The first results from the American space probe orbiting Mars reveal what may have been a massive ocean.

Geophysicist Maria Zuber explains how the probe is helping to unravel the mystery of Mars (3' 31")

The findings from the Global Surveyor, just published in the journal Science, show two very different landscapes.

There are highland mountains and valleys in the southern hemisphere and very flat land in the north.

Scientists believe that ancient ocean waters could possibly have shaped the northern part of the red planet's surface.

The mission will last two years

According to geophysicist Maria Zuber of the Massachusetts Institute of Technology, the northern hemisphere rises and falls by only 165 feet over hundreds of miles.

Ms Zuber said: "We now understand that a lot of Mars is the smoothest place in the solar system for which we have measured topography. "The only thing that comes close is the heavily sedimented floors of Earth's oceans, it's actually flatter than that.

"That does not tell us that Mars once had an ocean but it certainly gives us a basis for trying to understand the origin further."

The probe has also found evidence that the red plant once had a very vigorous magnetic field.

Over the next two years, Global Surveyor is expected to provide as much information about the planet as all previous missions combined.

The probe's launch at Cape Canaveral

Launched from Cape Canaveral, Florida, in November 1996, the probe reached Mars after a journey of 466 million miles in 308 days.

Unlike Mars Pathfinder, which landed a robotic rover on the planet, Surveyor did not touch down on Mars.

Instead it is studying the planet from 235 miles above the surface.

It carries a camera, a laser to measure the height of hills and valleys, and instruments to determine the composition of the planet surface and measure the planet's magnetic field.

It will map the entire surface of Mars which will help scientists decide suitable areas for landing probes in the future.

By learning more about the red planet scientists also hope to understand more about Earth and its climate.

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