Nature, distribution and origin of CO2 on Enceladus

Abstract

We present the first map of CO2 at the surface of Enceladus using data obtained by the Cassini Visible-Infrared Mapping Spectrometer (VIMS). In order to measure the weak and narrow CO2 absorption band depths around 4.26 µm, we improved: (1) the calibration of VIMS spectra; (2) the calculation of geographic coordinates; and (3) the projection techniques. We averaged multiple observations of a given area to obtain a signal to noise ratio high enough to map the CO2 abundances. CO2 is reliably detected mostly in the South Polar Region. This region includes active faults (Tiger Stripes), the highest observed surface temperatures, and locations of active plume eruptions. The occurrence here of CO2 is consistent with an endogenic origin controlled by tectonics. Both pure CO2 ice and complexed CO2 are detected from the positions of absorption bands between 4.27 and 4.24 µm. The highest concentrations of CO2 are between the main active faults of the South Polar Region, where the surface temperature is low. Pure CO2 ice deposits at the surface of Enceladus are best modeled by the formation of gas pockets below the icy crust and above the surface of the internal ocean. These pockets eventually release cold CO2 gas (∼70 to ∼119 K) at low-velocity (seeping) between the Tiger Stripes [Matson et al., 2018, Icarus, 302, 18–26]. CO2 clathrate hydrates may form in the ocean and may be subsequently released when a CO2 gas pocket blows out and erupts. Other mechanisms may contribute to reinforcing the anti-correlation of the CO2 distribution (of any type) with respect to the location of the Tiger Stripes, such as successive sublimation of CO2 and condensation on colder areas, and partial frost cover by H2O releases from plume eruptions.