Objectives

Comets

Comets and planets have long been thought of as having an inter-related origin. It is the fact that comets are active that leads to the hypothesis that they were involved in the Solar System formation process. This activity is produced by the sublimation of ices (mostly water ice) from the surfaces of small, irregular-shaped, solid, nuclei. The presence of ices (including many that are far more volatile than water ice such as carbon monoxide) suggests that these objects have not been significantly thermally processed since their formation. The possibility that the Solar System still contains remnants that have hardly been altered since the completion of the planetary formation process is a driver for the detailed study of these objects and was the motivation for the European Space Agency’s (ESA) Rosetta mission to
comet 67P/Churyumov-Gerasimenko. Many current theories on planet formation use the concept of pebble accretion in which cm-sized objects (pebbles) are brought together to produce the cores of gas giant planets.

Assuming comets are relics of this process, they should reveal the properties of these pebbles and indeed some authors have attempted to interpret high resolution observations of the nucleus of 67P acquired by instruments on the Rosetta spacecraft in those terms. However, these interpretations are contentious and can be challenged. Nonetheless, this has inspired modelling and prediction of the internal structure which has reached a level of sophistication that can be tested by experiment.

Through analysis of the Rosetta mission it has become clear that we do not understand how ices and refractory materials are combined within the cometary nucleus. Additionally, there remains no clear understanding of how activity proceeds in the surface layer (beyond the now trivial statement that ice sublimation drives dust loss
through gas drag). Clearly, these two issues are linked and the implications are profound because the physico-chemical relationship between ices and refractories is a key uncertainty in planetary formation modelling.

The Mission

The conclusion is that in situ study of nucleus material at sub-centimetre scales is the only way to constrain, through experiment, the properties of objects that are currently assumed to be
evidence of the initial starting point for the planetary formation process. The implication is that future missions to comets must be capable of analysing the physico-chemical relationships of cometary matter, in situ, and with limited influence on the integrity of that material.

THz Spectroscopy

THz as a technique has been chosen over optical microscopy because ice is rarely seen on the
surfaces of comets while THz frequencies provide deeper penetration into a sample. Hence, it may not be necessary to expose sub-surface ice (e.g. by digging). THz radiation is also strongly attenuated by H2O (a phenomenon used for in vivo medical applications) making water-contrast imaging of samples feasible. THz nonetheless maintains adequate spatial resolution for the task as opposed to ground penetrating radar techniques. The experimental study will be supported and enhanced by investigation of the influence of the physico-chemical structure of the surface layer of a cometary nucleus on the gas emissions from the nucleus. While numerical studies have been made before, three novel aspects will be explored. Firstly, experiments at THz and sub-THz frequencies can be performed on gas outflow from cometary analogues to support interpretation of Rosetta datasets and evaluate if constraints on the porosity and structure of the surface layer can be made by observing directly and modelling the gas flow. Secondly, we can link the sub-surface properties of the nucleus to the coma gas dynamics.
The gas properties are an observable for which a detailed under-exploited Rosetta dataset (the MIRO dataset) exists. The structure of the surface layer through which the sublimated gas flows before escaping the surface alters the gas temperature and velocity distribution function. Both of these properties influence the gas flow kilometres above the surface where they impact measurements made at Rosetta. Thirdly, the resulting simulations can be used to prepare the ground for a novel experiment to fly on a future mission to actually test surface layer models and thereby attempt to place stronger constraints on planetesimal formation mechanisms. This would include resolving uncertainties in the refractory to ice ratio, the ice-refractory mixing scale lengths, the pore sizes within the structure(s), exploring how activity can be monitored.

The two experimental setups COCoNuT and WEEVIL provide investigations on a proof-of-concept level to lay the groundwork for a possible future instrument for an in-situ comet exploration mission.