Mars spacecraft makes parting gift with pictures of the 'squirting gun'

THE newly defunct Mars Global Surveyor has delivered a stunning farewell gift to scientists studying the planet - crystal clear images of surface changes that point to the recent presence of flowing water.

The photographs provide the strongest evidence yet that liquid water, essential for sustaining life, exists on the barren landscape of the 4.5 billion-year-old planet. Until now, only ice had been discovered in remote polar regions, suggesting the presence of water long ago.

Experts hailed the discovery as the "holy grail" of Mars exploration. "You've heard of a smoking gun; this is what we call a squirting gun," said Professor Ken Edgett, head of a team of scientists at the California-based Malin Space Science Systems laboratory that operated the spacecraft's high-resolution camera.

"We saw there was evidence of water billions of years ago, creating channels and things like that. Today we're talking about water that exists on Mars right now. No-one expected what we have today."

The images are among the last of more than 240,000 captured by the orbiting surveyor, which spent seven years photographing the surface of Mars from low altitude before ceasing transmission last month. NASA believes the craft had reached the end of its operational life.

Dr Edgett and colleagues from the San Diego laboratory compared recent images of thousands of gullies and craters with those taken several years ago.

During their analysis of the images taken of two gullies in 2004 and 2005, they found bright, light-coloured deposits on the crater walls that were not present in the original photographs from 1999 and 2001. The deposits, possibly mud, salt or frost, were left by water cascading through, the scientists believe.

"This is material that flowed down the slope as it encountered different obstacles," Dr Edgett said.

"People have been asking for six years what could flow through these gullies and the consensus is liquid water."

The photographs also provided evidence of at least 20 newly formed, football stadium-sized craters, probably caused by meteorites or other space debris striking Mars at speeds of 5km per second.

The findings were announced at a NASA briefing in Washington yesterday.

The timing is significant because it comes one day after NASA's announcement of plans to build a permanent base on the Moon, which would be a crucial staging post for a proposed manned mission to Mars within the next three decades.

Yet not all experts are convinced that the surface changes indicate water. Oded Aharonson, a professor of planetary science at the California Institute of Technology, called the interpretation "compelling", but said the deposits could also have been left there by a flow of dust.

Dr Edgett said that was unlikely. "These regions are far away from any dust flow," he said. "When we disrupt dust on the surface of Mars, that surface usually darkens. These [deposits] are brighter."

The latest photographs add to previous clues that the Global Surveyor has provided about the existence of moisture on Mars. Its mineral-mapping infrared spectrometer found concentrations of a mineral that often forms under wet conditions, fine-grained hematite. This discovery led to selection of a hematite-rich region as the landing site for NASA's Mars Exploration Rover Opportunity, which has been exploring the surface since January 2004.