In general, signs of a habitable environment on Mars include evidence of liquid water. Geologists look for what are called "hydrous minerals," which are minerals that have water or some component of water built into their crystal structure. Examples are clays and sulfates. Additionally, geologists search for minerals that don't have water in their crystal structure, but form in the presence of liquid water, such as hematite.
Occasionally, aspects such as rock grain sizes can also give away the presence of water in the ancient past, such as when Curiosity found conglomerates - sedimentary rocks composed of various sizes of gravel-sized rock fragments - shortly after its landing.
"We are looking for interesting shapes and chemistries that are correlated as possible signs of life," Ken Williford, deputy project scientist of the Mars 2020 rover, told Seeker. "That's what will guide us as we choose our locations to sample."
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Williford added that Mars 2020 is not tasked to look for current life. NASA and other agencies have identified "special regions" on Mars where briny water likely flows on its surface, such as recurring slope lineae in craters. That said, it's tough to sterilize a spacecraft perfectly, so officials don't want to accidentally introduce Earth microbes in these environments. Also, as one of Mars 2020's goals is to collect the caches for a future sample-return mission, another concern is reducing the chance of bringing Mars microbes back to Earth.
Mars 2020 will have its project critical design review late this month, where traditionally engineers work from detailed drawings to see if there are any significant issues in the design. Given that Mars 2020 is closely based on Curiosity, however, Williford said that he expects the review will also include a discussion on how spare hardware can be best integrated into the design, and what final testing would be needed to prove that hardware still has the capabilities needed for the mission.
Image (top): A 2004 picture of a rock outcrop nicknamed "Longhorn" taken by the Spirit rover in Gusev Crater. The Columbia Hills region (which includes Gusev) is one of the sites being considered for the Mars 2020 mission. Image Credit: NASA/JPL-Caltech/Cornell