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At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Venus bowshock precursor

Significant ion and electron flux enhancements immediately upstream of the Venus bowshock have been observed by the Electron Temperature Probe on the Pioneer Venus Orbiter. It is shown that mass loading of the solar wind by oxygen ions (without the effect of MHD turbulence) accounts for only about 3.5 percent of the observed effect. Evidence is presented that turbulence may play a nonnegligible role in the ion pickup process in front of the Venus bowshock.

Fontheim, Ernest G.↗
NASA NTRS1990

The red and green lines of atomic oxygen in the nightglow of Venus

O(1D) and O(1S), the excited states that give rise to the atomic oxygen red and green lines, are produced in the Venus nightglow in dissociative recombination of O2(+). The emissions should also be excited by precipitation of soft electrons, the suggested source of the 'auroral' emission features of atomic oxygen at 1304 and 1356 A, which have been reported from observations of the Pioneer Venus Orbiter Ultraviolet Spectrometer. No emisison at 6300 or 5577 A was detected, however, by the visible spectrophotometers on the Soviet spacecraft Veneras 9 and 10; upper limits have been placed on the intensities of these features. The constraints placed on models for the auroral production mechanism by the Venera upper limits by modeling the intensities of the red and green lines in the nightglow are evaluated, combining a model for the vibrational distribution of O2(+) on the nightside of Venus with rate coefficients recently computed by Guberman for production of O(1S) and O(1D) in dissociative recombination of O2(+) from different vibrational levels. The integrated overhead intensities are 1 - 2 R for the green line and about 46 R for the red line.

Fox, J. L.↗
NASA NTRS1990

Evidence for lightning on Venus

Lightning in the clouds on Venus should most nearly resemble intra-cloud discharges on earth. Intra-cloud discharges are weaker, shorter and more frequent than cloud-to-ground discharges and cause more slowly varying luminosity. Terrestrial lightning also has both geographic and local time orderings. At Venus, there is much evidence for lightning similar to terrestrial lightning. The Venera landers saw electromagnetic radiation much like sferics from terrestrial lightning. Pioneer Venus also saw such signals leaking out into the night ionosphere. These signals have a strong local time dependence not unlike terrestrial lightning. The local time distribution helps explain the mixed results of optical surveys. The successful observation was on the evening side where there is much apparently lightning-generated electromagnetic radiation; the unsuccessful observations were on the morning side where such plasma waves, and hence lightning discharges, appear to be rare.

Russell, C. T.↗
NASA NTRS1989

Terrestrial spreading centers under Venus conditions - Evaluation of a crustal spreading model for Western Aphrodite Terra

The model of Reid and Jackson (1981) for terrestrial spreading centers is applied to Venus conditions. On the basis of spreading rate, mantle temperature, and surface temperature, the model predicts both isostatic topography and crustal thickness. The model and Pioneer Venus altimetry and gravity data are used to test the hypothesis of Head and Crumpler (1987) that Western Aphrodite Terra is the location of crustal spreading on Venus. It is concluded that a spreading center model for Ovda Regio in Western Aphrodite Terra could account for the observed topography and line-of-sight gravity anomalies found in the Pioneer data.

Sotin, C.↗
NASA NTRS1990

Upward ion flow in ionospheric holes on Venus

The nature of ion flow within two ionospheric holes on the nightside of Venus was investigated using ion composition measurements made by the ion mass spectrometer on the Pioneer Venus Orbiter. A comparison of the altitude profiles of the observed ion densities with those expected under diffusive equilibrium conditions indicates that the major ions O(+), NO(+), and O2(+), as well as the minor ions H(+) and He(+) flow upward and away from Venus along the axes of the holes. This result agrees with a quantitative evaluation of the ion flow speeds appearing in expressions derived from the equations for conservation of mass and momentum of the ions and electrons. The analysis shows that all ion species flow upward in the holes because the upward force produced by the plasma pressure gradient exceeds all downward forces. However, the nature of the ion source required to maintain such flow is not known.

Hartle, R. E.↗
NASA NTRS1990

International Ultraviolet Explorer observations of Venus SO2 and SO

Results of recent International Ultraviolet Explorer observations of Venus made on January 20, 1987 and April 2 and 3, 1988 are presented along with a reanalysis of the 1979 observations (Conway et al., 1979). The observations indicate that the amount of sulfur dioxide at the cloud tops of Venus declined by a factor of 8 + or - 4 from 380 + or - 70 ppb in 1979 to 50 + or - 20 ppb in 1987 and 1988. These values are consistent with the Pioneer Venus results. Absorption features of sulfur monoxide are identified for the first time, and the SO mixing ratio above the cloud level is estimated to be 20 + or - 10 ppb for 1979. This is consistent with photochemical models by Winick and Stewart (1980) and Yung and DeMore (1982) and with the upper limit from Wilson et al. (1981).

