The European Space Agency's orbiting Mars Express spacecraft spied some interesting water-related features in this picture of the summit of a Martian volcano called Hecates Tholus, the northernmost volcano of a volcano group called Elysium.

On the flanks of Hecates Tholus are several flow features -- lines radiating outwards. ESA scientists said yesterday these were likely created by water sometime in the distant past.

Most scientists agree that Mars was once warmer and wetter. Water likely carved many of the canyons seen today on the planet, and lakes or oceans may have been widespread. Water migth also have collected in volcanic or impact craters. [See Mars Waterworld Imagined.]

What scientists don't know is how long the water lasted, whether any remains at or near the surface, and whether it might ever have spawned life. Some scientists speculate the flows of water might have been eruptive, and the wet periods brief and hellish.

A "significant" NASA announcement expected at 2 p.m. ET today could greatly clarify all this.

The volcano is 3.3 miles (5.3 kilometers) tall. The caldera is 6.2 miles (10 kilometers) wide and 1,970 feet (600 meters) deep. Bowl-shaped calderas are common to terrestrial volcanoes, too. Multiple caldera collapses are evident in the Mars Express photo.

The image center is located at 150° East and 31.7° North. North is at the top.

Zoom in on Mars Express' view of Olympus Mons, the solar system's largest volcano.

-- Robert Roy Britt

Credits: ESA/DLR/FU Berlin (G. Neukum)