PASADENA, Calif. -- As NASAs Mars Global Surveyor heads into its fourth year in orbit around the Red Planet, mission scientists released a recent wide-angle image snapped by the spacecraft of three apparent water-carved valleys.

The major valley systems -- in the image, from left to right, Dao, Niger and Harmakhis -- lie east of the Hellas plains. Each valleys dimensions vary, but each is roughly 0.6 miles (1 kilometer) deep with widths ranging from 5 miles (8 kilometers) to 25 miles (40 kilometers).

Scientists believe torrents of water carved the valleys at some distant point in the Martian past, scouring the planets surface with a sudden outburst. In each case, the water flowed toward the bottom of the image.

In the case of Dao Vallis, the site may be targeted by future robotic missions to Mars that would explore spots where the sapping effect of ground water may have contributed to the formation of the valley system.

Indeed, scientists recently highlighted Dao Vallis as one of multiple locations on Mars where liquid water may have flowed in the comparatively recent geological past.

Furthermore, Dao Vallis lies close to the Hadriaca Patera volcano on Mars.

That proximity may have made -- or still make -- for the mixing of sources of volcanic energy and liquid water. That raises for scientists the tantalizing possibility of finding evidence of hydrothermal activity on Mars -- conditions that could favor the harboring of life.

The image, captured September 13 by the probes Mars Orbiter Camera, is about 500 miles (800 kilometers) across, with north toward the left. The Sun illuminates the scene from the lower left.

The Mars Global Surveyor, launched in November 1996, is currently NASAs lone functioning spacecraft at Mars. However, it will be joined on October 20, 2001 by a new probe, the 2001 Mars Odyssey.

The Odyssey orbiting spacecraft will probe the composition of Mars, detect water and ice buried beneath its surface and study the planets radiation environment.