Na, Chan Y.↗
NASA NTRS1990

The Magellan Venus radar mapping mission

The NASA Magellan Venus Radar Mapper spacecraft, which will be placed into orbit around Venus on August 10, 1990, is described and its mission is discussed. The orbiter's 12-cm wavelength, multimode radar system is examined and the applications of its modes are addressed. In the SAR mode, it can image most of the Venus surface at a resolution of better than 300 m, approaching 120 m over more than half the planet. In the altimeter mode, the radar will determine topographic relief to a vertical accuracy of better than 50 m averaged over a surface resolution cell approximately 10 km in diameter. In the radiometer mode, the radar receiver can determine the surface radio emission brightness temperature with an absolute accuracy of 20 K, at a resolution of 2 K. The nature of the data products and the archiving plans are also considered.

Saunders, R. S.↗
NASA NTRS1989

The Magellan Venus mapping mission

The April 1989 Magellan (MGN) Mission to Venus will initiate a new phase in the exploration of the solar system. In addition to being the first U.S. planetary mission in 10.5 years, it will also be the first such mission to use the Space Shuttle and IUS 2-Stage as launch vehicles. Upon arrival at Venus the spacecraft will begin a systematic mapping of the surface of that planet using side-looking SAR. This paper discusses some of the interesting trade-offs in mission design for this mission in the areas of launch and injection, interplanetary cruise, Venus Orbit insertion, and mapping. The Magellan mapping strategy is discussed briefly along with a few special experiments being considered for the MGN extended mission.

Cutting, E.↗
NASA NTRS1990

Barotropic instability with divergence - Theory and applications to Venus

IR images of Venus reveal a curious double-lobed hot spot in the polar region. Elson (1982) has suggested that this dipole represents a barotropic instability associated with a high-latitude jet. Unfortunately, the classical theory of barotropic instability cannot predict temperature variations. This paper generalizes the theory to include horizontal divergence, vertical motions, and temperature variations, and applies it to the stratosphere of Venus. The fastest-growing barotropic instability in the nominal model matches the observed dipole in period and horizontal temperature pattern. The accompanying wind variations are comparable to the speed of the mean jet, indicating strong nonlinear effects. It is concluded that the Venus dipole may represent the self-limited stage of a barotropic instability with divergence.

Dobrovolskis, Anthony R.↗
NASA NTRS1990

A statistical study of ions and magnetic fields in the Venus magnetotail

A statistical characterization is made of the combined ion and magnetic field properties of the Venus magnetosheath and magnetotail, on the basis of plasma and magnetic field data from 223 Pioneer Venus orbits; no assumptions are made as to existing regions or their plasma and field characteristics. Plasma is found to flow tailward in all locations, and the magnetotail is highly draped. Weak magnetic field asymmetries are associated with the plasma dropouts. A high-E/q plasma population, previously interpreted as planetary-pickup ions, is found asymmetrically both within the tail and in the adjacent sheath. The Venus tail is filled with plasma that is primarily shocked solar wind, at fluxes that are sometimes undetectable; the tail coexists with a photoion population which generates asymmetries in the bulk plasma and magnetic field properties.

Moore, K. R.↗
NASA NTRS1990

Geologic evidence of hotspot activity of Venus - Predictions for Magellan

A number of distinctive types of geologic features have been identified on Venus that are interpreted to be related to thermal plumes including domal rises, coronae, and major composite shield volcanoes. The basic characteristics of these features as well as their distribution are documented. The three types of features have related morphologies and are interpreted to represent a continuum of features formed by mantle plumes at scales from 100s to over 1000 km. The Artemis structure, located in Aphrodite Terra, is proposed to be a large corona. If crustal spreading processes are operating on Venus, hotspot features should form chains on the surface as seen in terrestrial ocean basins. On the basis of current data on hotspot-related feature distribution on Venus, no clear evidence exists for hotspot chains. The complete distribution of hotspot features in Magellan data will be used to understand better the relationship between interior processes and surface features, as well as to provide a test for the crustal spreading hypothesis.

