Published online 25 September 2008 | 455, 575 (2008) | doi:10.1038/455575a


Ancient water sites for next rover

Planetary scientists shortlist top landing sites on Mars.

The next Mars rover could end up down in the delta if a group of geologists and astrobiologists get their way.

The two top-ranked destinations for the next Mars rover are craters thought to contain ancient bodies of water. The walls of Holden Crater were breached by a flood that filled a lake in the centre.NASA/JPL/ARIZONA STATE UNIV.

The Mars science community has ranked a landing site called Eberswalde Crater as the most tantalizing destination for the Mars Science Laboratory (MSL), the US$2-billion rover that is due to launch in 2009. The crater seems to contain the remnants of a meandering river that spilled into a lake more than 3 billion years ago and piled up delta sediments — a prime target for MSL's instruments and their search for past or current microbial life. "If you go to any lake bed on Earth, that's where you find fossils," says James Rice, an astrogeologist at Arizona State University in Tempe who is a chief advocate for the site.

Scientists met last week to evaluate seven favoured sites (see slideshow, which includes a map showing all the sites) at a workshop in Monrovia, California. At the end of the meeting 104 paper ballots were cast, based only the scientific potential of each site — graded in 11 cate­gories including the diversity of minerals likely to be present and the potential of the site to preserve evidence of life. At a meeting in November, the engineering team will rate the sites technically, comparing the risks of landing the 900-kilogram rover within 20-by-25-kilometre ellipses and driving it to the most interesting spots within a Martian year — nearly two Earth years — of operation.

Eberswalde Crater contains an ancient river delta.K. LEWIS, CALTECH & NASA/JPL/UNIV. ARIZONA

Engineers are worried about Eberswalde and the next most highly ranked site, Holden Crater, because they sit in more southerly latitudes of Mars. The rover will use a lubricant that might not work as well during winter temperatures at those latitudes.

But Rice and others are optimistic about the Eberswalde site's chances — it's made quite a comeback after almost being knocked out of consideration at a workshop last year. Rice says the site shows clear signs of ancient river channels, some 100 metres wide, that emptied into a lake 150 metres deep over a period of perhaps a million years. The Holden Crater site lacks a delta but contains eroded sections of a lake bed that the rover could access more quickly than the delta at Eberswalde, says John Grant, of the Smithsonian Institute in Washington DC, who is co-chair of the landing site steering committee. Grant, an advocate for Holden Crater, says its walls were later breached by a massive flood that added to the volume of the lake.

But one of the two top sites will probably face elimination at the next meeting because of the concerns over their similar southerly latitude, says Grant. After a workshop next April, the committee will present recommendations to NASA science chief Edward Weiler, who will make the final decision on the target for the autumn 2009 launch. 

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