Dwarf Planets as the Most Populous Class of Planet
Dwarf planets should form whenever the surface density of a protoplanetary disk is low enough, and as a transient stage during planet formation in more massive disks. In terms of physical attributes (hydrostatic shape, presence of atmospheres, internal oceans, active geology, satellites) there is no clear dividing line bewteen dwarf planets and larger, "regular" planets. In our Solar System, all presently recognized dwarf planets (Eris, Pluto, Haumea, Makemake, Ceres) and former dwarf planets (Triton) are icy, although whether Ceres is a differentiated ice-rich body or a somewhat porous, hydrated rocky body can be debated. Regardless, it is only a matter of time (and data) before the dwarf planets outnumber the 8 "classical" planets. In this talk I will review the question of dwarf planet composition in the Kuiper Belt, including the key role of the solar C/O ratio, the evidence for differentiation (rock core formation) and compositional diversity among these bodies, and the possibility for active cryovolcanism such as may be observed by the New Horizons mission when it reaches the Pluto system in 2015.
Icy Dwarf Planet Surface Compositions
With the discovery of each additional small icy planet in the outer Solar System, opportunities for comparative planetology expand. This talk will focus on the surfaces of these bodies, investigating what recent observational data have to say about their compositions and the processes which act on them. Visible and infrared reflectance spectroscopy offers an especially valuable probe of surface compositions, textures, and thermal states and will thus be the primary focus of this talk, although other observational constraints will also be mentioned. It is useful to compare objects within broad compositional classes. One such class consists of worlds having surface veneers of CH4 and other volatile ices which evolve on seasonal time scales and are unstable to photolytic and radiolytic degradation. Another has surfaces composed of nearly pure, crystalline H2O ice, at temperatures cold enough that amorphous ice might be expected to be the dominant phase. Still others have surfaces apparently composed of dark, inert, organic and silicate materials. Differences and similarities between members of these compositional classes offer clues to their origins and to the processes which modify their surfaces.
Time of Formation and Chemical Alteration of Planetesimals, Icy Satellites, and Dwarf Planets in the Outer Solar System
We consider various scenarios for the early chronology of outer solar system icy objects (e.g., planetesimals, satellites, dwarf planets) depending on the time at which these objects formed with respect to the production of calcium-aluminum inclusions. The latter is our time of reference for computing the amount of short-lived radioisotopes accreted in these objects. We especially focus on hydrothermal activity that could have taken place in icy planetesimals and the consequences on the early history of bigger objects depending on the time and duration of accretion, i.e. whether or not short-lived radioisotopes were still in significant abundance in planetesimals when icy satellites and dwarf planets formed. Chemical alteration as a result of 26Al-triggered differentiation has been studied in the case of meteorite parent bodies, but the consequences of such a phenomenon in the case of outer Solar system objects has not been thoroughly addressed. However, various recent observations suggest that the outer Solar system could have formed in a few My after the beginning of the Solar system. In such conditions meteorite parent bodies and icy objects (from planetesimals to large icy objects) could have had a similar early history. Early melting is accompanied by hydrothermal circulation and resulting aqueous alteration and redistribution of major elements between the rock phase and the volatile phase. This can result in partial hydration of the silicate phase, formation of salt compounds in small objects from which molecular hydrogen can easily escape, as well as leaching of long-lived radioisotopes from the rock phase. Melting can also result in the destabilization of clathrate hydrates and thus degassing of major species predicted by cosmochemical models, with implications for the diversity of compositions of planetesimals in the early outer Solar System. We consider several classes of planetesimals, characterized by their size, time of formation, initial rock mass fraction, and volatile composition. The smallest ones are not affected by short-lived radioisotope decay. The medium-sized planetesimals (in the 5-20 km range) are affected by partial melting, while planetesimals several tens of km in radius could be fully differentiated before they accreted into larger objects. We expect that the disruption of differentiated planetesimals during the accretion could lead to chemical segregation. All these processes can have consequences for the long-term evolution of the larger bodies (e.g. salts affect the melting temperature of ice shells, hydrated silicates affect heat transfer through rocky cores). We will present some implications relevant to meteorite parent bodies, small icy satellites, and dwarf planets. Acknowledgements: This work has been conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Copyright 2009 California Institute of Technology. Government sponsorship acknowledged.
Pluto Insolation and the South Polar Cap
Pluto's south polar cap is a puzzle. The planet's southern cap may be brighter than the north, even though it was the south pole which faced the Sun on Pluto's recent approach to perihelion. One would think that the brighter pole would be the one which received less insolation: volatiles would be expected to sublimate from the sunny south and condense in the north, enlarging the north polar cap with fresh and bright frost. Thus the north pole should be brighter than the south. However, it may be the other way around, although the evidence is not entirely clear. One suggested explanation of the (possible) paradox is that the south polar cap has, over the last several million years, received less insolation than the north, accumulating a larger supply of volatiles. However, expressing the solar insolation in terms of Pluto's orbital elements clearly shows that both the north and south poles have received nearly the same amount of sunlight over the past several million years. Hence any difference between the polar caps cannot be ascribed to a difference in the amount of long-term insolation received at each pole. Thus any difference between the poles, if there is one, must invoke conditions peculiar to Pluto's climate system, rather than rely on insolation alone.