A team of scientists studying possible mission scenarios for a planned 2020 Mars mission modeled on NASA's Curiosity rover has recommended a spacecraft equipped with instruments designed to look for traces of past life in the red planet's frigid crust, agency managers said.

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The as-yet-unnamed Mars 2020 rover, expected to cost nearly $2 billion when launch costs are included, also would test technologies for collecting and caching core samples that could be returned by a future robotic mission for detailed analysis on Earth.

NASA unveiled initial plans for the Mars 2020 rover in December 2012 and the following January, a Science Definition Team made up of leading planetary scientists and engineers began studying possible mission designs based on four basic requirements.

"The first objective was to explore an astrobiologically relevant ancient environment on Mars to decipher the geologic processes and history, including its past habitability," said Jack Mustard, chairman of the committee and professor of geological sciences at Brown University.

"The second key objective was to assess the biosignature preservation potential within the selected geologic environment and search for potential biosignatures," or traces of past life.

The third objective "was to demonstrate significant technical progress towards the future return of scientifically selected, well-documented samples to Earth," Mustard said. The fourth goal was to provide a testbed for human spaceflight technologies.

"We are recommending a mission concept for a science-focused, highly-mobile rover to explore and investigate in detail a site on Mars that likely was once habitable," Mustard said. "Our preferred mission concept employs new, in situ science instrumentation in order to seek these signs of past life had it been there.

"We're not looking for the life that must have been there, because we don't know the answer to that question. But we're saying given as many signs as we've had, including what Curiosity is finding, that there is promise that had life been there, it had left a mark in the geologic record."

The new rover will be based on Curiosity's general design and the "sky crane" landing technology that successfully lowered the heavy rover to the surface from a rocket-powered backpack.

But one key difference involves the new rover's drilling system. Curiosity is equipped with an impact drill designed to bore into rocks to collect powdered samples for detailed chemical analysis. The new rover likely will be equipped with a drill system that will collect core samples that could be placed in a cache for possible return to Earth.

No such sample return mission is currently on the books and it's not clear when such a mission might be launched. But perfecting the technology is considered a key step toward one day getting samples back to Earth for detailed laboratory analysis.

In recent Mars missions, NASA has studiously avoided talking about direct searches for past or present life. Instead, the agency has focused on a "follow the water" strategy aimed at determining whether Mars ever hosted environments habitable to life as it is known on Earth.

That answer to that question now seems clear, scientists say. Habitable environments did, in fact, exist on Mars, but it's not yet clear when they were present or whether they lasted long enough for life to evolve.

The Mars 2020 rover will be equipped with state-of-the-art instruments to look for signs of past life in a yet-to-be-selected location where scientists see clear evidence of past habitability. But the rover will not be looking for signs of existing life.

"To go and look for simple organisms or not-so-simple organisms that are living within that toxic, harsh environment, we just think it's a foolish investment of the technology at this time," Mustard said.

But given past habitable environments, any life that did exist may have left detectable remains, or traces, across vast stretches of time that would be easier to find.

"To the best evidence that a good segment of the planetary science community understands, that period of time on Mars was in its ancient past when habitable environments were common and have left a number of records we can see from orbit," Mustard said. "We would like to sample those to see if it left biosignatures.

"If that biosignature happens to be a dinosaur-type bone, we probably wouldn't need to return that sample, we would recognize that with our current capabilities," he joked. "But our understanding is that it's likely to be microbial, and that's a darn hard measurement to make, and a darn hard measurement to convince the skeptical science community that that is indeed the case."

The Science Definition Team included members who pushed for instruments to look for currently existing life, "but the feeling was, on the basis of the scientific evidence we have to date, the most logical steps were to look for the ancient signs of life that would be preserved in the rock record," Mustard said.

Lindy Elkins-Tanton, director of the Carnegie Institution's Department of Terrestrial Magnetism, said it was a matter of maximizing the potential science return.

"If we were only looking for what microbes could be found on the surface in this place right now, that's like a tiny snapshot of the history of Mars and the possibility of life," she said. "But if we look back through the rock record, we're basically integrating over time and maximizing our chances of finding results."

NASA managers now will evaluate the Science Definition Team's report before asking the science community to submit proposals for scientific instruments.

"Here's the bottom line," said Elkins-Tanton. "Questions drive science. When it comes to NASA and the community together planning a flagship mission, the time, the commitment and the support needed, we should only be seeking to answer the very biggest questions.

"And one of the very biggest questions for all of humankind is are we alone? And that is the question we're hoping to make really big advances with this Mars 2020 mission."