Multi-instrument Rosetta data and model comparison for the innermost coma of 67P for the period around equinox (May 2015)

Abstract

From August 2014 to September 2016 ESA's Rosetta spacecraft escorted comet 67P/Chury-umov-Gerasimenko (hereafter 67P) on its journey into the inner solar system and out again. The mission provides, via various dust and gas instruments, unprecedented data on the nature of cometary activity. The determination of the activity distribution on the surface of a comet is a key goal of any mission to investigate the interaction of the comet with the Sun. As the ice sublimates the gas expands into space it fills the near-nucleus environment. Individual sources of activity have been observed on the surface but it remains uncertain where the bulk of the mass is lost and how the processes that are involved work in detail. There are several reasons for this. First, imaging experiments use the dust as a proxy for the gas activity. Because the optical depth of the dust is orders of magnitude below 1 in all but a few cases, it is not possible to trace dust filament back to the source against the backdrop of the illuminated surface. Second, remote sensing instruments detecting gas emission (i.e. infrared and sub-mm spectrometers) may suffer with limited spatial and temporal resolution. In addition, the spectra lines may be optically thick and the line-of-sight direction usually cuts through inhomogeneous coma (in density or temperature) which further complicates their interpretation considerably. However, as we will show, with good a-priori estimates of coma structures spectral lines can be accurately inverted to provide constraints of the gas coma down to a few hundreds of meters above the surface (e.g. MIRO). The in-situ instruments (e.g. ROSINA, or GIADA) must consider possible biases due to the spacecraft position relative to the nucleus and respective illumination conditions on the surface. For instance, the frequent use of terminator orbits by Rosetta introduced a significant problem because the measured local densities are at points remote from what we assume to be the main direction of outflow, namely near the sunward direction. In addition, the possible inhomogeneities of the outgassing at the surface cannot be detected due to the fact that the rapid gas expansion smoothens the coma. Therefore, measurements taken tens of kilometers above the nucleus surface are rather insensitive and provide only ambiguous results. The difficulties described above show the need for predictive models that can reproduce multiple measurements in one self-consistent framework. We will present results from our study of diverse Rosetta data sets (including OSIRIS, VIRTIS, MIRO, and ROSINA), constraining the gas emission into the coma and to establish whether the data enable us to reach appropriate conclusions on the activity distribution on the nucleus surface. The models can be used on the one hand to constrain certain properties of the activity and on the other hand they provide clues on the limits of the interpretations of some of the available datasets. We focus here on the time around May 2015 (equinox). While this period is a few months prior to perihelion, the spacecraft was close to the comet, providing a relatively high spacial resolution of the remote sensing observations such that, in principle, they can be more easily linked with the in-situ measurements.