02.21.2006
Water Ice Found on a Small Portion of the Comet's Nucleus By Lucy McFadden
In a paper appearing in Science Express on Feb. 2, 2006, an article by Sunshine et al. reports on the Deep Impact science team's finding of a small area of water ice on the surface of Tempel 1. This is the first time that water ice has been observed on the surface of a comet. Past efforts with the near-IR spectrometer on Deep Space 1 mission flying past comet Borrelly and from the ground of comets far from the sun and not enshrouded with coma, have yielded no evidence of water ice on their surface.
As a comet approaches the Sun, it releases gas and dust in its immediate vicinity forming the coma and obscuring the nucleus from view unless spacecraft can get at close range. Deep Impact did just that. Imaging with the two cameras, the HRI and MRI showed small regions that were about 30% brighter than surrounding areas. After scaling the images to an average value of the nucleus, three discrete areas on the nucleus are brighter in the ultraviolet and darker in the near-infrared. When Co-Investigator Dr. Jessica Sunshine looked at the spectra in that region, after subtracting a thermal component, what was left was the spectral signature of water ice, in the form of absorption bands at 1.5 and 2.0 µm. Absorption bands at these wavelengths are diagnostic of water ice. The combination of the relative colors and the spectra make a powerful case that there is water ice at these specific locations on Tempel 1.
Given that the spectrometer has a two dimensional detector, it is possible to make a map of Tempel 1 at the wavelength of the ice absorption bands. That map shows that the bright regions in the UV are correlated with dark regions in the near-IR where water ice absorbs light. Since the visible images have a higher spatial resolution, we use those images to calculate the extent of ice on Tempel 1's surface. That turns out to be a small fraction of the surface, only 0.5%. Next, the temperature map is combined with the color map, showing that two of the three regions are colder regions of the nucleus. Stereo images show the largest area of ice to be a depression 80 meters below surrounding areas. Never the less, the temperatures in this region are 285 -295 K, significantly above the ~200K at which ice would sublimate in space at the location of Tempel 1.
What is significant is that the extent of this ice on Tempel 1's surface is not sufficient to produce the observed abundance of water and its by-products in the comet's coma. The team thus concludes that there are sources of water from beneath the comet's surface that supply the cometary coma as well.
Also important is that the particle size of the water ice, is greater than the icy grains in the coma, and is probably recondensed onto the comet's surface. It is therefore probably not a primary block of cometary material which would be called a cometesimal.
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