High-luminosity white dwarfs.
High luminosity white dwarfs possible evolution from planetary nebulae, discussing plasma neutrino processes
Chiu, H.-Y.↗
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The structure and evolution of Jupiter - The fluid contraction stage
The complete evolution of a contracting star of Jovian mass consisting of a convective adiabatic homogeneous fluid is determined using stellar structure methods, improved model atmosphere calculations, and substantially improved thermodynamic properties for hydrogen and hydrogen-helium fluids. The model atmospheres are calculated in the form of time-averaged vertical temperature structures, including all relevant sources of opacity and a solar energy deposition component, and the thermodynamic properties are modified to obtain better agreement with Monte Carlo results for metallic fluids. The resultant gravitationally contracting evolutionary models are found to have two phases: an early stellar phase similar to a typical low-mass pre-main-sequence body and a later phase constituting an approach to a degenerate-dwarf cooling curve. The first phase is shown to have high luminosities and internal temperatures, while the second gives excellent agreement with the observed radius and luminosity of Jupiter. Analysis indicates that the equation of state and superadiabaticity have the strongest influence on evolution over planetary time scales.
Graboske, H. C., Jr.↗
The Galilean satellites of Jupiter - Four worlds
A broad survey is presented of our current knowledge of the Galilean satellites. Attention is given to the physical properties (size, masses, densities, and rotation) and to the surface properties (albedo, surface markings, composition, and physical state) of the satellites. In particular, Io's atmosphere is considered with emphasis on the sodium, hydrogen and sulfur clouds and the ionosphere. The atmospheres of the other satellites are examined briefly and consideration is given to models of planetary origin, evolution and interior structure.
Johnson, T. V.↗
The vagaries of variable viscosity convection
The wavelength dependence of convection with a temperature-dependent viscosity is studied using two-dimensional finite difference models. A steady-state solution with a horizontal wavelength of eight times the depth is found to transport heat more efficiently than a solution with exactly the same parameters but with a wavelength of twice the depth. Only at the longer wavelengths is it possible for the top high viscosity boundary layer to participate more fully in the flow. Using models with a constant heat flux resulted in a decrease in the interior temperature of approximately 30% for the longer wavelength. It is also demonstrated that a parameterization using only a constant viscosity thermal structure and an interior viscosity is inadequate to parameterize the variable viscosity calculations described. This suggests that parameterized convection schemes may not accurately resolve the thermal evolution of planetary interior unless they can correctly include the effects of vertical viscosity structure.
Daly, S. F.↗
Tectonics of the Galileo Regio on Ganymede
The surface of Ganymede consists of dark cratered terrain, and groved terrain. The dark cratered terrains form polygonal units, the largest of which is Galileo Regio, the surface of which is transected by furrows, smooth floored valleys bounded by relatively sharp parallel ridges. The most apparent of them are grouped together and form an apparently arcuate system of subparallel furrows which was mapped using Voyager pictures and plotted on a map using a stereographic projection. With this kind of projection, the main furrow system is not arcuate, but rectilinear. Observations strongly suggest that the Galileo Regio furrow systems are not of impact origin and appear to be irrelevant to discussions about the basins' morphology or evolution of planetary lithosphere determined from multiring structures.
Thomas, P. G.↗
Planetary surface processes: A review
The Earth's surface was shaped by a variety of processes, including volcanism, tectonism, impact cratering, and gradation. Solar System exploration and geological mapping showed that these processes operate on all the terrestrial planets. Gradation is a complex process that begins with weathering and erosion, continues with transport of weathered debris, and ends with deposition. Gradation works through the agents of gravity, wind, and water, and can shed light on planetary surface evolution. Rates of surface erosion vary with planet and must be considered in assessing ages of surface units based on impact crater frequency distributions. The products of gradation may substantially influence remote sensing signatures for determination of composition and must be taken into account in interpreting such data.
Greeley, R.↗
Studies in Aeolian geology
The objective of the research was to assess the significance of aeolian (windblown) processes in the evolution of planetary surfaces. The approach was to use wind tunnel simulations, field studies of possible analogs, and analyses of spacecraft data.
Greeley, Ronald↗
Comparative planetology of early intense cratering and other cratering effects
Intense early cratering in the first few hundred million years of the solar system, its decline to the present rate, and the consequent effects on the evolution of planetary surfaces were studied.
