An image that shows an area deep within the boundary of the seasonal south polar frost cap of Mars. The bright region on the left side of the picture has been known for centuries as the Mountains of Mitchel. MESSAGE BOARD Destination Mars  INTERACTIVE Destination Mars Pathfinder Visual Galaxy Mars Global Surveyor - in VRML

September 22, 1999
Web posted at: 5:30 p.m. EDT (2130 GMT)

(CNN) -- A patch of frost near Mars south polar cap caught the eye of an Ohio astronomer 150 years ago staring through a telescope at the red planet.

Now the frosty strip has showed up again in a recent image taken by NASA's Mars orbiter, and it still has scientists puzzling over why it hasn't melted away.

The bright region is known as the "Mountains of Mitchel" for the astronomer Ormsby McKnight Mitchel, credited with first observing it. Typically, the frost in this area is left behind after the rest of the polar cap recedes each spring -- which now has sprung at Mars.

Michel thought the area was mountainous because it seemed analogous to the snow that is left on Earth's mountain ranges in late spring and into summer.

Mars Global Surveyor snapped a shot of the area a few months ago and took laser altimeter readings that showed the region really isn't so mountainous.

Instead the feature is located in the cratered, southern highlands -- it's elevated but not significantly.

So how come the frost sticks around?

"I think some of it is the standard explanation that snow lasts longer on slopes that face away from the sun," said Dr. Mike Ravine, an advanced project manager at Malin Space Science Systems.

A narrow angle camera view of a 1.9-mile wide area along the edge of the hilly Gigas Sulci terrain.

That company operates the camera on Global Surveyor, as well as cameras on two missions en route to Mars -- Climate Orbiter and Polar Lander.

The south-facing shady slopes explanation probably is not the whole story, he said.

The frost at the so-called mountains seems whiter than it appears at other places on the seasonal polar cap.

"There could be something with the local weather patterns, something associated with that particular topography. And maybe you're getting less dust there," Ravine said.

Less dust would mean more light was reflected up from the feature.

Ravine said it's all speculation for now, but you've got to be curious about a frosty feature that has lasted 150 years at Mars.

It's usually a sub-zero planet, but even the region around the "Mountains of Mitchel" typically defrosts in the spring.

Geology of the Gigas Sulci

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Another pair of images recently released shows geologic features along the eastern edge of the Gigas Sulci -- an area within a volcanic region called western Tharsis.

A wide-angle view shows the location of a higher resolution image, which looks like a deep scar along the edge of the Gigas Sulci -- rough, hilly terrain that is cut by troughs that resemble knife gashes.

The wide-angle image, spanning about 115 km (71 miles), also shows several impact craters, including one with a tear-dropped tail formed by wind.

The close-up, illuminated from the left, shows a 3 km wide area along the edge of the Gigas Sulci, which consists of steep terrain interrupted by smooth, possibly dusted, valleys.

The edge of the hilly terrain is cut by a deep trough caused by faulting in the center of the valley, Ravine said.

Boulders on the slopes of the valleys are the size of small buildings, he said.