NASA's Curiosity rover is preparing to drill the Martian surface and is driving towards a flat rock with pale veins that may hold clues to a wet history on the Red Planet.
It's the most highly anticipated milestone since the six-wheel, nuclear-powered rover landed near the Martian equator five months ago on its two-year prime mission, investigating whether the planet ever offered an environment favourable for microbial life.
If the rock meets rover engineers' approval when Curiosity rolls up to it in coming days, it will become the first to be drilled for a sample during the mission, NASA said.
"Drilling into a rock to collect a sample will be this mission's most challenging activity since the landing. It has never been done on Mars," said Mars Science Laboratory project manager Richard Cook of NASA's Jet Propulsion Laboratory.
"The drill hardware interacts energetically with Martian material we don't control. We won't be surprised if some steps in the process don't go exactly as planned the first time through," Cook said in a NASA statement.
Curiosity first will gather powdered samples from inside the rock and use those to scrub the drill. Then the rover will drill and ingest more samples from this rock, which it will analyse for information about its mineral and chemical composition.
The chosen rock is in an area where Curiosity's Mast Camera (Mastcam) and other cameras have revealed diverse unexpected features, including veins, nodules, cross-bedded layering, a lustrous pebble embedded in sandstone, and possibly some holes in the ground.
The rock chosen for drilling is called "John Klein" in tribute to former Mars Science Laboratory deputy project manager John W Klein, who died in 2011.
The target is on flat-lying bedrock within a shallow depression called "Yellowknife Bay".
The terrain in this area differs from that of the landing site, a dry streambed about 500 meters to the west.
Curiosity's science team decided to look there for a first drilling target because orbital observations showed fractured ground that cools more slowly each night than nearby terrain.
"The orbital signal drew us here, but what we found when we arrived has been a great surprise," said Mars Science Laboratory project scientist John Grotzinger.
"This area had a different type of wet environment than the streambed where we landed, maybe a few different types of wet environments," said Grotzinger.