A bumper crop of "blueberries" has been found around the rim of Mars's Victoria Crater by NASA's Opportunity rover. The discovery suggests that when the rover descends into the crater in a few months, it may find a bathtub ring of the tiny concretions – an ancient high-water mark suggesting the area was altered by ground, not surface, water.

Blueberries are thought to have formed by the precipitation of iron-bearing minerals when groundwater rose up through layers of sediment. The spheres have been seen scattered across the surface almost everywhere Opportunity has crossed on its 7-kilometre voyage from its landing site to the nearly kilometre-wide Victoria Crater.

But they have become much smaller and sparser along the way, dropping from 5 millimetres wide to less than 1 mm across. Since the rover neared the crater's rim, however, that trend has reversed.