Engineering topics

Gladstone, G. Randall

Publications and source records attributed to Gladstone, G. Randall.

30 records · Page 2

NASA NTRS2000

Photochemistry Saturn's Atmosphere: Effects of an Influx of External Oxygen - 2

We use a one-dimensional diurnally averaged model of photochemistry and diffusion in Saturn's stratosphere to investigate the influence of extraplanetary debris on atmospheric chemistry. In particular, we consider the effects of an influx of oxygen from micrometeoroid ablation or from ring-particle diffusion; the contribution from cometary impacts, satellite debris, or ring vapor is deemed to be less important. The photochemical model results are compared directly with Infrared Space Observatory (ISO) observations to constrain the influx of extraplanetary oxygen to Saturn. From the ISO observations, we determine that the column densities of CO2 and H2O above 10 mbar in Saturn's atmosphere are (6.3 +/- 1) x 10(exp 14) and (1.4 +/- 0.4) x 10(exp 15)/ square cm, respectively; our models indicate that a globally averaged oxygen influx of (4+/-2) x 10(exp 6) O atoms /sq cm/s is required to explain these observations. Models with a locally enhanced influx of H20 operating over a small fraction of the projected area do not provide as good a fit to the ISO H2O observations. If volatile oxygen compounds comprise one-third to one-half of the exogenic source by mass, then Saturn is currently being bombarded with (3 +/- 2) x 10(exp -16) g/square cm/s of extraplanetary material. To reproduce the observed CO2/H2O ratio in Saturn's stratosphere, some of the exogenic oxygen must arrive in the form of a carbon-oxygen bonded species such as CO or CO2. An influx consistent with the composition of cometary ices fails to reproduce the high observed CO2/H2O ratio, suggesting that (i) the material has ices that are slightly more carbon-rich than is typical for comets, (ii) a contribution from an organic-rich component is required, or (iii) some of the hydrogen-oxygen bonded material is converted to carbon-oxygen bonded material without photochemistry (e.g., during the ablation process). We have also reanalyzed the 5-micron CO observations of Noll and Larson and determine that the CO lines are most sensitive to the 100- to 400-mbar column density for which we derive a range of (0.7-1.5) x 10(exp 17)/square cm; the CO observations do not allow us to distinguish between an external or internal source of CO on Saturn. If we assume that all the extraplanetary oxygen derives from a micrometeoroid source, then the unfocused dust flux at 9.5 AU must be (i) (1 +/- 0.7) x 10(exp -16) g/square cm/s if interstellar grains are the source of the external oxygen on Saturn, (ii) (4 +/- 3) x 10 (exp -17) g/ sq. cm./s if IDPs on randomly inclined, highly eccentric orbits (e.g., Halley-type comet grains) are the source of the external oxygen, or (iii) (2+/- 1.4) x 10(exp -18)/g/sq cm/s if IDPs on low inclination, low eccentricity orbits (e.g., Kuiper-belt grains) are the source of the external oxygen. These estimates can be used in combination with future Cassini dust detection data to determine the ultimate source of the dust at Saturn's distance from the Sun.

Moses, Julianne I.↗

NASA NTRS1999

High-Resolution Spectroscopy of Auroras on Jupiter and Saturn/Earth Dayglow

The purpose of the grant was to allow the researcher to: (1) reduce and analyze Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometers (ORFEUS) II observations of Jupiter (200s) and Saturn (1200s); (2) Reduce and analyze selected ORFEUS-II Earth Far Ultraviolet (FUV) airglow data; (3) Modify existing scripts for simulating Earth FUV airglow emissions to model a subset of the ORFEUS data.

Gladstone, G. Randall↗

NASA NTRS1998

Ly(alpha) Photolysis in the Primitive Solar Nebula

This is the final report for the third year of work on this project. Our proposal was to quantitatively investigate the importance of photochemistry in the solar nebula. In the generally accepted theory for the chemical evolution of the primitive solar nebula, Prinn and Fegley argued that photochemistry is unimportant, and that thermochemistry controls the relative abundances of molecular species throughout the planet-forming region. They provided useful estimates of the chemical energy available to the solar nebula from a variety of sources, and established that even the small photolysis rate due to starlight is more important than the photolysis rate from direct sunlight (although small, the UV flux from starlight could have processed a non-negligible fraction of the solar nebula. The reason for this is that the opacity of the disk was so large that direct sunlight could only penetrate to 0.1 AU or so, despite the expectation that the protosun, if comparable to a T-Tauri star, would be emitting up to 10(exp 4) more H I Ly(alpha) photons than the current sun. We developed a Monte Carlo resonance fine radiative transfer code, capable of accurately calculating the radiation field of H I Ly(alpha), He I 584 A, and He II 304 A emissions throughout the nebula and the nearby interstellar medium in which it is embedded. We applied the code to two appropriate models of the primitive solar nebula. Our model provided the photolysis rates of various species over the entire surface layer of the nebula, and from this we evaluated the importance of UV photochemistry due to backscattered solar UV resonance line emissions on different parts of the nebula. The results discussed below were presented.

