Near-surface speeds of cometary refractories

When a cometary nucleus approaches the sun, the volatiles sublimate and carry along embedded dust particles. On the first few kilometres this dust is accelerated by the gas drag, while further out, solar radiation pressure and gravity take over, shaping the comets outer coma and tail. While the dust dynamics after leaving the nucleus is reasonably well understood, the emission process as such is not. It is closely related to the way in which dust and ice are intermixed in the cometary surface and how the surface material is structured, which are also not well understood.In recent years, indications from multiple directions have emerged that standard gas drag is not sufficient to accelerate the refractory particles to the speeds measured in the cometary coma, especially for the largest particles emitted. Using different imaging sequences from the Rosetta mission and different analysis techniques, Pfeifer et al. (2024), Lemos (2024) and Shi et al. (2024) find that decimetre- to metre-sized chunks leave the surface or an unresolved region very close to it at speeds of 0.5 1 m/s.We review the observational evidence for such a non-zero initial speed and discuss potential implications for understanding the structure and thermophysical properties of the near-surface layers.References:Pfeifer, M. et al. (2024), accepted for publication in Astronomy Astrophysics, arXiv:2402.18613Lemos, P. et al (2024), accepted for publication in Astronomy Astrophysics, arXiv:2405.08261Shi, X. et al. (2024), ApJ Letters, 961:L