CO2 and 12C:13C Isotopic Ratios on Phoebe and Iapetus
Abstract
Cassini VIMS has obtained spatially resolved 0.35 to 5.1 micron reflectance spectra of Saturn's satellites beginning with the Phoebe fly-by in 2004 and a close fly-by of Iapetus in 2007. Both surfaces contain relatively abundant CO2. The new (2016 RC19) calibration of VIMS has provided a significant increase in the data quality, such that isotopic absorption bands in CO2 are now well defined. CO2 on Saturn's icy satellites is trapped (Cruikshank et al., 2010, Icarus v206 p561; Pinilla-Alonso et al. 2011, Icarus v211, p75i), predominantly in the dark material (Clark et al. 2012, Icarus v218 p831). Clark et al. modeled the CO2 abundance as 2.8% on Iapetus and 3.7% on Phoebe. The main 12CO2 band in VIMS spectra on Iapetus occurs at 4.253 microns and Phoebe at 4.266 microns. The 13CO2 absorption is strong on Phoebe at 4.367 microns and weak on Iapetus at 4.387 microns. Converting the Phoebe, Iapetus, and a lab reflectance spectrum (of trace H2O-CO2 mixture on a diffuse substrate), we derive preliminary values for the ratio of the equivalent widths of the 12C and 13C absorptions as 19±2 on Phoebe, 82±8 on Iapetus, and 98±10 for the laboratory spectrum. These ratios are related to the 12C/13C ratio, but there may be effects due to intra-molecular and inter-molecular coupling that will contribute to systematic errors in the isotopic abundances derived using equivalent-width measurements that we've yet to quantify. We Believe the effects are small, and will be attempting to quantify them in the future. For comparison, the terrestrial value of the 12C/13C ratio is 90.17, and vibrational coupling may explain the slightly high lab mixture result. The local interstellar medium is 69±15 (Boogert et al., 2000, A&A). Because the CO2 bands on Phoebe and Iapetus dark material have different positions, and because the observed 13C absorption strengths are so different, the surface evolutions must be different. The large enrichment in 13C on Phoebe argues for significant processing, e.g. due to radiation from UV and/or thermal processes. The CO2 in the Phoebe dust is apparently lost on its way from Phoebe to Iapetus. The different band positions and isotopic band strengths on Iapetus argues for an endogenic source for the CO2 rather than from Phoebe, perhaps a sampling of the original solar nebula or interstellar medium.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.P42A..07C
- Keywords:
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- 6008 Composition;
- PLANETARY SCIENCES: COMETS AND SMALL BODIESDE: 6024 Interiors;
- PLANETARY SCIENCES: COMETS AND SMALL BODIESDE: 6040 Origin and evolution;
- PLANETARY SCIENCES: COMETS AND SMALL BODIESDE: 6055 Surfaces;
- PLANETARY SCIENCES: COMETS AND SMALL BODIES