Spirit used its rock abrasion tool to grind below the rock surface and reveal cracks filled with apparent minerals, an indicator of water action familiar to geologists studying Earth rocks.
The water most probably was present when the magma was crystallizing into rock. It could have come up with the volcanic magma or the magma could have interacted with ground water, becoming infused with it, Arvidson said.
Scientists are unable to determine when the water may have been present, but the evidence suggests it was during the long-ago formation of the rocks.
Scientists making the historic announcement about Opportunity's discovery earlier this week could not say whether there had been standing surface water or even an ocean there, but data showed water had flowed or percolated through those rocks.
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 noted that the resulting photograph "looks a little bit like a Mickey Mouse silhouette."
The tool on each rover has performed beyond expectations, he said, showing side-by-side photographs of an unused, clean tool and a later image of the tool covered with martian dust.
Jim Bell, lead scientist for the rovers' panoramic cameras, also said Opportunity had photographed a solar eclipse caused by the passage of the martian moon Deimos across the sun, but scientists were waiting for images to be sent to Earth.
Opportunity, perched near the rim of the tiny crater in which it landed, did send home new views of the horizon, showing what is believed to be its discarded heat shield about a quarter-mile away. The shield sits next to a big divot it apparently made when it hit the surface.
The new panoramas show extremely flat terrain over which the rover bounced, swaddled in air bags, before it rolled into its tiny crater like a golf ball into a hole. One image showed a bounce mark on the crater rim, with a big rock in the background.
