Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “sublimation”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

A numerical simulation of climate changes during the obliquity cycle on Mars

A one-dimensional seasonal energy balance climate model of the Martian surface is developed. The model shows the importance of using short-period diurnal and seasonal variations of solar irradiance instead of yearly-averaged quantities. The roles of meridional heat transport and greenhouse warming are shown to be important. The possible existence of hysteresis cycles in the formation and sublimation of permanent deposits during the course of the obliquity cycle is demonstrated.

Francois, L. M.↗
NASA NTRS1990

Zonally averaged thermal balance and stability models for nitrogen polar caps on Triton

Voyager four-color imaging data of Triton are analyzed to calculate the bolometric hemispheric albedo as a function of latitude and longitude. Zonal averages of these data have been incorporated into a thermal balance model involving insolation, reradiation, and latent heat of sublimation of N2 ice for the surface. The current average bolometric albedo of Triton's polar caps is 0.8, implying an effective temperature of 34.2 K and a surface pressure of N2 of 1.6 microbar for unit emissivity. This pressure is an order of magnitude lower than the surface pressure of 18 microbar inferred from Voyager data (Broadfoot et al., 1989; Conrath et al., 1989), a discrepancy that can be reconciled if the emissivity of the N2 on Triton's surface is 0.66. The model predicts that Triton's surface north of 15 deg N latitude is experiencing deposition of N2 frosts, as are the bright portions of the south polar cap near the equator. This result explains why the south cap covers nearly the entire southern hemisphere of Triton.

Stansberry, John A.↗
NASA NTRS1990

Reexamination of data from the asteroid/meteoroid detector

A reexamination of the results of the Pioneer 10 and 11 Asteroid Meteoroid Detector, or Sisyphus, was carried out in the light of a recently derived theory characterizing interplanetary matter and the Zodiacal Light (ZL). Sisyphus measured individual meteoroids from reflected sunlight and ZL between meteoroid events. The results were questioned because meteoroid orbits could not be calculated as intended and the ZL as computed from individual meteoroids did not agree with values determined from the ZL mode and from the other ZL sensor on the spacecraft. It is first shown that, independent of any explanation, the measurements are, with high probability, valid and strongly correlated with the ZL. The model which explains the strange behavior of the Sisyphus instrument also resolves the enigma why the three dust experiments on the Pioneer 10 and 11 spacecraft produced extreme disparate results for the distribution and orbits of meteoric particles and the ZL. The theory based primarily on these measurements requires a population in the inner solar system of cold meteoroid material composed mainly of volatile molecules. These meteoroids in orbits of high eccentricity are called cosmoids. They are impulsively disrupted from solar heating, resulting in order of magnitude increases in optical cross section. The dispersed particles, predominantly micron sized, scatter most of the ZL and supply the polarization. The sublimation time in sunlight for micron sized particles of volatile composition opposes the gravitational flux increase expected in approaching the sun. The other two Pioneer 10/11 dust experiments were: the Imaging Photopolarimeter for the ZL, and the Meteoroid Detection Experiment that measured penetration of 25 micron (Pioneer 10) and 50 micron (Pioneer 11) thick walls of pressurized gas cells.

Soberman, Robert K.↗
NASA NTRS1991

Silicon carbide, a semiconductor for space power electronics

After many years of promise as a high temperature semiconductor, silicon carbide (SiC) is finally emerging as a useful electronic material. Recent significant progress that has led to this emergence has been in the areas of crystal growth and device fabrication technology. High quality single-crystal SiC wafers, up to 25 mm in diameter, can now be produced routinely from boules grown by a high temperature (2700 K) sublimation process. Device fabrication processes, including chemical vapor deposition (CVD), in situ doping during CVD, reactive ion etching, oxidation, metallization, etc. have been used to fabricate p-n junction diodes and MOSFETs. The diode was operated to 870 K and the MOSFET to 770 K.

