Rather appropriately for the time of year, NASA's EPOXI team says that the mission's recent encounter with comet Hartley 2 observed a flurry of snow streaming out from the comet.
The 'snowflakes' range from golf-ball-sized particles right up to fluffy basketball-sized chunks, steaming from the comet's rocky ends. At the same time, a different process is causing water vapor to escape from the comet's smooth mid-section.
"When we first saw all the specks surrounding the nucleus, our mouths dropped," said Pete Schultz, EPOXI mission co-investigator at Brown University. "Stereo images reveal there are snowballs in front and behind the nucleus, making it look like a scene in one of those crystal snow globes."
Scientists compared the new data to data from comet Tempel 1, observed during a flyby in 2005.
"This is the first time we've ever seen individual chunks of ice in the cloud around a comet or jets definitively powered by carbon dioxide gas," said Michael A'Hearn, principal investigator for the spacecraft at the University of Maryland. "We looked for, but didn't see, such ice particles around comet Tempel 1."
The new findings show Hartley 2 acts differently to Tempel 1 or the three other comets with nuclei imaged by spacecraft. Carbon dioxide appears to be a key to understanding Hartley 2 and explains why the smooth and rough areas scientists saw respond differently to solar heating, and have different mechanisms by which water escapes from the comet's interior.
Data show the smooth area of comet Hartley 2 looks and behaves like most of the surface of comet Tempel 1, with water evaporating below the surface and percolating out through the dust. However, the rough areas of Hartley 2, with carbon dioxide jets spraying out ice particles, are very different.
"The carbon dioxide jets blast out water ice from specific locations in the rough areas, resulting in a cloud of ice and snow," said Jessica Sunshine, EPOXI deputy principal investigator at the University of Maryland. "Underneath the smooth middle area, water ice turns into water vapor that flows through the porous material, with the result that close to the comet in this area we see a lot of water vapor."
Engineers have checked to see whether the ice particles peppered the spacecraft itself. So far, they've found nine occasions when particles may have hit the spacecraft without damaging it.
"The EPOXI mission spacecraft sailed through the Hartley 2's ice flurries in fine working order and continues to take images as planned of this amazing comet," said Tim Larson, EPOXI project manager at JPL.
Scientists are working on a more detailed analysis to determine how long the cometarysnow storm has been active, and whether the differences in activity between the comet's middle and ends are the result of how it formed some 4.5 billion years ago or are caused by more recent evolutionary effects.