How habitable was Mars in the past? Since NASA's Curiosity rover touched down on Mars in August 2012, it has helped answer a few of these questions in the area surrounding its equatorial landing site of Gale Crater.

Most notably, in March 2013, Curiosity investigators announced extensive evidence of a lake bed or river system in a region that NASA dubs "Yellowknife Bay." The environment, which could be a favorable spot for microbes, includes minerals such as clays that are formed in waters that once existed there. The waters themselves were probably not too salty or acidic, geologic evidence shows, which gives further credence thatlife could have been possibleon the Red Planet.

Curiosity is now ascending its prime target Mount Sharp (Aeolis Mons). NASA isnt going to stop there, however. The agency is readying a successor rover to follow on the heels of Curiosity. [Curiosity Arrives at Mount Sharp (Video)]

Mars 2020, as its currently called, will have improved instruments over Curiosity. The new rover is heavily based on the Curiosity design, and as with its predecessor it will be able to search for habitable environments.

But Mars 2020 would also look directly for evidence of life, something Curiosity was not designed to do. This will make choosing a landing site crucial, since it would involve finding a spot where water or volcanic activity was present in the past. These processes provide energy for microbes.

"It will be a multi-year, hundreds of people effort tochoose the landing site for 2020," said Jim Bell, a planetary scientist at Arizona State Universitys School of Earth and Space Exploration.

"There are lots of great places to go. The finalist sites for Curiosity are already listed for consideration," he added.

These sites include Holden Crater, which scientists suspect may have been a lake system, andEberswalde Crater, a possible ancient lake bed.

Mars 2020s success will depend heavily on the seven instruments the rover is expected to carry to the Red Planet. The shortlisted instruments will have capabilities that range from taking pictures, to doing chemical composition analysis of the surface, to probing for organics, chemicals and carbon dioxide. [NASA's 2020 Mars Rover in Pictures]

The seven instruments are:

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Life on Mars? NASA's next rover aims to find out.