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At least 793 records · Page 44
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NASA NTRS1986

Local variation of haze thickness on the equatorial region of Venus

The horizontal distribution of haze particles in the equatorial region of Venus was investigated using selected polarimetry maps obtained by the Orbiter Cloud Photopolarimeter (OCP) aboard the Pioneer Venus Orbiter, with five of these maps having been acquired on five consecutive terrestrial days. Nine sets of OCP polarimetry maps (PM) similar to PM 16 employed by Kawabata et al. (1980) were selected, and 935 nm data points were extracted to construct a cloud-haze model, from which the haze fraction f(h) within the cloud and the corresponding optical thickness tau(h) were determined for the period from December 1978 to October 1980. The results indicate that, in this period, the upper cloud layer of Venus had haze layers with tau(h) of about 0.02. Furthermore, the submicron particles present within the principal cloud contributed approximately 6 percent to the total scattering cross section per unit volume. Although no systematic time variations in the derived values of tau(h) and f(h) were noticeable, the consecutive five day observations indicated a presence of a large-scale horizontal inhomogeneity in haze particle distribution.

Kawabata, Kiyoshi↗
NASA NTRS1986

Dissipative tides - Application to Venus' lower atmosphere

A linearized planetary scale wave model is used to investigate the effects of thermal and mechanical damping on atmospheric tides. When the damping rate is comparable to the frequency of solar diurnal forcing the circulation consists of three parts: a classical vertically propagating 'atmospheric tide' in the upper atmosphere, a simple thermally direct subsolar-to-antisolar circulation or 'Halley cell' in most of the lower atmosphere, and finally, a reversed 'anti-Halley cell' near the surface. The near-surface circulation produces horizontal divergence near the subsolar point. While tides are a frequently encountered phenomenon (Venus, earth, and Mars), there is so far no observational evidence of a Halley circulation in any planetary atmosphere. A subsolar-antisolar circulation might be possible in Venus' slowly rotating lower atmosphere if the mechanical dissipation time scale is of the order of or less than a Venusian day. Such a circulation could be a factor in maintaining the superrotation of Venus' upper atmosphere.

Covey, Curt↗
NASA NTRS1987

Pioneer Venus and near-earth observations of interplanetary shocks

Twenty-three transient interplanetary shocks observed near earth during 1978-1982, and mostly reported in the literature, have also been identified at the Pioneer Venus Orbiter spacecraft. There seems to be a fairly consistent trend for lower shock speeds, farther from the sun. Shock normals obtained using the Pioneer Venus data correspond well with published values from near earth. By referring to the portion of the Pioneer Venus plasma data used here from locations at longitudes within 37 deg of earth, it is found that shocks are weaker at earth, compared with those closer to the sun.

Mihalov, J. D.↗
NASA NTRS1987

Limits on modes of lithospheric heat transport on Venus from impact crater density

Based on the observed density of impact craters on the Venus surface obtained from Venera 15-16 radar images, a formalism to estimate the upper bounds on the contributions made to lithospheric heat transport by volcanism and lithospheric recycling is presented. The Venera 15-16 data, if representative of the entire planet, limit the average rate of volcanic resurfacing on Venus to less than 2 cu km/yr (corresponding to less than 1 percent of the global heat loss), and limit the rate of lithospheric recycling to less than 1.5 sq km/yr (and probably to less than 0.5 sq km/yr), corresponding to 25 percent (and to 9 percent) of the global heat loss. The present results indicate that heat loss at lithospheric levels in Venus is dominated by conduction.

Grimm, Robert E.↗
NASA NTRS1987

Venus 'lightning' signals reinterpreted as in situ plasma noise

This paper presents detailed evidence in support of the earlier suggestion by Taylor et al. (1985, 1986) that the 100-Hz noise (or LW events) detected by the Pioneer Venus Orbiter across the nightside of Venus should not be unconditionally interpreted as evidence of 'lightning' at Venus. It is shown that the statistical behavior of the electric field noise is consistent with the interpretation that the noise source is local to the spacecraft; the LW events result from the plasma instabilities through which the Orbiter passes. It is also shown that the occurrence of the LW events attributed to lightning decreases significantly at altitudes below about 180 km. The noise is most frequently observed near 180-200 km; the irregularities in this region are interpreted as a result of the nightside interaction of the solar wind/IMF with the ionosphere.

