Snow Queen Showing Her Age
Close-up Images of ‘Snow Queen’ Show Changes
A distinctive hard-surface feature called "Snow Queen" beneath NASA’s Phoenix Mars Lander visibly changed sometime between mid-June and mid-July, close-up images from the Robotic Arm Camera show.
Cracks as long as 10 centimeters, or about four inches, have appeared. 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," Phoenix co-investigator Mike Mellon of the University of Colorado, Boulder, said. "We might expect to see additional changes in the next 20 sols."
Mellon, who has spent most of his career studying permafrost, said long-term monitoring of Snow Queen and other icy soil cleared by Phoenix landing and trenching operations is unprecedented for science. 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, but only RAC can view Snow Queen near a strut under the lander.
The fact that RAC is attached to the robotic arm is both an advantage and a disadvantage. The advantage is that RAC can take close-ups of Snow Queen, while the Surface Stereo Imager can’t see Snow Queen at all from the topside of the spacecraft. The disadvantage is that the robotic arm has so many tasks to perform that RAC can’t be used for monitoring trench ice at some opportune times.
Also, RAC hasn’t been used to take up-close images of other icy places under the spacecraft cleared on landing because it would require the robotic arm to make a difficult and complex series of moves.
"I’ve made a list of hypotheses about what could be forming cracks in Snow Queen, and there are difficulties with all of them," Mellon said.
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, Mellon said.
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," Mellon said.
"Another possibility is that these fractures were already there, and they appeared because ice sublimed off the surface and revealed them," he said.
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," Mellon said.
Phoenix scientists and engineers spent the weekend examining how the icy soil on Mars interacts with the scoop on the lander’s robotic arm, while trying different techniques to deliver a sample to one of the instruments.
"It has really been a science experiment just learning how to interact with the icy soil on Mars — how it reacts with the scoop, its stickiness, whether it’s better to have it in the shade or the sunlight," said Phoenix Principal Investigator Peter Smith of the University of Arizona.
The team tried two methods over the weekend to pick up and deliver a sample of icy soil to a laboratory oven of the Thermal and Evolved-Gas Analyzer (TEGA). In both cases, most of the sample stuck inside the lander’s inverted scoop. Images returned early Monday showed a small amount of soil reached the screened opening, but other data indicated that not enough had been funneled into the oven for beginning an analysis of the composition.
Samples obtained last Friday and Sunday contained material churned up from a hard layer by the motorized rasp on the scoop. That layer is believed to include water ice mixed with the soil.
Sunday’s attempt to deliver a sample to cell number zero of Phoenix’s TEGA instrument used more vibration with a motor inside the scoop and held the scoop upside down over the opened doors for longer than was used on Friday. The team plans to keep gaining experience in handling the icy soil while continuing with other Phoenix studies of the soil and the atmosphere.
Smith said, "While we continue with determining the best way to get an icy sample, we intend to proceed with analyzing dry samples that we already know how to deliver. We are going to move forward with a dry soil sample."