As NASA's next-generation Mars rover approaches completion, mission planners are moving ever-closer to making a critical decision on where exactly on the red planet to send its $2.1 billion robotic science lab.

In mid-October, a consortium of more than 150 scientists came together for the fourth and final time to argue for and against potential landing sites for the Mars 2020 rover. Since 2014, the group has narrowed down the list of possible locations from over 30 to just four. Their final selections, ranked according to a specific set of criteria, will be sent for consideration to NASA scientist Thomas Zurbuchen. He's expected to make a final decision by the end of the year.

So what do we know about these potential sites? To make it this far in the selection process, all four sites had to host an "astrobiologically-relevant ancient environment" with "geologic diversity that has the potential to yield fundamental scientific discoveries." They also had to have the potential for significant water resources (water-rich hydrated minerals, ice/ice regolith or subsurface ice) that might be utilized for future exploration missions.

One other requirement that's new to the exploration of Mars: the sites must also yield potentially rich samples for a first-ever return trip back to Earth. During the course of its time on Mars, the 2020 rover will collect and cache up to a dozen samples for retrieval at a later date.

Now that we know the requirements, below is a bit more about each of the finalists.

Jezero crater

Sediments carried by ancient water channels fan out into Jezero Crater delta. This image, captured in November 2011, combines information from two instruments on NASA's Mars Reconnaissance Orbiter: the Compact Reconnaissance Imaging Spectrometer for Mars
Sediments carried by ancient water channels fan out into Jezero Crater delta. This image, captured in November 2011, combines information from two instruments on NASA's Mars Reconnaissance Orbiter: the Compact Reconnaissance Imaging Spectrometer for Mars and the Context Camera. (Photo: NASA/JPL-Caltech/MSSS/JHU-APL)

Jezero crater spans some 30 miles across and is believed to have been flooded at one point. As shown in the photo above, the crater contains the remains of a fan-delta deposit rich in clays. Extensive studies of Jezero's surface features using the Mars Reconnaissance Orbiter have also led scientists to believe the lake was long-lived and, therefore, may have been a prime hotspot for life.

"The delta and nearby outcrops expose clays and other materials whose properties make them favorable for preserving organics and (or) other biogenic signatures," the Mars 2020 Landing Site Steering Committee writes. "In addition, there are carbonate-bearing rocks whose origin may relate to past weathering and overlying cratered and possibly volcanic rocks on the crater floor that could be used to help constrain Martian chronology."

Bottom line: If Mars once hosted life, it's possible that remnants of it have been preserved in the clay deposits of Jezero crater.

Northeast Syrtis

The bedrock around NE Syrtis Major is more than 4 billion years old and contains a diversity of hydrated minerals.
The bedrock around NE Syrtis Major is more than 4 billion years old and contains a diversity of hydrated minerals. (Photo: NASA/JPL-Caltech/University of Arizona)

A site of considerable mineral diversity, the northeast edge of Syrtis Major (which is also home to Jezero crater), would allow easy access for the Mars 2020 rover to examine clays, carbonate-bearing rocks and other deposits bearing the hallmarks of a once warm and wet region.

Because NE Syrtis was once volcanically-active, it's thought that the combination of water and heat could have provided a rich environment for life to flourish. Weathering has also exposed various types of rock formations that could enable the rover to analyze and collect samples from different point in Mars' history. Unlike other potential landing sites, the Mars rover would not have to travel far to begin new and useful science.

"The regions of interest are more clustered in Northeast Syrtis," UT Austin geoscientist Tim Goudge told Wired.

Bottom line: NE Syrtis has both large carbonate deposits and exposed strata that may offer both proof of former life and insight into Mars' rich geological history.

Midway

Located near Jezero crater, Midway may have been shaped by both ancient surface drainage and/or glacial activity.
Located near Jezero crater, Midway may have been shaped by both ancient surface drainage and/or glacial activity. (Photo: NASA/JPL/University of Arizona)

Earlier this summer, scientists pouring over data from the various candidate sites came to the conclusion that it might be possible for the Mars 2020 rover to visit more than just one location. To that end, they set their gaze upon Midway, a region that contains the same enticing morphology of NE Syrtis, while also being within striking distance (17 miles) of Jezero crater.

"The community prefers a mega-mission," Bethany Ehlmann, a planetary scientist at the California Institute of Technology in Pasadena, told Nature. "If we’re going to do sample return, it has to be a sample cache for the ages."

While Midway is attractive, there's still much uncertainty about whether or not the vehicle will last long enough to reach Jezero. Since landing in 2012, NASA's Curiosity rover has only traveled a little over 11 miles. The 2020 rover benefits from new technology and a bit of a bump in speed (1.65 inches per second vs. Curiosity's 1.5), as well as more robust wheels for handling Mars' rough terrain, but it would still take a little over two years (or almost the duration of its primary mission) to make it to the rim of Jezero.

"The further away you land from your gold mine, the higher the risk you might not get there," Ray Arvidson, a planetary geologist at Washington University in St. Louis, Missouri, told ScienceMag about his worry of not reaching Jezero.

Bottom line: Midway is attractive because of the potential diversity of sample sites possible at both Syrtis and Jezero. Whether the rover will succeed in traveling the distance and navigating Mars' tricky surface features remains a giant worry.

Columbia Hills

Gusev Crater in Columbia Hills on Mars is rich in  opaline silica, carbonates and other aqueous phases.
Gusev Crater in Columbia Hills is rich in opaline silica, carbonates and other aqueous phases. (Photo: NASA/JPL-Caltech)

Columbia Hills, located within the 103-mile-wide Gusev crater, are perhaps the safest bet of the four landing sites for one big reason: we've visited them before. In 2004, the Mars Exploration Rover Spirit touched down within Gusev and proceeded to travel to the base of Columbia Hills. Researchers are interested in following up on promising science started with Spirit (the rover went silent in 2010 after becoming stuck in a sand trap), which indicated the presence of promising carbonates, opaline silica and sulfates.

According to James Rice, co-investigator and geology team leader on the Mars Exploration Rover Project, landing the 2020 rover near Columbia Hills would also provide a rare opportunity to investigate the final resting place of Spirit.

"By this time Spirit would have been exposed to the Martian environment for over 15 years," Rice wrote in the final report. "Thereby, making for an excellent long duration exposure experiment providing long-term data on the Martian environment, including weathering, micrometeorites, and its effects on materials degradation and other systems (including power, propulsion and optics). This data will aid in the design of surface systems, equipment and structures for the future robotic and manned exploration of the planet."

Bottom line: Columbia Hills offers a familiar location with promising outcrops likely formed by ancient mineral springs. Investigation of Spirit offers potential value for future exploration.

Where to?

The landing site for the Mars 2020 rover is expected to be chosen by the end of 2018.
The landing site for the Mars 2020 rover is expected to be chosen by the end of 2018. (Photo: NASA)

At the end of the three-day summit, participants were asked to rate on a scale of 1-5 the four landing sites against the predetermined criteria. From the 158 votes tallied, Jezero crater came in first, with both NE Syrtis and Midway very close behind. Columbia Hills, meanwhile, scored the lowest.

"Interestingly, the Midway and Jezero crater sites were assessed the highest (and received the most votes for high potential) with respect to extended mission criteria," the committee reported, "perhaps reflecting the interest in possible extended mission opportunities between the two sites."

These recommendations will now be sent to NASA, with administrators there left to puzzle over where exactly in Syrtis Major the Mars 2020 rover will make its descent.

"It's a huge service to the community to hold these workshops," Goudge told Discover Magazine. "But I'm not jealous of the ones making that decision."

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.