The US space agency's Mars rover Opportunity has begun investigating rocks along the rim of the large crater it is perched by on the Red Planet.
Lion Stone is Opportunity's first science target at Endurance Crater
The rover has conducted scientific tests on Lion Stone, a 30cm-long rock by the edge of Endurance Crater.
Opportunity has been making a three-week trip to Endurance Crater, which could reveal even more about the history of liquid water on Mars.
Scientists must now decide whether to risk sending the rover into the crater.
There is a real possibility, they say, that it might not be able to get out.
Opportunity has taken microscopic images of Lion Stone and readings with its Moessbauer spectrometer and its alpha-particle X-ray spectrometer.
On Friday, the rover will grind into Lion Stone with its rock abrasion tool (Rat).
Scientists hope to learn whether the rocks differ significantly in their geological make-up from those Opportunity encountered at its landing site in Eagle Crater.
Those rocks yielded compelling evidence that there was once a large body of water at Meridiani Planum, the flat Martian region the rover is currently exploring.
Opportunity also carried out remote sensing of possible targets for investigation around the crater with its mini-thermal emission spectrometer (Mini-Tes) instrument.
And the rover took pictures of the heatshield that protected it on descent through the Martian atmosphere in January.
The heatshield thumped into the ground 250 metres (about 820 feet) south of Endurance Crater.
Mission scientists have said that it may be a legitimate science target because it has undoubtedly dug the "biggest fresh hole" on the plain of Meridiani.