P13C-1327
Geodynamics and Rate of Volcanism on Massive Earth-like Planets
We provide estimates of volcanism versus time for planets with Earth-like composition and masses up to 25 times Earth, as a step toward predicting atmospheric mass on extrasolar rocky planets. We use a thermal evolution model, calibrated against Earth, in combination with standard melting models, to explore the dependence of convection-driven decompression mantle melting on planet mass. We find:- (1) volcanism is likely to proceed on massive planets with plate tectonics over the main-sequence lifetime of the parent star; (2) crustal thickness (and melting rate normalized to planet mass) is weakly dependent on planet mass; (3) stagnant lid planets can have higher rates of melting than their plate tectonic counterparts early in their thermal evolution, but melting shuts down after a few Gyr; (4) plate tectonics may not operate on high mass planets because of their production of buoyant crust which is difficult to subduct; and (5) melting is necessary but insufficient for efficient volcanic degassing; volatiles partition into the earliest, deepest melts, which may be denser than the residue and sink to the base of the mantle on young, massive planets. Magma must also crystallize at or near the surface, and the pressure of overlying volatiles must be fairly low, if volatiles are to reach the surface.
P13C-1328
Thermal structure and evolution of tidally-locked Super Earths
Over 260 extrasolar planets have been discovered, many of which are massive (often many times the mass of Jupiter) and orbiting very close to their parent star. Of particular interest to researchers, however, are the handful of discovered planets that are within 20 Earth masses, due to their potential for habitability. We present a model of the internal temperature structure of such tidally-locked 'Super Earth' exoplanets. The planets of interest have a terrestrial, rocky composition, with a hot side facing its star at all times, and a cold side facing away. Heat circulation through an atmosphere is assumed to be negligible due to the planets' proximity to the star, which causes potential atmospheres to be evaporated; therefore, the primary modes of heat transfer within the planet are convection and conduction, with absorption on the hot side of the flux from the star, and black-body radiation to space in all directions. We have modified a spherical axisymmetric version of the finite element code ConMan (SSAXC), which was first created to model the internal thermal evolution of Earth's mantle. The results from this code are plotted and a thermal profile, with potentially molten rock on one side and very cool rock on the other with a temperature gradient connecting the two, allows us to determine where potentially habitable regions would exist on the planet. Finally, an approximation of what typical mantle rocks would be melted at predicted temperatures and pressures are compared with these plotted internal gradients.
P13C-1329
Magma Ocean Lifetimes
The early evolution of a terrestrial planet may play an important role in its subsequent thermal evolution. Terrestrial super-Earths are expected to evolve similarly to terrestrial planets: quickly cooling from a magma ocean state, in which they experience high surface heat fluxes and temperatures. We study the solidification of magma oceans in terrestrial super-Earths and determine how long this phase lasts in the presence and absence of an atmosphere (grey and water-vapor). In the blackbody approximation the timescales are of order 105 years, while in a grey approximation the timescales can be extended to a few million years for an optically thick atmosphere (τ =100). We find that a 10 earth-mass planet takes about twice as long to cool as a one earth-mass planet. This early epoch is the most amenable to direct observation due to the high planetary heat flux; depending on how long it lasts it might be possible to directly detect super-Earths in this hot state. Development of more sophisticated atmospheric models may allow the inference of outgassing products from spectral signatures.
