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Comet and Asteroid Re-Shaping through Activity

Periodic Reporting for period 4 - CAstRA (Comet and Asteroid Re-Shaping through Activity)

Período documentado: 2021-11-01 hasta 2023-08-31

Comets and asteroids are remnants from the formation of planets in the early time of our solar system. While they likely represent some of the least altered samples of material in the protoplanetary disc, these bodies have been and continuously are subject to various processes that change them. An observable manifestation of such processes is called activity, the release of gas and dust to space. Key goals of this project are to obtain a comprehensive overview of the various processes underlying activity in comets and asteroids, investigate how ice is stored in them, and characterise the ejected dust. The project relies on data from the European Space Agency's Rosetta mission that visited comet 67P/Churyumov-Gerasimenko between 2014 and 2016, and on Earth-based telescope observations of active asteroids, complemented by numerical modelling of dust physical and compositional properties and dynamics. An additional aim of the project is to understand to which extent it is possible to generalise Rosetta results to a wider sample of comets or even asteroids.
A comprehensive numerical model has been developed to describe the interaction of light with cometary matter. The model simultaneously calculates the optical properties of large aggregated dust-particles, their thermally excited radiation and processes related to solid-state heat conduction and phase change. The model has been has been applied to interpret optical and near-infrared measurements by OSIRIS and VIRTIS. The measurements were found to be consistent with large aggregated particles composed of large silicate grains embedded in smaller carbon-rich grains.
To study the motion of debris emitted from comet 67P, two algorithms have been developed to track the motion of individual dust particles or boulders flying in the coma in either a series of short exposures, or a single long exposure. This paved the way to studying the source regions and ejection circumstances of the material.
Telescope time on Hubble and the Very Large Telescope of the European Southern Observatory has been awarded to project members to observe several active asteroids and comets. A multi-year series of Hubble observations allowed us to constrain the mutual orbit of the binary main-belt comet 288P and identify its active component, which has led to the serendipitous discovery of a third component to this system. Hubble images of interstellar comet 2I/Borisov led us to conclude that activity in this object was subject to both a diurnal and a seasonal cycle, similar to Rosetta target 67P.

These results are published in peer reviewed articles, e.g.
Markkanen et al. (2018), ApJ, 868, L16.
Markkanen & Agarwal (2019), A&A, 631, A164.
Markkanen & Agarwal (2020), A&A, 643, A16.
Kim et al. (2020), ApJL 895, L34.
Agarwal et al. (2020), A&A 643, A152.
Pfeifer et al. (2022), A&A, 659, A171.
Lemos et al. (2023), MNRAS 519, 5775.
We find that cometary activity likely is distributed inhomogeneously across the cometary surface and with time. Some regions eject decimetre-sized chunks while particles from other regions are much smaller. The large chunks seem to leave the surface or near-surface coma layer with a speed of about 1 m/s whose origin is not yet understood. While particles larger than about a centimetre are able to conserve embedded water ice for times long enough for Rosetta to observe them, we do not find strong evidence of outgassing from the motion of the large chunks.
We find that the diurnal activity variation in interstellar comet 2I/Borisov is similar to that in Rosetta target comet 67P.
The main-belt asteroid 288P is likely a triple system. It probably formed by splitting of a precursor body whose rotation was driven to a critical frequency by asymmetric emission of thermal radiation, such that centrifugal forces exceeded gravity. However, we also find that this splitting likely was not the cause of the current sublimation of water ice that must have recently been excavated from deeper layers.
Credits: ESA/Rosetta/MPS for OSIRIS Team and NASA, ESA und J. Agarwal (MPS)