Areas with water ice are shown in blue (upper portion of comet surface). Note: Impact occurred at the bottom, right hand side of the comet as shown in this image

Newswise — Scientists for Deep Impact, the University of Maryland-led NASA mission that made history when it smashed into a comet this past July 4th, have added another first to their growing list: the first finding of water ice on the surface of a comet.

By analyzing data and images taken prior to impact, Deep Impact scientists have detected water ice in three small areas on the surface of Comet Tempel 1. This is the first time ice has been detected on the nucleus, or solid body, of a comet. The findings are published today in the online version of the journal Science.

“These results show that there is ice on the surface, but not very much and definitely not enough to account for the water we see in the out-gassed material that is in the coma [the cloud of gas and dust that surrounds the comet],” said lead author Jessica Sunshine of Science Applications International Corporation

“These new findings are significant because they show that our technique is effective in finding ice when it is on the surface and that we can therefore firmly conclude that most of the water vapor that escapes from comets is contained in ice particles found below the surface,” said Deep Impact Principal Investigator Michael A’Hearn of the University of Maryland.

Where’s the Ice?

Through observations of ice grains and water vapor in the coma of comets, scientists have long known that ‘dirty snowballs,’ as comets are sometimes described, must indeed contain substantial amounts of water ice. However, prior to Deep Impact they didn’t have any knowledge about how such ice was distributed between the surface, subsurface and inner core of a comet’s nucleus.

In the Science article, the authors say that prior to Deep Impact there existed few observations of nuclei not obscured by the coma. Among previous cometary missions, the most notable of such observations was the Deep Space-1 mission to comet Borrelly, which searched unsuccessfully for evidence of water ice and other volatiles on that comet’s surface. Limited ground based observations of possibly bare cometary nuclei have also failed to find clear evidence of surface ice.

The fact that the Deep Impact team found water on the surface, but only in a few scattered places, all but eliminates the possibility that there is a lot of undetectable surface ice “just hiding in the surface darkness,” explained Sunshine.

The surface ice that the team detected (see figure) was not located where the impact later occurred. This means, Sunshine explained, that the water ice and water vapor the team already had found in analyses of material ejected by the July 4 impact must have come from ice located close to, but not on, the surface of the comet.

Bright Spots in a Dark Landscape

As a comet approaches the Sun, it releases gas and dust forming a cloud (coma) that obscures the nucleus from view unless spacecraft can get very close. Deep Impact did just that. The Deep Impact science team used four types of data in their search for ice on the mostly coal black surface of Tempel 1.

First, images from Deep Impact’s high resolution and medium resolution instruments (the HRI and MRI) showed three small regions that were about 30 percent brighter than surrounding areas. After scaling the images to the average brightness value of the nucleus, these three discrete areas on the nucleus where found to be brighter in the ultraviolet and darker in the near-infrared, a combination that is consistent with water ice. In addition, Sunshine’s analysis of the spectra of light emitted and absorbed in those regions showed the distinctive spectral signature of water ice. The combination of the relative colors and the spectral signature make a powerful case that there is water ice at these specific locations on Tempel 1.

Using visual images and spectral mapping of the impact side of the surface of Tempel 1, the team found that the patches of surface ice represented only 0.5 percent of the total observed surface.

Team member Olivier Groussin, a University of Maryland research scientist, made a temperature map and combined it with the color map to show that two of the three ice patches regions were in colder regions of the nucleus. Stereo images show the largest area of ice to be a depression 80 meters below surrounding areas.