NASA’s next Mars rover — the first to gather rock samples meant to come back to Earth — should dream big and visit as many different places on the red planet as possible, scientists concluded on 18 October.

That would probably include some combination of Jezero crater, which was once home to river deltas and a lake; Northeast Syrtis, which contains some of the most ancient rocks on Mars; and Midway, a compromise option located between those two. Project scientists have proposed visiting both Jezero, for the river and lake sediments that might retain signs of past life, and Midway, for the ancient rocks.

The two are approximately 28 kilometres apart — an ambitious but achievable distance.

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

The Columbia Hills region, which NASA’s Spirit rover explored between 2004 and 2011, ranked far lower in the scientists’ poll despite having silica deposits similar to those formed by hot springs. (The ice-covered region near Mars’s south pole where ESA’s Mars Express recently detected what could be a buried lake was not on the list.)

The decision about where to send the 2020 rover ultimately rests with NASA’s science chief, Thomas Zurbuchen, who will choose in the coming months.

“I would be excited about any sample back,” says Meenakshi Wadwha, a planetary scientist at Arizona State University in Tempe. “But we have the luxury of being able to choose between good sites.”

Rock the vote

Slated to launch in July 2020, the US$2.4-billion rover will be the first from any nation to collect Mars rocks and stash them for a future mission to bring back to Earth. The geology of the landing site has to be intriguing enough — and the potential scientific discoveries there great enough — to make the mission worth the investment.

NASA has not planned how it would retrieve the rocks collected by the 2020 rover. But the agency gathered Mars experts in Glendale, California, from 16 to 18 October to hash out the merits of four finalists for the rover’s landing site .

Jezero, Northeast Syrtis, and Midway came in remarkably close to one another in votes by 158 scientists at the workshop. The researchers ranked the sites using several criteria, such as the potential of samples at data collected at each site to answer crucial scientific questions about Mars.

The idea of visiting Jezero and then Midway — or the other way around — emerged in the past year as mission scientists debated how to get the most out of the rover’s journey. “It is ambitious as heck,” says John Mustard, a planetary scientist at Brown University in Providence, Rhode Island.

Midway contains ancient rocks similar to those at Northeast Syrtis, within striking distance of the rivers-and-lake system at Jezero.

Perilous path

Sending a rover to both Jezero and Midway would require gambling that the vehicle will last that long. Its primary mission is 1.25 Mars years (2.35 Earth years); during that time it is expected to travel on the order of 15 kilometres. That would get the rover most of the way around the Jezero site, if it started in there, and possibly even to the crater’s rim. But then it would face a trek across a dune-strewn region to Midway. “The reward is absolutely worth the risk,” says Mustard.

NASA’s Curiosity rover, the agency’s biggest and most powerful to date, has traveled more than 19 kilometres since it landed on Mars in 2012. The engineers developing the 2020 rover expect it to be able to travel faster than Curiosity, in part because of new technology that improves its ability to navigate on its own.

One major question is how many rock samples the rover will collect, and from where. The 2020 rover is equipped with 42 pencil-thin sample tubes, five of which will be reserved as spares in case there is a problem as it tries to drill and gather material with its robotic arm. That leaves 37 tubes to be filled with the most precious extraterrestrial rocks ever collected.

“Sooner or later, somebody is going to have to decide whether these samples are worth bringing back,” project scientist Ken Farley, of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, told the meeting. “I don’t want to fail because we have not been ambitious enough to make the cache scientifically worthy.”

Treasure trove

At the workshop, project scientists laid out options for what might fill those 37 tubes. They range from chunks of the lake deposits at Jezero, to enormous blocks of rocks from the crater rim there, to the ancient rocks at Midway. The nuclear-powered rover has several possible paths to navigate the 28 kilometres of dune fields between Jezero and Midway. Driving that distance would take an estimated 401 Martian days, says deputy project scientist Katie Stack Morgan of JPL.

Still unknown is where the rover might stash its precious samples. One possibility is that it could collect two identical sets of samples at Jezero, depositing one there and carrying a second set on its way to Midway, Farley told the meeting. That would leave open the possibility of retrieving the samples at Jezero if something went wrong with the rover on its way to Midway.

NASA has not yet decided whether or how it might fetch the samples, although tentative plans call for a mission to launch in the late 2020s. “We’re actually serious about bringing these samples back,” Zurbuchen told the meeting. “That’s what we’re here for.”