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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.

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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

A model of the ion wake of Mars

Observations from the Phobos spacecraft suggest that, for the most part, the Martian magnetotail is induced instead of intrinsic. Test particle techniques are used to determine how the associated ion wake might appear to detectors behind Mars if it is produced primarily by planetary ions picked up by the solar wind in the dayside exosphere. The results suggest that low energy pickup ions populate the inner wake, while a more energetic component is present in the outer magnetosheath and adjacent solar wind.

Luhmann, J. G.↗
NASA NTRS1990

Upstream waves at Mars - Phobos observations

The region upstream from the Mars subsolar bow shock is surveyed for the presence of MHD wave phenomena using the high temporal resolution data from the Magma magnetometer. Strong turbulence is observed when the magnetic field is connected to the Mars bow shock in such a way as to allow diffuse ions to reach the spacecraft. Also weak waves are observed at the proton gyro frequency. These waves are left-hand elliptically polarized and may be associated with the pick-up of protons from the Mars hydrogen exosphere.

Russell, C. T.↗
NASA NTRS1990

Evidence for magnetospheric effects on the sodium atmosphere of Mercury

Monochromatic images of Mercury at the sodium D2 emission line showed excess sodium emission in localized regions at high northern and southern latitudes and day-to-day global variations in the distribution of sodium emission. These phenomena support the suggestion that magnetospheric effects could be the cause. Sputtering of surface minerals could produce sodium vapor in polar regions during magnetic substorms, when magnetospheric ions directly impact the surface. Another important process may be the transport of sodium ions along magnetic field lines toward polar regions, where they impact directly on the surface of Mercury and are neutralized to regenerate neutral sodium atoms. Day-to-day variations in planetary sodium distributions could result from changing solar activity, which can change the magnetosphere in time scales of a few hours. Observations of the sodium exosphere may provide a tool for remote monitoring of the magnetosphere of Mercury.

Potter, A. E.↗
NASA NTRS1991

Modeling the coma of 2060 Chiron

Observations of comet-like activity and a resolved coma have established that 2060 Chiron is a comet. Determinations of its radius range from 65 to 200 km. This unusually large size for a comet suggests that the atmosphere of Chiron is intermediate to the tightly bound, thin atmospheres typical of planets and satellite and the greatly extended atmospheres in free expansion typical of cometary comae. Under certain conditions it may gravitationally bind an atmosphere that is thick compared to its size, while a significant amount of gas escapes to an extensive exosphere. These attributes coupled with reports of sporadic outbursts at large heliocentric distances and the identification of CN in the coma make Chiron a challenging object to model. Simple models of gas production and the dusty coma were recently presented but a general concensus on many basic features has not emerged. Development was begun on a more complete coma model of Chiron. The objectives are to report progress on this model and give the preliminary results for understanding Chiron.

Boice, D. C.↗
NASA NTRS1991

Revived interest in the lunar atmosphere

The recent discoveries of Na and K in the lunar atmosphere are emphasized in a review of lunar atmospheric observations which identifies possible directions of research. Observations are briefly reviewed through the Apollo era which identified the atmosphere as a surface-boundary exosphere, and identified atmospheric constituents included He and Ar which rose and fell by a factor of 20 during the lunar diurnal cycle. Ar-36 and -40 were also identified as were the sources of ionospheric source and sink processes including the solar EUV flux. Na and K are discussed which were identified in a study of the spectroscopic ring effect, and the effects of elemental interaction with the lunar surface can be approximated in laboratory experiments. Observations and measurements of the lunar surface are important for the continued investigation of lunar and other planetary mechanisms.

Morgan, Thomas H.↗
NASA NTRS1992

Mass loading at Titan and comets

Present understanding of the cometary mass loading process (the addition of newborn ions to high speed magnetized plasma flows) is explored and the cometary lesson is applied to the environment of Titan. Mass loading is the dominant physical process controlling the interaction of the solar wind with comets, and it might play an important role in the interaction of Titan's neutral torus and exosphere with Saturn's magnetosphere. Freshly born ions follow cycloidal trajectories, resulting from the superposition of gyration and E/B drift. In the plasma frame, the resulting velocity space distribution is a ring beam distribution at comets and a ring distribution at Titan. This difference in free energy is expected to generate significantly different levels of low frequency broad band electromagnetic turbulence and, thus, result in important differences in the implanted ion distributions. It is predicted that mass loading creates a hot, equatorially trapped plasma population at Titan.

