Nasa finds new evidence of water on Mars

Nasa's Spirit rover has found evidence of past water activity in a volcanic rock on the other side of Mars from where its twin, Opportunity, discovered signs that ground there had once been drenched.

The amount of water at Spirit's site in Gusev Crater would have been much less than at Opportunity's site, said Ray Arvidson, deputy principal investigator of the rover mission, on Friday.

The findings came from a study of a rock dubbed "Humphrey" that Spirit came across en route to a big crater named "Bonneville", Dr Arvidson said.

Spirit used its rock-abrasion tool to grind below the rock surface and reveal cracks filled with apparent minerals, an indicator of water action.

The water was most likely present when the magma was crystallising into rock. It could have come up with the volcanic magma or the magma could have interacted with ground water, becoming infused with it, Dr Arvidson said.

Scientists are unable to say when the water may have been present but evidence suggests it was during the formation of the rocks.

Making the historic announcement about Opportunity's discovery earlier this week, scientists could not say whether there had been standing surface water or even an ocean there, but data showed water had flowed or percolated through the rocks.

Dr Arvidson said there was much less water indicated by "Humphrey".

"I don't think it was a ground water percolation necessarily, but probably water that came up with the magma," he said.

At one point Spirit created a familiar image on "Humphrey" when the rock-abrasion tool left three circular scars in the rock face. Scientist Stephen Gorevan said the resulting photograph looked "a little bit like Mickey Mouse".

Each rover has performed beyond expectations, he said.

Jim Bell, lead scientist for the rovers' panoramic cameras, also said that Opportunity had photographed a solar eclipse caused by the passage of the Martian moon Deimos across the sun, but scientists were still waiting for the images to be sent to Earth.

Opportunity did send home new views of the horizon, showing what is believed to be its discarded heat shield about a quarter of a mile away.

The new panoramas show the flat terrain over which the rover bounced, swaddled in air bags, before it rolled into its tiny crater. One image showed a bounce mark on the crater rim, with a big rock in the background.

"Follow the bounce marks back into the far field and you can see that one of them is right next to that rock, so not only did we get incredibly lucky to get this hole-in-one in the crater, but on the way into this crater we hit the only rock around," Dr Bell said.