Astrogeology Helps Choose Where to Land on Mars

24 October 2018

Where would you send NASA’s next Mars rover? That was the question that hundreds of scientists, including USGS Astrogeology’s Robin Fergason, Ken Herkenhoff, and Ryan Anderson set out to answer last week. The Mars 2020 rover represents the beginning of an ambitious plan to collect rock and soil samples from the surface of Mars and return them to Earth for detailed analysis, raising the stakes on the selection of the best landing site possible.

Astrogeology has a long history of providing the most reliable data possible to NASA to aid in the assessment of landing sites, and the Mars 2020 mission is no exception. Astrogeology has been involved in almost every aspect of site characterization for Mars 2020. Topographic data and thermal analyses from the USGS are used, along with images and other data, to identify areas of the landing sites that are dangerous, such as cliffs or craters or fields of sand dunes from which the rover would be unable to escape. Mars 2020 will use a new technology that will allow it to autonomously avoid hazards by comparing its view of the surface with images provided by USGS Astrogeology in real-time as it is landing.

The meeting last week was the culmination of years of discussion and was focused on the science potential of three finalists that emerged from previous meetings: Columbia Hills, Northeast Syrtis, and Jezero. A fourth site, Midway, located between Northeast Syrtis and Jezero was also added to the mix.

Fans of Mars exploration may recognize the name Columbia Hills. The Mars Exploration Rover Spirit, which landed in Gusev crater in 2004, spent much of its mission exploring the Columbia Hills, and its discoveries there are what make the site an appealing candidate for sample return. Spirit discovered carbonate rocks that may hold clues to the early evolution of the martian atmosphere, volcanic rocks that could provide a precise numerical age for the martian surface, and most importantly, abundant evidence for volcanic and hydrothermal activity. Hydrothermal settings on Earth are known as habitats for life, and silica deposits found in hydrothermal springs are excellent at capturing and preserving microbes. Silica deposits discovered by the Spirit rover look uncannily similar to those found on Earth, so they may represent an ideal sample to collect from Mars to answer the question of whether life ever arose there.

Advocates for the Columbia Hills site at last week’s meeting said that knowing that there are excellent samples to collect at the site means that the Mars 2020 mission is more likely to be successful. However, others argued that there is a risk that any discoveries made in the Columbia Hills might be difficult to link to the rest of Mars, because it is unclear how the Colombia Hills rocks are related to other geologic units on Mars. There was also concern that returning to a site that has been explored before when we have only seen a handful of places on the surface of Mars could limit the potential for new discoveries as compared to visiting new sites.

Northeast Syrtis would represent one such new site. It is in the rugged, cratered highlands that represent some of the oldest rocks on Mars. Even better, it is part of an entire region where data from satellites in orbit around Mars reveals that there is a predictable sequence of rocks, with extremely ancient weathered rocks on the bottom, carbonate-bearing rocks on top of those, and erosion-resistant cap rocks on top of that. Included in the ancient rocks at the bottom of this sequence are huge blocks of rock ejected from the nearby Isidis basin when it was formed by a giant impact. Farther afield from the landing site, but close enough to be reached during an extended mission are layered sulfate minerals that likely formed in water, and the extensive Syrtis Major lava flows.

Advocates for Northeast Syrtis point to the diverse, ancient rocks that could be sampled that are part of a broader regional, or potentially global, geologic sequence. They also argue that the hydrothermal alteration that formed some of the minerals observed in these rocks would provide a long-lasting habitable environment for life, making them good targets to help answer the question of life on Mars. However, some scientists questioned how well we understand the rocks at Northeast Syrtis and worry that landing there might result in a mission that is inefficient at first as the science team tries to get their bearings and figure out where to go first. There were also concerns about how well the rocks at Northeast Syrtis could preserve the evidence for ancient life, and the extended mission at Northeast Syrtis would bring the rover into rough terrain where a subsequent mission to retrieve the collected samples could have trouble landing.

Jezero crater is in the same region as Northeast Syrtis, but its main attraction is a spectacular fan-shaped deposit on the northwestern crater floor, thought to be a delta of sediment formed by a river emptying into a lake that once filled the crater. On Earth, deltas are rich in organic materials collected from throughout the river’s watershed and concentrated in the finest sediments. The floor of Jezero also contains an extensive surface with a volcanic composition which, if it is a lava flow, could be analyzed to determine its age. Jezero also shows evidence for concentrations of carbonate minerals, especially along the base of the crater rim near the delta, right at the elevation expected for the former lake, making some scientists hopeful that the carbonates could have been deposited at the shore of the lake. Such deposits on Earth are good at preserving microbes.

Scientists in favor of Jezero as the landing site for Mars 2020 point to the delta deposits and carbonates as high priority samples for answering the question of life on Mars, and the crater floor samples as providing constraints on the age of the martian surface. They also say that the delta provides a clear target of known origin so that the rover can hit the ground running for more efficient operations. However, some scientists were skeptical of Jezero, expressing concern about uncertainty in the age of the delta and whether the crater floor might be sedimentary rather than volcanic rock and therefore less useful for dating the surface.

Midway, just to the west of Jezero and north of Northeast Syrtis, was the final site discussed. It was suggested by the Mars 2020 project as an alternative to Northeast Syrtis. Midway has basically the same rocks as Northeast Syrtis (one benefit of the rocks being consistent across the region), but has the added advantage that, in theory, the rover could drive from Jezero to Midway or vice versa in its extended mission. This idea of a “megamission” was presented as an ambitious compromise solution between Jezero and Northeast Syrtis.

The meeting ended with an anonymous vote considering the science potential of each site, the samples that might be collected there, and the confidence in that assessment of science potential. The extended mission options for each site were also evaluated based on the same criteria. In the end, Jezero, Midway, and Northeast Syrtis ended up with essentially the same scores, and Columbia Hills was ranked lowest of the four sites.

This result, the distillation of years of work by the Mars science community, will be used by the Mars 2020 project to decide which site to recommend to NASA headquarters, where the final decision will be made in the next few months after weighing the science with other considerations.

We are extremely lucky to live in a time where we have such a wealth of data that we can have such sophisticated debates about the merits of landing sites on another planet, and it is always exciting to be involved. No matter where Mars 2020 ends up going, Astrogeology will be there, helping to ensure a safe landing and participating in the mission itself through our involvement in several of the scientific instruments.