Comet Siding Spring from Mars Reconnaissance Orbiter CRISM
CRISM acquired the first image (left) at 18:16 UTC, just prior to the comet's closest approach to Mars at 18:27. The second image was taken 37 minutes later. The comet -- traveling at 55 kilometers per second -- had traversed one-third of the way across the Martian sky between those imaging times. The scale of the left image is approximately four kilometers per pixel; for the right image, it is about five kilometers per pixel. The images show the inner part of the cloud of dust, called the coma, that is generated around the nucleus by the warmth of the sun. The solid nucleus itself is not resolved. CRISM observed 107 different wavelengths of light in each pixel. Here, only three colors are shown.
The three colors represent the amount of signal in three regions of the visible and near-infrared spectrum. For each pixel, the median of 850-980 nanometers is displayed as red, 720-850 nanometers as green, and 500-630 nanometers as blue.