On the interaction of sublimating gas with cometary bodies

Jordan K Steckloff, Purdue University

Abstract

Sublimation of volatiles is a defining process of comet nuclei, and profoundly affects their dynamics, structure, and appearance. Central to understanding the processes by which comets formed and subsequently evolved is a careful computation of this sublimation pressure as a function of heliocentric distance. Unlike previous efforts, I develop a thermodynamic method to numerically compute the sublimation pressure of any species from limited knowledge of its physical properties. I then describe a novel cometary disruption mechanism in which this sublimation pressure induces differential stresses within the body of the nucleus that exceed its material strength, resulting in structural failure and breakup of the nucleus. I show that this mechanism is consistent with the behavior of Comet ISON (C/2012 S1), and use it to estimate the cohesive strength of ISON’s nucleus, a first for a Long-Period Comet. Sublimating volatiles can also generate sublimative torques that alter the rotation state of the nucleus. However, computing these torques requires high-resolution information on the shape and activity of the nucleus, which is available only for the few nuclei visited by spacecraft. To remedy this, I develop a novel framework based on the YORP Effect (the torques asteroids experience by emitting thermal photons from their asymmetric shapes) to study the effects of sublimative torques on populations of cometary bodies. I take advantage of the similar manner in which surfaces emit both thermal photons and sublimating molecules to derive numerical relationships that describe sublimative torques by appropriately scaling the YORP torque equations. I then use this framework to explain the formation of dust striae (long linear features in the tails of Long-Period Comets that align with the Sun), which has remained an enigma for more than a century. I show that the observed ∼10-100 m chunks ejected from comet nuclei experience sublimative torques that spin them up to the point of disruption, forming the observed striae. Sublimative torques can also significantly affect nuclei themselves, and cause large avalanches that excavate buried supervolatile ices. The activity of Comet 103P/Hartley 2 is dominated by CO2 driven sublimation at the tip of its bilobate nucleus. This CO2 ice responds to the nucleus’s diurnal cycle, and must therefore be very near the surface. However, CO2 ices were expected to have receded deep below Hartley 2’s surface during its ∼10 million year migration from the Kuiper Belt to the Jupiter Family, suggesting that these ices were somehow brought to the surface. I map the gravitational slopes of Hartley 2’s surface as a function of rotation period, and show that large avalanches capable of excavating these CO2 ices set in at a rotation period of ∼11 hours, and are entirely confined to the regions of the nucleus exhibiting CO2 driven activity. This suggests that a period of fast rotation activated this CO2 activity. At the rate of spin-down observed by EPOXI, this avalanche likely occurred between 1984 and 1991, and would have significantly brightened the comet, consistent with its discovery in 1986. Furthermore, this mechanism allows me to date nearly all terrains imaged by EPOXI, a first for a comet.

Degree

Ph.D.

Advisors

Melosh, Purdue University.

Subject Area

Astrophysics|Physics|Astronomy

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