When the $2.5 billion dollar Mars Science Laboratory (MSL) rover Curiosity was designed and built, its landing site was not known in advance. Rather, a series of open-invitation workshops was convened to solicit input from the scientific community to help evaluate potential sites.
Think of the selection process as the science fair to end all science fairs. Many elements of the site selection process for MSL trace their heritage to procedures developed during the Apollo era. Here we trace some of the connections to past site selection activity and envision the future through similar series of workshops planned for the European Space Agency’s (ESA) ExoMars rover (slated for launch in 2018) as well as NASA’s Mars 2020 rover (see Figure 1).
The primary incentive for participating in the process is information: Scientists who volunteer their time are rewarded by new data on their favored locales on Mars. This, in turn, spurs more research and fuels more curiosity about Mars.
Engineering Constraints Based on Environmental Factors
The most basic dictates as to where and when a spacecraft can land are termed “engineering constraints,” and they are primarily determined by mission safety. Safety trumps science and for good reason. If a mission cannot land safely, it will be over before it has begun.
Selection of landing sites is therefore governed by a set of mission-specific engineering constraints that are unique to a given mission’s particular architecture. These constraints provide the broad framework of potentially acceptable places to land, and scientists must abide by these engineering constraints in selecting the most scientifically relevant landing sites.
For the Apollo missions, the need to maintain constant Earth-Moon communications plus orbital constraints of the command module limited the initial site selection to an “Apollo Operational Zone” that encompassed ±45° longitude and ±5° latitude (i.e., the equatorial nearside of the Moon) [Beattie and El-Baz, 1970]. In an analogous fashion on Mars, potential landing sites for the solar-powered Mars Exploration Rovers (MERs) were similarly confined to an equatorial zone extending from 10°N to 15°S latitude due to insolation and concomitant power constraints [Golombek et al., 2003]. Although the MSL rover is powered by a radioisotope thermoelectric generator, it too had a range restricted to ±30° latitude for thermal considerations [Golombek et al., 2012].
Yet not all engineering constraints are hard limits; some, such as latitude, may be somewhat flexible. For example, the initial MSL latitude constraints were ±60° north or south from the equator of Mars [Golombek et al., 2006]; later, these were reduced to ±30°. Although the nuclear power source of MSL does not depend on sunlight to operate, the spacecraft has a preferred operational temperature range. Colder temperatures mean more power would need to be diverted to the heaters to keep various sensitive electronic components warm, and hence, less power would be available for science and mobility (meaning smaller traverse distances, slower movement rates, and fewer samples analyzed by the instruments).
Landing System Constraints
In addition to power and thermal constraints, many of the key parameters that determine if a given site is accessible for landing stem from the incoming spacecraft trajectory and landing system. For example, for the Mars Pathfinder, MER, and MSL missions, a landing site had to be at or below an elevation of 0.0, −1.3, and −1.0 kilometers, respectively (as Mars has no sea level, elevations are reported relative to an average reference elevation). Sites above these elevation limits would have insufficient column density of atmosphere to ensure full deployment of the parachutes used to slow the spacecraft during entry, descent, and landing (EDL) processes.
Following the precedents of the Apollo landings on the Moon, the totality of trajectory parameters during the EDL process is used to define the three-sigma landing dispersion ellipse (or landing ellipse for short), which is the probability envelope for a given region within which the spacecraft has a 99.7% chance of landing. For MSL, the landing ellipse was initially about 20 by 25 kilometers.
The area of MSL’s landing ellipse shows that as EDL technology improved, the size of the landing ellipse shrank. The long axes of the landing ellipses for the Viking, Pathfinder, and MER spacecraft, in contrast to MSL, were 300, 200, and 82 kilometers, respectively [Soffen, 1977; Golombek et al., 1997; Golombek et al., 2003].
