Washington, July 30 : NASA's Phoenix Mars Lander have discovered visible changes in Martian ice, with cracks appearing in a distinctive hard-surface feature called "Snow Queen".
The feature visibly changed sometime between mid-June and mid-July, close-up images from the Robotic Arm Camera show.
The visible change is that cracks as long as 10 centimeters, or about four inches, have appeared.
Also, a seven-millimeter (less than one-third inch) pebble or clod not seen there before has popped up on the surface, and some smooth texture on Snow Queen has subtly roughened.
Phoenix's Robotic Arm Camera, or RAC, took its first close-up image of Snow Queen on May 31, 2008, the sixth Martian day, or sol, after the May 25 landing.
Thruster exhaust blew away surface soil covering Snow Queen as Phoenix landed, exposing a hard layer comprising several smooth, rounded cavities.
"Images taken since landing showed these fractures didn't form in the first 20 sols of the mission," said Phoenix co-investigator Mike Mellon of the University of Colorado, Boulder. "We might expect to see additional changes in the next 20 sols," he added.
According to researchers, it's the first chance to see visible changes in Martian ice at a place where temperatures are cold enough that the ice doesn't immediately sublimate, or vaporize, away.
Phoenix scientists discovered that centimeter-sized chunks of ice scraped up in the Dodo-Goldilocks trench lasted several days before vanishing.
The Phoenix team has been watching ice in the Dodo-Goldilocks and Snow White trenches in views from the lander's Surface Stereo Imager as well as RAC.
"I've made a list of hypotheses about what could be forming cracks in Snow Queen, and there are difficulties with all of them," said Mellon.
One possibility is that temperature changes over many sols, or Martian days, have expanded and contracted the surface enough to create stress cracks. It would take a fairly rapid temperature change to form fractures like this in ice.
Another possibility is the exposed layer has undergone a phase change that has caused it to shrink.
An example of a phase change could be a hydrated salt losing its water after days of surface exposure, causing the hard layer to shrink and crack.
"I don't think that's the best explanation because dehydration of salt would first form a thin rind and finer cracks," said Mellon.
"Another possibility is that these fractures were already there, and they appeared because ice sublimed off the surface and revealed them," he added.
As for the small pebble that popped up on Snow Queen after 21 sols, it might be a piece that broke free from the original surface or it might be a piece that fell down from somewhere else.
"We have to study the shadows a little more to understand what's happening," said Mellon.