It is a year since the first of two robot rovers touched down on Mars.
The US space agency's (Nasa) Spirit vehicle led the way when it landed on 3 January 2004, followed three weeks later by the Opportunity robot.
Both rovers, which are equipped with a suite of geology tools, have uncovered direct evidence that the Red Planet was once drenched in liquid water.
Their initial 90-day mission has been extended twice and they are expected to continue working for some time yet.
"The rovers are both in amazingly good shape for their age," said Jim Erickson, the rover project manager from Nasa's Jet Propulsion Laboratory.
"The twins sailed through the worst of the Martian winter with flying colours, and spring is coming.
"Both rovers are in strong positions to continue exploring, but we can't give you any guarantees."
Spirit came down in a near-equatorial region of Mars known as Gusev Crater. There it has discovered a mineral in the bedrock called goethite which forms only in the presence of water.
Spirit's twin - Opportunity - which arrived on 24 January is exploring the other side of the planet at Meridiani Planum, a flat region about the size of Oklahoma.
Its discoveries have perhaps been the more dramatic. In particular, the rover has found jarosite, an iron sulphate mineral which, together with other evidence, suggests Meridiani was once the location for an acidic lake.
The results from both vehicles have encouraged scientists to believe the Red Planet may indeed have had the necessary conditions early in its history to harbour life.
Science magazine described the rover mission as the "breakthrough" of 2004
Opportunity is currently driving towards the heatshield that protected it during descent through the Martian atmosphere.
Rover team members hope to determine how deeply the atmospheric friction charred the protective layer. "With luck, our observations may help to improve our ability to deliver future vehicles to the surface of other planets," Erickson said.
Spirit is exploring the Columbia Hills within the Gusev Crater. It is investigating phosphorus-rich rocks which could have been the explosive products of a volcano or a meteoroid impact.
The flow of robotic missions to Mars will continue this year with the despatch in August of yet another orbiter, the Mars Reconnaissance Orbiter.
The next mission to land on the planet will probably be Nasa's Phoenix laboratory in 2008.
The Europeans, who expect to start searching for sub-surface water on Mars shortly with their Mars Express orbiter, hope to send a lander before the end of the decade.