Hartmann, William K.↗
LYMAN - The far ultraviolet explorer
The LYMAN FUSE mission concept for far ultraviolet astronomy is presented. The wavelength window from 100 to 1200 A provides access to a wide range of important scientific problems in cosmology, galactic structure, stellar evolution, and planetary magnetospheres, which cannot be studied in any other way. The LYMAN FUSE Phase A study is examining in detail mission operations, instrumentation technology, the construction of the instrument module, and the interfaces between the Instrument Module and the Explorer Platform Mission. Most of the mission observing time will be allotted through a competitive Guest Observer program analogous to that in operation for the IUE.
Moos, Warren↗
The impact history of the solar system - Implications for the origins of atmospheres
The present consideration of the roles played by impacts in the origin and evolution of planetary and satellite atmospheres notes that while small impacts of volatile-rich planetesimals can contribute to a body's volatile inventory, large impacts blow away existing primordial atmospheres and generate main-body outgassing; these processes would have been in greatest competition during the end-stage of accretion. The uncertainties in solar system cratering-record interpretation and event-chronology yield only order-of-magnitude estimates. Temporal fluctuations in the impact rate may be very large, notably in the case of cometary sources, where a factor-of-2 variation is common, and a factor-of-100 variation is entirely possible.
Weissman, P. R.↗
The dynamic instability of adiabatic blast waves
Adiabatic blastwaves, which have a total energy injected from the center E varies as t(sup q) and propagate through a preshock medium with a density rho(sub E) varies as r(sup -omega) are described by a family of similarity solutions. Previous work has shown that adiabatic blastwaves with increasing or constant postshock entropy behind the shock front are susceptible to an oscillatory instability, caused by the difference between the nature of the forces on the two sides of the dense shell behind the shock front. This instability sets in if the dense postshock layer is sufficiently thin. The stability of adiabatic blastwaves with a decreasing postshock entropy is considered. Such blastwaves, if they are decelerating, always have a region behind the shock front which is subject to convection. Some accelerating blastwaves also have such region, depending on the values of q, omega, and gamma where gamma is the adiabatic index. However, since the shock interface stabilizes dynamically induced perturbations, blastwaves become convectively unstable only if the convective zone is localized around the origin or a contact discontinuity far from the shock front. On the other hand, the contact discontinuity of accelerating blastwaves is subject to a strong Rayleigh-Taylor instability. The frequency spectra of the nonradial, normal modes of adiabatic blastwaves have been calculated. The results have been applied to the shocks propagating through supernovae envelopes. It is shown that the metal/He and He/H interfaces are strongly unstable against the Rayleigh-Taylor instability. This instability will induce mixing in supernovae envelopes. In addition the implications of this work for the evolution of planetary nebulae is discussed.
Ryu, Dongsu↗
Origins of the rings of Uranus and Neptune. II - Initial conditions and ring moon populations
Catastrophic fragmentation of the ring moons of Uranus and Neptune occurs in approximately 10 exp 8 years. The fate of the debris following a fragmenting impact is central to understanding the evolution of these satellites and the hypothesized origin of rings from their debris. In this paper the possible effects of the velocity distribution of fragments following a catastrophic fragmentation on satellite diminution via a collisional cascade is examined. Fragment velocities are critical in the evolution of the collisional cascade because of the possibility of reaccretion following disruption. The fragment velocity distribution is used to calculate the initial phase space distribution of the new ring particles. This provides a physically realistic initial condition for simulations of the collisional evolution of planetary rings.
Colwell, Joshua E.↗
Orbital evolution of the PSR1257+12 planetary system
The detection of orbital perturbation effects in the PSR1257+12 timing data would provide irrefutable confirmation that planets are indeed orbiting the pulsar. Here we give an overview of how perturbation effects are expected to affect the orbital elements of the two planets over the next few years. In particular, we give a simple calculation of resonant perturbations, including the nonlinear effects which could be important if sin i less than about 0.1. We also present a new analysis of the effects of close encounters between the two planets and we discuss their detectability.
Rasio, Frederic A.↗
Trans-Global Mission Architectures
Mission operations include the utilization of both space and ground resources to achieve mission objectives. Future architectures will make the spacecraft a node on a distributed system, thus expanding the scope of missions beyond the global scale. The history and evolution of planetary mission operations are outlined, together with the current global involvement in planetary missions. The modular nature and reuse of the supporting ground data systems, and the inclusion of automation and dedicated software in space missions, are discussed. A trans-global mission architecture is presented, and consists of an extension of a layered reusable mission operations architecture to create an open ground/space operations system. Concurrent mission engineering with such trans-global structures is discussed.