Gombosi, Tamas I.↗
NASA NTRS1991

Non-LTE models of Titan's upper atmosphere

Models for the thermal structure of Titan's upper atmosphere, between 0.1 mbar and 0.01 nbar are presented. The calculations include non-LTE heating/cooling in the rotation-vibration bands of CH4, C2H2, and C2H6, absorption of solar IR radiation in the near-IR bands of CH4 and subsequent cascading to the nu-4 band of CH4, absorption of solar EUV and UV radiation, thermal conduction and cooling by HCN rotational lines. Unlike earlier models, the calculated exospheric temperature agrees well with observations, because of the importance of HCN cooling. The calculations predict a well-developed mesopause with a temperature of 135-140 K at an altitude of approximately 600 km and pressure of about 0.1 microbar. The mesopause is at a higher pressure than predicted by earlier calculations because non-LTE radiative transfer in the rotation-vibration bands of CH4, C2H2, and C2H6 is treated in an accurate manner. The accuracy of the LTE approximation for source functions and heating rates is discussed.

Yelle, Roger V.↗
NASA NTRS1991

The upper atmosphere of Uranus

Voyager measurements of the upper atmosphere of Uranus are analyzed and developed. The upper atmosphere of Uranus is predominantly H2, with at most 10 percent He by volume, and the dominant constituent of the exosphere is H. The thermosphere is warm, with an asymptotic isothermal temperature of about 800 K. Atomic hydrogen at this temperature forms an extensive thermal corona and creates gas drag that severely limits the lifetime of small ring particles. The upper atmosphere emits copious amounts of UV radiation from pressures greater than 0.01 microbar. The depth of this emission level imposes a powerful constraint on permissible emission mechanisms. Electron excitation from a thin layer near the exobase appears to violate this constraint. Solar fluorescence is consistent with the observed trend in solar zenith-angle variation of the emissions and is absent from the night side of the planet. On Uranus, it accounts for the observed Lyman beta to H2 bands intensity ratio and an important fraction of the observed intensity (about 55 percent).

Strobel, Darrell F.↗
NASA NTRS1992

The evidences of waves in the atmospheres of Venus and Mars

In the last 15 years, distributions of state properties and velocities measured in the atmospheres of Venus and Mars have indicated the presence of waves of varied types occurring from the lower atmosphere to the exosphere. This paper gives an organized picture of observations which indicate and characterize wave phenomena in the Venusian and Martian atmospheres, to illustrate how widely they occur, and to emphasize the probable importance of waves to the dynamics. Some of the Pioneer Venus Probe observations presented have not been previously discussed in the context of waves.

Seiff, Alvin↗
NASA NTRS1992

Electrons in the ionosphere of Titan

A theoretical model of the spatial and energy distribution of electrons in the ionosphere of Titan has been constructed using the two-stream electron transport method and the electron energy equation. The calculated electron spectra show abrupt decreases that can contribute to the 'bite-out' signature observed by the Voyager 1 plasma science instrument. Energy deposition rates in the exosphere of Titan by photoelectrons and magnetospheric electrons are calculated. Calculated N2 EUV airglow emission rates lead to the conclusion that airglow emission due to photoelectron impact is much more important than airglow emission due to magnetospheric electron interactions. Thermal electron temperatures at Titan are calculated for the first time. The electron gas remains well thermalized with the neutral atmosphere for radial distances from the center of Titan less than 3500 km. For radial distances beyond about 4000 km, energy transport dominates the energetics, and the electrons are almost isothermal along magnetic field lines.

Gan, L.↗
NASA NTRS1992

Upstream waves at Mars

Weak, about 0.15 nT, narrow band emissions at the proton gyro frequency are observed by the Phobos magnetometer MAGMA, upstream from the bow shock of Mars. These waves are left-hand elliptically polarized. They may be associated with the pick up of protons from the Martian hydrogen exosphere. Strong turbulence, similar to that observed at the terrestrial bow shock, is found on occasion in the upstream region when the IMF connects to the bow shock. On two occasions this turbulence occurred when the spacecraft crossed the orbit of Phobos. This coincidence raises the possibility that material in the orbits of Phobos interacts with the solar wind in such a way to either affect the direction of the IMF or to cause instabilities in the solar wind plasma. However, since on a third occasion these waves did not occur, these waves may be shock associated rather than Phobos associated.

