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

Our aim is to investigate early activity (July 2014) of 67P/CG with 3D coma and radiative transfer modeling of 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 Hapi and Inhotep independently, 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 somewhat sensitive to the location of the terminator in the coma. At last, fitting the MIRO observations we show that the Imhotep region (possible extended source of H$_{2}$O due to CO$_{2}$ activity) contributes only a small 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 Hapi region seems required to fit the MIRO line shapes. This is consistent with earlier 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 July 2014, which may illustrate that a more sophisticated coma model or more accurate temperature/velocity distribution is needed.