Here is a riddle: What takes more than 60 locations, 5 years, and 150 scientists to decide? The landing site for the Mars Science Laboratory (MSL) rover Curiosity. Picking the landing site for a spacecraft to land on another planet is always serious business. And the job of finding the best location for Curiosity to set down on Mars was no exception.
Curiosity’s mission is geared towards understanding whether Mars could have ever been habitable. And recent data from NASA’s orbiting spacecraft (Odyssey and the Mars Reconnaisance Orbiter) and the Mars Exploration Rovers suggests the planet has had a long and complicated history of changing environmental conditions and landscapes. Combine that with the fact that the landing site could be anywhere between 30 degrees north and south of the equator and below an elevation of 0 kilometers (relative to the Martian datum) and there is a lot of territory to consider.
This map of Mars shows all of the landing sites proposed for the Mars Science Laboratory (red dots) and the four final candidate sites (blue dots). From the four final sites of Eberswalde crater, Gale crater, Holden crater, and Mawrth Vallis, Gale eventually was selected as the landing site. The white shaded areas are more than 30 degrees north and south of the equator and off limits to MSL because of seasonally harsh (cold) conditions expected there. The black areas are above 0 kilometer in elevation and too high to be considered for landing.
The vast majority of the sites proposed for consideration (Figure 1) were within the general bounds outlined above and many possess attributes making them attractive as possible landing sites. Moreover, the design of the rover enables consideration of a variety of sites. So science merit became the major discriminator of which site would eventually win out.
Over a series of workshops, the science community and MSL science team came together to discuss and evaluate the various proposed sites. The diverse expertise represented at the workshop coupled with ample discussion time ensured each site got a good look. As the process went along, more and more sites were dropped from consideration as potential issues were identified. Finally, four sites remained, all of which were deemed satisfactory for MSL and each with a substantial group of science advocates. These four sites include a relict river delta in Eberswalde crater, a 5 kilometer (3.1 mile) thick section of layered rocks in Gale crater, ancient alluvial and possible lake beds in Holden crater, and ancient sequence of clay-bearing rocks near Mawrth Vallis (Figure 2). The four sites became the focus of intense study and discussion at the final two workshops, with efforts geared towards understanding how the rocks in and near the sites were emplaced and whether they might be accessible to Curiosity once on the ground. As data related to the sites poured in and evaluations went on, the four final sites have become arguably the best imaged and studied locations on the surface of Mars. In the end, there was no “smoking gun” that was found to rule out any of the four final candidate sites and the community reiterated their satisfaction with any one of them. Much more information about each of the proposed landing sites can be found on Marsoweb.
Summaries of each of the final four candidate landing sites for the Mars Science Laboratory. The left column shows the regional context of each of the four sites (labeled on the left and discussed above) with colors representing the elevation of the land surface (purple lowest and red highest). The middle column shows examples of high priority science targets for exploration near the ellipse (yellow box in left column shows the location of each) and the right column shows science targets within each target landing ellipse (white box in left column shows the location of each). At Eberswalde crater, Curiosity would land on the crater floor and probe ancient river and possible lake beds on the way to a large delta on the western wall of the crater. At Gale crater, the site chosen as the landing site for Curiosity, landing will occur on an alluvial fan near the northern wall of the crater and the rover will than traverse to a thick stack of layered rocks to the south. At Holden crater, landing would take place on broad alluvial fans flanking the western wall of the crater and the rover would traverse down to underlying and finely layered rocks that may have been deposited in a lake. At Mawrth Vallis, landing would occur directly on a layered sequence of clay-bearing rocks that extend regionally across the surface. The images comprising the panels in the middle and right columns are from the HiRISE camera on the Mars Reconnaissance Orbiter. The scale bars in each panel indicate distance in kilometers.
The Curiosity science team then met and considered all of the information related to the sites. Both science potential and risks to rover landing and traversing were considered. In the end, Gale crater was selected as the landing site because the thick section of rocks (Figure 2) was deemed likely to enable study of changing conditions on Mars over a time when the abundance and duration of water on the surface was decreasing over time. As water is an important factor in evaluating potential habitability, the chance to access the rocks that record the changes from relatively wetter to drier present an opportunity to learn a great deal about Mars as a planet and its potential to support life.
Curiosity lifts off towards the Red Planet late in 2011 and will arrive at Mars in mid-2012. In the days and months leading up to landing at Gale crater, the MSL science team will continue to pore over existing and new images to plan the best path towards rocks they feel hold the clues to understanding Mars’ habitability. Once on Mars and on the move, Curiosity will provide images and information from its science payload of instruments that will enable all of us to follow along in the excitement of exploration and learn more about how one of our neighboring planets evolved over time.