Cometary Comae-Surface Links:The Physics of Gas and Dust from the Surface to a Spacecraft

A comet is a highly dynamic object, undergoing a permanent state of change. These changes have to be carefully classified and considered according to their intrinsic temporal and spatial scales. The Rosetta mission has, through its contiguous in-situ and remote sensing coverage of comet 67P/Churyumov-Gerasimenko (hereafter 67P) over the time span of August 2014 to September 2016, monitored the emergence, culmination, and winding down of the gas and dust comae. This provided an unprecedented data set and has spurred a large effort to connect in-situ and remote sensing measurements to the surface. In this review, we address our current understanding of cometary activity and the challenges involved when linking comae data to the surface. We give the current state of research by describing what we know about the physical processes involved from the surface to a few tens of kilometres above it with respect to the gas and dust emission from cometary nuclei. Further, we describe how complex multidimensional cometary gas and dust models have developed from the Halley encounter of 1986 to today. This includes the study of inhomogeneous outgassing and determination of the gas and dust production rates. Additionally, the different approaches used and results obtained to link coma data to the surface will be discussed. We discuss forward and inversion models and we describe the limitations of the respective approaches. The current literature suggests that there does not seem to be a single uniform process behind cometary activity. Rather, activity seems to be the consequence of a variety of erosion processes, including the sublimation of both water ice and more volatile material, but possibly also more exotic processes such as fracture and cliff erosion under thermal and mechanical stress, sub-surface heat storage, and a complex interplay of these processes. Seasons and the nucleus shape are key factors for the distribution and temporal evolution of activity and imply that the heliocentric evolution of activity can be highly individual for every comet, and generalisations can be misleading

Calathus: A sample-return mission to Ceres

Ceres, as revealed by NASA's Dawn spacecraft, is an ancient, crater-saturated body dominated by low-albedo clays. Yet, localised sites display a bright, carbonate mineralogy that may be as young as 2 Myr. The largest of these bright regions (faculae) are found in the 92 km Occator Crater, and would have formed by the eruption of alkaline brines from a subsurface reservoir of fluids. The internal structure and surface chemistry suggest that Ceres is an extant host for a number of the known prerequisites for terrestrial biota, and as such, represents an accessible insight into a potentially habitable “ocean world”. In this paper, the case and the means for a return mission to Ceres are outlined, presenting the Calathus mission to return to Earth a sample of the Occator Crater faculae for high-precision laboratory analyses. Calathus consists of an orbiter and a lander with an ascent module: the orbiter is equipped with a high-resolution camera, a thermal imager, and a radar; the lander contains a sampling arm, a camera, and an on-board gas chromatograph mass spectrometer; and the ascent module contains vessels for four cerean samples, collectively amounting to a maximum 40 g. Upon return to Earth, the samples would be characterised via high-precision analyses to understand the salt and organic composition of the Occator faculae, and from there to assess both the habitability and the evolution of a relict ocean world from the dawn of the Solar System.The attached document is the authors’ final accepted version of the journal article provided here with a Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) Creative Commons Licence. You are advised to consult the publisher’s version if you wish to cite from it.

Dust in the Inner Coma of Comet 67P/Churyumov-Gerasimenko

The main goal of the work presented in this PhD thesis is to investigate the dust dynamics in the near-nucleus coma of comet 67P/Churyumov-Gerasimenko (67P) and to gain a deeper understanding of the physical processes governing it. In our analyses, we study numerical simulation results of gas and dust comae in combination with image analysis of Rosetta OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) data. To simulate the gas and dust comae, we first calculate the 3D gas flow field using the Direct Simulation Monte Carlo (DSMC) approach. We use either a simplified spherical nucleus or the complex shape of the nucleus of 67P as a source. Surface temperatures and gas production rates are calculated with a thermal model. We use H2O outgassing in simulations with one gas species only and combine H2O and CO2 in simulation cases with a multispecies gas coma. In a second step, we simulate the motion of dust particles through our simulation domain by solving an equation of motion including gas drag and nucleus gravity. Finally, we generate synthetic images for direct comparison with OSIRI data by calculating column densities and using a light scattering model to compute the image brightness. We present results from coma simulations corresponding to observations acquired on 11. April 2015 at 67P. In a series of four OSIRIS images we studied the dayside-to-nightside coma brightness ratio (DS/NS ratio) and found it to be surprisingly low at an average 2.49 ± 0.18. Simulated values from purely insolation-driven water outgassing greatly exceed the measured DS/NS ratios. We investigate gravity dominated particles on fall-back trajectories and direct nightside activity as possible explanations and conclude that direct nightside activity is needed to explain the observations. In a follow-up study, we look at a multi-species coma of H2O and CO2 in a model that includes heat conduction into the nucleus interior. We find that such a model can reproduce the correct nightside activity that matches the observations to first order. We also investigate the effect of the dynamic motion of dust particles on the dust size distribution at different locations in the inner coma. We find that the dust size distribution is uniform above the active dayside of the nucleus but shows deviations towards the nightside coma. In various simulation models we show that the transition from uniform to non-uniform local dust size distributions is dependent on the production rate but happens beyond the terminator region in the coma. We conclude that the results from Rosetta experiments sampling dust were not affected by deviations in the local dust size distribution

On understanding multi-instrument Rosetta data of the innermost dust and gas coma of comet 67P/Churyumov-Gerasimenko - results, strengths, and limitations of models

Numerical models are powerful tools for understanding the connection between the emitted gas and dust from the surface of comets and the subsequent expansion into space where remote sensing instruments can perform measurements. We will present such a predictive model which can provide synthetic measurements for multiple instruments on board ESA’s Rosetta mission to comet 67P/Churyumov-Gerasimenko (hereafter 67P). We will demonstrate why a multi instrument approach is essential and how models can be used to constrain the gas and dust source distribution on the surface