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Are Centaurs Comet Nuclei?

Published onJun 01, 2020
Are Centaurs Comet Nuclei?

Centaurs are an interesting type of small bodies found between the orbits of Jupiter and Neptune. Three Centaurs, 95P/Chiron, 174P/Echeclus, and 166P/NEAT, have shown some sort of activity and are classified as comets, in addition to their minor planet designations. However, is activity a sufficient criterion for comet classification and does this apply to the remainder of the 500 or so known Centaurs? Of course, the major problem is that we have no clear definition of a comet, or more properly, comet nucleus. Detailed information of several comet nuclei has been gathered by space probes, so the ancient definition of a comet as a small body that shows signs of an active coma (usually dust) needs to be revisited and is long overdue. A new definition should take into account physical and chemical properties, dynamics, time and place of origin, activity mechanisms, and other important aspects. Where would Centaurs fit into this scheme? Compared to comet nuclei in the Kuiper Belt where temperatures are low enough to preserve volatile ices of CO2, NH3, and CH3OH (like Arrakoth recently visited by the New Horizons mission), Centaurs are found at distances where these ices can actively sublimate. Do large Centaurs, including those with ring systems (i.e., 2060 Chiron and 10199 Chariklo), represent a class of transition objects between comets (with extensive exospheres) and bodies with thin, tightly bound atmospheres typical of some planets and large satellites? Are these large Centaurs dwarf planets? The greater mass of large Centaurs introduces nontrivial gravitational effects on the surrounding gas and dust. By comparing the ratio of gravitational potential energy to the kinetic energy of gas molecules (and dust grains), an expected size limit can be set on bodies with surface-bound exospheres and those with bound dust comae or rings. Possible outburst mechanisms for comets, including the Pressurized Obstructed Pores (POP) model and the Cometary Outburst Model by Avalanche (COMA), likely also apply to Centaurs. Large Centaurs, using Chiron as a prototype, represent an interesting and unique type of small Solar System bodies, intermediate between comets and planets, and are worthy of spacecraft exploration. In this paper, the characteristics of Centaurs are discussed from a cometary perspective.

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