Landing Site Selection for Mars Exploration Rovers

Can a decision-support system assist the process of selecting landing sites on Mars and other destinations?

Selecting an extraterrestrial landing site is an extremely complex process that balances how scientifically interesting a location is with how conducive it is to safe landing and surface operation. The science value of candidate sites is assessed by scientists from a wide spectrum of disciplines, including geologists, climatologists, exobiologists, chemists, and physicists. Their recommendations must be integrated and evaluated against the mission's engineering constraints.

The places most appealing to scientists (the side of a cliff, for example, on which the planet's history is recorded in layers of sedimentary rock) are often the most frightening to engineers charged with the robot's safety. And conversely, the benign areas that delight engineers (such as a large, flat plain devoid of large, pointy rocks and chasms) tend to trigger yawns in their scientist colleagues. The best overall choices reflect compromise between the two competing interests.

In the case of the Mars Exploration Rovers, Spirit and Opportunity, this involved a two-year process in which teams of scientists and engineers debated the relative merits and pitfalls of the many possible targets before the mission science team reached a final decision.

We are in the process of designing an automated system to help decision-makers make well-informed choices in a much shorter amount of time, and possibly with a wider range of options to consider. Our approach involves quantifying the characteristics of an extraterrestrial landscape and producing a map of the most promising high-value, low-risk destinations. In this case study, we used available information about Mars, and sought to produce results similar to those of the team that actually determined where Spirit and Opportunity would go.

This study is part of a larger work in progress, which seeks to optimize the return on investment of conducting science on the surface of Mars (or any extraterrestrial target). Our overall approach (as further detailed in Methodology) is to derive all components of a mission from the top-level science goals, to quantify and manage risk, and to develop analytic tools to help decision-makers.

Unexpectedly, the most difficult part of this task turned out to be compiling and coordinating the data that have been produced over the years by Mars orbiters and surface explorers. Nevertheless, we did manage to gather a considerable amount of information from various instruments aboard the Viking landers, Pathfinder, Mars Global Surveyor, and Odyssey.

The above chart illustrates the engineering and environmental constraints we used to determine the safety of a potential landing site, the objectives we employed to determine the desirability of landing sites from a scientific perspective, and the sources of our data.

These maps illustrate three sets of data regarding the Martian surface. (We are currently gathering additional information to improve the process). Note that each is a map of the entire Martian surface, with the poles at top and bottom, and the equator stretching across the middle.

The first map shows the planet in visible light. The second is a digital elevation map. The third illustrates gamma ray spectrometer readings that indicate the presence of hydrogen, a marker for water and water ice. Locating areas of past or present water is a prime objective of NASA's Mars Exploration Program, on the premise that they are the most likely places to find evidence of past or present life.

Here we see evaluations of the relative landing risk and science value of areas across the Martian surface. The first image shows that there is enough sunlight for solar panels only in a band near the equator. In the second image, red indicates highlands, which are undesirable for landing because there isn't enough atmosphere above them to enable a parachute to reduce the lander's speed sufficiently. In the third image, bright green indicates locations with a good probability of water near the surface. The fourth image shows the expected utility of the various locations, computed from the information contained in the first three. Here, red indicates undesirable locations (either because of landing risk, absence of sufficient sunlight, or low probability of finding water), green indicates moderately good locations, and yellow indicates the best locations.

In this image, the top 5% of locations with high science value and relatively low risk are highlighted in yellow. The best of these locations are shown in green.

To see how our results contrast with those of the real MER panel, compare this image with the one below:

On this Martian map, rectangles indicate an early set of candidate sites selected for further analysis by the team that actually determined where Spirit and Opportunity would land. We see that there is considerable overlap between their selections and ours, but that our map also highlights a cluster of locations at left which was not selected on the MER team's map. We intend to interview some of these scientists and engineers to learn whether they considered and rejected that cluster (and if so, why), or whether it might have been useful to them if these areas could have been called to their attention. This review process will help us to refine our automated selection process, to make it as useful a tool as possible for the people who ultimately must decide where to send our explorers, both robotic and human.