SAN FRANCISCO (Reuters) - On spring days on Mars, powerful geysers sometimes spew carbon dioxide “steam” and dust to great heights, a phenomenon unlike anything ever seen on earth, scientists said on Tuesday.

These eruptions can be so strong that the falling dirt creates fan-shaped patterns extending hundreds of yards metres.

“Here’s a place that looks wildly different than anything on Earth,” NASA scientist Candice Hansen said at a meeting of the American Geophysical Union.

Mars and Earth are similar in that both are small, rocky planets with seasons caused by tilts in their axes, although the Martian year is twice as long as ours. But the seasonal geysers illustrate one result of their vast differences in climate.

In winter, the southern pole of Mars is -200 degrees F (-129 C), so cold that carbon dioxide in the atmosphere freezes to form a layer of dry ice about 20 inches thick.

On spring days, dry ice warmed by sunlight begins to turn into gas, some of which is trapped between the planet’s surface and the remaining ice. When the pressure grows strong enough, the gas erupts through cracks or vents like a steamy jet, Hansen said.

As the day goes on and the planet’s surface warms, the eruptions become larger. By midday, the gas also carries dust, which by evening has fallen on the surface in long fan-shaped patterns.

“I would call it a dust plume or a gas jet,” Hansen said. “We’ve never actually caught one in action, but the fans that they leave behind are certainly impressive.”

Images of these dust patterns have been captured by NASA’s Mars Reconnaissance Orbiter, which has taken high-resolution photos of the planet for the past year, Hansen said.

The orbiter is scheduled to keep gathering images through the end of 2008, when it will have recorded about 1 percent of the red planet’s topography.