P54A-01
Earth Orbit, Period, and Temperature - Past and Future
The Earth climate, its recent history and near future (+/- 100 years) are under intense scrutiny because of temperature changes ascribed to green house effects. However, long-term temperature changes since the formation of the solar system (-4.5Byr) and for the extended future are not known. The present paper addresses the magnitude and the rate of Earth temperature changes for this time range, and compares the results to present observations. The model is based on the cohesion of the solar system which is determined by mass (solar radiative and solar wind)and gravity loss since the formation of the solar system, and the resulting expansion of planetary orbits.(1) This model has previously successfully predicted the why and when of the transition from water to ice on Mars. (2) After the formation of the solar system (-4.5 Byr) the Earth orbit was at 1.38E08 km (presently 1.50E08 km) and the Earth period was 0.89 years. In the future (+4.5 Byr), they are predicted to be 1.64E08 km and 1.15 yr. At -4.5 Byr, present temperatures of -50, zero, and + 50 C were higher at -40.5, 11.7, and 63.9 C, respectively. It is predicted that in +4.5 Byr these temperatures will have decreased to -60.0, -12.2, and 35.5 C. In the past million years, the present -50, 0, and +50 C temperatures were about 0.03C higher, and will be about 0.03 C lower in another million years. These results indicate that temperature changes due to changes of solar-Earth orbital interactions do not significantly contribute to the observed Earth global warming observations. (1) I. H. Leubner, ‘Stability of planetary Orbits', AGU 2006 Fall Meeting, San Francisco, Ca, Dec. 11-15, 2006 (2) I. H. Leubner, ‘Mars Orbit and Temperature: Why and When an Early wet Mars', AGU 2004 Fall Meeting, San Francisco, Ca, Dec. 14, 2004 http://www.RocherResearch.us
P54A-02
Mass- radius relationships for Super-Earths and Water-rich planets: an estimate of the uncertainties.
The investigation of planetary systems opens the perspective of discovering new types of planets, unknown in our Solar System. In particular, the recent discovery of exoplanets with intermediate masses (5 Earth masses or more) is a confirmation of the existence of Super-Earths. One important aspect of these planets is that they are potentially habitable, in the sense of bearing a significant amount of liquid surface water. In addition, recent planet formation models predict that intermediate mass planets exist at various distances from their parent star. These models predict a substantial migration extent for the planets, which therefore are likely to start their formation beyond the ice-line. Hence, they contain a large amount of water. Our goal is to investigate if the confrontation of future data to current understanding of M-R relationships will allow for the distinction between these two families. We have studied the relationship between mass and radius up to 100 Earth mass. The bulk composition of the planetary body is described using four parameters: [Fe/Si], [Mg/Si], the silicate Mg number, and the amount of water. The model allows us to investigate a large range of putative exoplanets depending on their silicate composition, the amount of water, and temperature conditions. The accuracy of M-R relationships is determined by investigating the dependency of M-R scaling laws on compositional effects, temperature variation, and the choice of equation of state. It will be shown that important radii variations are observed depending on the chosen parameters and equation of state, which induces large uncertainties on M-R relationships above 20 Earth masses. Nonetheless, a distinction between these two families and planets with thick atmospheres such as Uranus will be possible.
P54A-03
Evolution of Super-Earths
Super-Earths are the newest class of planets discovered with masses in the 1-10 M\oplus range. Determining their physical properties is of interest to better design detection projects and to further our understanding of Earth. Additionally, among them we might find true Earth analogs. To understand their physical properties it is crucial to understand their structure and evolution. We model the evolution of super-Earths from their hot initial state of magma ocean convection to the transition into plate tectonics and eventually stagnant lid. The timescales between the different states depend on the mass of the planet which determines the size, density, gravity, heat flow and therefore, the Rayleigh number. The magma ocean state is the most favorable for direct detection (TPF, Darwin) because of the high surface temperatures due to large mantle convective transport and the presence of a thick atmosphere. The transition to plate tectonics is possible due to the large convective stresses underneath thin lithospheric plates, with both being a result of large Rayleigh numbers. For a given planetary mass there is a threshold heat flow limit below which the convective stresses are not enough to overcome the plate's resistance to deformation and plate tectonics is unlikely to occur. Super-Earths above this threshold are likely to exhibit plate tectonics and therefore are good candidates in the search of habitable planets.
P54A-04
Tidal Heating Susceptibility in Short Period Terrestrial Exoplanets
We have analyzed the general tidal heating of terrestrial class exoplanets to identify the orbital range where tides dominate planetary heat flux and to analyze possible limiting mechanisms. Calculations show that for Earth- mass planets in short period orbits similar to those of the Hot Jupiters (roughly 1 to 20 days), the potential exists for extreme tidal heating many orders of magnitude beyond what is observed in our solar system, and far in excess of each given planet's radiogenic heating. The long-term eccentricities needed to support such tides may come from indirect secular perturbations or directly from mean-motion resonances with Hot Jupiters. In particular, a 2:1 resonance between a gas giant and terrestrial companion has the potential to create a unique type of supertidal world. We compare the results of various basic methods for estimating tidal heating, including classical frequency independent methods as well as frequency and temperature dependent viscoelastic methods based on Maxwell and Burgers rock rheologies. Equilibrium surface and interior temperatures have been calculated using parameterized convection models for both hypothetical planets and observed exoplanets such as GJ876d, GJ581c and GJ581d. While hotspots are likely, we find that tidal heating will have a negligible impact on the global surface temperatures of these planets unless they exist in unusually low insolation environments such as around red dwarf stars. Results show that equilibrium tidal heat production for Earth-mass planets in the range of Earth's current radiogenic and secular cooling background heat flow (40 TW) results in partial melting at the base of an Earth-like lithosphere. We find the subsequent advection of segregated melt from this layer is insufficient to produce a surface magma ocean below a tidal forcing of 300,000 TW and may thus represent a stable layer configuration. High tidal heat outputs such as this are predicted by models without melting, but may be suppressed by the onset and growth of partial melting and the resulting drop in overall planetary viscosity. The effects of various phenomenology central to these planets such as heliocentric spin-orbit resonances and atmospheric stability are also addressed.
