥� says Bruce Jakosky, a geologist at the University of Colorado in Boulder, on the latest results. He says warmth from the sun during the southern summer may have thinned the surface coating of CO₂ ice to a few millimetres 鈥� enough to make the water detectable. The findings may illuminate how water is transported through the planet鈥檚 atmosphere, and how the climate changes through the seasons.

But just as water was confirmed at the south pole, the ESA team may also have glimpsed it escaping into space. ASPERA, an instrument that detects atoms and charged particles, has found that oxygen ions, most likely from water vapour, are being stripped from the atmosphere by charged particles that make up the solar wind. Unlike Earth, Mars cannot fend off the solar wind as it lost its global magnetic field nearly 4 billion years ago when the planet鈥檚 molten core probably cooled and solidified. The mission鈥檚 scientists now hope to find out if the solar wind is the main culprit behind the dearth of water on Mars today.

Pictures and videos from the High Resolution Stereo Camera, which has now imaged 1.87 million square kilometres of the surface, also highlight the planet鈥檚 watery past. Sinuous channels and pits similar to those eroded by water on Earth, as well as rocky debris from former glaciers that evaporated or melted away, have been imaged in full colour and 3D at resolutions of just a few metres. 鈥淚 think we can say, yes, there was water acting on the surface of Mars,鈥� says Gerhard Neukum, the camera鈥檚 lead scientist.

The images reveal several intriguing features, including dust spilling over the lip of a 3-kilometre-deep caldera on the Albor Tholus volcano (below), and mysterious bluish splotches along some valleys (left). Neukum says these could be water-related sediments and hopes OMEGA may soon identify them.

鈥淎s wonderful as the data is, it鈥檚 very preliminary,鈥� says project scientist Agustin Chicarro. 鈥淭hings can only improve from now on.鈥� Testing of six of the spacecraft鈥檚 seven instruments will continue until 20 February, while the final device, a radar that will probe the top five kilometres of the crust, will be activated on 20 April.