Shape evolution of cometary nuclei via anisotropic mass loss

Abstract

Context. Breathtaking imagery recorded during the European Space Agency's Rosetta mission confirmed the bilobate nature of comet 67P/Churyumov-Gerasimenko's nucleus. Its peculiar appearance is not unique among comets. The majority of cometary cores imaged at high resolution exhibit a similar build. Various theories have been brought forward as to how cometary nuclei attain such peculiar shapes.Aims. We illustrate that anisotropic mass loss and local collapse of subsurface structures caused by non-uniform exposure of the nucleus to solar irradiation can transform initially spherical comet cores into bilobed ones. Methods. A mathematical framework to describe the changes in morphology resulting from non-uniform insolation during a nucleus' spin-orbit evolution is derived. The resulting partial differential equations that govern the change in the shape of a nucleus subject to mass loss and consequent collapse of depleted subsurface structures are solved analytically for simple insolation configurations and numerically for more realistic scenarios. Results. The here proposed mechanism is capable of explaining why a large fraction of periodic comets appear to have peanut-shaped cores and why light-curve amplitudes of comet nuclei are on average larger than those of typical main belt asteroids of the same size.

Cometary Nuclei—From Giotto to Rosetta

“…Less suggestive is the indication of onion shell-like structures of both lobes separately ). If true, it would confirm that the bilobate shape was formed by the merger of two bodies rather than by erosion (Jutzi and Asphaug 2015;Vavilov et al 2019). No differences of the physical and chemical properties of the bodies were found.…”

The Phenomenon of Shape Evolution due to Solar Driven Outgassing for Analogues of Small Kuiper Belt Objects

“…The overarching question is whether solar driven loss may alter significantly the overall shape (even at large heliocentric distances), is it possible for a "neck" or other concavity be driven purely by sublimation, and does homogeneous mass loss smooth out or deepen pre-existing concavities, and similar questions. In contrast to the recent work of Vavilov et al (2019), we try to approach the solar driven mass loss in a general manner, rather than trying to fit a particular cometary observation. We attempt to explore it as a phenomena valid for any ice, orbits, shapes, etc, and explicitly avoid any assumption on the (ad-hoc) prior localization of activity.…”

“…1, we will investigate the evolution of five, what we consider, "general" shapes. 1) a spherical shape, 2) an oblate body rotating around the axis of the maximum moment of inertia that is often regarded to be an empirical shape model for solar system small bodies, 3) an elongated shape which could be evolved from 1) and 2) (Vavilov et al 2019), 4) a concave shape exhibiting a small initial concavity to study its evolution under different starting conditions, and finally, 5) a bilobed shape where the initial configuration resembles that of a contact binary. In addition, we consider two cases of spin-axis for each body, a) the long axis rotation and b) the short axis rotation.…”

“…Another possible scenario responsible for nucleus shape modification is sublimation (Jewitt 2009;Jewitt et al 2017). Most recently, simulations done by Vavilov et al (2019) present the nucleus shape evolution processes from a sphere to varieties of irregular structures including bilobed ones due to anisotropic mass loss. Their work assumes a nucleus with higher production rates in the center region of the comet such that the bilobed/concaved structure can be produced.…”

“…In this work, we aim to extend and improve on the study of Vavilov et al (2019) in the effort to understand the limits of possible solutions of long-term sublimation driven mass loss in more realistic way. We combine spin and orbits states with idealized outgassing model of CO at large heliocentric distance acting over long time scales.…”