Powell, J. Anthony↗
NASA NTRS1990

Weathering and erosion of the polar layered deposits on Mars

The Martial polar layered deposits are widely believed to be composed of water ice and silicates, but the relative amount of each component is unknown. The conventional wisdom among Mars researchers is that the deposits were formed by periodic variations in the deposition of dust and ice caused by climate changes over the last 10 to 100 million years. It is assumed here that water ice is an important constituent of the layered deposits, that the deposits were formed by eolian processes, and that the origin and evolution of the north and south polar deposits were similar. Weathering of the layered deposits by sublimation of water ice can account for the geologic relationships in the polar regions. The nonvolatile components of the layered deposits appears to consist mainly of bright red dust, with small amounts of dark dust or sand. Dark dust, perhaps similar to the magnetic material found at the Viking Lander sites, may perferentially form filamentary residue particles upon weathering of the deposits. Once eroded, these particles may saltate to form the dark dunes found in both polar regions.

Herkenhoff, K. E.↗
NASA NTRS1991

Triton's global heat budget

Internal heat flow from radioactive decay in Triton's interior along with absorbed thermal energy from Neptune total 5 to 20 percent of the insolation absorbed by Triton, thus comprising a significant fraction of Triton's surface energy balance. These additional energy inputs can raise Triton's surface temperature between approx. 0.5 to 1.5 K above that possible with absorbed sunlight alone, resulting in a factor of approx. 1.5 to 2.5 increase in Triton's basal atmospheric pressure. If Triton's internal heatflow is concentrated in some areas, as is likely, local effects such as enhanced sublimation with subsequent modification of albedo could be quite large. Furthermore, indications of recent albedo change on Triton suggest that Triton's surface temperature and pressure may not now be in steady state, further suggesting that atmospheric pressure on Triton was as much as 10 times higher in the recent past.

Brown, R. H.↗
NASA NTRS1991

Inner coma imaging of Comet Levy (1990c) with the Hubble Space Telescope

Observations of comet Levy were carried out with the Hubble Space Telescope (HST) on UT 27 Sep. 1990. The comet was imaged with the Wide Field Camera (WFC) through both red and blue filters, which were selected to isolate continuum emission peaking sharply at the nucleus. The longest exposures (4 sec) through the red filter had sufficient signal to noise that image deconvolution could be used to recover virtually the full spatial resolution of HST. These images reveal a fan-shaped inner coma in which the sunward-facing hemisphere is significantly brighter than the tailward hemisphere, consistent with volatile sublimation occurring primarily on the dayside of the nucleus.

Weaver, Harold A.↗
NASA NTRS1991

Modeling the surface and interior structure of comet nuclei using a multidisciplinary approach

The goal was to investigate the structural properties of the surface of comet nucleus and how the surface should change with time under effect of solar radiation. The basic model that was adopted was that the nucleus is an aggregate of frosty particles loosely bound together, so that it is essentially a soil. The nucleus must mostly be composed of dust particles. The observed mass ratios of dust to gas in the coma is never much greater than unity, but this ratio is probably a much lower limit than that of the nucleus because it is vastly easier to remove the gaseous component by sublimation than by carrying off the dust. Therefore the described models assumed that the particles in the soil were frost covered grains of submicron basic size, closely resembling the interstellar grains. The surface properties of such a nucleus under the effects of heating and cooling as the nucleus approaches and recedes from the Sun generally characterized.

Odell, C. R.↗
NASA NTRS1990

A kinematic determination of the structure of the double ring planetary nebula NGC 2392, the Eskimo

Slit spectra and existing 'velocity cube' data have been used to determine the structure of the double ring PN NGC 2392. The inner shell is a stellar wind-sculpted prolate spheroid with a ratio of axes of 2:1 and the approaching end of the long axis pointed 20 deg from the line of sight in P.A. = 200 deg. The outer ring is caused by an outer disk with density dropping off with distance from the central star and with distance from its plane, which is the same as the equatorial band of high density in the inner shell. The outer disk contains a ring of higher density knots at a distance of 16 arcsec and is losing material through free expansion, forming an outer envelope of increasing velocity. Forbidden S II spectra are used to determine the densities in all of the major regions of the nebula. It is argued that the filamentary cores at the centers of the knots seen in the outer ring originate in the sublimation of bodies formed at the same time as the parent star.

