Alfred McEwen, a mission scientist and University of Arizona professor of planetary science, said the black-and-white photos, taken by three cameras and received on Friday, show deep channels and layered surface debris around the planet's midsection, features that probably were formed by water.

The images were taken from an altitude of 2489km above the surface, about three times higher than the orbiter will be when it formally begins its science mission in November.

The spacecraft, which reached orbit on March 10, is to map about 1% of the Martian surface for future landings by robotic probes and human astronauts.

The resolution of the test images is comparable to those captured by the less powerful cameras of three other orbiters circling Mars, but the tests show that cameras survived the seven-month space trip that began in August.
   
Nasa scientists will use the images to calibrate the cameras, and will later combine the images to create broader view and to add colour.

They are available for viewing at www.jpl.nasa.gov.

Atmosphere study

Over the next seven months, the orbiter will "aerobrake", dipping into Mars' atmosphere and gradually changing its elliptical orbit into a near-circle about 300km above the planet's surface.

In the lower orbit, scientists will be able to distinguish surface objects as small as one metre wide, McEwen said.

The Mars Reconnaissance Orbiter has a suite of onboard instruments to map the planet's subsurface minerals, monitor its atmosphere and look for evidence of subsurface ice or water.

"It's not a matter of finding water on Mars but learning its importance in climate change ... and clearly it has been important to shaping the landscape"

Alfred McEwen
a mission scientist

"At this point we have an idea that water is probably abundant on Mars in the form of ice," McEwen said.

"It's not a matter of finding water on Mars but learning its importance in climate change ... and clearly it has been important to shaping the landscape."

The orbiter's first mission is to find landing sites for the Phoenix Mars Lander, set to arrive on Mars in May 2008 to dig for subsurface water ice, and for the 2009 arrival of the Mars Science Laboratory, a larger version of the twin robotic geologists Spirit and Opportunity, which have been traversing the planet's surface since 2004.
 
The Mars Reconnaissance Orbiter has the most advanced and powerful instruments of any of the four science satellites circling the planet and will return more than 10 times the quantity of data than all other probes combined, McEwen said.