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

Abstract

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 A PREPRINT -FEBRUARY 28, 2019 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.

Gas terminal velocity from MIRO/Rosetta data using neural network approach

“…The synthetic spectra database is generated by applying our a 1D spherical symmetric general non-LTE forward model based on the accelerated lambda iteration (ALI), which has previously applied to the MIRO measurements (Marshall et al 2017;Marschall et al 2019) and also validated for icy-moon atmospheres (Yamada et al 2018). The 1D model radial profiles of density follow the Haser distribution, whereas velocity and temperature profiles are parameterized through two variables hyperbolic tangent function and a power law, similarly to Lee et al (2015) and Zhao et al (2019). The only additional step is to vary the free variables to create a database that samples the necessary range of densities and velocities as described below.…”

“…The Microwave Instrument on the Rosetta Orbiter (MIRO) measured spectral radiance from several preselected rotational transitions of key molecular species (H 2 O, CO, CH 3 OH, NH 3 ) in the 548-579 GHz frequency interval (Gulkis et al 2007). These measurements contain information on the column density within the MIRO beam and can be used to estimate the respective production rates, Q[H 2 O] [molec s −1 ], which have been measured since the early arrival at the comet 67P/Churyumov-Gerasimenko (Gulkis et al 2015;Lee et al 2015;Biver et al 2015;Zhao et al 2019). Marshall et al (2017) used a method of pre-calculated interpolation tables to invert H 2 O line areas of the entire MIRO dataset to study spatial Tables with the estimated velocities are available at the Open Science Foundation link: http://dx.doi.org/10.17605/OSF.IO/XMN32 distributions of activity in greater detail.…”

Effect of the 3D distribution on water observations made with the SWI

“…The LIME code has recently been applied to study 3D effects on the H 16 2 O line shape during the early activity of comet 67/Churyumov-Gerasimenko (67P/CG) measured from Rosetta (Zhao et al 2020). However, LIME has not yet been applied to gravitationally bound atmospheres of moons or planets.…”