Taylor, Harry A., Jr.↗
NASA NTRS1987

Magnetotails at unmagnetized bodies - Comparison of Comet Giacobini-Zinner and Venus

It is found that the near ionopause environs play a crucial role in the tail formation process at both Venus and G-Z and that draping at the two very different sized bodies occurs on ionopause scale sizes. On the other hand, ion densities, downtail mass fluxes, tailward J x B forces, and lobe betas are factors of about 10,000, 50, 100, and 20 times greater in the G-Z tail than in Venus', while bulk flow speeds and ion temperatures are factors of about 15 and 240 times lower. These large quantitative differences in the properties within the two magnetotails are attributable to the significantly greater upstream mass loading of the solar wind by the extended neutral atmosphere at G-Z (comets in general) compared to the gravitationally bound atmosphere of Venus.

Mccomas, D. J.↗
NASA NTRS1988

A bibliography of dunes: Earth, Mars, and Venus

Dunes are important depositional landforms and sedimentary environments on Earth and Mars, and may be important on Venus. The similarity of dune forms on Earth and Mars, together with the dynamic similarity of aeolian processes on the terrestrial planets indicates that it is appropriate to interpret dune forms and processes on Mars and Venus by using analog studies. However, the literature on dune studies is large and scattered. The aim of this bibliography is to assist investigators by providing a literature resource on techniques which have proved successful in elucidating dune characteristics and processes on Earth, Mars, and Venus. This bibliography documents the many investigations of dunes undertaken in the last century. It concentrates on studies of inland dunes in both hot and cold desert regions on Earth and includes investigations of coastal dunes only if they discuss matters of general significance for dune sediments, processes, or morphology.

Lancaster, N.↗
NASA NTRS1987

Barotropic instability of midlatitude zonal jets on Mars, earth and Venus

A linearized, nondivergent, barotropic vorticity model on a sphere is used to intercompare the fastest growing, barotropically unstable wave modes computed for zonal jets at high latitudes in the middle atmospheres of Venus, earth, and Mars. The model is briefly described, and the choice of a range of zonal jet parameters - primarily Rossby numbers and the jet width - appropriate to Venus and Mars is discussed. The results are presented and compared with those found by Elson (1982) and Hartmann (1983) for nondivergent, barotropically unstable modes in relatively broad, midlatitude zonal jets in planetary atmospheres. Some specific examples appropriate to Venus and Mars are presented.

Michelangeli, Diane V.↗
NASA NTRS1988

Venus mesosphere and thermosphere. III - Three-dimensional general circulation with coupled dynamics and composition

The three-dimensional structure and circulation of Venus' upper mesosphere and thermosphere is examined by means of a modification of NCAR's earth thermosphere general circulation model, using the parameterizations from an earlier two-dimensional Venus model that included eddy diffusion and wave drag. Many of the observed Venus thermosphere features are found to be reproduced by the model, which thereby serves as a benchmark on which to incorporate additional minor constituents and test new self-consistent parameterizations for wave drag and superrotation.

Bougher, S. W.↗
NASA NTRS1988

Evidence of lightning and volcanic activity on Venus - Pro and con

It is argued that the impulsive 100-Hz noise bursts detected with the use of the electric field antenna on the Pioneer Venus Orbiter (PVO) have plasma wave charcteristics that can only be explained if they are whistler mode signals of a type that can be produced by atmospheric discharges. It is further argued that these data are evidence for lightning and volcanic activity on Venus. A reply contends that the PVO electric field measurements are unrelated to either the lower atmosphere or the surface of Venus.

Scarf, Frederick L.↗
NASA NTRS1988

Deformational models of rifting and folding on Venus

Features of presumed tectonic origin on Venus are reviewed, and lithospheric strength envelopes are derived based on laboratory measurements of the deformational properties of crustal and subcrustal rocks, extrapolated to conditions appropriate to Venus. Models for rifting and folding are developed that use this lithospheric structure and take into account both brittle and ductile yielding as well as finite elastic strength. For both rifting and folding, structures with characteristic widths and spacings are predicted whose size depends on the thickness of the lithosphere, density contrast, and elastic properties of the layer. Finally, the model predictions are compared with the widths and spacings of observed tectonic features, and it is concluded that they are consistent with a relatively strong mantle layer separated from a thin brittle surface layer by a ductile lower crust. These results allow constraints to be placed on the crustal thickness and thermal gradient on Venus.

