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Multi-instrument analysis of Rosetta data – Establishing a new paradigm for cometary activity

Periodic Reporting for period 1 - MiARD (Multi-instrument analysis of Rosetta data – Establishing a new paradigm for cometary activity)

Reporting period: 2016-03-01 to 2017-05-31

The MiARD project is using data from the Rosetta space mission to better understand the origin and evolution of comets, and their role in the solar system (including the possibility that they may have indirectly influenced the emergence of life, by delivering volatile species such as water to the Earth).

The broad objectives of the MiARD project are to use an interdisciplinary multi-instrument approach to the analysis of data from the Rosetta mission, in order to make progress towards answering some fundamental questions about comets and the formation and evolution of the solar system. In doing this, the project will also derive information that is expected to be useful in risk mitigation for cometary and cometary-related impacts with Earth and man-made objects.
The fundamental questions are:
• How did our solar system and other planetary systems form?
• How did life develop on Earth?
• How unique are these processes?
We do not expect to obtain definitive answers to these questions within the MiARD project! The project will, however, proceed in a structured way to obtain information required to address these questions. We begin by deriving an accurate and detailed shape model of the comet, a pre-requisite for subsequent studies of the strength of cometary materials, and for numerical modelling of the activity (outgassing of the comet). The nature of the cometary surface, its strength, and the extent of material loss are key to understanding the evolution of the comet 67P/Churyumov-Gerasimenko (and by extension, other comets), and to what extent the current state of the comet can tell us about its past. In the second half of the project, we will focus on comparing what is known about 67P/Churyumov-Gerasimenko with information from other comets, and on considering the scientific case for a cometary nucleus sample return mission. We also focus on quantifying non-gravitational forces that perturb cometary orbits, and the trajectories of material (dust) ejected from comets, with respect to the risk to Earth and spacecraft.
The MiARD project work has led to the most accurate, complete and highest resolution shape models available for comet 67P/Churyumov-Gerasimenko. The generation of these improved shape models of the comet is of central importance to the project as the global shape models are used as inputs for the temperature and activity modelling (WP2 and WP4), and also in the processing of spectral datasets used for line-of-sight compositional and temperature measurements. Both the global shape model and local DTMs are being used to make deductions about the strength and erosion rate of the cometary materials. It is therefore unsurprising that the three first papers from the project to have been submitted for publication in a peer-reviewed journal are concerned with the effects of topography on outgassing rates and the surface evolution of the comet.
Analysis of data from the PHILAE lander, together with constraints derived from the shape model and laboratory analogues, has allowed the project to place numerical bounds on the material strength of different areas of the comet (in general very weak, just tens of Newtons per square metre, but with stronger icy layers). This information will be of use in interpreting the topography of the comet, in models of formation of the comet, and for any future cometary missions involving a lander, sampling of the comet or attempts to perturb a comet's trajectory. Quantitative stress maps for the comet have been derived (taking into account both self-gravity and thermal effects) and show that the highest stresses are in the neck of the comet, and may correlate with highly active regions.
'Activity' modelling of the outgassing from the comet has been successful so that the agreement between numerical models and observed data is now rather good. The extent of the agreement for different model assumptions has been tested statistically, so that some physical models can be ruled out. This work has also shown that there is unlikely to be a unique solution, and that it is not possible for example to distinguish between completely homogenous degassing, and degassing from small-scale more active sites that are distributed more or less uniformly.
The MiARD project has developed the most accurate and complete shape model of comet 67P, and used this shape model to support better physical and numerical models of the activity (out-gassing) of the comet, including the sub-surface flow of gases through the porous cometary surface. Data from Rosetta (images and the derived shape model) has been combined with PHILAE lander observations and laboratory measurements on analogue cometary materials and processes to constrain the tensile and compressive strengths of cometary materials. Better constraints on the composition of the comet have been obtained by combining mass spectral data (from instruments on Rosetta and PHILAE) with spectroscopic data (Rosetta).
A high resolution digital terrain model of the Agilkia region on comet 67P/Churyumov-Gerisimenko