Author(s): David C. Catling [1]
In Soviet times, Russians would joke, "Is there life on Mars? No, not there either". Indeed, by the late 1970s, NASA's Viking mission had revealed a cold, dry planet, hostile to life [1]. Then, in the 1990s, came the confluence of two strands of science: a heated debate about possible traces of ancient life in a martian meteorite [2] and, in biology, increasing evidence that microbes can thrive in extreme environments. The idea of life on Mars re-emerged. The question of where to look for signs of past or present life on Mars has been guided by the principle that all life, as we know it, requires liquid water. Consequently, when NASA launched two rovers to Mars last year, one of the rovers, named Opportunity, was targeted at Terra Meridiani, a region where orbital reconnaissance had indicated abundant crystalline haematite (Fe2 O3 ), a mineral whose precursors usually form in water [3, 4, 5, 6].
On 25 January 2004, Opportunity landed in a 22-m-diameter crater in Meridiani. There the rover's camera stared across dark basaltic sand towards an outcrop, 0.5 m tall and 12 m long, of light-toned sedimentary rock near the crater rim [7]. Surprisingly, the haematite was found to be in the form of spheres, typically a few millimetres in diameter and embedded in rock outcrops like "blueberries in a muffin" (in the words of Steve Squyres, the lead rover scientist). But as the outcrops crumble away through wind erosion, the haematite spheres, which are more resistant to erosion, are falling to the ground. Haematite spheres or their broken fragments litter the plains of Terra Meridiani and represent leftovers of a large area of rock that has vanished in the wind.
NASA's rover team has interpreted the haematite spheres as concretions, which are hard, compact mineral accumulations of different composition from their surrounding rock. Concretions form from water that carries dissolved minerals through soft sediments or porous rock: minerals precipitate around a nucleus in layers that can incorporate or replace surrounding sediment. On Earth, nucleation is sometimes attributed to microbial processes [8]. Concretions occur in a variety of terrestrial rocks, but is there anything like the martian blueberries?
On page 731 of this issue, Chan...