Media playback is unsupported on your device Media caption Drive with Nasa's next Mars rover through Jezero Crater

It could be easier to detect the signs of ancient life on Mars than it is on Earth, say scientists connected with Nasa's next rover mission.

The six-wheeled robot is due to touch down on the Red Planet in 2021 with the specific aim of trying to identify evidence of past biology.

It will be searching for clues in rocks that are perhaps 3.9 billion years old.

Confirming life on Earth at that age is tough enough, but Mars may have better preservation, say the researchers.

It comes down to the dynamic processes on our home world that constantly churn and recycle rocks - processes that can erase life's traces but which shut down on the Red Planet early in its history.

"We don't believe, for example, that Mars had plate tectonics in the way Earth has had for most of its history," said Ken Williford from Nasa's Jet Propulsion Laboratory (JPL) in California.

"Most of Earth's rock record has been destroyed by subduction under the ocean crust. But even the rock left at the surface is heated and squeezed in ways it might not have been on Mars.

"So, paradoxically, it may well be that the older rocks on Mars are better preserved than much younger rocks on Earth," he told BBC News.

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Image copyright NASA/JPL/JHUAPL/MSSS/BROWN UNIVERSITY Image caption Jezero Crater shows strong evidence from orbit of past water activity

The new rover will be dropped into the near equatorial Jezero Crater, which satellite observations suggest once held a deep lake.

Scientists hope that if microbes lived in or around this body of water, signatures of their presence will be retained in sediments that can be easily drilled today.

A key target will be the carbonate deposits that seem to line what would have been the palaeo-lake's shoreline.

"Carbonates are a type of mineral that precipitates out of water and what's really great about that process is that when they precipitate out - they trap everything that's in the water. So, everything that's living there can be trapped inside the mineral," explained Briony Horgan from Purdue University in Indiana.

Media playback is unsupported on your device Media caption Briony Horgan: "Jezero Crater is one of the best places on Mars to see carbonates"

The dream scenario would see the rover stumble upon formations that look like stromatolites. These are carbonate dome structures that on Earth have been built by microbial mats.

The rover will choose the most enticing spot along the putative shoreline and drill samples that can be packed away in a canister and left on the ground for a later pick-up.

Nasa and its European counterpart, Esa, are currently planning a joint venture to retrieve the rover's up-to-40 samples, probably in the early 2030s.

Drs Williford and Horgan were discussing the future rover's prospects here at the American Geophysical Union (AGU) Fall Meeting in Washington DC - the largest annual gathering of Earth and space scientists.

Image copyright NASA Image caption The new rover follows the design template of the Curiosity robot which landed on Mars in 2012

The vehicle will be a close copy of the one-tonne Curiosity robot that Nasa landed in Gale Crater in 2012.

It will use the same "Skycrane" technology that put the previous machine down with such great precision - but with an important add-on. Engineers have developed an on-the-fly mapping system called Terrain-Relative Navigation which ought to bring even greater accuracy to the landing process.

The expectation is that this system will place the rover right up against rocks that record the delta that fed the lake with water.

Ken Farley, the mission's chief scientist, told the AGU meeting that the route the rover will take after landing had already been planned.

The robot will be equipped with a sophisticated navigation system that will give it the autonomy to work out the best, most direct course between waypoints.

This should dramatically speed up the arrival to different science targets. "In good terrain we'll be driving more than a 100m a day," Dr Farley told BBC News.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos