LOS ANGELES -- The next rover to Mars should hunt for signs of ancient life and gather rocks that a future mission could bring back to Earth for the first time, a team of scientists appointed by the U.S. space agency said Tuesday.

The scientists' new report outlines ambitious goals for a mission to Mars that NASA wants to launch in 2020.

The plan marked the first concrete step toward returning a piece of Mars to Earth, but NASA said it's unclear how or when the cache would be retrieved.

"We're not signing up to a timetable or a commitment for a follow-on mission," said NASA sciences chief John Grunsfeld, adding that it's up to future planners to decide the next steps.

NASA has the ultimate say on what the future rover will accomplish within its $1.5 billion budget, excluding the cost of the launch vehicle.

The rover will be modeled after Curiosity, which captivated the world last year with its daring controlled-crash landing near the Martian equator.

Despite the successful touchdown, the $2.5 billion mission ran over budget and faced technical problems during development. To save money, engineers will dust off Curiosity's blueprints and reuse spare parts where possible.

The future rover would build on discoveries of past Mars missions. Spirit and Opportunity, launched in 2004, uncovered plenty of geologic evidence of past water. Curiosity found a habitable environment where microbes could thrive.

Scientists want Curiosity's successor to carry high-tech instruments that can peer at rocks on a microscopic level in search of chemical clues that might have been left behind by microbes, if they existed.

Since the Martian surface is a harsh environment with no signs of water, the panel said it didn't make sense to look for current life.

That would be a "foolish investment," said Brown University planetary geologist John Mustard, who headed the NASA-appointed team.