P13C-1330
Chemistry and Structure of Super-Earths
The chemical composition and structure of super-Earths can be understood from data and models for the chemical evolution of our galaxy, our solar system and models of formation and interiors of planets. Because the relative proportions of most rock-forming elements in stars of our galaxy are rather constant the mineralogy of solid grains that formed in the proto-planetary disks can be estimated from a calculation of condensation of a solar composition gas. Such a calculation shows that the minerals forsterite (Fo) and FeNi metal are the two most abundant solids from which Earth-like planets accrete, and therefore studying the behavior of Fo-metal mixtures at ultrahigh pressures (5-10 TPa) and temperatures is essential for modeling the internal structures of the largest super-Earths. The most critical parameter is the metal/silicate ratio of the planet. What determines the metal to silicate ratio is not well established, but it is most likely related to the amount of metal oxidized by water in planetesimals. Earth-like planets are, by definition, volatile depleted, but do have variable patterns of intermediate to highly volatile elements that are of secondary importance for understanding the chemistry of super-Earths. The deep interiors of the largest super-Earths may be in a WDM (warm dense matter) state which is an atomic fluid that may dissolve most components normally residing in the silicate mantle. We are trying to characterize this state using the high energy density lasers (ZBL and Z-Petawatt) at Sandia National Laboratories. Models of such planets with atomic fluid cores may be substantially different from the standard super-Earth models that have been discussed up to now. In anticipation of the measurement of radii (R) and masses (M) of super-Earths in the coming years (e.g., with NASA's Kepler mission) the standard super-Earth models yielded a scaling law of R ∝ M0.262 - 0.274 for Super-Earths. This law may have to be modified for planets with WDM cores.
P13C-1331
Inferring the Composition of a super-Earth
In anticipation of the numerous transiting super-Earths that will be detected in the near future, we have constructed a tool to infer the internal composition of super-Earths given their masses and radii. This publicly available tool is based on the model constructed by Valencia et al. and considers planets with variable amounts of Fe/Si ratio, H2O content and the addition of a Hydrogen/Helium envelope above the solid interior. Different compositional ensembles can fit the same mass and radius values. We present the interactive interface we developed that shows all possible compositions that fit the measurements, including uncertainties that come from both, model and data. Results show that the precision in radius affects the interpretation of the data more significantly than the precision in mass. On average, planet radius measurements to better than 5%, combined with mass measurements to better than 10%, are needed to distinguish between a volatile-rich or rocky composition.
P13C-1332
Snowball Planets: A Possible Type of Water-Rich Terrestrial Planet in Extrasolar Planetary Systems
Existence of liquid water on the planetary surface is essential for life. However, terrestrial planets with abundant water have multiple climate modes, including an ice-free, a partially ice-covered, and a globally ice- covered state, even when the incident flux from the central star and the abundance of greenhouse gasses in the atmosphere are the same. This multiplicity of climate mode is derived from large difference in the albedo of ice and water. Recent geological studies have revealed that the Earth experienced global glaciations ("snowball Earth" events) in its history. In the snowball glaciations, liquid water is thought to have existed under the ice shell because of geothermal heat flow from the Earthfs interior. By analogy with the snowball glaciations, I discuss the conditions for an extrasolar terrestrial planet which is covered with ice but has an internal ocean for the timescale of planetary evolution owing to geothermal heat flow from the planetary interior. I show that liquid water can exist if the planetary mass and the water abundance are comparable to the Earth, although a planet with a mass of <0.4 Me (Me is the Earth's mass) would not be able to maintain the internal ocean. Liquid water would be absolutely stable for a planet with a mass of >4 Me (i.e., super- Earth), irrespective of planetary orbit and luminosity of the central star. It is therefore implied that super-Earth inevitably have liquid water either on its surface (for the ice-freee or partially ice-covered modes) or beneath the ice (for the globally ice-covered mode). Searches for terrestrial planets in extrasolar planetary systems should consider such a "snowball planet", which is a possible type of water-rich terrestrial planet other than an Earth-like "ocean planet". Because a snowball planet is much brighter than (more than twice) an ocean planet with the same size, it would be a good target for the astronomical observation in the future.
P13C-1333
Ocean-bearing planets near the ice line: How far does the water's edge go?