A smaller landing ellipse permits landing in a larger variety of candidate sites. Gale crater, the eventual landing site for MSL, had been considered as a potential MER site but was dropped from consideration because it could not accommodate the larger MER landing ellipse. The nominal landing ellipse size for Mars 2020 is identical to MSL, while the ExoMars ellipse is 100 by 15 kilometers [Vago et al., 2013], which is comparable in size to MER and Mars Phoenix landing ellipses.
A final element of consideration is the roughness within the landing ellipse and approach pathway. A rough or rocky landing site presents a significant hazard to any landing system.
Although the presence of a manned crew significantly increases the degree of complexity of a mission, in the case of Apollo 11, the ability of its commander to make last-minute course corrections during descent turned a potential failed landing into a successful one. Such a feat would be difficult to replicate with an automated landing system—mission planners could only mitigate this risk for unmanned spacecraft sent to Mars by avoiding placing landing ellipses in rocky areas. Hence, there is a growing demand for high-resolution photographs of potential landing sites to certify that they are nearly rock free.
Site Selection Workshops
During the Apollo program, NASA established an advisory body known as the Group for Lunar Exploration Planning [Beattie and El-Baz, 1970]. It had expert astronomers, geologists, geochemists, geophysicists, and engineers who were charged with selecting scientific instruments for successive missions. Within this group, a landing site selection committee took all of the scientific objectives into account in the selection process and incorporated all relevant engineering constraints. Site selection during the Apollo years was not a static activity but rather a continuous process, with later mission selections planned with flexibility to incorporate data returned by previous missions.
Starting with the Mariner photographic missions in the late 1960s, the exploration of Mars began during the preparation for the Apollo landings. Because of this temporal overlap, the site selection for the first spacecraft to land on Mars, Viking 1 in 1976, benefited greatly from the procedures of site selection developed during the Apollo lunar landings. By default, the process was limited to a small group of scientists who were heavily involved with NASA’s efforts.
A larger group of scientists and engineers participated in the open site selection process for the Mars Pathfinder mission [Golombek et al., 1997], and a still larger group participated in the process for the MER missions [Golombek et al., 2003; Grant et al., 2004]. The growing inclusiveness of the site selection process has increased with the advent of the Internet, that is, the facilitation of instant, worldwide communications with all those interested in planetary exploration. The broad community input into the selection process is a testament to this exceptional development.
For the MSL mission, five open public workshops were convened between 2006 and 2011 by its landing site steering committee to solicit and evaluate potential landing sites [Grant et al., 2011; Golombek et al., 2012]. Of the sites proposed, several dozen were later designated for special targeting by existing spacecraft orbiting Mars, including the Mars Reconnaissance Orbiter (MRO), giving workshop participants access to high-resolution data on specific locations on Mars.
These targeted observations were key to getting scientists involved with the workshops. High-resolution cameras and spectrometers on MRO and other orbiters are enormously powerful, but their detailed views of the surface come with a trade-off: limited spatial coverage. For example, it is estimated that the High Resolution Imaging Science Experiment (HiRISE) will only be able to view less than 2% of the Martian surface at its maximum resolution, even after spending 8 years in orbit with near-continual operations.
Thus, targeting orbital cameras to potential landing sites on Mars allows scientists to get a detailed picture of a favorite location that otherwise would not be scrutinized—this in itself can be a real boon to participating scientists. Indeed, this is one of the main incentives used to get the participants to donate time and effort to a thoughtful site selection process.
Picking Future Mars Landing Sites
The first landing site workshop for the ESA’s ExoMars 2018 rover was held 26−28 March 2014 in Madrid, Spain. Sites under consideration for NASA’s Mars 2020 rover received a similar first airing at a workshop that was held 14−16 May 2014 in Washington, D. C.
What kind of site do we want to visit next on Mars? First and foremost, the chosen locale must align with the scientific objectives of the mission. ExoMars and Mars 2020 have related but distinct science objectives, and each will be targeted at a site to maximize its scientific return. The ExoMars rover will have the capability to drill up to 2 meters into the subsurface and will search for evidence of organics from an environment that is shielded from surface radiation. The Mars 2020 rover will characterize an astrobiologically relevant ancient environment and search for potential biosignatures, and it is tasked with assembling a diverse cache of Mars samples to be returned to Earth by a future mission.
