Comet 108P/Ciffreo: The Blob

Short-period comet 108P/Ciffreo is known for its peculiar double morphology, in which the nucleus is accompanied by a co-moving, detached, diffuse 'blob'. We report new observations of 108P/Ciffreo taken with the Hubble Space Telescope and the Nordic Optical Telescope and use them to determine the cause of this unusual morphology. The separation and the longevity of the blob across several orbits together rule out the possibility of a single, slow-moving secondary object near the primary nucleus. We use a model of coma particle dynamics under the action of solar gravity and radiation pressure to show that the blob is an artifact of the turn-around of particles ejected sunward and repelled by sunlight. Numerical experiments limit the range of directions which can reproduce the morphology and explain why the co-moving blob appearance is rare.