248 research outputs found
Solid state convection models of lunar internal temperature
Thermal models of the Moon were made which include cooling by subsolidus creep and consideration of the creep behavior of geologic material. Measurements from the Apollo program on seismic velocities, electrical conductivity of the Moon's interior, and heat flux at two locations were used in the calculations. Estimates of 1500 to 1600 K were calculated for the temperature, and one sextillion to ten sextillion sq cm/sec were calcualted for the viscosity of the deep lunar interior
The Survival Rate of Ejected Terrestrial Planets with Moons
During planet formation, a gas giant will interact with smaller protoplanets
that stray within its sphere of gravitational influence. We investigate the
outcome of interactions between gas giants and terrestrial-sized protoplanets
with lunar-sized companions. An interaction between a giant planet and a
protoplanet binary may have one of several consequences, including the delivery
of volatiles to the inner system, the capture of retrograde moons by the giant
planet, and the ejection of one or both of the protoplanets. We show that an
interesting fraction of terrestrial-sized planets with lunar sized companions
will likely be ejected into interstellar space with the companion bound to the
planet. The companion provides an additional source of heating for the planet
from tidal dissipation of orbital and spin angular momentum. This heat flux
typically is larger than the current radiogenic heating of the Earth for up to
the first few hundred million years of evolution. In combination with an
atmosphere of sufficient thickness and composition, the heating can provide the
conditions necesary for liquid water to persist on the surface of the
terrestrial mass planet, making it a potential site for life. We also determine
the possibility for directly detecting such systems through all-sky infrared
surveys or microlensing surveys. Microlensing surveys in particular will
directly measure the frequency of this phenomenon.Comment: 4 pages, 2 figures, Accepted to ApJ
Studies of heat source driven natural convection
Natural convection energy transport in a horizontal layer of internally heated fluid with a zero heat flux lower boundary, and an isothermal upper boundary, has been studied. Quantitative information on the time-mean temperature distribution and the fluctuating component of temperature about the mean temperature in steady turbulent convection are obtained from a small thermocouple inserted into the layer through the upper bounding plate. Data are also presented on the development of temperature at several vertical positions when the layer is subject to both a sudden increase and to a sudden decrease in power input. For changes of power input from zero to a value corresponding to a Rayleigh number much greater than the critical linear stability theory value, a slight hysteresis in temperature profiles near the upper boundary is observed between the heat-up and cool-down modes
Structure and phase boundaries of compressed liquid hydrogen
We have mapped the molecular-atomic transition in liquid hydrogen using first
principles molecular dynamics. We predict that a molecular phase with
short-range orientational order exists at pressures above 100 GPa. The presence
of this ordering and the structure emerging near the dissociation transition
provide an explanation for the sharpness of the molecular-atomic crossover and
the concurrent pressure drop at high pressures. Our findings have non-trivial
implications for simulations of hydrogen; previous equation of state data for
the molecular liquid may require revision. Arguments for the possibility of a
order liquid-liquid transition are discussed
The viscosity of Miranda
Voyager 2 images of Miranda revealed a significant history of geological activity. Overlying an apparently ancient cratered terrain are assemblages of concentric ridges, scarps, and dark banded material. The problems that evolutionary thermal and structural modes of Miranda must face, to provide a convincing explanation for such topographic complexity, are examined
Protostellar disks and the primitive solar nebula
The objective is to obtain quantitative information on the turbulent transport of mass, angular momentum, and energy under the conditions that characterize the solar nebula, by direct numerical calculations. These calculations were made possible by research conducted on supercomputers (Cray XMP and Cray 2) by the Ames Computational Fluid Dynamics Branch. Techniques were developed that permitted the accurate representation of turbulent flows over the full range of important eddy sizes. So far, these techniques were applied (and verified) primarily in mundane laboratory situations, but they have a strong potential for astrophysical applications. A sequence of numerical experiments were conducted to evaluate the Reynold's stress tensor, turbulent heat transfer rate, turbulent dissipation rate, and turbulent kinetic energy spectrum, as functions of position, for conditions relevant to the solar nebula. Emphasis is placed on the variation of these properties with appropriate nondimensional quantities, so that relations can be derived that will be useful for disk modeling under a variety of hypotheses and initial conditions
The potential for tidally heated icy and temperate moons around exoplanets
Moons of giant planets may represent an alternative to the classical picture
of habitable worlds. They may exist within the circumstellar habitable zone of
a parent star, and through tidal energy dissipation they may also offer
alternative habitable zones, where stellar insolation plays a secondary, or
complementary, role. We investigate the potential extent of stable satellite
orbits around a set of 74 known extrasolar giant planets located beyond 0.6 AU
