New pictures ofMars released this week shed light on the composition of the planet's polar icecap, reveal and odd dust storm embedded in a volcano's crater, and show astrangely off-center impact crater.

The pictureswere produced by the Mars Orbiter Camera on NASA's Mars Global Surveyorspacecraft, which this month began its second year of an extended mission.

One of thenewly released pictures has helped researchers better understand how sand anddust mix with water ice on the Red Planet.

Ice layers

The north poleof Mars is known to contain water ice. But because the pole spends about sixmonths of the year in darkness, it is hard to photograph. In summer, clouds anddust storms often obscure the polar region.

So during astretch of good weather in April last year, the orbiter produced an image thatshows exposed layers of ice at the Chasma Boreale. Darker, lower layers areolder, scientists say. The new photo suggests that these layers includeconsiderable amounts of sand, while the upper layers lack sand and instead maybe a mixture of ice and dust.

The lowerlayers appear to contribute sand to dune fields that surround the polar cap,said scientists at Malin Space Science Systems, which operates the camera.

Weather

Anotherpicture, taken on the southern hemisphere's first day of spring -- June 19 of2001 -- shows an unusual spiral-shaped cloud within the 68 mile-wide (110 km)caldera of the Arsia Mons volcano.

Arsia Mons isnearly large enough to cover the state of New Mexico.

The cloudprobably consisted mostly of fine dust grains, scientists said. Its spin mayhave been induced by winds off the inner slopes of the volcano's caldera wallsresulting from the temperature differences between the walls and the calderafloor, or by a vortex as winds blew up and over the caldera.

Mars isroutinely buffeted by dust storms and other wildweather. A recent dust storm grew to enshroudthe entire planet.

Impact

Another picturereveals a small crater in the Tharsisregion that researchers have been hunting for years. It was first seen inViking Orbiter images from the 1970s as a dark patch surrounded by rays oflighter material. But several attempts to get higher-resolution images inrecent years failed.

Finally, inJune 2001, researchers imaged the object. Three photos were combined with anold Viking image to produce the picture released this week. It reveals a craterthat is just 130 meters (427 ft) across, but which is clearly caused by a meteoriteimpact, as suspected. The surrounding rays represent material that was blastedout during the impact.

This crater isonly one-tenth the size of the famous Meteor Crater in northern Arizona.

Interestingly,the newly imaged crater is not located at the center of the bright rays. Therays point to the center of a bright area, not the crater. Further, the darkmaterial ejected from the crater--immediately adjacent to the crater rim is notcontinuously connected to the larger pattern of rays.

The offset ofthe crater from the center of the rays suggests that the meteor struck at anangle, most likely from the bottom/lower right.

Still, thestrange geometry of the rays is quite different from that seen for raysassociated with impact craters on theMoon and other airless bodies, scientists said. One possible explanation isthat they resulted from disruption of dust on the Martian surface by windsgenerated by the shock wave as the meteor plunged through the Martianatmosphere before it struck the ground.

Features likethese will soon be investigated by two spacecraft at once, as the MarsOdyssey probe begins its own science operations this month.

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