Are the Circular, Dark Features on Comet Borrelly Albedo Variations or Craters?

Details Are the Circular, Dark Features on Comet Borrelly Albedo Variations or Craters? Are the Circular, Dark Features on Comet Borrelly Albedo Variations or Craters?

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p11a0345n&link_type=abstract

American Geophysical Union, Fall Meeting 2002, abstract #P11A-0345

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6000 Planetology: Comets And Small Bodies, 6055 Surfaces And Interiors, 6061 Remote Sensing

Scientific paper

The highest resolution images of Comet P/19 Borrelly taken by the Miniature Integrated Camera and Spectrometer (MICAS) on Deep Space 1 show several dark circular features which upon first inspection might be construed as craters (Soderblom et al., 2002). However, it is also reasonable to suggest that these are localized albedo differences and not the product of shadowing variations from a depression in the local topography. To distinguish this we conducted a photometric analysis of the three most prominent of these features using six of the highest quality MICAS images from the DS1 flyby. The phase angle variation in this data set is from 51 to 75 degrees. The lower spatial resolution images were re-scaled at resolution equivalent to the highest resolution image. The integrated flux in each of the three circular features was measured. We find that the integrated I/F increased as phase angle exhibiting a photometric behavior similar to the higher albedo surrounding terrain. This is inconsistent with the behavior or a shadowed region where increases in spatial resolution should cause a decrease in I/F. Two control regions that were just beyond the terminator (and hence in permanent shadow but still exhibiting flux from the coma) were also measured in each image. The control regions showed no increase in I/F with decreasing phase angle as expected for a region in permanent shadow. We also made photometric scans through the center of each circular feature from the terminator direction toward the limb direction. These were searched for changes in symmetry of the transect with phase variation as might be expected if these features were craters. The scans showed no pronounced asymmetry and no changes in symmetry were found with change in phase angle. In summary we find that: 1) The dark circular features follow the photometric behavior of the rest of the object and not that of shadowed areas 2) The darkest parts of the circular features have an integrated reflectance that is well above the coma 3) The brightness of the dark circular features increases with increasing spatial resolution (if they were craters then they should decrease) 4) The contrast does not fade with decreasing angle of incidence 5) Profiles through craters do not show bright lips or pronounced asymmetry from terminator to limb We conclude from these measurements that the dark circular features seen on comet Borrelly are not craters but are low albedo regions on an otherwise topographically uniform localized area. This work carried out at JPL and USGS under contract with NASA. Soderblom et al., 2002. Science, 296, 1087-1091

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