Anderson, Barbara V.↗
Thermal History and Mantle Dynamics of Venus
One objective of this research proposal is to develop a 3-D thermal history model for Venus. The basis of our study is a finite-element computer model to simulate thermal convection of fluids with highly temperature- and pressure-dependent viscosities in a three-dimensional spherical shell. A three-dimensional model for thermal history studies is necessary for the following reasons. To study planetary thermal evolution, one needs to consider global heat budgets of a planet throughout its evolution history. Hence, three-dimensional models are necessary. This is in contrasts to studies of some local phenomena or local structures where models of lower dimensions may be sufficient. There are different approaches to treat three-dimensional thermal convection problems. Each approach has its own advantages and disadvantages. Therefore, the choice of the various approaches is subjective and dependent on the problem addressed. In our case, we are interested in the effects of viscosities that are highly temperature dependent and that their magnitudes within the computing domain can vary over many orders of magnitude. In order to resolve the rapid change of viscosities, small grid spacings are often necessary. To optimize the amount of computing, variable grids become desirable. Thus, the finite-element numerical approach is chosen for its ability to place grid elements of different sizes over the complete computational domain. For this research proposal, we did not start from scratch and develop the finite element codes from the beginning. Instead, we adopted a finite-element model developed by Baumgardner, a collaborator of this research proposal, for three-dimensional thermal convection with constant viscosity. Over the duration supported by this research proposal, a significant amount of advancements have been accomplished.
Hsui, Albert T.↗
Reflectance Spectroscopy and Lunar Sample Science: Finally a Marriage After Far Too Long an Engagement
Inferences about the igneous and impact evolution of planetary bodies are based upon spectral remote sensing of their surfaces. However, it is not the rocks of a body that are seen by the remote sensing, but rather the regolith, that may contain small pieces of rock but also many other phases as well. Indeed, recent flybys of objects even as small as asteroid Ida have shown that these objects are covered by a regolith. Thus, spectral properties cannot be directly converted into information about the igneous history of the object. It is imperative to fully understand the nature of the regolith, particularly its finer fraction termed "soil," to appreciate the possible effects of "space weathering" on the reflectance spectra. We have initiated a study of our nearest, regolith-bearing body, the Moon, as "ground truth" for further probes of planetary and asteroidal surfaces. the foundation for remote chemical and mineralogical analyses lies in the physics underlying optical absorption and the linking of spectral properties of materials measured in the laboratory to well understood mineral species and their mixtures. From this statement, it is obvious that there should be a thorough integration of the material science of lunar rocks and soils with the remote-sensing observations. That is, the lunar samples returned by the Apollo missions provide a direct means for evaluation of spectral characteristics of the Moon. However, this marriage of the remote-sensing and lunar sample communities has suffered from a prolonged unconsummated betrothal, nurtured by an obvious complacency by both parties. To make more direct and quantitative links between soil chemistry/mineralogy and spectral properties, we have initiated a program to (1) obtain accurate characterization of the petrography of lunar soils (in terms relevant to remote analyses), coupled with (2) measurement of precise reflectance spectra, with testing and use of appropriate analytical tools that identify and characterize individual mineral and glass components. It is the finest-sized fractions of the bulk lunar soil that dominate the observed spectral signatures.
Taylor, Lawrence A.↗
Overview of the Keck Interferometer
This is a presentation about the Keck Interferometer which is being constructed on top of Mauna Kea, Hawaii. This includes using the world's largest telescopes for optical and near-infrared astronomy, the twin 10 meter Keck telescopes. The two Keck telescopes, in conjunction with four proposed outrigger telescopes, will be used as an interferometer to conduct observations as part of NASA's Origins Program. These observations will address a variety of topics, including the origin and evolution of planetary systems. This presentation reviews the key features of the interferometer, and the specifications of the telescopes that will be used. It shows diagrams of the site, and the basement layout. It also reviews the science for which the interferometer will be used.
vanBelle, Gerard↗
Mars Surveyor Project Landing Site Activities
The Mars Surveyor Program --now a cooperative program led by NASA and CNES along with other international partners -- is underway. It has the primary science objective of furthering our understanding of the biological potential and possible biological history of Mars and has the complementary objective of improving our understanding of martian climate evolution and planetary history The missions will develop technology and acquire data necessary for eventual human Exploration. Launches of orbiters, landers and rovers will take place in 2001 and in 2003; in 2005 a complete system will be launched capable of returning samples to Earth by 2008. A key aspect of the program is the selection of landing sites. This abstract 1) reports on the status of the landing site selection process that begins with the 2001 lander mission and 2) outlines be opportunities for the Mars community to provide input into the landing site selection process.
Gulick, Virginia C.↗