One way to narrow the potential slate of candidate sites is by inferred mode of deposition or formation. Among the highest-priority sites are those with subaqueous sediments or hydrothermal deposits [Mustard et al., 2013]. These formative processes are inferred from both morphological and mineralogical evidence.
For example, some of the clearest morphological indicators of past aqueous activity are channel deposits indicative of past fluvial activity or the terminal fan or delta deposits present within basins (such as in Holden crater in Mars’ southern highlands or at nearby Eberswalde delta). While there is some spectroscopic evidence for limited alteration phases present at these sites, the strength of these spectral features is dwarfed by the spectral signatures of the massive clay deposits in the Mawrth Vallis region (possibly formed by volcanic ash or the weathering of rocks to form soil). Other sites of interest include clay-chloride sites such as East Margaritifer Terra and sites with potential hydrothermal alteration near Nili Fossae. See Figure 1 for these sites’ locations on Mars.
An attractive element of the list of candidate reference sites identified by McLennan et al.  is that they contain both aqueous or sedimentary rocks and igneous rocks. Access to igneous material from a widespread volcanic unit would provide an important calibration point for understanding the Martian timescale. A more complete preview of some of the potential sites is provided in the additional supporting information in the online version of this article.
Some caution is warranted, however, when attempting to select a landing site on the basis of its perceived mode of formation. The selection of Gusev crater as a MER landing site was motivated by evidence for past lacustrine activity, but evidence for paleolacustrine processes turned out to be buried beneath subsequent lava flows. Fortuitous discoveries in the nearby Columbia Hills instead indicate that Gusev is a prime example of a site with past hydrothermal activity, but such a connection could not be established from orbital data alone.
A more pragmatic division of potential landing sites is between those for which the primary scientific objectives lie entirely within the landing ellipse (dubbed “land-on” sites) versus sites where the science objectives that motivated its selection are located outside the landing ellipse (called “go-to” sites). Such a distinction factors in to considerations about potential EDL technology improvements.
Lessons Learned From MSL
Just as the later Apollo missions benefited from the experiences gained in the early mission, so too has the Mars exploration program built upon past experience. With MSL, despite favorable placement of the landing ellipse within Gale crater, it appears unlikely that the rover will achieve its primary science objectives in the central mound [e.g., Thomson et al., 2011] before the end of its primary mission. This is due in part to its extended commissioning phase and the rover’s relatively slow rate of movement (i.e., including pauses for stationary surface science operations). MSL will still likely attain this goal during an extended mission. However, this drawn-out pace of exploration should give the scientific community pause in its conception of long rover traverses at future landing sites.
Relevant to the characterization of potential sites as land-on versus go-to, a critical element for the Mars 2020 landing site selection process will be the potential addition of new EDL technologies. For example, triggering the parachute to open using a measure of the range to the ground rather than the current velocity-based trigger could reduce the size of the landing ellipse from 25 by 20 kilometers to 18 by 13 kilometers, thus enabling placement of the ellipse closer to targets of interest that are too rough for a direct landing and reducing necessary traverse distances.
In addition to this “range trigger” option, there are some additional considerations of enabling maneuverability of the spacecraft during the descent phase [Mustard et al., 2013]. Landings conducted with these enhanced capabilities could potentially be more tolerant of hazards (e.g., isolated areas with steep slopes or rocks), thus opening up more sites for potential consideration.
Being involved in site selection is a tremendous opportunity for all students of science and engineering; anyone with an interest in planetary exploration is welcome to pitch in and help choose a future landing site on Mars. Whether the next missions to Mars are sent to previously proposed sites or some as yet unidentified locales remains to be determined, but it is never too early to start planning ahead.
J. T. was supported in part for this work by NASA MDAP grant NNX11AN01G. We thank John Grant for helpful discussions and Bethany Ehlmann, Joseph Michalski, and an anonymous reviewer for insightful reviews that improved the manuscript.