O'Dell, C. R.↗
NASA NTRS1990

Triton's geyser-like plumes - Discovery and basic characterization

One model for the mechanism driving the plumes of the four active geyser-like eruptions observed by Voyager 2 on Triton is a heating up of nitrogen ice in a subsurface greenhouse environment, where nitrogen gas pressurized by solar heating explosively vents to the surface carrying clouds of ice and dark particles into the atmosphere. A temperature increase of less than 4 K above the ambient surface value of 38 + or - 3 K suffices to drive the plumes to 8-km altitude. Each eruption may last a year or more, over the course of which 0.1 cu km of ice is sublimed.

Soderblom, L. A.↗
NASA NTRS1990

Energy sources for Triton's geyser-like plumes

Four geyser-like plumes were discovered near Triton's south pole in areas now in permanent sunlight. Because Triton's southern hemisphere is nearing a maximum summer solstice, insolation as a driver or a trigger for Triton's geyser-like plumes is an attractive hypothesis. Trapping of solar radiation in a translucent, low-conductivity surface layer (in a solid-state greenhouse), which is subsequently released in the form of latent heat of sublimation, could provide the required energy. Both the classical solid-state greenhouse consisting of exponentially absorbed insolation in a gray, translucent layer of solid nitrogen, and the 'super' greenhouse consisting of a relatively transparent solid-nitrogen layer over an opaque, absorbing layer are plausible candidates. Geothermal heat may also play a part if assisted by the added energy input of seasonal cycles of insolation.

Brown, R. H.↗
NASA NTRS1990

Scatterers in Triton's atmosphere - Implications for the seasonal volatile cycle

Nitrogen and methane ices on the surface of Triton, Neptune's largest satellite, are exchanged between the summer and winter hemispheres on a seasonal time scale. Images of the satellite's sky obtained by the Voyager 2 spacecraft show the presence of several types of scattering materials that provide insights into this seasonal cycle of volatiles. Discrete clouds, probably composed of N2 ice particles, arise in regions of active sublimation. They are found chiefly poloward of 30 deg S in the southern, summer hemisphere. Haze particles, probably made of hydrocarbon ices, are present above most, but not all places. Recent snowfall may have occurred at low southern latitudes in places where they are absent. The latent heat released in the formation of the discrete clouds may have a major impact on the thermal balance of the lower atmosphere. Triton may have been less red at the time of the Voyager flyby than 12 years earlier due to recent N2 snowfall at a wide range of latitudes.

Pollack, James B.↗
NASA NTRS1990

Energetics of molecular-beam epitaxy models

The removal energies of constituent atoms from various unreconstructed semiconductor surfaces are calculated using a Green function method. An efficient difference-equation approach within the second-neighbor tight-binding model is employed. For a compound AB, binding energies for the A and B atoms on the (111), (-1 -1 -1), (100), and (110) surfaces are calculated. Analyses are made of the energy to remove an atom from the nearly full surface and from the nearly empty surface. Results are presented for Si, GaAs, CdTe, and HgTe; and the surface sublimation energies are found to depend on surface coverage and do not display a simple linear relationship to the number of bonds broken, as is often assumed in modeling growth by MBE.

Krishnamurthy, Srinivasan↗
NASA NTRS1990

The atmospheric abundance of SO2 on Io

The IUE satellite has obtained near-UV spectra of Io with sufficient resolution to ascertain the east, or leading and west, or trailing hemispheres' dayside atmosphere SO2 abundance. The derived geometric albedos are compared with various model albedos that might result from proposed SO2 atmospheres, as well as from localized, sublimation- or volcanism-generated atmospheres. A homogeneous-layer alternative atmosphere is introduced whose upper limit on the average SO2 column density for both hemispheres implies that a collisionally thick SO2 atmosphere of intermediate density may have been present on Io's dayside during the present observations.