Stofan, Ellen R.↗
NASA NTRS1990

A post-Pioneer Venus reassessment of the Martian dayside ionosphere as observed by radio occultation methods

Global characteristics of the dayside ionosphere of Mars were investigated by reanalyzing the electron-density dayside altitude profiles obtained by Mariners 4, 6, 7, and 9 and the Viking 1 and 2. The properties of both the electron density peaks and the topside profiles with the behavior expected for a Chapman layer in the Mars ionosphere were compared with those observed at Venus with the Pioneer Venus. The results yield an improved picture of the solar zenith angle dependences of both the peaks and the scale heights of the ionosphere of Mars, and their comparison with Venus at solar minimum. It is shown that, under similar conditions where the incident solar wind dynamic pressure exceeds the peak ionospheric thermal pressure, the Martian dayside ionosphere peaks at higher altitudes in the flanks and has a greater scale height. Thus, Martian and Venusian ionospheres would present different obstacles to solar wind.

Zhang, M. H. G.↗
NASA NTRS1990

Venus - A total mass estimate

Reductions of four independent blocks of Pioneer Venus Orbiter Doppler radio tracking data have produced very consistent determinations of the GM of Venus (the product of the universal gravitational constant and total mass of Venus). These estimates have uncertainties that are significantly smaller than any values published to date. The value of GM is also consistent with previously published results in that it falls within their one-sigma uncertainties. The value of 324858.60 + or - 0.05 cu km/sec sq is the best estimate.

Sjogren, W. L.↗
NASA NTRS1990

Magellan radar to reveal secrets of enshrouded Venus

Imaging Venus with a synthetic aperture radar (SAR) with 70 percent global coverage at 1-km optical line-pair resolution to provide a detailed global characterization of the volcanic land-forms on Venus by an integration of image data with altimetry is discussed. The Magellan radar system uses navigation predictions to preset the radar data collection parameters. The data are collected in such a way as to preserve the Doppler signature of surface elements and later they are transmitted to the earth for processing into high-resolution radar images. To maintain high accuracy, a complex on-board filter algorithm allows the altitude control logic to respond only to a narrow range of expected photon intensity levels and only to signals that occur within a small predicted interval of time. Each mapping pass images a swath of the planet that varies in width from 20 to 25 km. Since the orbital plane of the spacecraft remains fixed in the inertial space, the slow rotation of Venus continually brings new areas into view of the spacecraft.

Saunders, R. Stephen↗
NASA NTRS1990

The spin vector of Venus

Analysis of radar observations from 1964 through 1983 yields the following values for the components of the spin vector of Venus: P = 243.026 + or - 0.006 d (retrograde); alpha = 272.75 + or - 0.09 deg; and delta = 67.10 + or - 0.09 deg, where the standard errors quoted are three- to five-fold larger than the statistical standard errors and encompass the changes in results obtained by various tests designed to expose possible systematic errors. These values demonstrate conclusively that the spin state of Venus is not in resonance with the relative orbital motions of Venus and earth.

Shapiro, I. I.↗
NASA NTRS1991

Lithospheric and atmospheric interaction on the planet Venus

Lithospheric and atmospheric interaction in the planet Venus are discussed. The following subject areas are covered: (1) manifestation of exogenic processes using photogeological data; (2) the chemical composition and a chemical model of the troposphere of Venus; (3) the mineral composition of surface rock on Venus; and (4) the cycles of volatile components.

Volkov, Vladislav P.↗
NASA NTRS1990

Magellan orbits Venus

Various events surrounding Magellan's orbit of Venus are recounted. Significant events include the successful firing of a solid rocket motor while the spacecraft was behind Venus to transfer it from a solar-centered trajectory to an orbit around the planet. The spacecraft orbits Venus every 3.26 hours at a maximum altitude of 8500 km and minimum altitude of 291 km in an elliptical orbit. The successful August 16 test of the synthetic-aperture radar system is discussed, noting that it produced two strips, each about 20 km x 16,000 km, revealing details as small as 120 m. Two anomalies causing a delay in the start of mapping operations and subsequent breaks in the communication link with earth for 14.5 hours and 17.7 hours are discussed. Protective measures directed from the spacecraft's ROM during breach of contact are listed, and possible causes of the anomalies are suggested, such as solar activity or hardware or software elements, although the actual cause is not yet known.

Mclaughlin, W. I.↗
NASA NTRS1990

The surface of Venus

NASA's Magellan spacecraft first revealed the true nature of Venus's surface when it began to transmit photos in September 1990, and it continues its planned mission to map the whole surface of the planet. The very sharp images obtained and processed to date leave the overall impression that Venus is a dynamic world that has been shaped by processes fundamentally similar to those that have taken place on earth, but often with dramatically different results. Magellan's synthetic aperture radar reveals details as small as 120 meters across, one tenth the size of those detected previously. Venus will be mapped eight times over the next five years, which will improve the resolution and fill in missing areas. A number of images are provided along with explanatory descriptions.

Saunders, R. Stephen↗