Browsing by Author "Marschall, Raphael"

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Limitations in the determination of surface emission distributions on comets through modelling of observational data - A case study based on Rosetta observations
(2020-08-01)
Marschall, Raphael
;
Liao, Ying
;
Thomas, Nicolas
;
Wu, Jong-Shinn
;
機械工程學系
The European Space Agency's (ESA) Rosetta mission has returned a vast data set of measurements of the inner gas coma of comet 67/Churyumov-Gerasimenko. These measurements have been used by different groups to determine the distribution of the gas sources at the nucleus surface. The solutions that have been found differ from each other substantially and illustrate the degeneracy of this issue. It is the aim of this work to explore the limitations that current gas models have in linking the coma measurements to the surface. In particular, we discuss the sensitivity of Rosetta's ROSINA/COPS, VIRTIS, and MIRO instruments to differentiate between vastly different spatial distributions of the gas emission from the surface. We have applied a state of the art 3D DSMC gas dynamics code to simulate the inner gas coma of different models that vary in the fraction of the surface that contains ice and in different sizes of active patches. These different distributions result in jet interactions that differ in their dynamical behaviour. We have then produced synthetic measurements of Rosetta's gas instruments. By comparing the different models we probe the limitations of the different instruments to variations in the emission distribution. We have found that ROSINA/COPS measurements by themselves cannot detect the differences in our models. While ROSINA/COPS measurements are important to constrain the regional inhomogeneities of the gas emission, they can by themselves not determine the surface-emission distribution of the gas sources to a spatial accuracy of better than a few hundred metres (similar or equal to 400 m similar to 50 MEN. Any solutions fitting the ROSINA/COPS measurements is hence fundamentally degenerate, be it through a forward or inverse model. Only other instruments with complementary measurements can potentially lift this degeneracy as we show here for VIRTIS and MIRO. In particular, we find that MIRO is the only instrument that can distinguish between most of our models. Finally, as a by-product, we have explored the effect of our activity distributions on lateral flow at the surface that may be responsible for some of the observed aeolian features.
The Dust-to-Gas Ratio, Size Distribution, and Dust Fall-Back Fraction of Comet 67P/Churyumov-Gerasimenko: Inferences From Linking the Optical and Dynamical Properties of the Inner Comae
(2020-06-24)
Marschall, Raphael
;
Markkanen, Johannes
;
Gerig, Selina-Barbara
;
Pinzon-Rodriguez, Olga
;
Thomas, Nicolas
In this work, we present results that simultaneously constrain the dust size distribution, dust-to-gas ratio, fraction of dust re-deposition, and total mass production rates for comet 67P/Churyumov-Gerasimenko. We use a 3D Direct Simulation Monte Carlo (DSMC) gas dynamics code to simulate the inner gas coma of the comet for the duration of the Rosetta mission. The gas model is constrained by ROSINA/COPS data. Further, we simulate for different epochs the inner dust coma using a 3D dust dynamics code including gas drag and the nucleus' gravity. Using advanced dust scattering properties these results are used to produce synthetic images that can be compared to the OSIRIS data set. These simulations allow us to constrain the properties of the dust coma and the total gas and dust production rates. We determined a total volatile mass loss of (6.1 +/- 1.5) center dot 10(9)kg during the 2015 apparition. Further, we found that power-laws withq=3.7-0.078+0.57are consistent with the data. This results in a total of5.1-4.9+6.0 center dot 109kg of dust being ejected from the nucleus surface, of which4.4-4.2+4.9 center dot 109kg escape to space and6.8-6.8+11 center dot 108kg (or an equivalent of14-14+22cm over the smooth regions) is re-deposited on the surface. This leads to a dust-to-gas ratio of0.73-0.70+1.3for the escaping material and0.84-0.81+1.6for the ejected material. We have further found that the smallest dust size must be strictly smaller than similar to 30 mu m and nominally even smaller than similar to 12 mu m.