Washington, Aug 3 , The question of whether there could be life on Mars has long excited imaginations, and the Phoenix mission could put scientists one step closer through its planned analysis of ice on the planet.

The Martians of science-fiction lore aren't roaming the Red Planet, but scientists want to see if microscopic life is possible on Earth's nearest neighbour.

To find out more, the Phoenix will use a robotic arm to dig up ice on Mars' northern plains and use instruments to analyse the samples for elements that could help support life.

"Mars is a great opportunity. We always have exciting missions, but missions to Mars just seem to capture the imagination," NASA's launch director Chuck Dovale said.

The spacecraft is scheduled for takeoff Saturday, toward the beginning of a window till Aug 24 that positions Earth for a clear shot at Mars. Because the planets are only properly aligned every 26 months, another launch window won't be open until 2009.

Saturday's planned launch is to take place before dawn to maximise the chances of clear weather in a season that often brings afternoon thunderstorms to NASA's Kennedy Space Centre in Florida.

"While there are challenges getting it landed on Mars, my biggest challenge is getting it out of Florida," forecaster Joel Tumbiolo joked.

The probe is due to reach Mars May 25 and will examine an area near the north pole, where earlier missions showed evidence of ice.

Phoenix will descend using a heat shield and a parachute to slow its speed to about 215 km per hour. The last bit of the descent will be controlled through special rocket thrusters.

In the $420 million mission, Phoenix's robotic arm will use a scoop shovel to dig as far as 1 metre into a mixture of ice, rock and dust at its landing place. The region's temperature ranges from 0 degrees Celsius to minus 100 degrees.

"Phoenix investigates the recent Odyssey discovery of near-surface ice in the northern plains on Mars," scientist Peter Smith of the University of Arizona said. "Our instruments are specially designed to find evidence for periodic melting of the ice and to assess whether this large region represents a habitable environment for Martian microbes."

A series of miniature "ovens" will heat the material to see if it releases chemicals that provide clues to its chemical composition, in what University of Arizona scientist William Boynton described as identifying baking cookies by the aroma.

Phoenix will also shoot laser beams into the atmosphere to study cloud structure and other elements. After the onset of the Martian winter, the probe will continue to function as a weather station.

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