P54A-05
Interiors of the Four Giant Planets
We apply the method of gravity sounding that we used for Saturn (Anderson and Schubert, Science 317, 7 September 2007) to the solar system's other three giant planets and obtain an empirical equation of state EOS for each. We suggest that each EOS so obtained can be used as a reference model for more detailed calculations that use the physics of materials thought to exist in the four giant planets. As expected the planets fall into two classes. The first for Jupiter and Saturn is characterized by a change in the EOS at about 3 Mbar half way into the planet, where the transition from molecular to metallic hydrogen occurs. In the second class for Uranus and Neptune large cores are inferred from the gravitational data. Even though the outer planets break down into two classes, their bulk properties suggest that they each have unique characteristics related to their formation. The mean densities in units of kg per cubic meter are 1327 for Jupiter, 686 for Saturn, 1177 for Uranus and 1552 for Neptune. The respective mean radii are 69,894 km, 58,255 km, 26,017 km, and 25,066 km. The normalized mean moment of inertia (2/5 for a uniform sphere) can be obtained from the inferred density distribution, along with a mapping of the errors in the three measured zonal gravitational coefficients through the software system used for the gravity sounding. The results are 0.265668(3) for Jupiter, 0.222794(35) for Saturn, 0.216196(4) for Uranus, and 0.230700(250) for Neptune, where the numbers in parentheses indicate the standard error in the last place. The EOS of the planet in the outer envelope of all four planets can be approximated by a polytrope with index of 1.795 for Jupiter, 1.631 for Saturn, 2.023 for Uranus, and 1.842 for Neptune. This EOS is valid to a pressure of about 100 kbar. The pressure at a density level of 100 kg per cubic meter is 28.10 kbar for Jupiter, 19.17 kbar for Saturn, 7.22 kbar for Uranus, and 6.40 kbar for Neptune. Depths at the 100 kbar pressure level, the limit of validity of the polytrope, are 4890 km for Jupiter, 8160 km for Saturn, 5070 km for Uranus and 3885 km for Neptune.
P54A-06
Formation of Jupiter via Core Nucleated Accretion Within a Dissipating Protoplanetary Disk
We model the effect of gradual dissipation of the protoplanetary disk on Jupiter's growth. The planet's structure is computed using a Henyey-type stellar evolution code. Previous simulations of this process have taken the radius of the planet to be approximately one Hill sphere radius, RH. Recent 3-D hydrodynamic simulations show that only gas within about 0.25 RH remains bound to the planet, with the more distant gas participating in the shear flow of the protoplanetary disk. Therefore, we computed Jupiter models for which the planet's radius is 0.25 RH. Results indicate that the smaller radius increases the time required for planetary growth by ~ 15%. Observations suggest that the typical lifetime of massive disks around young stellar objects is ~ 3 Myr. To account for the dissipation of such disks, we computed Jupiter models for which the surface gas density is decreased on this time scale and the maximum rate of gas flow onto the planet is taken from 3-D hydrodynamic simulations. Results of these simulations will also be presented.
P54A-07
Consistent Kinetics for Hydrocarbon Synthesis in Outer Planet Atmospheres
Recombination reactions with low energy barriers are important steps to form larger hydrocarbons in reducing atmospheres such as Titan, and statistical rate theory calculations are applied here to generate low pressure and temperature rate constants consistent with other laboratory determinations and calculated potential surface parameters. Multiple decomposition channels, sometimes leading to smaller products, must be considered. Tunneling plays an important role in H atom additions to unsaturated species. Results will be presented for the formation of benzene, butene, allyl radical, vinyl radical, and the CN + C2H4 / H + C2H3CN / HCN + C2H3 reaction system. Research supported by NASA Outer Planets Research Program.
P54A-08
The Runaway Greenhouse on Massive or Tide-Locked Extrasolar Planets
The discovery of a "large Earth" in Gliese 581 has sparked considerable interest in the habitability of such planets, and the effect of various hypothetical atmospheres on habitability. A vast range of fundamental climate issues remain to be studied, particularly with regard to the long-term climate evolution and lifetime of biosphere. In this contribution I will discuss the implications of the large surface gravity and tide-locked rotation on the runaway greenhouse phenomenon which would pose a habitability crisis if the planet has a substantial water reservoir. In particular, I will show that high surface gravity increases the Kombayashi-Ingersoll limit and makes it harder for a runaway greenhouse to occur; moreover, the slope of the fundamental energy balance curve makes it easier for such a planet to remain habitable over a wide range of stellar luminosity, even in the absence of a CO2 weathering thermostat. The reasoning is elementary, and relies largely on the fact that the surface pressure of water vapor is determined by surface temperature via Clausius-Clapeyron, while the corresponding mass (which determines infrared opacity) is related to the surface pressure divided by surface gravity, in accord with hydrostatics. I will also make some remarks about the novel aspects of the operation of the greenhouse effect in cases like Gliese 581, resulting from the substantial infrared component of the stellar radiation. Finally, I will point out that the runaway greenhouse threshold is determined by the global energy balance, not the dayside energy balance, owing to the strong heat transports that would result from the pressure gradient implicit in a "dayside only" runaway. Prospects for a nightside glaciation with dayside torrid zone, separated by a habitable zone near the ice margin at the terminator, will be discussed. The relavant physics is demonstrated using a range of simplified climate models.