If 20 to 40 ton cosmic snowballs pelt Earth as claimed by Frank and Sigwarth [1997a>, dark pixels will be produced in the 130.4 nm images of dayglow obtained by the Ultraviolet Imager (UVI) on the Polar spacecraft. Examination of the UVI images has revealed that dayglow images are indeed spotted with single and multiple dark pixels. But is a snowball the only explanation for these dark pixels? To learn more about the dark pixels, we have examined the calibration images obtained from the same camera just before the instrument was launched. We find that dark pixels similar to those in dayglow images also exist in calibration images. This strongly indicates that the source of the dark pixels is instrumental. For further verification, a statistical analysis found the dark pixels from dayglow and calibration images have nearly identically shaped occurrence patterns. We have also looked for evidence of spacecraft wobble which demonstrates that the source of a bright or dark feature in the images is external to the camera, but found none for dark pixels. Finally, we studied the bright streaks that frequently appear in UVI images, sometimes comet-like in appearance. These trails are ionization tracks produced by cosmic rays or other penetrating energetic particles interacting with our camera. We conclude that the source of the dark pixels in dayglow images is internal to the camera system and there is no scientific evidence from UVI that snowballs pelt Earth. ¿ 1997 American Geophysical Union