Gladstone, G. Randall↗

NASA NTRS1995

Post-SL9 Sulfur Photochemistry on Jupiter

We have modeled the photochemical evolution of the sulfur containing species that were observed in Jupiter's stratosphere after the Shoemaker Levy 9 (SL9) impacts. We find that most of the sulfur is converted to S8 in the first few days. Other important sulfur reservoirs are CS, whose abundance increases markedly with time, and possibly H2CS, HNCS, and NS, whose abundances depend on kinetic reaction rates that are unknown at the present. We discuss the temporal variation of the major sulfur compounds, make abundance and compositional predictions useful for comparison with observations, and discuss the possible condensation of sulfur containing species.

Moses, Julianne I.↗

NASA NTRS1995

Nitrogen and Oxygen Photochemistry following SL9

The collision of Shoemaker Levy 9 (SL9) with Jupiter caused many new molecular species to be deposited in the Jovian stratosphere. We use a photochemical model to follow the evolution of the impact derived species. Our results regarding the nitrogen and oxygen compounds are presented here. NH3 photolysis initiates the nitrogen photochemistry. Much of the nitrogen ends up in N2, nitrogen-sulfur compounds, and HCN, but NH3 and nitriles such as C2H3CN may also exist in observable quantities for a year or so after the impacts. Oxygen species survive for a long time in the Jovian stratosphere. The only major oxygen containing compounds that exhibit dramatic changes in the lower stratosphere in the first year following the impacts are SO, SO2, and OCS - H2O, CO2, and CO are comparatively stable. We discuss the important photochemical processes operating on the nitrogen and oxygen species in the Jovian stratosphere, make prediction concerning the temporal variation of the major species, and identify molecules that might act as good tracers for atmospheric dynamics.

Moses, Julianne I.↗

NASA NTRS1994

Hubble Space Telescope Goddard high-resolution spectrograph H2 rotational spectra of Jupiter's aurora

We have observed the emission spectrum from Jupiter's north auroral atmosphere with 0.57 A spectral resolution over 1204-1241 A. Bright emissions have been detected from 50 deg to 60 deg latitude at locations consistent with 6 to 30 R (sub J) auroral ovals, with much fainter emissions away form the auroral ovals. The emission spectrum is well fitted by both laboratory spectra and theoretical models of optically thin electron excited H2, with added Doppler-broadened Lyman Alpha emission. The observed Lyman Alpha emission wings extend more than 1 A from line center and appear correlated in strength with the H2 brightness. Individual rotational lines in the H2 Werner band system are resolved, allowing a determination of the H2 rotational temperature at the altitude of the emission. We derive best-fit temperatures from 400-450 to 700-750 K, with the auroral emission layer temperature changing either across the auroral oval or over several days' time. These observations demonstrate for the first time the ability to measure the observed rapid H2 temperature variations across Jupiter's auroral atmosphere.

Clarke, John T.↗

NASA NTRS1994

EUVE photometric observations of the Moon

During its all-sky survey, the Extreme Ultraviolet Explorer (EUVE) satellite observed the Moon several times at first and last quarters, and once immediately following the Dec. 10 1992 lunar eclipse. We present here a portion of this data, in the form of extreme-ultraviolet (EUV) images of the Moon and derived geometric albedos. From the EUVE photometer data we obtain average geometric albedos of 0.15% (+/- 0.03%), 3.1% (+/- 0.3%), and 3.5% (+/- 0.3%), over wavelength intervals of 150-240 A, 400-580 A, and 550-650 A, respectively. An upper limit geometric albedo of 0.13% is obtained for the wavelength interval 75-180 A. Also, using previously published ROSAT data, we estimate a lunar geometric albedo of 0.014% (+/- 0.002%) over the wavelength interval 50-80 A. These EUV albedos (and previously published far-ultraviolet albedos) are well fit by the scaled reflectivities of SiO2 and Al2O3. Over the wavelength ranges of the EUVE photometers, the observed brightness of the Moon seems to be largely consistent with reflected sunlight rather than X-ray fluorescence. Since the L- and M-shell fluorescence signal is expected to be carried by only small number of emission lines, however, it will require EUV observations of higher spectral resolution (approximately less than 5 A) to determine their exact contribution, if any, to the lunar EUV spectrum.

