Planetary scientists want NASA to collect samples of Martian rocks for later return to Earth (Image: NASA/JPL-Caltech/Cornell University)

Bringing Mars rocks to Earth should be a top priority for NASA in the coming decade, says a high-level panel of planetary scientists. It also recommended a mission to Jupiter’s icy moon, Europa, thought to harbour an ocean of liquid water beneath its surface.

But if the agency cannot afford such multi-billion-dollar “flagship” missions, they should be delayed in favour of smaller missions, the panel says. The panel’s recommendations are not binding, but the report is likely to strongly influence NASA’s decisions.

As its name implies, the Planetary Science Decadal Survey is organised just once every 10 years by the National Academy of Sciences. It ranks the importance of missions to study planets and other objects in the solar system, excluding the sun and Earth. The report, released on Monday, covers the decade from 2013 to 2022.


Searching for life

It recommends that NASA make a Mars sample return mission its top priority among its large planetary science missions.

Bringing Martian rocks back to Earth would allow scientists to study them with a much wider array of instruments than can be packaged on a Martian rover. That in turn would allow scientists to better search for signs of past or present life, the report says.

“If we are to advance our fundamental knowledge of whether life has existed elsewhere in the solar system, we have to bring samples back,” agrees Jack Mustard of Brown University in Providence, Rhode Island, who was not a member of the panel. “We could go on for another 50 years sending rovers, but I think the next ultimate step in our understanding will come from sample return.”

Because of budget limitations, the panel recommended moving forward only with the first stage of such a sample return mission in the coming decade – a rover called Mars Astrobiology Explorer-Cacher (MAX-C) that would find and collect the samples. A second mission could later be sent to launch these samples into Mars orbit, while a third mission would bring them back to Earth. MAX-C is estimated to cost $3.5 billion but the panel hopes NASA can tweak its design to shave $1 billion off that price tag.

Subsurface ocean

The panel’s second-highest priority for large missions is a probe that would orbit Europa. It would assess the moon’s potential for hosting life by trying to confirm the presence of an ocean and determining how far the ocean lies below the moon’s icy surface. The mission, estimated to cost $4.7 billion, would also study the surface to help identify interesting landing sites for future missions.

NASA has already been studying the concept as part of a possible joint mission with the European Space Agency, which would supply an orbiter for Ganymede, another icy moon of Jupiter with a possible ocean beneath its surface.

Other large missions that the panel recommended, though at a lower priority than the Mars sample return and Europa missions, include:

• An orbiter and atmospheric probe for Uranus

• An orbiter for Saturn’s icy moon, Enceladus, which may have a reservoir of liquid water beneath its surface

• An orbiter to study Venus’s climate

The panel also encouraged moving forward with a small mission called the Mars Trace Gas Orbiter, which would try to determine the source of methane observed on Mars, which some scientists say could be due to life, though it could also be produced by water-rock interactions that do not involve life. NASA and the European Space Agency are already planning to launch such a mission in 2016.

The report made a short list of the most important medium-size missions to fly at the next opportunity, including:

• A comet surface sample return

• A sample return from the giant impact scar at the moon’s south pole, which could test the idea that a barrage of impactors struck the inner solar system 3.9 billion years ago

• An atmospheric probe for Saturn

• A mission to the so-called Trojan asteroids that share Jupiter’s orbit

• Venus explorer that would drop into the planet’s atmosphere

How many of the large, medium, and small missions actually fly depends on NASA’s uncertain future budgets. If money turns out to be really tight, the panel recommended postponing some or all of the large missions rather than eliminating the small- and medium-sized missions.