A leading theory for giant planet formation involves the accretion of a solid core, probably ice-rich, that in turn accretes a massive mantle of hydrogen-helium gas from a primordial disk. The relative timing of core formation and disappearance of nebular gas in a few millions of years is critical; the correlation between heavy element abundance in stellar photospheres and their propensity to host giant planets is cited as support for the theory. Conversely, systems that are relatively heavy element-poor or lose their gas earlier should contain either "failed" cores or a set of icy planetary embryos that did not accrete. Indeed, Uranus and Neptune may represent similar embryos that were scattered by Jupiter into the outer disk where they efficiently accreted planetesimals rich in volatiles with low condensation temperatures. We propose that a region straddling the "snowline" (3-5~AU for solar-mass stars) could frequently be inhabited by one or more water ice-rich, super-Earth-mass objects that accreted only a modest amount of nebular gas. We predict that metal-poor bulge and halo stars are more likely to host such objects. Current and future microlensing surveys will be able to determine the population of Earth-mass planets in this range of semimajor axes and test this hypothesis. If they are sufficiently frequent, the nearest examples will be detectable by the Space Interferometer Mission and perhaps a visible-light Terrestrial Planet Finder mission. We show that retention of a ~1~bar hydrogen-helium atmosphere is sufficient to maintain a surface water ocean, depending on semimajor axis and thermal history, and that sufficiently massive, "naked" ice planets can have interior oceans a la Europa. Planets with more substantial (>200~bar) atmospheres will be devoid of a liquid water phase at the surface. The existence of a surface water ocean could be inferred by the absence of highly soluble molecules such as NH3 or SO2 in the atmosphere. Objects with such oceans, although outside the conventional habitable zone, could nevertheless conceivably support life.
P13C-1334
Climates of Oblique Exoplanets
A previous paper (Dobrovolskis 2007; Icarus 192, 1-23) showed that eccentricity can have profound effects on the climate, habitability, and detectability of extrasolar planets. This complementary study shows that obliquity can have comparable effects. The known exoplanets exhibit a wide range of orbital eccentricities, but those within several million km of their suns are generally in near-circular orbits. This fact is widely attributed to the dissipation of tides in the planets, which is particularly effective for solid/liquid bodies like "Super-Earths". Along with friction between a solid mantle and a liquid core, tides also are expected to despin a planet until it is captured in the synchronous resonance, so that its rotation period is identical to its orbital period. The canonical example of synchronous spin is the way that our Moon always keeps nearly the same hemisphere facing the Earth. Tides also tend to reduce the planet's obliquity (the angle between its spin and orbital angular velocities). However, orbit precession can cause the rotation to become locked in a "Cassini state", where it retains a nearly constant non-zero obliquity. For example, our Moon maintains an obliquity of about 6.7° with respect to its orbit about the Earth. For comparison, stable Cassini states can exist for practically any obliquity up to 180° for planets of binary stars, or in multi-planet systems with high mutual inclinations, such as are produced by scattering or by the Kozai mechanism. This work considers planets in synchronous rotation with circular orbits. For obliquities greater than 90°, the ground track of the sub-solar point wraps around all longitudes on the surface of such a planet. For smaller obliquities, the sub-solar track takes the figure-8 shape of an analemma. This can be visualized as the intersection of the planet's spherical surface with a right circular cylinder, parallel to the spin axis and tangent to the equator from the inside. The excursion of the sub-solar point in latitude is equal to the obliquity β, while the corresponding libration in longitude is smaller (±arcsin(tan2(β/2))). Obliquity thus affects the distribution of insolation over the planet's surface, particularly near its poles. For β = 0, one hemisphere bakes in permanent sunshine, while the opposite hemisphere experiences eternal darkness. As β increases, the region of permanent daylight and the antipodal realm of endless night both shrink, while a more temperate area of alternating day and night spreads in longitude, and especially in latitude. The regions of permanent day or night disappear at β = 90°. The insolation regime passes through several more transitions as β continues to increase toward 180°, but the surface distribution of insolation remains non-uniform in both latitude and longitude.