MONTREAL - Canadian science will play a key role in the hunt for signs of life on Mars as part of a NASA mission launching next weekend.

Slated to blast off Saturday, the Mars Science Laboratory rover will focus on what one Canadian physicist describes as the red planet's Holy Grail.

The University of Guelph's Ralf Gellert is lead scientist on the Alpha Particle X-Ray Spectrometer (APSX), which is Canada's contribution to the NASA "Curiosity" rover.

The job of APSX's sensor, which is the size of a soupcan and works off the end of the rover's robotic arm, is to analyze chemical elements in Martian rocks and soil once it lands in August 2012.

Gellert says the mission will focus on clay spotted on the surface by Mars Express, an orbiting satellite launched by the European Space Agency in 2003.

"Clay is something like the Holy Grail of Mars exploration," he said in a recent interview.

Alain Berinstain, the Canadian Space Agency's director of science development, explains that clay is a basic mineral which exists on Mars and on Earth.

"And everywhere you find clay, you might not find water there today, but at least there has been some in the past. And remember: wherever there's water on Earth there's life," he said in an interview.

"So if you follow the water on Mars, then definitely you're going to come closer to finding habitable zones or one day actually finding evidence of life itself."

Berenstain says APSX is one of 10 instruments on Curiosity, a motorized field geologist and geochemist.

APSX was funded by the CSA and MacDonald, Dettwiler and Associates (MDA) was the prime subcontractor.

Gellert says there's a very good possibility that the analysis carried out by the roving science lab will provide a better idea whether there was life on Mars.

"If life arose on two different planets (Earth and Mars), then it's very likely -- if the conditions are right -- that life can form also on other planets," he added.

"If we are lucky we might find remnants that point to old bacteria or organisms."

Paul Fulford, the project manager for MDA, says APSX has flown on two previous Martian missions.

It was used on "Sojourner," a rover that landed on Mars in 1997 on the Pathfinder spacecraft, and again on the Mars Explorer Rover (MER) mission.

NASA's MER mission involved two small rovers -- "Spirit" and "Opportunity" -- that touched down on the Martian surface in 2003.

One is still wandering around and carrying out experiments, but communications with the other were lost earlier this year.

Nadeem Ghafoor, MDA's manager of planetary initiatives, says the exploration of Mars has been challenging.

"It's been a hard obstacle for the Americans and Russians to crack and that's why you're seeing almost as many failures -- probably more than successes -- as far as missions go," he said.

The website "Universe Today" has reported that two-thirds of all missions to Mars have failed in some way.

The Russians recently launched the Phobos-Grunt satellite, which is supposed to visit one of the moons of Mars and bring back samples.

But after being blasted into space, it got stuck in orbit around the Earth and scientists have been working feverishly to gain control of the satellite and send it on its way.

If they fail, Phobos-Grunt could come crashing back to Earth and may burn up in the atmosphere.

Ghafoor says there's still a lot of exploring to be done before humans actually set foot on Mars.

"Everybody agrees that getting a better understanding of the conditions for habitability on Mars is very important," he said.

"And getting down safely to the Martian surface as soon as we can to get samples back -- those are some of the key missions I think that are going to happen in our lifetime."

Ghafoor predicts that children who are just now starting school will be the future scientists or researchers who will be working in space.

"We think they'll be the people who will be looking at how to put together the human Mars missions," he said.

The rover was originally scheduled to take off Friday, but officials delayed the launch by a day because of rocket trouble.