DRAG - Dynamics of Rarefied Gas

Cometary nuclei are very irregular in shape and their dust emissions are highly non-isotropic. With a limited amount of data from ground based observations, it is difficult to constrain model parameters that describe the dynamics within the coma. Because of their small size (typically <10 km in diameter), cometary nuclei and their innermost coma is best investigated by spacecraft. Our main goal is to create models that describe spacecraft observations, which can eventually help us to understand the dynamic phenomena in cometary comae and link them to their surfaces.

Numerical simulations of the outflow from cometary nuclei tend to be simplified assuming axisymmetric geometry with respect to the Sun and solving the Navier-Stokes equations to derive a gas and dust distribution (e.g. Keller et al., 1994; Crifo and Rodionov, 1999). However, these solutions are not valid for intermediate flow regimes. For the gas outflow, we use the Direct Simulation Monte Carlo (DSMC) approach (Bird, 1994; Davidsson and Skorov, 2004; Crifo et al., 2002a, 2005), which has significant advantages in that low gas outflow rates can be examined. This has been of particular importance for the Rosetta mission which closely followed comet 67P/Churyumov-Gerasimenko (67P/C-G) as it approached the inner solar system from about 3.6 AU, through to perihelion and on its way back to the outer solar system, covering a wide range of gas outflow regimes. We develop coma simulations to help us interpret measurements from different instruments onboard of Rosetta (ROSINA, VIRTIS and MIRO).

For a detailed modelling of the dust outflow, image analysis is also needed. Therefore, we analyse a large set of observations by the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) onboard Rosetta in order to investigate the dust emissions from the nucleus of comet 67P/C-G. A numerical model of the dust emission has been established which allows us to input the emission parameters seen at the source and extrapolate them to simulate OSIRIS’s observations.

Effects of the complex topography of comet nuclei on the flow field can only be studied in a 3D-DSMC simulation. A collaboration with the group of J.-S. Wu (National Chiao Tung Uni., Taiwan) has been established and boosted considerably through an SNSF-funded exploratory workshop in Bern in January 2013. They have developed a DSMC code named PDSC++, which has been parallelized and is capable of running 2D, 2D-axisymmetric and 3D flow fields.

The 67P/C-G's shape model of Lowry et al. (2012) was used for the first runs with a homogeneous outgassing and indicate that the code runs successfully within a manageable time. Considerable time has been invested in optimizing the grid and reducing the collision distance to mean free path (mfp) ratio by implementation of the transient adaptive subcell method (Su et al., 2010). A high-resolution shape model of comet 67P/CG (SHAP7 by Preusker et al., 2017) is now available and has recently been used to model dust-gas outflow from smaller surface features.

Optical remote-sensing instruments on cometary missions probe the structure of the inner coma by measuring sunlight scattered by emitted dust. Modelling of the ejection and acceleration of gas and dust can provide the link between the observable dust distribution and processes occurring at the surface of the nucleus that are driven by the sublimation of ice when it is exposed to solar irradiation.

ESAs (European Space Agency) spacecraft Rosetta observed comet 67P/C-G’s nucleus and coma from August 2014 to September 2016. During the mission, the spacecraft escorted the comet from around 3.6 AU from the Sun through to perihelion (at about 1.24 AU) when water production rates reached around 3.5e28 molecules per second as measured by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) (Hansen et al. 2016). In order to interpret Rosetta’s measurements, a model for the gas-dust interaction of comet 67P/C-G is necessary. Preliminary simulations of the outgassing of comet 67P/C-G have been done using 3D-DSMC (Direct Simulation Monte Carlo) method. By setting up models and implementing simulations, the properties of gas and dust (ex. the gas flow-field in the innermost coma and the dust acceleration by gas drag) of the comet can be thus well studied.

A purely insolation-driven model for the surface H2O outgassing does not fit to Rosetta's observations between August-September 2014. Instead, a stronger source in the region of the nucleus named Hapi is needed to match ROSINA's and OSIRIS's data (Marschall et al. 2016). The role of cliffs has also been studied. However, in this case non-unique solutions have been found (Marschall et al. 2017). 

