Distribution and nature of CO2 on Enceladus

Abstract:

We present the first global mapping and analysis of CO₂ on the surface of Enceladus, and we report the largest concentrations of free CO₂ on the southern polar region using the Visual and Infrared Mapping Spectrometer (VIMS) on Cassini. Free CO₂ ice and complexed CO₂ were already reported near the South Pole (Brown et al., Science, 2006; Hansen, LPSC, 2010). Our work focuses on determining the amount, location and molecular state of CO₂ on Enceladus, which could help identify and model geophysical processes that currently occur in the interior. One hypothesis for bringing heat and chemicals to the surface is a warm subsurface ocean containing dissolved gases, mostly CO₂ (Postberg F. et al., Nature, 2009). Therefore, our observations are consistent with erupted and condensed materials onto Enceladus’ surface (Matson et al., Icarus, 2012; Matson et al. AGU Fall meeting 2015). Free CO₂ ice absorbs at 4.268 µm (Sandford and Allamandola, 1990) and CO₂ complexed with other molecules absorbs at 4.247 µm (Chaban et al., Icarus, 2007). The Enceladus case is complicated because both free and complexed CO₂ are present, and the absorption band of interest is shallow and close to the instrument detection limit. Many of the few Enceladus VIMS data sets have significant and sometimes unusual noise, which we attempted to avoid or remove. We utilized all VIMS data sets available that were collected over ten years of the Cassini mission as a way to improve the detection statistics and signal to noise. We also used wavelengths near 2.7 µm where CO₂ has a narrow absorption as a filter to help identify CO₂-rich areas. Finally, we selected observations that have spatial resolution better than 100 km in order to create a map that can be compared with the largest fractures, known as Tiger Stripes, in the southern polar region.