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Abstract

Context. The MIRO instrument’s remote sensing capability is integral to constraining water density, temperature, and velocity fields in the coma of 67P/Churyumov-Gersimenko.

Aims. Our aim is to quantify how much water density originates from the facets of the shape model within the field of view of MIRO versus the water contribution from all the other facets. This information is crucial to understanding the MIRO derived coma production rates and their relation to the nucleus characteristics, and to understanding the spatial resolution of the measurements.

Methods. This study relies on a detailed 3D nucleus shape model, illumination conditions, and the pointing information of the viewing geometry. With these parameters we can evaluate the relative contribution of water density originating from facets directly inside the MIRO beam and outside the beam as a function of distance along the MIRO line of sight. We also calculate the ratio of in-beam versus out-of-beam water gas number density.

Results. We demonstrate that despite the rather small MIRO field of view there is only a small fraction of molecules that originate from facets within the MIRO beam. This is true for the nadir, but a similar conclusion can also be applied to the limb observing geometry.

Conclusions. The MIRO instrument cannot discriminate active from inactive regions directly from observations. This study also suggests that the beam averaged solar incidence angle, local time, and mean normal vectors are not necessarily related to molecules within the MIRO beam. These results also illustrate why the 1D spherical Haser model can be applied with relative success to analyzing the MIRO data (and generally any Rosetta measurements). The future possibilities of constraining gas activity distribution on the surface should use 3D codes extracting information from the MIRO spectral line shapes which contain additional information. The results presented here are applicable to remote sensing instruments on board Rosetta.