Six days after landing on Mars, the U.S. Opportunity rover has rolled off its landing platform onto the ground, ready to collect scientific data.

Mission controllers at the Jet Propulsion Laboratory in California ordered the six-wheeled Opportunity to make a three-meter drive down the lander's ramp.

"It's on the surface, we've done our first movement, and we're no longer tied to any part of what we brought with us from Earth," explained mission manager Jim Erickson. He says Opportunity's immediate science objectives will be to map the shallow crater in which it landed, study exposed bedrock within the crater, and eventually crawl over the low crater rim onto the Plains of Meridiani.

Opportunity and its twin rover Spirit on the other side of Mars are looking for evidence that water once flowed on the planet, suggesting it could have supported life.

Mr. Erickson says that before Opportunity left the lander, it remotely sensed an iron compound called hematite on the plains, a mineral that usually forms in water.