Cometary Comae-Surface Links

Abstract

A comet is a highly dynamic object, undergoing a permanent state of change. These changes have to be carefully classified and considered according to their intrinsic temporal and spatial scales. The Rosetta mission has, through its contiguous in-situ and remote sensing coverage of comet 67P/Churyumov-Gerasimenko (hereafter 67P) over the time span of August 2014 to September 2016, monitored the emergence, culmination, and winding down of the gas and dust comae. This provided an unprecedented data set and has spurred a large effort to connect in-situ and remote sensing measurements to the surface. In this review, we address our current understanding of cometary activity and the challenges involved when linking comae data to the surface. We give the current state of research by describing what we know about the physical processes involved from the surface to a few tens of kilometres above it with respect to the gas and dust emission from cometary nuclei. Further, we describe how complex multidimensional cometary gas and dust models have developed from the Halley encounter of 1986 to today. This includes the study of inhomogeneous outgassing and determination of the gas and dust production rates. Additionally, the different approaches used and results obtained to link coma data to the surface will be discussed. We discuss forward and inversion models and we describe the limitations of the respective approaches. The current literature suggests that there does not seem to be a single uniform process behind cometary activity. Rather, activity seems to be the consequence of a variety of erosion processes, including the sublimation of both water ice and more volatile material, but possibly also more exotic processes such as fracture and cliff erosion under thermal and mechanical stress, sub-surface heat storage, and a complex interplay of these processes. Seasons and the nucleus shape are key factors for the distribution and temporal evolution of activity and imply that the heliocentric evolution of activity can be highly individual for every comet, and generalisations can be misleading.

An Update of the Correlation between Polarimetric and Thermal Properties of Cometary Dust

“…The first four comets were observed in two observing modes with a few and a few tens of thousand kmsized apertures at r H 1 au. Their data points thus represent the dust properties in the dust tail/trail coma region, free from any significant collisions and ice sublimations (Marschall et al 2020). Together with the results of Bonev et al (2008) showing that the dust radial profile reaches an equilibrium at ∼1,000 km from the nucleus at r H ∼ 1 au, the aperture differences in such an order might sample more or less similar coma dust.…”

The Reactivation of Main-Belt Comet 259P/Garradd (P/2008 R1)

“…For example, while volatile depletion and mantling would be expected to cause overall declines in activity strength over time (e.g., Kossacki & Szutowicz 2012;Hsieh et al 2015a), processes such as sinkhole collapses and expansion of active areas via sublimation-driven erosion, as observed on 67P/Churyumov-Gerasimenko by the Rosetta spacecraft (e.g., Vincent et al 2015), could conceivably produce occasional increases in activity strength. If active sites are localized (e.g., Hsieh et al 2004;Yu et al 2020), variable global or local seasonal effects due to rotational axis precession or topographical surface evolution between active apparitions (e.g., Gutiérrez et al 2016;El-Maarry et al 2019;Lai et al 2019;Marschall et al 2020) could also conceivably alter MBC activity strength from orbit to orbit. Continued investigation of activity evolution in MBCs to ascertain the relative frequencies of different evolutionary trends will be extremely valuable for illuminating the significance of different processes in driving the evolution of MBC surfaces.…”

Past and Future Comet Missions

“…The color, polarization, and radial brightness profile of the dust coma of 67P indicated evolution of particle properties as dust moved outward, interpreted as fragmentation of these particles at distances beyond where Rosetta was measuring their properties in situ (Boehnhardt et al 2016;Rosenbush et al 2017). A combination of these large-scale observations and inner-coma spacecraft measurements is required to build a full model of how the cometary coma develops from the nucleus (Marschall et al 2020). On the largest scales, cometary tails reveal both comet dust and gas properties and how their induced magnetospheres interact with the solar wind (see chapter by Goetz et al in this volume).…”