Rotationally Induced Surface Slope-Instabilities and the Activation of CO2 Activity on Comet 103P/Hartley 2

TL;DR

This study proposes that a recent rapid rotation episode caused avalanches on comet 103P/Hartley 2, exposing CO2 ice and activating surface activity, explaining its unique diurnal CO2-driven activity and surface features.

Contribution

It introduces a novel explanation linking recent rapid rotation to surface avalanches and CO2 activation on Hartley 2, supported by observational and modeling evidence.

Findings

Rapid rotation likely induced avalanches exposing CO2 ice.

Surface debris deposits correspond to observed mounds and coma chunks.

Rotation slowdown correlates with surface feature redistribution.

Abstract

Comet 103P/Hartley 2 has diurnally controlled, CO2-driven activity on the tip of the small lobe of its bilobate nucleus. Such activity is unique among the comet nuclei visited by spacecraft, and suggests that CO2 ice is very near the surface, which is inconsistent with our expectations of an object that thermophysically evolved for ~45 million years prior to entering the Jupiter Family of comets. Here we explain this pattern of activity by showing that a very plausible recent episode of rapid rotation (rotation period of ~11 [10-13] hours) would have induced avalanches in Hartley 2's currently active regions that excavated down to CO2-rich ices and activated the small lobe of the nucleus. At Hartley 2's current rate of spindown about its principal axis, the nucleus would have been spinning fast enough to induce avalanches ~3-4 orbits prior to the DIXI flyby (~1984-1991). This coincides with Hartley 2's discovery in 1986, and implies that the initiation of CO2 activity facilitated the comet's discovery. During the avalanches, the sliding material would either be lofted off the surface by gas activity, or possibly gained enough momentum moving downhill (toward the tip of the small lobe) to slide off the nucleus. Much of this material would have failed to reach escape velocity, and would reimpact the nucleus, forming debris deposits. The similar size frequency distribution of the mounds observed on the surface of Hartley 2 and chunks of material in its inner coma suggest that the 20-40 meter mounds observed by the DIXI mission on the surface of Hartley 2 are potentially these fallback debris deposits. As the nucleus spun down from a rotation period of ~11 hours to 18.34 hours at the time of the DIXI flyby, the location of potential minima, where materials preferentially settle, migrated about the surface, allowing us to place relative ages on most of the imaged terrains of the nucleus.