Evidence for fresh frost layer on the bare nucleus of comet Hale--Bopp at 32 AU distance

TL;DR

This study presents evidence of a fresh frost layer on comet Hale--Bopp's nucleus at 32 AU, showing increased albedo after activity ceased, likely due to icy grain redeposition in a cold environment.

Contribution

It provides the first observational evidence of a fresh frost layer on a giant comet's nucleus at large heliocentric distances, highlighting processes affecting such bodies beyond typical comet activity zones.

Findings

Comet Hale--Bopp's activity ceased between 2007 and 2009 at 28 AU.

Post-activity albedo increased to 8.1%, higher than any known comet.

Albedo was smaller before activity, indicating surface changes.

Abstract

Here we report that the activity of comet Hale--Bopp ceased between late 2007 and March, 2009, at about 28 AU distance from the Sun. At that time the comet resided at a distance from the Sun that exceeded the freeze-out distance of regular comets by an order of magnitude. A Herschel Space Observatory PACS scan was taken in mid-2010, in the already inactive state of the nucleus. The albedo has been found to be surprisingly large (8.1$\pm$0.9%{}), which exceeds the value known for any other comets. With re-reduction of archive HST images from 1995 and 1996, we confirm that the pre-perihelion albedo resembled that of an ordinary comet, and was smaller by a factor of two than the post-activity albedo. Our further observations with the Very Large Telescope (VLT) also confirmed that the albedo increased significantly by the end of the activity. We explain these observations by proposing gravitational redeposition of icy grains towards the end of the activity. This is plausible for such a massive body in a cold environment, where gas velocity is lowered to the range of the escape velocity. These observations also show that giant comets are not just the upscaled versions of the comets we know but can be affected by processes that are yet to be fully identified.