Abstract
Abstract Solar system bodies with surface and subsurface volatiles will show observational evidence of activity when they reach a temperature where those volatiles change from solid to gas and are released. This is most frequently seen in comets, where activity is driven by the sublimation of water, carbon dioxide, or carbon monoxide ices. However, some bodies (notably the asteroid (3200) Phaethon) show initiation of activity at very small heliocentric distances, long after they have reached the sublimation temperatures of these ices. We investigate whether the sodium present in the mineral matrix could act as the volatile element responsible for this activity. We conduct theoretical modeling which indicates that sodium has the potential to sublimate in the conditions that Phaethon experiences, depending on the mineral phase it is held in. To test this, we then exposed samples of the carbonaceous chondrite Allende to varying heating events, similar to what would be experienced by low-perihelion asteroids. We measured the change in sodium present in each sample and find that the highest temperature samples show a significant loss of sodium from specific mineral phases over a single heating event, comparable to a day on the surface of Phaethon. Under specific thermal histories possible for Phaethon, this outgassing could be sufficient to explain this object’s observed activity. This effect would also be expected to be observed for other low-perihelion asteroids as well and may act as a critical step in the process of disrupting small low-albedo asteroids.
Highlights
The small bodies of the inner solar system are generally grouped into two phenomenological classes: comets and asteroids
The main difference distinguishing these two populations is whether or not the body is observed to show activity in the form of emission of gas and dust. This divide is usually traced to whether the object contains frozen subsurface volatile materials such as water, carbon dioxide, or carbon monoxide and is used as a proxy for where in the solar system the object formed with respect to the protoplanetary ice line of each molecule
We propose that the driver of the activity on Phaethon at perihelion is the volatilization of sodium bound in minerals such as sodalite and nepheline distributed in the matrix
Summary
The small bodies of the inner solar system are generally grouped into two phenomenological classes: comets and asteroids. The main difference distinguishing these two populations is whether or not the body is observed to show activity in the form of emission of gas and dust This divide is usually traced to whether the object contains frozen subsurface volatile materials such as water, carbon dioxide, or carbon monoxide and is used as a proxy for where in the solar system the object formed with respect to the protoplanetary ice line of each molecule. This clear distinction has been called into question by a newly recognized population of objects. Referred to by various publications as Active Asteroids, Main Belt Comets, or Active Main Belt Objects, this population can be divided into objects showing activity from impact events, objects with activity consistent with disruption after rotational spin up, and objects with activity consistent with the presence of subsurface water ice
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