Martian 'pebbles' don't prove watery past

NASA probe could be walking on broken glass.

NASA's Spirit and Opportunity rovers continue to send back photos from Mars, some offering tantalising geological evidence that water once flowed across the red planet's surface. But researchers caution that there are other explanations.

Opportunity hit the headlines last week when it sent home pictures of smooth, "round rocks":../multimedia/movie1.html on the surface of Meridiani Planum, the flat plain where it landed. Some scientists think the rocks may have been eroded by water, just like river pebbles.

But others say they could have been created when molten rock was hurled into the air by a volcanic eruption. This could have frozen as round droplets in the thin Martian atmosphere before falling to the ground. Such processes are well known on Earth.

The rocks could also be droplets of glass similarly formed during meteorite impacts, says Peter Schultz, a geologist at Brown University in Providence, Rhode Island1. "It's too early to conclude that Meridiani held water," says Schultz's colleague John Mustard.

The best way to resolve the issue is to bring samples back from Mars, he says. Water-eroded rocks would probably be solid, while rocks from volcanoes or meteorite strikes would be filled with bubbles. "If the gas bubbles are sulphurous, [the rocks] are likely to be volcanic. But if they contain atmospheric gases, that's good evidence that the glass was formed when a meteorite hit the surface," says Mustard. NASA's first Mars return mission is planned for 2011.

Making tracks

Since landing at the end of January, Opportunity has sent back "snapshots":../multimedia/m2.html of the surface, drawn up a "mineral map":../multimedia/m3.html of the immediate vicinity and trundled over to an interesting rocky outcrop for closer inspection.

On the opposite side of the planet, Opportunity's twin, Spirit, is in the Gusev Crater. Last Friday the rover finally "drilled":../multimedia/m4.html into the football-shaped rock nicknamed Adirondack, which it has been parked in front of since computer memory problems froze its arm almost three weeks ago.

The rover will analyse material from the 2.65-millimetre-deep "hole":../multimedia/m5.html to assess Adirondack's geologic history, before driving to a nearby crater 250 metres to the northeast. But Mustard doubts whether Spirit will find anything worthwhile.

"Gusev has accumulated so much dirt over billions of years, I don't think the site will tell us a lot," he says.

Both landers were designed to keep on roving and taking photos for another two to three months.

Beagle rests in pieces

Meanwhile, Europe's Mars Express orbiter continues to take novel "pictures":../multimedia/m6.html of the surface. But the Beagle 2 lander has been officially declared dead. The British-led team gave up on its lander late on Friday after failing to make contact since Christmas day. The scientists are convinced that Beagle hit its planned landing zone, but the orbiting Mars Global Surveyor saw no debris on the surface during a recent flyby.

Undeterred, Colin Pillinger has said that he plans to launch several more Beagle 'pups' to Mars in 2007.


  1. 1

    Schultz, P. H. & Mustard, J. F. Impact melts and glasses on Mars. Journal of Geophysical Research, published online, doi:10.1029/2002JE002025 (2004).

Download references


Related links

Related links

Related links in Nature Research

Mars Special

Related external links

Mars In Focus

NASA: Mars Rovers

ESA: Mars_Express

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Peplow, M. Martian 'pebbles' don't prove watery past. Nature (2004).

Download citation


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.