914 research outputs found
Silicon Tetrafluoride on Io
Silicon tetrafluoride (SiF4) is observed in terrestrial volcanic gases and is
predicted to be the major F - bearing species in low temperature volcanic gases
on Io (Schaefer and Fegley, 2005b). SiF4 gas is also a potential indicator of
silica-rich crust on Io. We used F/S ratios in terrestrial and extraterrestrial
basalts, and gas/lava enrichment factors for F and S measured at terrestrial
volcanoes to calculate equilibrium SiF4/SO2 ratios in volcanic gases on Io. We
conclude that SiF4 can be produced at levels comparable to the observed
NaCl/SO2 gas ratio. We also considered potential loss processes for SiF4 in
volcanic plumes and in Io's atmosphere including ion-molecule reactions,
electron chemistry, photochemistry, reactions with the major atmospheric
constituents, and condensation. Photochemical destruction (tchem ~ 266 days)
and/or condensation as Na2SiF6 (s) appear to be the major sinks for SiF4. We
recommend searching for SiF4 with infrared spectroscopy using its 9.7 micron
band as done on Earth.Comment: 16 pages, 2 figures, 1 table; Icarus, in pres
Predictions of the atmospheric composition of GJ 1132b
GJ 1132 b is a nearby Earth-sized exoplanet transiting an M dwarf, and is
amongst the most highly characterizable small exoplanets currently known. In
this paper we study the interaction of a magma ocean with a water-rich
atmosphere on GJ 1132b and determine that it must have begun with more than 5
wt% initial water in order to still retain a water-based atmosphere. We also
determine the amount of O2 that can build up in the atmosphere as a result of
hydrogen dissociation and loss. We find that the magma ocean absorbs at most
~10% of the O2 produced, whereas more than 90% is lost to space through
hydrodynamic drag. The most common outcome for GJ 1132 b from our simulations
is a tenuous atmosphere dominated by O2, although for very large initial water
abundances atmospheres with several thousands of bars of O2 are possible. A
substantial steam envelope would indicate either the existence of an earlier H2
envelope or low XUV flux over the system's lifetime. A steam atmosphere would
also imply the continued existence of a magma ocean on GJ 1132 b. Further
modeling is needed to study the evolution of CO2 or N2-rich atmospheres on GJ
1132 b.Comment: 14 pages, 11 figures, accepted at Ap
Godunov-Type Upwind Flux Schemes of the Two-Dimensional Finite Volume Discrete Boltzmann Method
A simple unified Godunov-type upwind approach that does not need Riemann solvers for the flux calculation is developed for the finite volume discrete Boltzmann method (FVDBM) on an unstructured cell-centered triangular mesh. With piecewise-constant (PC), piecewise-linear (PL) and piecewise-parabolic (PP) reconstructions, three Godunov-type upwind flux schemes with different orders of accuracy are subsequently derived. After developing both a semi-implicit time marching scheme tailored for the developed flux schemes, and a versatile boundary treatment that is compatible with all of the flux schemes presented in this paper, numerical tests are conducted on spatial accuracy for several single-phase flow problems. Four major conclusions can be made. First, the Godunov-type schemes display higher spatial accuracy than the non-Godunov ones as the result of a more advanced treatment of the advection. Second, the PL and PP schemes are much more accurate than the PC scheme for velocity solutions. Third, there exists a threshold spatial resolution below which the PL scheme is better than the PP scheme and above which the PP scheme becomes more accurate. Fourth, besides increasing spatial resolution, increasing temporal resolution can also improve the accuracy in space for the PL and PP schemes
Atmospheric Chemistry of Venus-like Exoplanets
We use thermodynamic calculations to model atmospheric chemistry on
terrestrial exoplanets that are hot enough for chemical equilibira between the
atmosphere and lithosphere, as on Venus. The results of our calculations place
constraints on abundances of spectroscopically observable gases, the surface
temperature and pressure, and the mineralogy of the surface. These results will
be useful in planning future observations of the atmospheres of
terrestrial-sized exoplanets by current and proposed space observatories such
as the Hubble Space Telescope (HST), Spitzer, James Webb Space Telescope
(JWST), Terrestrial Planet Finder, and Darwin.Comment: 35 pages, 4 figures, 3 tables; 1 appendix; submitted to ApJ; version
A Unified and Preserved Dirichlet Boundary Treatment for the Cell-Centered Finite Volume Discrete Boltzmann Method
A new boundary treatment is proposed for the finite volume discrete Boltzmann method (FVDBM) that can be used for accurate simulations of curved boundaries and complicated flow conditions. First, a brief review of different boundary treatments for the general Boltzmann simulations is made in order to primarily explain what type of boundary treatment will be developed in this paper for the cell-centered FVDBM. After that, the new boundary treatment along with the cell-centered FVDBM model is developed in detail. Next, the proposed boundary treatment is applied to a series of numerical tests with a detailed discussion of its qualitative and quantitative properties. From the results, it can be concluded that the new boundary treatment is at least first-order accurate for a variety of Dirichlet boundary conditions (BCs). It can handle both the velocity and density Dirichlet BCs in a unified way and further realize some BCs that the conventional lattice Boltzmann model fails to simulate. In addition, such a boundary treatment can incorporate different lattice models without changing its framework, and it can preserve the Dirichlet BCs up to machine accuracy in different situations
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