GLOBAL MAPPING OF CO2 ON ENCELADUS

Abstract:

We present the first global map of CO₂ on Enceladus. The purpose is to determine whether CO₂ is associated to fractures and eruptions, and if it formed recently.

Cassini observed tectonic features and plumes on Enceladus, which could be caused by a warm subsurface ocean containing dissolved gases. CO₂ should be one of these gases (Postberg F. et al., Nature, 2009), and some of it should be erupted and condensed onto the surface (Matson et al., Icarus, 2012). Validation of this hypothesis could be done by determining the amount, location and molecular state of the CO₂.

Free CO₂ ice and complexed CO₂ were reported on Enceladus (Brown et al., Science, 2006; Hansen, LPSC, 2010) from analysis of Cassini Visual and Infrared Mapping Spectrometer (VIMS) data, and on other Saturn icy satellites (Cruikshank et al., Icarus, 2010 ; Filacchione et al., Icarus, 2010). Complexed CO₂ has also been found from Galileo Near-Infrared Mapping Spectrometer (NIMS) spectra on the icy Galilean satellites (McCord et al., Science, 1997 and JGR, 1998), apparently due to both interior outgassing and radiation processing.

CO₂ has an asymmetric stretching mode that creates an absorption band, the wavelength position of which is sensitive to the nature of molecular associations between CO₂ and their neighbors. Free CO₂ ice absorbs at 4.268 µm for (Sandford and Allamandola, 1990) and CO₂ complexed with other molecules absorbs at shorter wavelengths, around 4.25 µm or shorter (Chaban et al., Icarus, 2007). In VIMS spectra of Enceladus, this stretching mode absorption band is near the instrument detection limit. We utilized all VIMS data sets available that had significant spatial resolution to increase the statistics of the observations for any given location and improve the signal to noise.

CO₂ has also a smaller absorption at 2.7 µm, although it occurs in a range of wavelength that has higher signal-to-noise ratio by several magnitudes, because the surface of Enceladus (mostly H₂O ice) has higher albedo. We used both absorptions in the range 4.25-4.268 µm and at 2.7 µm to identify CO₂.

Our results confirm free CO₂ is concentrated on the southern polar region, which is consistent with release from the eruptions, and is compatible with CO₂ present in the interior.

We now focus on detecting variations of band position at 4.25-4.268 µm, in order to map free and complexed CO₂.