When NASA's Phoenix spacecraft settled in Mars' northern latitudes on 25 May, there was little doubt that it would land amid a quilted pattern of troughs created over time by ice contracting and cracking below the surface. The surprise is that Phoenix is surrounded by 'polygons' much smaller than expected. "I'm not in a panic," says mission scientist Mike Mellon of the University of Colorado at Boulder. "But I'm intrigued that there's something we can learn here about Mars that we didn't know before." Other surprises may arise this week. As Nature went to press, Phoenix had scooped its first handful of martian soil and was expected to unleash a battery of geochemical instruments on soil and ice samples. But the polygons - with their direct analogies in permafrost regions on Earth - offer one of the mission's earliest mysteries.
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