Gladstone, G. Randall↗

NASA NTRS1993

Primary results from the Berkeley EUV airglow rocket spectrometer: O I and N2 FUV/EUV dayglow in the thermosphere and lower exosphere

The Berkeley extreme-ultraviolet airglow rocket spectrometer (BEARS) made spectroscopic measurements of far and extreme UV, atomic oxygen emissions from a Black Brandt XII (12.041 WT) sounding rocket launched from Wallops Island, Virginia, on September 30, 1988. BEARS' primary instrument, a near-normal Rowland mount spectrometer, measured several atomix oxygen and molecular nitrogen dayglow features at high spectral resolution (1.5 A): O I (989, 1027, 1304, and 1356 A); and N2 Lyman-Birge-Hopfield (4, 0) and (3, 0) bands at 1325 and 1354 A. The instrument collected over 800 s of data spanning altitudes of 150 - 963 km with look directions between 95 deg and 125 deg from zenith. We have analyzed the data using electrons and radiative transport models in a forward modeling approach. The model and data are generally in good agreement. However, there are some discrepancies, which are discussed in terms of remote sensing capabilities and improvements to the models.

Cotton, Daniel M.↗

NASA NTRS1993

Optically thick cascade to the O I 3s 3S state in the Earth's thermosphere

The electron impact excitation cross section for the O I (1304 A) emission is enhanced in the terrestrial thermosphere owing to an optically thick cascade mechanism. Values for the contribution of the cascade have been calculated and deduced from measured 1304-A airglow emissions to between o and 200%. We have developed a model to calculate a large portion of the optically thick cascade contribution, including contributions from the radiative entrapment of the O I (1040, 1027, and 989 A) lines. High-resolution spectroscopic data from a sounding rocket experiment are analyzed using this model. The optically thick cascade contribution for the three lower lying states was evaluated to be 24.5% and compares well with recent calculations. The data, however, support a total contribution of 40%. The 15.5% difference is likely due to the contribution from higher Rydberg states.

Cotton, Daniel M.↗

NASA NTRS1993

Sounding rocket observation of a hot atomic oxygen geocorona

A sounding rocket measurement of the ultraviolet, atomic oxygen dayglow reveals an excess of emission compared to standard thermospheric model calculations at exospheric altitudes. We explore two explanations for this discrepancy: a breakdown of the radiative transfer model due to nonlocal thermal equilibrium (non-LTE) conditions above the exobase and a hot atomic oxygen geocorona. In particular, the effects of non-LTE on the 3P2, 1, 0 sublevel populations are modeled, and a hot O component in the upper thermosphere and lower exosphere is added to investigate the effects on the modeled emissions. For both cases, the data are reanalyzed and compared with the results using a standard LTE model. A hot O geocorona having a peak density of 10(exp 6)/cc at 550 km and a temperature of 4000 K is consistent with the data and appears to be the most reasonable explanation of the high-altitude enhanced emissions observed in the data.

Cotton, Daniel M.↗

NASA NTRS1988

UV resonance line dayglow emissions on earth and Jupiter

The similarities and differnces between atomic resonance line emissions on earth and Jupiter are studied. For earth, the scattering of the conservative atomic oxygen triplet transition at 1304 A and the nonconservative atomic oxygen sextuplet at 989 A is considered. For Jupiter, the scattering of the conservative atomic hydrogen doublet at 1216 A and the nonconservative atomic hydrogen doublet at 1026 A are addressed. Models are presented for the intensities of the emission features as seen from a distance of several planetary radii, using the same observational geometry for both earth and Jupiter. Variations of the line-integrated emissions across the disk and near the limb of each planet are examined in detail. Line profiles for the emission near the limb and at disk-center are also studied. The models reveal the importance of including inhomogeneities and temperature variations of the atmosphere in scattering models, and indicate that outer planet emissions previously interpreted as 'electroglow' may be solely due to resonant scattering of solar emissions.

Gladstone, G. Randall↗

NASA NTRS1988

Why is Pluto bright? Implications of the albedo and lightcurve behavior of Pluto

Several mechanisms accounting for Pluto's high geometric albedo in the presence of CH4 and various radiations are investigated, in light of the hypothesis that the planet must replenish its surface with clean volatiles on a time-scale of less than 20,000 years. Pluto's high albedo is explained by a CH4 snow cover created by orbital atmospheric sublimation and freeze-out cycles. As a result of the 250-year atmospheric regeneration time-scale, large surface CH4 crystals will be deprived of sufficient time for growth; it is predicted that the albedo decline will reverse following a 7-17 year phase lag after perihelion.

Stern, S. Alan↗