Gerig, S-B., R. Marschall, N. Thomas, I. Bertini, D. Bodewits, B. Davidsson, M. Fulle, W.-H. Ip, H.U. Keller, M. Küppers, F. Preusker, F. Scholten, C.C. Su, I. Toth, C. Tubiana, J.-S. Wu, H. Sierks, C. Barbieri, P.L. Lamy, R. Rodrigo, D. Koschny, H. Rickman, J. Agarwal, M.A. Barucci, J.-L. Bertaux, G. Cremonese, V. Da Deppo, S. Debei, M. De Cecco, J. Deller, S. Fornasier, O. Groussin, P.J. Gutierrez, C. Güttler, S.F. Hviid, L. Jorda, J. Knollenberg, J.-R. Kramm, E. Kührt, L.M. Lara, M. Lazzarin, J.J. Lopez Moreno, F. Marzari, S. Mottola, G. Naletto, N. Oklay, & J.-B. Vincent, (2017), On Deviations from Free-Radial Outflow in the Inner Coma of Comet 67P/Churyumov-Gerasimenko, Icarus, submitted.

Marschall, R., S. Mottola, C.C. Su, Y. Liao, M. Rubin, J.S. Wu, N. Thomas, K. Altwegg, H. Sierks, W.-H. Ip, H.U. Keller, J. Knollenberg, E. Kührt, I.L. Lai, Y. Skorov, L. Jorda, F. Preusker, F. Scholten, J.-B. Vincent, and the OSIRIS and ROSINA teams, (2017), Cliffs vs. Plains: Can ROSINA/COPS and OSIRIS data of comet 67P/Churyumov-Gerasimenko in autumn 2014 constrain inhomogeneous outgassing?, Astron. Astrophys., accepted (27 June 2017).

Liao Y., C.C. Su, R. Marschall, J.S. Wu, M. Rubin, I.L. Lai, W.-H. Ip, H.U. Keller, J. Knollenberg, E. Kührt, Y. Skorov, and N. Thomas, (2016), 3D Direct Simulation Monte Carlo Modelling of the Inner Gas Coma of Comet 67P/Churyumov-Gerasimenko: A Parameter Study, Earth, Moon, and Planets, accepted, 28 February 2016.

Marschall, R., C.C. Su, Y. Liao, N. Thomas, K. Altwegg, H. Sierks, W.-H. Ip, H.U. Keller, J. Knollenberg, E. Kührt, I.L. Lai, M. Rubin, Y. Skorov, J.S. Wu, L. Jorda, F. Preusker, F. Scholten, A. Gracia Berná, A. Gicquel, G. Naletto, X. Shi, and J.-B. Vincent, (2016), Modelling of the observations of the inner gas and dust coma of comet 67P/Churyumov-Gerasimenko – First results, Astron. Astrophys., accepted , 27 February 2016

Liao, Y., C.C. Su, S. Finklenburg, M. Rubin, W.-H. Ip, H.U. Keller, J. Knollenberg, E. Kührt, L.I. Lai, Y. Skorov, N. Thomas, J.S. Wu, and Y.S. Chen, (2014), 3-D DSMC Simulations of Comet 67P/Churyumov-Gerasimenko, Lunar and Planetary Institute Science Conference Abstracts, 45, 1764.

Finklenburg, S., N. Thomas, C.C. Su , and J.-S. Wu, (2014), The spatial distribution of water in the inner coma of comet 9P/Tempel 1: Comparison between models and observations, Icarus, accepted, 24 March 2014.

Finklenburg, S. and N. Thomas, (2014) Relating in situ gas measurements to the surface outgassing properties of cometary nuclei, Planetary and Space Science, accepted, 11 February 2014, doi:10.1016/j.pss.2014.02.005.

Finklenburg, S., N. Thomas, J. Knollenberg, and E. Kührt, (2011), Comparison of DSMC and Euler Equations Solutions for Inhomogeneous Sources on Comets, 27th International Symposium on Rarefied Gas Dynamics, 2010 AIP Conf. Proc. 1333, 1151-1156, doi: 10.1063/1.3562799.

Thomas, N. (2009) The nuclei of Jupiter family comets: A critical review of our present knowledge, Planetary and Space Science, 57, 1106-1117.