Enceladus' near-surface CO₂ gas pockets and surface frost deposits

Abstract

Solid CO₂ surface deposits were reported in Enceladus' South Polar Region by Brown et al. (2006). They noted that such volatile deposits are temporary and posited ongoing replenishment. We present a model for this replenishment by expanding on the Matson et al. (2012) model of subsurface heat and chemical transport in Enceladus. Our model explains the distributions of both CO₂ frost and complexed CO₂ clathrate hydrate as seen in the Cassini Visual and Infrared Mapping Spectrometer (VIMS) data. We trace the journey of CO₂ from a subsurface ocean. The ocean-water circulation model of Matson et al. (2012) brings water up to near the surface where gas exsolves to form bubbles. Some of the CO₂ bubbles are trapped and form pockets of gas in recesses at the bottom of the uppermost ice layer. When fissures break open these pockets, the CO₂ gas is vented. Gas pocket venting is episodic compared to the more or less continuous eruptive plumes, emanating from the "tiger stripes", that are supported by plume chambers. Two styles of gas pocket venting are considered: (1) seeps, and (2) blowouts.

The presence of CO₂ frost patches suggests that the pocket gas slowly seeped through fractured, cold ice and when some of the gas reached the surface it was cold enough to condense (i.e., T ∼70 to ∼119 K). If the fissure opening is large, a blowout occurs. The rapid escape of gas and drop in pocket pressure causes water in the pocket to boil and create many small aerosol droplets of seawater. These may be carried along by the erupting gas. Electrically charged droplets can couple to the magnetosphere, and be dragged away from Enceladus. Most of the CO₂ blowout gas escapes from Enceladus and the remainder is distributed globally. However, CO₂ trapped in a clathrate structure does not escape. It is much heavier and slower moving than the CO₂ gas. Its motion is ballistic and has an average range of about 17 km. Thus, it contributes to deposits in the vicinity of the vent. Local heat flow indicates that gas pockets can be located as deep as several tens of meters below the surface. Gas pockets can be reused, and we explore their life cycle.