Mars scientists today nominated eight enticing targets for NASA’s next rover, due for launch in 2020. More than 100 planetary scientists narrowed down a list of 21 sites with a vote at the conclusion of a 3-day workshop in Monrovia, California. The researchers were keen to find sites that could preserve signs of life in rocks which would be sampled by the rover and, they hope, eventually returned to Earth.

The top vote getter was Jezero crater, which contains a relic river delta that could have concentrated and preserved organic molecules. “The appeal is twofold,” says Bethany Ehlmann, a planetary scientist at the California Institute of Technology (Caltech) in Pasadena. “Not only is there a delta, but the rocks upstream are varied and diverse.”

Second on the list was Columbia Hills, a region explored by the rover Spirit before it expired in 2010. Spirit found silica deposits that may have been deposited by a hydrothermal system—one that could have nourished life billions of years ago.

Several other favored sites sit at the edge of Isidis Basin, an impact structure that created deep troughs with significant carbonate deposits, which could help explain how Mars lost its once-thick carbon dioxide atmosphere. That atmosphere “was either lost to space, or it has to be sequestered down in the rock as carbonates,” Ehlmann says. “We can explore one of those paths.”

The new $1.5 billion rover will be very similar to Curiosity, which has been exploring Gale crater since 2012. But there are key differences. The main one is that the 2020 rover is expected to drill and collect more than 30 pencil-sized rock cores to be stored in a sample “cache.” Later missions could then land a small rover that would fetch and deliver the cache to a rocket that would return the samples to Earth for analysis.

The 2020 rover also has a slimmed down payload compared to Curiosity. It won’t have, for instance, a mass spectrometer analysis instrument of the type that has at times bogged down Curiosity. That should enable mission scientists to assemble a cache more quickly. The “sky crane” system, which delivered Curiosity to the surface, will also be used again, though engineers are considering enhancements that could allow zones of confident targeting to shrink by more than 50%, to ellipses as small as 13 kilometers by 7 kilometers.

A new sample collection plan

In recent months, mission planners have also shifted to a new tactic in the way that the cache of samples will be assembled. Previously, the rock cores were to be put inside a single football-sized container that, at the conclusion of the mission, would be placed on the surface for future pickup. Now, the rover will put cores into sealed metal tubes that can be placed directly on the ground in a “depot.” That tactic would allow the rover to return to the depot at multiple points throughout the mission to deposit more tubes, rather than waiting until the end of the mission to place the single large cache on the ground, says Ken Farley, the mission’s project scientist at the Caltech. “There was no success until we got that package off the rover,” he says. “This provides an opportunity to get [the samples] off the rover in a way that’s staged through time.” The metal tubes will have to be coated to protect the samples inside from heat for a decade or more, Farley says.

It is expected that the rover will have to travel to different areas to assemble a cache worth returning. Scientists want two main types of samples. First, they want rocks that stand a decent chance of preserving biosignatures. Second, they want igneous rocks, which can be dated on Earth and can help scientists understand the way that Mars formed.

To satisfy the first requirement—preservation of biosignatures—some scientists prefer sites like Jezero and Eberswalde, which contain relic river deltas, where organic material may have been concentrated by water, and preserved as the sediments were squeezed into stone. But sites with hydrothermal systems have also become popular among the scientists—sites such as Columbia Hills and those near Isidis Basin. The mode of organic preservation in these sites would be different, says Ehlmann, a co-investigator on the 2020 rover’s mast camera instrument. “You look in the precipitated veins [of rock], where organisms may have been entombed by mineral formations.”

The eight sites will be studied until the next selection workshop in January 2017, when the number of candidates will be winnowed to four, says John Grant, a planetary scientist at the Smithsonian Institution in Washington, D.C., and co-chair of the site selection process. New sites can still be considered, and mission engineers have yet to weigh in on the technical feasibility of landing the rover in the nominated sites.