Russell, C. T.↗
NASA NTRS1992

Lunar base activities and the lunar environment

The Moon is an attractive site for astronomical observatories and other facilities because of the absence of a substantial lunar atmosphere and the stability of the lunar surface. The present lunar atmosphere is sufficiently transparent that there is no significant image distortion due to absorption or refraction. This thin atmosphere results from a combination of small sources and prompt losses. The major source that has been identified is the solar wind, whose total mass input into the lunar atmosphere is approximately 50 gm/sec. The major components of the solar wind are light elements (H and He) that promptly escape from the lunar surface by exospheric evaporation (Jeans' escape). The principal atmospheric loss mechanism for heavier gases is photoionization within a period of weeks to months, followed by immediate loss to the solar wind. Lunar base activities will modify the lunar atmosphere if gas is released at a larger rate than that now occurring naturally. Possible gas sources are rocket exhaust, processing of lunar materials, venting of pressurized volumes, and astronaut life support systems. For even modest lunar base activity, such sources will substantially exceed natural sources, although effects are expected to be localized and transient. The Apollo database serves as a useful reference for both measurements of the natural lunar environment and its modification by lunar base activities.

Vondrak, Richard R.↗
NASA NTRS1992

Stellar lyman alpha emission and the local interstellar medium

Under the auspices of this ADP program, a systematic study was made of IUE archival images in order to extract spectra of the Lyman alpha region and to measure the stellar Lyman alpha flux for as many late-type stars as possible. The Lyman alpha resonance line is a powerful cooling channel for the hot chromospheres of solar-type stars, but has not been studied before in any systematic fashion across the H-R diagram. A major deterrent which has limited the use of Lyman alpha in the study of stellar chromospheres is the contamination of this spectral feature caused by the scattering of solar Lyman alpha photons in the Earth's exosphere. This scattered light is monochromatically imaged through the entrance slot of the IUE telescope and is superposed onto the stellar spectrum. In all but the shortest exposures with IUE, this 'geocoronal emission' overwhelms the stellar flux and makes it impossible to directly measure the strength of the stellar chromospheric feature. The IUESIPS processing contains no provision for correcting standard G.O. output products for this contamination. The first task was to develop a scheme for removing the geocoronal flux, specifically from low-dispersion spectra taken with the Short-Wavelength Camera of IUE. The strategy adopted was to fit a 'sky model' to the spatially-resolved geocoronal emission observed through the large science aperture of the telescope, using the spectral orders on either side of the central ones where the stellar emission is concentrated. The model emission was then subtracted from the observed image, leaving behind the corrected stellar Lyman alpha emission. The details of this fitting procedure are described. Having devised a successful method for removing the unwanted geocoronal emission, the correction procedure was applied to 366 archival images which, from inspection of the photowrites in the IUE browse file, seemed especially promising. In this survey, Lyman alpha emission were eventually detected in the spectra of 227 stars representing a wide range in age, temperature, and luminosity throughout the cool half of the H-R diagram. Previously fewer than 30 such stars had been measured, and an order of magnitude increase in the numbers of stars having Lyman alpha flux measurements is provided. Multiple measurements were made for 52 stars and upper limits on chromospheric flux were derived for another 48 stars.

Simon, Theodore↗
NASA NTRS1992

The Coronal Ultraviolet Berkeley Spectrometer (CUBS)

We describe an instrument package to remotely measure thermospheric, exospheric, and plasmaspheric structure and composition. This instrument was flown aboard the second test flight of the Black Brant XII sounding rocket on December 5, 1989, which attained an apogee of 1460 km. The experiment package consisted of a spectrophotometer to measure He I 584 A, O II 834 A, O I 989 A, hydrogen Lyman beta (1025 A), hydrogen Lyman alpha (1216 A), and O I 1304 A transitions, and a photometer to measure the He II 304 A emission. The optical design of the spectrophotometer was identical to that of the Berkeley Extreme Ultraviolet (EUV) Airglow Rocket Spectrometer payload, flown on September 30, 1988 aboard the maiden flight of the Black Brant XII rocket. We present the initial data analysis and describe directions we will go toward the completion of our study.

Bush, Brett C.↗
NASA NTRS1993

Maintaining the Na atmosphere of Mercury

The possible sources of the Na atmosphere of Mercury are calculatively studied. The likely structure, composition, and temperature of the planet's upper crust is examined along with the probable flux of Na from depth by grain boundary diffusion and by Knudsen flow. The creation of fresh regolith is considered along with mechanisms for supplying Na from the surface to the exosphere. The implications of the calculations for the probable abundances in the regolith are discussed.

