A diagram of the seven instruments expected to fly on Mars 2020. Image courtesy NASA.

　　(NASA,Oct 29, 2014)How habitable was Mars in the past? Since the 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 that life could have been possible on the Red Planet.

　　Curiosity is now preparing to ascend its prime target - Mount Sharp (Aeolis Mons). NASA isn't going to stop there, however. The agency is readying a successor rover to follow on the heels of Curiosity.

　　Mars 2020, as it's 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 to choose the landing site for 2020," said Jim Bell, a planetary scientist at Arizona State University's 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, and Eberswalde Crater, a possible ancient lake bed.

　　Picture zoom for scienceMars 2020's 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.

　　The seven instruments include:

　　+ Mastcam-Z, a camera system that can zoom, take panorama images or spectroscopic images. Principal investigator: Jim Bell, Arizona State University

　　+ SuperCam, an instrument that can sense organic compounds in rocks and regolith through mineralogy and chemical composition analysis. Principal investigator: Roger Wiens, Los Alamos National Laboratory, Los Alamos, New Mexico.

　　+ Planetary Instrument for X-ray Lithochemistry (PIXL), which is designed to look for elements in the Martian surface. Principal investigator: Abigail Allwood, NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.

　　+ Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC), which will examine the spectrum of surface samples to learn what they are made of, and possibly to find organic compounds. Principal investigator: Luther Beegle, JPL.

　　+ Mars Oxygen ISRU Experiment (MOXIE), a device that will try to produce oxygen from the atmosphere of Mars, which is made up of carbon dioxide. Principal investigator: Michael Hecht, Massachusetts Institute of Technology, Cambridge, Massachusetts.

　　+ Mars Environmental Dynamics Analyzer (MEDA), a sort of weather station that will provide information on conditions around the rover such as temperature, humidity, dust size and shape, and wind speed and direction. Principal investigator: Jose Rodriguez-Manfredi, Centro de Astrobiologia, Instituto Nacional de Tecnica Aeroespacial, Spain.

　　+ Radar Imager for Mars' Subsurface Exploration (RIMFAX), which will use radar to probe underground to see what geology is there. Principal investigator: Svein-Erik Hamran, Forsvarets Forskning Institute, Norway.

　　While many of these instruments are new technologies, Mastcam-Z stems from a proven technology on Mars. Predecessors of this instrument flew on Curiosity, as well as on the Spirit and Opportunity rovers, which landed on the Red Planet in 2004. While Spirit ceased transmissions in 2010, Opportunity is still roaming the surface and taking pictures with that instrument.

　　With Curiosity, it's very difficult to get stereo images from its pair of cameras, Bell said. To do that, investigators have to combine nine images from a wide-angle camera, and a single one from a narrow-angle camera of higher resolution.

　　"That's a lot of data volume and there's not a lot of bandwidth from Mars," Bell said.

　　The new Mastcam system is able to zoom, meaning that investigators can match the focal length between the cameras and make the stereo images. This is not only important for rover navigation, but also to direct the rover's science.

　　Pictures are an important public relations tool, but for the scientists it establishes relationships between outcrops and sand dunes, provides a view of layers of rock, and guides the investigators on where to probe next.

　　"You can't scoop everywhere, it takes weeks to do those activities, so you have to winnow places down using the cameras. Their resolution and color capability help identify the best possible places," Bell said.

　　Proof of life?Luther Beegle's instrument, SHERLOC, has been ongoing since about 1998, and was most recently funded under the Astrobiology Science and Technology Instrument Development Program grant that was awarded to Deputy PI Rohit Bhartia.

　　This time around, the investigators made sure to design the proposal to meet the Mars 2020 requirements, and received approval to go ahead.

　　The instrument auto-focuses on an image, then scans a laser beam across a 7 x 7 millimeter area. It performs fluorescence spectroscopy to identify organics and Raman spectroscopy to look at the vibrations of individual molecules.

　　All ringed organic molecules fluoresce in distinctive ways, which is where the search for organics comes in. If investigators detect the signal of organics using this instrument, it would be a first step to looking for evidence of current life on Mars. (Organics can be produced from both biological and non-biological processes, so they are not definitive proof that life exists.)

　　Beegle emphasized that even if the organics are living, the laser will not hurt them.

　　"At such low power we don't see any disruption of organic molecules. The number of photons we use is really small."

　　Before doing the scans, Beegle said it will be necessary to use an instrument to remove dust from the surface. Organics do not survive well under surface environmental conditions on Mars, but could cling to the surfaces underneath. The instrument is also designed to peer into drill holes that the rover does.

　　If organics are found, one key to habitability will be to see where they are located. For example, if the organics follow an individual geologic feature such as a vein, that could strengthen the case for life. But this would depend on what the instruments say, and what environment the rover is scanning.

　　Building oxygenMichael Hecht's instrument is something entirely new to Mars, but a similar technology was developed by Johnson Space Center for an earlier mission that never flew.

　　MOXIE has been in the works since the 1990s, when NASA was pursuing a "faster, better, cheaper" approach to Mars using small missions. A notable success to this approach was the Mars Pathfinder lander, but there were failures as well. One, called the Mars Polar Lander, never made it to the surface.

　　At that time, there was a sister lander to MPL in development for 2001. For that mission, Hecht was developing MECA, an instrument to study dust-related hazards to future astronauts, but NASA cancelled the mission late in development out of concern that it would meet a similar fate as MPL. Other stationary landers were planned for 2003 and 2005, but were replaced with the Spirit and Opportunity rovers instead.

　　"We were disappointed as scientists directly involved in the Mars Surveyor Program, but as Mars explorers really excited about how bold and daring those replacement missions were," Hecht recalled.

　　MOXIE builds on the predecessor instrument, called MIPP, but is more efficient after 20 years of development, Hecht said. It proposes to create 20 grams of oxygen per hour at 99.6 percent purity on Mars to operate for the equivalent of 50 Martian days or sols.

　　This instrument could eventually strengthen the search for habitability because it would make it easier for humans to do investigations on the Red Planet themselves.

　　One major obstacle to landing people on Mars is making sure they have enough fuel and oxygen to return. If MOXIE is successful in generating oxygen in the long term, this would be an encouraging step to making Martian colonies possible in the coming decades.

摘自：http://www.marsdaily.com/reports/Curiositys_Successor_Mars_2020_Will_Continue_Search_for_Habitability_999.html