Constraining spatial pattern of early activity of comet 67P/C–G with 3D modelling of the MIRO observations

Our aim is to investigate early activity (2014 July) of 67P/C–G with 3D coma and radiative transfer modeling of Microwave Instrument on the Rosetta Orbiter (MIRO) measurements, accounting for nucleus shape, illumination, and orientation of the comet. We investigate MIRO line shape information for spatial distribution of water activity on the nucleus during the onset of activity. During this period we show that MIRO line shape have enough information to clearly isolate contribution from ‘neck’ (Hapi) and bottom of large lobe (Imhotep), and compare it to the nominal case of activity from the entire illuminated surface. We also demonstrate that spectral line shapes differ from the 1D model for different viewing geometries and coma conditions relevant to this study. Specifically, line shapes are sensitive to the location of the terminator in the coma. At last, fitting the MIRO observations we show that the Imhotep region (possible distributed source of H2O sublimating from the icy grains in the coma lifted due to CO2 activities) contributes negligible fraction of the total number of water molecules into MIRO beam in the early activity. On the other hand, a strong enhancement of water activity from the ‘neck’ region seems required to fit the MIRO line shapes. This is consistent with earlier analysis of Rosetta results. Nevertheless, within the assumption of our coma and surface boundary conditions we cannot get a reasonable fit to all MIRO mapping observations in 2014 July. We provide discussion on how to enhance these results and resolve the found issues in the future