Sub-Surface Properties of Comet 67P/Churyumov-Gerasimenko Derived from Combined Thermal and Spectroscopic Data from the MIRO Instrument
Abstract
The Microwave Instrument on the Rosetta Orbiter (MIRO) [1] measures a) thermal emission from the nucleus of comet 67P/Churyumov-Gerasimenko in two continuum bands (centered near 190 GHz and 562 GHz), and b) eight molecular emission and absorption lines (of H2 16O, H2 17O, H2 18O, CH3OH, NH3, and CO) in the 562 GHz band with a spectral resolution of 44 kHz. Average thermal and dielectric properties of the nucleus have been derived from the continuum measurements in Ref. 2, 3 and 4, and coma properties were obtained in Ref. 2, 5 and 6. The reference publications relied on thermal models of the sub-surface regions of the nucleus for the analysis of continuum data, and on models of the coma density, gas velocity, and temperature, and on radiative transfer calculations for the analysis of the molecular lines. These two types of analyses were mostly conducted without explicit consideration for the coupling that exists between the thermal models and gas production models. Consequently, gas production rates derived from the spectral data analysis are not necessarily consistent with the temperature profiles obtained from the continuum data analysis, where sublimation of ices was not included. We will present results from a unified analysis of the continuum emission and the molecular lines for a set of observations in 2015, including two flybys with closest approach to the nucleus smaller than 10 km. Our analysis constrains the physical properties of the sub-surface material of the nucleus, such as its porosity, ice-to-dust ratio, thickness of the dust layer, and dust grain size, by using a thermal model for porous materials similar to the one described in Ref. 7, including sublimation of ice, and a combination of simple hydrodynamic and gas kinetic models to compute the resulting properties of the gas column probed with MIRO. Non-local thermal equilibrium radiative transfer calculations yield the spectral line shapes. As part of the analysis of the sub-surface nucleus materials, we will compare the model parameters used in the independent thermal and spectral, and combined model approaches.
- Publication:
-
European Planetary Science Congress
- Pub Date:
- October 2015
- Bibcode:
- 2015EPSC...10..386V