A Mars rover has found further evidence of past water activity on the Red Planet, the US space agency says.
The fresh signs were discovered by the Spirit vehicle after its tools bored a hole in volcanic rock.
The announcement comes days after its twin, Opportunity - which is exploring the other side of Mars - revealed that rocks there had once been drenched.
The rovers landed in January, and are looking for signs that the planet may once have been able to sustain life.
Spirit came across its water evidence when studying a dark volcanic rock, dubbed "Humphrey", in Gusev Crater.
The rock, which is about 60cm (2ft) tall, shows bright material in interior crevices and cracks that looks like minerals crystallised out of water, Dr Ray Arvidson of Washington University, St Louis, reported at a Nasa news briefing.
"If we found this rock on Earth, we would say it is a volcanic rock that had a little fluid moving through it," the deputy principal investigator for the rovers' science instruments added.
If this interpretation is correct, the fluid - water with minerals dissolved in it - may have been carried in the original magma that formed the rock or may have interacted with the rock later, he said.
The Nasa briefing stressed the water involved would have been much less than at Opportunity's site, Meridiani Planum.
"It is by no means the gobs of water at Meridiani, but again it demonstrates that when rocks are made on Mars, fluids are involved," Dr Arvidson said.
Rich in sulphur
On Tuesday, Nasa said Opportunity had found proof that parts of the planet would have had the conditions necessary to sustain life some time in the geologic past.
The evidence lay in Meridiani Planum' rocks which had come into contact with substantial quantities of water. This water had modified the rocks' appearance and chemistry.
The rover's instruments detected high levels of sulphate salts which on Earth would normally form in water or, after formation, be highly altered by long exposures to water.
Nasa has two rovers on opposite sides of Mars
"The only way you can form such large concentrations of salt is to dissolve it in water and allow the water to evaporate," mission scientist Dr Benton Clark said on Tuesday.
In particular, Opportunity found jarosite, an iron sulphate mineral which suggests that an acid-rich lake or hot-spring environment might have existed at Meridiani Planum.
"We believe at this place on Mars for some period in time... this was a ground water environment that would have been suitable for life," said Professor Steve Squyres, the rovers' lead scientist.
"That doesn't mean that life was there. We don't know that," he added.