US space agency NASA is pondering 28 potential missions focusing on finding life beyond Earth inside our solar system, a US researcher said Wednesday.

"Astrobiology and the search for life is really central to what we should be doing next in the exploration of the solar system," Steve Squyres, a researcher at Cornell University in Ithaca, New York, said in a telephone press briefing.

"We are looking for a total of 28 different missions.... They cover everything from Mercury landers to fly-by of objects in deep outer space of the solar system -- and they are particularly relevant to looking for life," he explained.

Among other objects of interest: a three-stage Martian mission that would bring Martian soil samples back to Earth, said Squyres, who worked on the Spirit and Opportunity rovers, still active six years after their arrival on the Red Planet.

"Those samples might reveal a great deal of whether Mars once or today (has) forms of life," he said.

Another part of the mission would be to look at the origin of methane in the planet's atmosphere, and whether or not it is organic, he said.

Methane breaks down quickly in the Martian atmosphere, but scientists discovered plumes of methane on Mars in 2003, raising the prospect that the planet is not entirely dead.

"So we will send a mission potentially to look at the sources of the methane and consider the possibility that they could be from biological origins," Squyres added.

Further out in the solar system, scientists are very interested in Europa, a moon of Jupiter, he said.

"It's believed that Europa may have an ocean of liquid water underneath its icy crust. And we are looking at a Europa robot mission that would have, among other things, the capability to use a radar system that can penetrate through that ice, find the ocean if it exists, identify places where you might go to the surface to learn more" about the ocean, Squyres said.

Likewise Titan, a moon of Saturn, has scientists' attention.

It is "a moon we know to be very rich in organic materials. We are looking long term at a mission that will study Titan in great deal, including an orbiter, a balloon in Titan atmosphere and a lander to land in some of the lakes of liquid methane," the Cornell scientist added.

Another mission under consideration is one to return samples from a comet, the researcher said. "Comets are believed to be rich in organic materials, rich in building blocks of life. Most of the organic material on earth was probably delivered to earth by comets."

The number of projects needs to be whittled down, he said, before a recommendation will be made on what missions NASA should prioritize.