Ballester, Gilda E.↗
NASA NTRS1990

The effect of volume phase changes, mass transport, sunlight penetration, and densification on the thermal regime of icy regoliths

The present quantitative modeling of convective, condensational, and sublimational effects on porous ice crust volumes subjected to solar radiation encompasses the effect of such insolation's penetration of visible bandpass-translucent light, but opaque to the IR bandpass. Quasi-steady-state temperatures, H2O mass fluxes, and ice mass-density change rates are computed as functions of time of day and ice depth. When the effects of latent heat and mass transport are included in the model, the enhancement of near-surface temperature due to the 'solid-state greenhouse effect' is substantially diminished. When latent heat, mass transport, and densification effects are considered, however, a significant solid-state greenhouse effect is shown to be compatible with both morphological evidence for high crust strengths and icy shell decoupling from the lithosphere.

Fanale, Fraser P.↗
NASA NTRS1991

Water vapor in the Orion Molecular Cloud

Infrared observations of interstellar gas-phase H2O in the spectrum of the BN object in Orion are reported. There are absorptions (S/N = 2-5) at the positions of four of the strong lines in the 000-001 nu3 vibration-rotation band. With an estimated excitation temperature of 150 K, the column density of gaseous H2O toward BN in the OMC-1 cloud is (2 + or - 1) x 10 to the 17th/sq cm. The intensities of the lines imply an ortho/para ratio of 1 + or - 0.5 indicating recent sublimation of H2O from low-temperature grains. The results give gas-phase abundance ratios of H2O/CO roughly 0.03 + or - 0.02 and HDO/H2O = 0.001-0.0001 toward BN. The velocities of the H2O absorptions agree with those of the ridge source and CO outflow, but the position along the line of sight is not well constrained. The gas/solid ratio is H2O(gas)/H20(ice) = 0.05 or less. Less than 1 percent of the oxygen is in H2O gas (assuming total cosmic abundance). Most of the H2O in the line of sight to BN, and by inference in quiescent regions of molecular clouds generally, is frozen on grains.

Knacke, R. F.↗
NASA NTRS1991

Dark-ray and dark-floor craters on Ganymede, and the provenance of large impactors in the Jovian system

The dark-floor and dark-ray craters on the icy Jovian satellite, Ganymede, may derive their visual characteristics from impactor contamination. It is presently hypothesized that the rays darken as a result of the near-surface concentration of impactor material; this could occur, first, due to magnetic sputtering while the rays are bright, and subsequently, once a critical albedo is reached, due to thermal sublimation into discrete icy and nonicy patches. Voyager visible spectra of dark rays indicate that most large-ray systems are 'redder' than grooved or cratered terrains, and are among the 'reddest' units on Ganymede. More than half of the recent impactors on Ganymede may have been reddish D-type asteroids or comets, accounting for the albedos and colors of dark terrains on both Ganymede and Callisto.

Schenk, Paul M.↗
NASA NTRS1991

Triton's global heat budget

Internal heat flow from radioactive decay in Triton's interior along with absorbed thermal energy from Neptune total 5 to 20 percent of the isolation absorbed by Triton, thus comprising a significant fraction of Triton's surface energy balance. These additional energy inputs can raise Triton's surface temperature between about 0.5 and 1.5 K above that possible with absorbed sunlight alone, resulting in an increase of about a factor of about 1.5 to 2.5 in Triton's basal atmospheric pressure. If Triton's internal heat flow is concentrated in some areas, as is likely, local effects such as enhanced sublimation with subsequent modification of albedo could be quite large. Furthermore, indications of recent global albedo change on Triton suggest that Triton's surface temperature and pressure may not now be in steady state, further suggesting that atmospheric pressure on Triton was as much as ten times higher in the recent past.

Brown, R. H.↗