from their parent stars - where moons should be long-lived with respect to
removal by stellar tides. Approximately 60% of these giant planets can sustain
satellites or moons in bands up to AU in width. For comparison, the
Galiean satellites extend to AU. We investigate the stellar
insolation that moons would experience for these exoplanet systems, and the
implications for sublimation loss of volatiles. We find that between 15 and 27%
of {\em all} known exoplanets may be capable of harboring small, icy, moons. In
addition, some 22-28% of all known exoplanets could harbor moons within a
``sublimation zone'', with insolation temperatures between 273 K and 170 K. A
simplified energy balance model is applied to the situation of temperate moons,
maintained by a combination of stellar insolation and tidal heat flow. We
demonstrate that large moons (M), at orbital radii
commensurate with those of the Galilean satellites, could maintain temperate,
or habitable, surface conditions during episodes of tidal heat dissipation of
the order 1-100 times that currently seen on Io. (Abridged).Comment: 28 pages, 8 Figures, AASTex, Accepted for publication in the
Astrophysical Journa
Nonlinear Outcome of Gravitational Instability in Disks with Realistic Cooling
We consider the nonlinear outcome of gravitational instability in optically
thick disks with a realistic cooling function. We use a numerical model that is
local, razor-thin, and unmagnetized. External illumination is ignored. Cooling
is calculated from a one-zone model using analytic fits to low temperature
Rosseland mean opacities. The model has two parameters: the initial surface
density Sigma_0 and the rotation frequency Omega. We survey the parameter space
and find: (1) The disk fragments when t_c,eff Omega = 1, where t_c,eff is an
effective cooling time defined as the average internal energy of the model
divided by the average cooling rate. This is consistent with earlier results
that used a simplified cooling function. (2) The initial cooling time t_c0 or a
uniform disk with Q = 1 can differ by orders of magnitude from t_c,eff in the
nonlinear outcome. The difference is caused by sharp variations in the opacity
with temperature. The condition t_c0 Omega = 1 therefore does not necessarily
indicate where fragmentation will occur. (3) The largest difference between
t_c,eff and t_c0 is near the opacity gap, where dust is absent and hydrogen is
largely molecular. (4) In the limit of strong illumination the disk is
isothermal; we find that an isothermal version of our model fragments for Q <
1.4. Finally, we discuss some physical processes not included in our model, and
find that most are likely to make disks more susceptible to fragmentation. We
conclude that disks with t_c,eff Omega < 1 do not exist.Comment: 30 pages, 12 figure
Mercury
Prior to the flight of the Mariner 10 spacecraft, Mercury was the least investigated and most poorly known terrestrial planet (Kuiper 1970, Devine 1972). Observational difficulties caused by its proximity to the Sun as viewed from Earth caused the planet to remain a small, vague disk exhibiting little surface contrast or details, an object for which only three major facts were known: 1. its bulk density is similar to that of Venus and Earth, much greater than that of Mars and the Moon; 2. its surface reflects electromagnetic radiation at all wavelengths in the same manner as the Moon (taking into account differences in their solar distances); and 3. its rotation period is in 2/3 resonance with its orbital period. Images obtained during the flyby by Mariner 10 on 29 March 1974 (and the two subsequent flybys on 21 September 1974 and 16 March 1975) revealed Mercury's surface in detail equivalent to that available for the Moon during the early 1960's from Earth-based telescopic views. Additionally, however, information was obtained on the planet's mass and size, atmospheric composition and density, charged-particle environment, and infrared thermal radiation from the surface, and most significantly of all, the existence of a planetary magnetic field that is probably intrinsic to Mercury was established. In the following, this new information is summarized together with results from theoretical studies and ground-based observations. In the quantum jumps of knowledge that have been characteristic of "space-age" exploration, the previously obscure body of Mercury has suddenly come into sharp focus. It is very likely a differentiated body, probably contains a large Earth-like iron-rich core, and displays a surface remarkably similar to that of the Moon, which suggests a similar evolutionary history
Terrestrial Planet Formation in Disks with Varying Surface Density Profiles
The ``minimum-mass solar nebula'' (MMSN) model estimates the surface density
distribution of the protoplanetary disk by assuming the planets to have formed
in situ. However, significant radial migration of the giant planets likely
occurred in the Solar system, implying a distortion in the values derived by
the MMSN method. The true density profiles of protoplanetary disks is therefore
uncertain. Here we present results of simulations of late-stage terrestrial
accretion, each starting from a disk of planetary embryos. We assume a
power-law surface density profile that varies with heliocentric distance r as
r^-alpha, and vary alpha between 1/2 and 5/2 (alpha = 3/2 for the MMSN model).
We find that for steeper profiles (higher values of alpha), the terrestrial
planets (i) are more numerous, (ii) form more quickly, (iii) form closer to the
star, (iv) are more massive, (v) have higher iron contents, and (vi) have lower
water contents. However, the possibility of forming potentially habitable
planets does not appear to vary strongly with alpha.Comment: 10 pages, 5 figures in emulateapj style. tp appear in Oct 20, 2005,
issue of Ap
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