Rover investigates deep-set rock

Peace (centre) may give a better idea of the history of Gusev Crater

The US space agency (Nasa) robot rover Spirit has been studying what could be its most important rock to date on the surface of the Red Planet.

The rock, dubbed Peace, was found in exposed bedrock at Columbia Hills, an area of elevated land at Spirit's Gusev Crater landing site.

The rock shows clear signs of having been altered by water in the past.

"This may be what the bones of this mountain are really made of," said rover chief scientist, Steve Squyres.

"It gives us even more compelling evidence for water playing a major role for altering the rocks here," the Cornell University researcher added.

Peace contains more sulphate salt than any other rock Spirit has examined, and the rover's abrasion tool, which cut a centimetre into Peace, shows the salt's presence goes deep inside.

Entry debris

Rover scientist Dr Ralf Gellert, of Max-Planck-Institut fur Chemie, Mainz, Germany, said: "Usually when we have seen high levels of sulphur in rocks at Gusev, it has been at the very surface.

"The unusual thing about this rock is that deep inside, the sulphur is still very high. The sulphur enrichment at the surface is correlated with the amount of magnesium, which points to magnesium sulphate."

Spirit's investigations reveal the rock contains significant amounts of the minerals olivine, pyroxene and magnetite, all of which are common in some types of volcanic rock.

The Peace's texture appears to be sand-size grains coated with a material loosely binding the rock together.

"It looks as if you took volcanic rocks that were ground into little grains, and then formed a layered rock with them cemented together by a substantial quantity of magnesium-sulphate salt," Dr Squyres said.

"Where did the salt come from? We have two working hypotheses we want to check by examining more rocks. It could come from liquid water with magnesium sulphate salt dissolved in it, percolating through the rock, then evaporating and leaving the salt behind.

"Or it could come from weathering by dilute sulphuric acid reacting with magnesium-rich minerals that were already in the rock. Either case involves water," he said.

Spirit's twin rover, Opportunity, which is sited on the opposite side of Mars, has been examining the heat shield that protected the spacecraft as it slammed into Mars' atmosphere a year ago.

This is the first time experts have been able to examine a heat shield after it entered another planet's atmosphere. Engineers expect the findings to aid design for future missions.

Without the heatshield (top left), Opportunity would never have survived the entry into Mars' atmosphere