The Mars rover Spirit has completed its first full set of scientific measurements with the instruments on its robotic arm, revealing mysterious hollow grains in the soil.The one-metre arm used its microscope to take a close-up image of an undisturbed patch of soil next to the NASA rover. It shows mostly sand-sized particles, but with a large number of apparently hollow spheres or tubes. The image resolution is about 30 microns per pixel - about the width of a human hair.Such grains were completely unexpected. But John Grotzinger, a geologist at the Massachusetts Institute of Technology, says they closely resemble formations he has seen in soils in the southwestern deserts of the US. "There are little tubes that build up by capillary action," he told New Scientist, as salty water evaporates from the nearly-dry soil.The Martian grains must also be strong enough to withstand the region's strong winds and perpetual scouring by dust devils - tornado shaped vortexes that can tower to heights of kilometres.

The pressure and temperature conditions at Spirit's landing site, Gusev crater, are close to the triple point of water, Grotzinger says. This means any water present could change phase between solid, liquid and gas in response to small changes in conditions, and it is an area whose temperatures vary widely during the daily and seasonal cycles.


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