Decimetre-sized Cometary Debris as a Key to Understanding Cometary Activity (DeCoDe)

• Applicant: Professorin Dr. Jessica Agarwal
• Subject Area: Astrophysics and Astronomy
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541767366

The goal of project DeCoDe is to investigate the emission of decimetre-sized chunks of surface material from comet 67P/Churyumov-Gerasimenko. In particular, the source regions of such chunks are to be identified, and their (initial) velocities and accelerations, and their size distribution are to be measured, as well as the time evolution of these quantities as solar energy input changes. The Rosetta mission has followed comet 67P during 2014 through 2016, and regularly obtained image sequences showing large boulders and their motion relative to the spacecraft. Some of these sequences have exemplarily been analyzed by two PhD students, including the development of the required software packages. With DeCoDe, these analyses shall be extended to the entire suitable data set available in the Rosetta archive. This will open a global perspective on the emission of large fragments from the cometary surface. The up to meter-sized fragments represent the large end of a size distribution of refractory particles ("dust") that at its small end extends down to micrometre scales. Most likely, the large chunks contain the bulk of the mass ejected from the comet, while the light--scattering cross section (relevant to Earth-based telescope observations) is concentrated in smaller particles. To investigate the decimeter-sized fragments, data from an in situ spacecraft are therefore required, and the Rosetta heritage is unique for this purpose. For all parts of the dust size distribution we currently do not have a consistent model describing the thermophysical processes in the upper cometary surface layers that lead to the emission of dust and larger chunks. A key problem is that on the one hand, sufficient gas pressure needs to build inside or below the surface to overcome material cohesion, but that on the other hand, the material that must enable this pressure to build, must also be breakable by it. To construct a consistent surface model, detailed constraints are required regarding the conditions under which different types of dust are emitted. DeCoDe will provide these constraints for the largest observed fragments ejected. Cometary nuclei formed in the young solar system in the region of the outer planets and were subsequently scattered to orbits beyond Neptune, potentially preserving much of their original volatile content and physical structure. By informing on the physical and compositional structure of cometary material, DeCoDe will represent a major milestone towards understanding how planetesimals, and ultimately planets formed.

• DFG Programme: Research Grants