Acceleration of Individual, Decimetre-sized Aggregates in the Lower Coma of Comet 67P/Churyumov-Gerasimenko

TL;DR

This study uses high-resolution images to measure the acceleration of decimetre-sized aggregates near comet 67P, revealing the significant role of gas drag and outgassing recoil forces in their dynamics and potential contribution to the comet's debris trail.

Contribution

First direct measurement of aggregate acceleration in the innermost coma of a comet, highlighting the effects of gas drag and recoil forces on particle trajectories.

Findings

Approximately 50% of aggregates accelerate away from the nucleus.

Accelerations are up to ten times stronger than local gravity.

At least 10% of aggregates escape the nucleus and feed the debris trail.

Abstract

We present OSIRIS/NAC observations of decimetre-sized, likely ice-containing aggregates ejected from a confined region on the surface of comet 67P/Churyumov-Gerasimenko. The images were obtained in January 2016 when the comet was at 2 AU from the Sun out-bound from perihelion. We measure the acceleration of individual aggregates through a two-hour image series. Approximately 50% of the aggregates are accelerated away from the nucleus, and 50% towards it, and likewise towards either horizontal direction. The accelerations are up to one order of magnitude stronger than local gravity, and are most simply explained by the combined effect of gas drag accelerating all aggregates upwards, and the recoil force from asymmetric outgassing, either from rotating aggregates with randomly oriented spin axes and sufficient thermal inertia to shift the temperature maximum away from an aggregate's subsolar region, or from aggregates with variable ice content. At least 10% of the aggregates will escape the gravity field of the nucleus and feed the comet's debris trail, while others may fall back to the surface and contribute to the deposits covering parts of the northern hemisphere. The rocket force plays a crucial role in pushing these aggregates back towards the surface. Our observations show the future back fall material in the process of ejection, and provide the first direct measurement of the acceleration of aggregates in the innermost coma (<2km) of a comet, where gas drag is still significant.