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.
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.
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.
John Grant is a geologist in the Center for Earth and Planetary Studies at the National Air and Space Museum and served as the co-chair of the Mars Landing Site steering committee for the Mars Science Laboratory.