Comets in the framework of the formation and evolution of the Solar System
Dr Alessandro Morbidelli, Observatoire de la Côte d’Azur, France
I will review the dynamical evolution of comets, from the primordial planetesimal disk to their current reservoirs (Oort cloud, Kuiper belt, Scattered disk) in the framework of models of formation and evolution of the Solar System. The emphasis will be on the expected differences/similarities in formation place, and therefore physical and chemical properties, among long period and short period comets, as well as between comets and primitive asteroids.
The collisional evolution of comets, associated with the dynamical history, will also be discussed, trying to answer the question: how primitive are comets?
Main Belt Comets and activity in other unusual places
Dr Henry Hsieh, Planetary Science Institute, USA
Main-belt comets (MBCs) are a recently identified class of small solar system objects that exhibit comet-like mass loss indicative of ice sublimation, yet have orbits indistinguishable from those of main-belt asteroids. These objects have drawn significant interest since their surprising discovery in the main asteroid belt (previously believed to consist entirely of inert bodies) due to (1) the implication that MBC activity is driven by the sublimation of volatile material (presumed to be water ice), (2) indications that most MBCs likely formed in situ in the main asteroid belt, and (3) the possibility that icy bodies from the asteroid belt were a significant primordial source of the Earth's current water inventory. With MBCs as potential tracers of the distribution and abundance of ice in the asteroid belt, we now have a potentially powerful means for probing the plausibility of the latter hypothesis, and also for gaining insights into and placing constraints on the formation of our solar system in general. I will review recent and ongoing research efforts aimed at determining the physical, dynamical, and thermal properties of both individual MBCs and the population as a whole, with the ultimate objective of achieving an improved understanding of small icy bodies in the inner solar system. I will also discuss the prospects for and implications of finding unexpected cometary activity in members of other potentially icy small body populations currently believed to consist largely or entirely of inert objects.
Dr Diane Wooden
Comet dust is primitive and shows significant diversity. Our knowledge of the properties of primitive particles has expanded significantly through microscale investigations of cosmic dust samples (IDPs and AMMs) and of comet dust samples (Stardust and Rosetta's COSIMA), as well as through remote sensing (spectroscopy and imaging) via Spitzer and via spacecraft encounters with 103P/Hartley 2 and 67P/Churyumov-Gerasimenko. Microscale investigations show that comet dust and cosmic dust are particles of unequilibrated materials, including aggregates of materials unequilibrated at submicron scales. We call unequilibrated materials "primitive" and we deduce they were incorporated into ice-rich (H2O-, CO2-, and CO-ice) parent bodies that remained cold, i.e., into comets, because of the lack of aqueous or thermal alteration since particle aggregation; yet some Stardust olivines suggest mild thermal metamorphism. Primitive particles exhibit a diverse range of: structure and typology; size and size distribution of constituents; concentration and form of carbonaceous and organic matter; D-, N-, and O- isotopic enhancements over solar; Mg-, Fe-contents of the silicate minerals; the compositions and concentrations of sulfides, and of less abundant mineral species such as chondrules, CAIs and carbonates. The unifomity within a group of samples points to: aerodynamic sorting of particles and/or particle constituents; the inclusion of a limited range of oxygen fugacities; the inclusion or exclusion of chondrules; a selection of organics. The properites of primitive particles imply there were disk processes that resulted in different comets having particular selections of primitive materials. The diversity of primitive particles has implications for the diversity of materials in the protoplanetary disk present at the time and in the region where the comets formed.
Summary of discussions, placing the Rosetta results in context, and the next steps in cometary science
Professor Michael F. A'Hearn, University of Maryland, USA
Rosetta has brought a wealth of new data about composition, structure, and processes on 67P/Churyumov-Gerasimenko (C-G). At this point the implications of the data are still being sorted but many things are already clear. There is an emerging consensus that Rosetta confirms our long-standing claims that except for the very near surface layers, the bulk of the nucleus is pristine, i.e., it has not changed in structure or composition since formation roughly 4.5 Gy ago. The one exception is the surface and near surface region which has been changed substantially both by removal of the surface and by redeposition of material from one portion of the nucleus to another.
The measurement of D/H in C-G as being higher than in any other comet was the trigger for dynamicists to rethink the origin of Jupiter-family vis a vis Oort-cloud comets, such that they formed in a very extended region common to both dynamical classes. The detection of super-volatiles requires that the bulk interior have formed very cold and remained cold since formation. The nuclear surface exhibited many new and unexpected features – hard areas below the surface and at the surface elsewhere, long cracks, some of which look like thermal fracturing, while others could be mechanical stresses, surface changes in the form of subsidence in areas that rapidly expand radially, higher dust/ice ratios than many of us expected, and extremely high porosity (>70% in the bulk nucleus) with no evidence for macroscopic voids.
This review will place the results from Rosetta, including those presented at this meeting, in the larger context of the ensemble of comets and look at where we go from here.