| アブストラクト |
Rocky and icy exoplanets are as yet poorly characterized. Nonetheless analogies with bodies in our own solar system may prove helpful in interpreting forthcoming data. In this talk, I will focus on three processes: accretion; heat transfer; and tides.
Accretion is important for four reasons. Via the release of gravitational energy, it exerts a strong control on the initial temperature and volatile inventory of the body. Because accretion is not 100% efficient, it can cause variations in the body’s bulk composition; the anomalous compositions of Mercury and the Moon are probably the result of accretionary processes. Ac- cretion can control the body’s initial spin and orbital states (Uranus is a possible example). And finally, smaller impacts post-dating the main accretionary and nebular blowoff epoch can have additional effects on bulk composition and (especially) atmospheric evolution.
There are four main sources of energy available to heat exoplanets. For close-in “roasters”, stellar luminosity may be dominant, yielding an equilibrium surface temperature in some cases above the silicate solidus. Close-in planets may also experience tidal heating, at a rate dependent on their internal structure (Io is a good example). Finally, radionuclides, both short- and long- lived, and gravitational energy during accretion and differentiation, will always play a role irrespective of semi-major axis.
In the first few Myrs, the dominant heat sources are accretion and (if present) short-lived nuclides. The heating rates generally result in prodigious melting and the formation of magma oceans. The lifetime of such oceans depends on whether and when they develop a flotation crust and/or thick atmosphere. Advection of heat by melting is a very efficient cooling mechanism; as long as melts are less dense than their solid surroundings, planets will cool relatively rapidly (tens of Myr) to a temperature profile approximating the solidus.
Over 100 Myr timescales, sub-solidus convection likely transfers most heat, and is roughly balanced by long-term radiogenic heat production. The details depend on whether plate tec- tonics operates or not, which appears to be sensitive to factors such as water content (consider Earth vs. Venus) and is hard to predict on purely theoretical grounds. The efficiency of melting and convection can be judged by measuring what fraction of the 40Ar produced has been out- gassed. Whether or not a planetary body develops a dynamo is usually thought to depend on how rapidly heat is extracted out of the core into the overlying mantle.
In many ways, the best analogues to close-in synchronous exoplanets are the satellites of our solar system. For instance, satellite formation and migration in the presence of a gaseous disk is probably similar to exoplanet formation, and results in similar mass ratios. However, the distribution of angular momentum is quite different between exoplanet and satellite systems.
In solar system satellites, tides can play a major role in both thermal evolution and orbit/spin state. For bodies in resonance, thermal evolution need not be monotonic (Enceladus is a possible example) and eccentricities will be time-dependent. In some cases, the orbital characteristics of a resonance may be used to directly ascertain interior characteristics of a planet/satellite such as its Love number. If a tidally-locked body occupies a Cassini state, information about its interior mass distribution may be derived by measuring its obliquity, and obliquity tides may result in heating and inclination damping. |