Killen, Rosemary M.↗
NASA NTRS1993

Light ion flow in the nightside ionosphere of Venus

The flow characteristics of the light ions H(+) and He(+) have been studied in the midnight region of the ionosphere of Venus. Measurements of ion composition, electron and ion temperatures and magnetic fields by instruments onboard the Pioneer Venus Orbiter have been used in rite electron and ion equations of conservation of mass and momentum to derive the vertical flow velocities of H(+) and He(+). When average height profiles of the measured quantities were used, H(+) was found to flow upward, accelerating to speeds of almost 1 km/s at the ion-exobase. In a similar fashion, He(+) was found to flow downward into the neutral atmosphere where it is readily quenched by charge transfer reactions. The polarization electric field played an important role in forcing H(+) upward, but did not contribute enough to the He(+) force balance to produce upward flow. At the ion-exobase, the outward electric polarization force on H(+) was shown to be five times the gravitational force. Using an analogy with the terrestrial ion-exosphere, H(+) was inferred to flow upward into the ionotail of Venus and accelerate to escape speeds. A planet averaged escape flux of 1.4 x 10 exp 7/sq cm/s was calculated, which is comparable to hydrogen loss rates estimated by other investigators.

Hartle, R. E.↗
NASA NTRS1977

Mariner 9 Ultraviolet Spectrometer Experiment: Mars Atomic Oxygen 1304-A Emission

Selected results are presented for an analysis of Mariner 9 1304-A data from the first 100 passes. Based on a comparison of limb data, the exospheric temperature appears lower on several Mariner 9 passes than when Mariner 6 and 7 encountered the planet in 1969. Temperatures of 300 and 350 K are considered in the atmospheric modeling. At 300 K, derived values of the atomic oxygen concentration are typically between 0.5 and 1 percent of the total density at 135 km, based on fitting theoretical intensities to limb data from several passes. Structure in the limb data below 200 km suggests the possibility that approximately 0.2 kR of the observed approximately 0.8 kR near 150 km is due to dissociative excitation of CO2. There is variability in limb profiles indicating changes in the O distribution with time. The variability does not appear to follow a recognizable pattern. Interpretation of the 1304-A disc data shows a correlation in intensity with the 10.7 cm solar flux and larger O concentrations in the afternoon than in the morning. The correlation in intensity with the 10.7 cm flux is probably due largely to variation in the solar 1304-A fluxes. Selected disc data show localized and random enhancements in the intensity.

Strickland, D. J.↗
NASA NTRS1993

Nitrates in SNCs: Implications for the nitrogen cycle on Mars

Nitrogen is the second most abundant constituent of the Martian atmosphere, after CO2, present at a level of ca. 2.7 percent. Several authors have hypothesized that earlier in the planet's history, nitrogen was more abundant, but has been removed by processes such as exospheric loss from the atmosphere. However, an alternative sink for atmospheric nitrogen is the regolith; model calculations have predicted that, via the formation of NOx, HNO2 and HNO3 in the lower layers of the Martian atmosphere, the regolith might trap nitrite and nitrate anions, leading to the build-up of involatile nitrates. Integrated over 4.5 x 10(exp 9) yr, such a mechanism would contribute the equivalent of a layer of nitrates up to 0.3 cm thick distributed across the Martian surface. Features in thermal emission spectra of the surface of Mars have been interpreted tentatively as emanating from various anions (carbonates, bicarbonates, sulphates, etc.), and the presence of nitrates has also been addressed as a possibility. The identification of carbonates in SCN meteorites has allowed inferences to be drawn concerning the composition and evolution of the Martian atmosphere in terms of its carbon isotope systematics; if nitrites, nitrates, or other nitrogen-bearing salts could be isolated from SNC's, similar conclusions might be possible for an analogous nitrogen cycle. Nitrates are unstable, being readily soluble in water, and decomposed at temperatures between 50 C and 600 C, depending on composition. Any nitrates present in SNC's might be removed during ejection from the planet's surface, passage to Earth, or during the sample's terrestrial history, by weathering etc. The same might have been said for carbonates, but pockets of shock-produced glass (lithology C) from within the EET A79001 shergottite and bulk samples of other SNC contain this mineral, which did apparently survive. Nitrates occurring within the glassy melt pockets of lithology C in EET A79001 might likewise be protected. Lithology C glass was therefore selected for nitrate analysis, first by non-destructive infra red spectroscopy, and then by stepped combustion.

Grady, Monica M.↗