Enlargement of depressions on comet 81P/Wild 2: Constraint based on 30-year cometary activity in the inner Solar System

TL;DR

This study investigates the formation of depressions on comet 81P/Wild 2 by analyzing activity data and modeling dust and water ejection, concluding most depressions predate the comet's current orbit and originated in the Kuiper belt.

Contribution

It provides a quantitative analysis linking cometary activity to depression formation, highlighting that most depressions preexisted and originated in the Kuiper belt.

Findings

Only 30% of depression volume is explained by current activity.

Most depressions likely formed before the comet's current orbit.

Dust-to-ice mass ratio estimated between 2 and 14.

Abstract

Context. The Stardust flyby mission to Jupiter-family comet (JFC) 81P/Wild 2 (hereafter, 81P) captured its dense quasicircular depressions. Nevertheless, the formation mechanism remains a subject of ongoing debate. Aims. We aimed to study how cometary activity contributed to the formation and enlargement of these depressions by analyzing Stardust flyby images and ground-based observation data. Methods. We calculated the time-dependent water production rate of 81P inside the snow line (<3 au) and compared it with the observational data. In addition, we estimated the fallback debris mass using an observation-based model, where a dust ejection from 81P was considered to reproduce ground-based observations of the dust tail. We compared the total excavated volume of water and dust with the total depression volume derived using the 81P shape model. Results. We find that the total excavated volume after 81P was injected into the inner Solar System accounts for up to only 30 % of the depression volume. This insufficiency suggests that a large portion (>70 %) of the depressions had already existed before the comet was injected into the current orbit. In addition, we estimate the dust-to-ice mass ratio for 81P to be 2-14. Conclusions. We suggest that most depressions were formed in the source region of the Kuiper-belt objects.