The pre-perihelion activity of dynamically new comet C/2013 A1 (Siding Spring) and its close encounter with Mars

TL;DR

This study tracked the activity of comet C/2013 A1 (Siding Spring) from 4.5 AU to perihelion at 1.4 AU, revealing how its gas production rates and activity patterns evolved as it approached Mars, driven by sublimation processes.

Contribution

It provides the first systematic observation of a dynamically new comet's activity from pre-perihelion to perihelion, highlighting the role of icy grain sublimation and nucleus heterogeneity.

Findings

Water production rate peaked at 1.5 x 10^28 molecules/s near Mars

Gas delivery to Mars was between 3.1-5.4 x 10^4 kg

Activity changes linked to CO2 depletion and nucleus heterogeneity

Abstract

We used the UltraViolet-Optical Telescope on board Swift to systematically follow the dynamically new comet C/2013 A1 (Siding Spring) on its approach to the Sun. The comet was observed from a heliocentric distance of 4.5 AU pre-perihelion to its perihelion at 1.4 AU. From our observations, we estimate that the water production rate during closest approach to Mars was 1.5 +/- 0.3 x 1E28 molecules/s, that peak gas delivery rates were between 4.5-8.8 kg/s, and that in total between 3.1-5.4 x 1E4 kg cometary gas was delivered to the planet. Seasonal and evolutionary effects on the nucleus govern the pre-perihelion activity of comet Siding Spring. The sudden increase of its water production between 2.46-2.06 AU suggests the onset of the sublimation of icy grains in the coma, likely driven by CO2. As the comet got closer to the Sun, the relative contribution of the nucleus' water production increased, while CO2 production rates decreased. The changes in the comet's activity can be explained by a depletion of CO2, but the comet's high mass loss rate suggests they may also reflect primordial heterogeneities in the nucleus.