Banerdt, W. Bruce↗
NASA NTRS1987

The average configuration of the induced Venus magnetotail

The interaction of the solar-wind flow with Venus is discussed as well as the morphology of magnetic-field-line draping in the Venus magnetotail. Emphasis is placed on the importance of the interplanetary magnetic field X-component in controlling the configuration of field draping in this induced magnetotail. The average magnetic configuration of this magnetotail is studied. A connection is made between the derived consistent plasma flow speed and density and the observational energy/charge range and sensitivity of the Pioneer Venus Orbiter plasma analyzer.

Mccomas, D. J.↗
NASA NTRS1988

Solar cycle changes in the morphology of the Venus ionosphere

Altitude profiles of median plasma density are presented for the central nightside Venus ionosphere for the solar cycle maximum (SCmax) and minimum (SCmin), constructed from Pioneer Venus retarding-potential-analyzer total ion density data. The curves demonstrate that, between the altitudes of 200 and 2000 km, the nightside ionospheric density during SCmin was smaller, by a factor of 10, than it was during SCmax. This observation, along with the Venera radio occultation measurements at SCmin, support the contention that the Venus SCmin ionosphere is typically confined to an altitude below 250 km at all solar zenith angles (SZAs), with possible exception of an SZA interval near the terminator, and is composed primarily of O2(+).

Knudsen, William C.↗
NASA NTRS1988

Galileo Earth-Venus trajectory correction maneuver design

As a result of delays in the Space Shuttle program and a re-evaluation of safety concerns which resulted in cancellation of the Shuttle Centaur Upper Stage program, the Galileo mission to Jupiter has undergone a substantial redesign effort. Use of the Inertial Upper Stage (IUS) for injection has necessitated a complicated Venus-Earth-Earth gravity assist trajectory in order to reach Jupiter. One flyby of Venus and two flybys of Earth are required to compensate for the lower injection energy available from the IUS. As the spacecraft was not originally designed for the environment within 1 AU of the Sun, modifications to the spacecraft and operating procedures have been developed. This paper describes some of these changes and how they have affected the design and implementation of trajectory correction maneuvers on the Earth-Venus leg of the mission. A strategy of biasing the IUS target at injection to control the statistical velocity distribution of a subsequent trajectory correction maneuver is discussed.

Wilson, M. G.↗
NASA NTRS1989

A heat-pipe mechanism for volcanism and tectonics on Venus

A heat-pipe mechanism is proposed for the transport of heat through the lithosphere of Venus. This mechanism allows the crust and lithosphere on Venus to be greater than 150 km. thick. A thick basaltic crust on Venus is expected to transform eclogite at a depth of 60 to 80 km; the dense eclogite would contribute to lithospheric delamination that returns the crust to the interior of the planet completing the heat-pipe cycle. Topography and the associated gravity anomalies can be explained by Airy compensation of the thick crust. The principal observation that is contrary to this hypothesis is the mean age of the surface that is inferred from crater statistics; the minimum mean age is about 130 Myr and this implies an upper limit of 2 cubic kilometers per year for the surface volcanic flux. If the heat-pipe mechanism was applicable on the Earth in the Archean it would provide the thick lithosphere implied by isotopic data from diamonds.

Turcotte, D. L.↗
NASA NTRS1989

Comet Halley: The view from Pioneer Venus

The plans to scan Halley's Comet at close range using the Pioneer Venus Orbiter are discussed. The composition of comets, their paths through space, and the history of comet encounters are examined. An ultraviolet spectrometer aboard the spacecraft will determine the composition of the gaseous coma and will measure the total gas production during its passage. The Pioneer Venus Orbiter will observe the comet for five weeks before solar interference with communications occurs as Venus passes on the far side of the Sun from Earth. Diagrams of the solar system and the relationship of the comet to the planets and the Sun are provided.

Source record↗
NASA NTRS1988

A precursor to the Venus bow shock

This paper describes the observations by the Pioneer-Venus-Orbiter Langmuir probe of the low-density Venus bow shock precursor, i.e., a region of electron and ion-current enhancements just upstream from the Venus bow shock. It is suggested that the precursor signatures represent the effects of a small population of energetic ions in this region. Several possible planetary sources of these ions are considered. The paper also discusses the Langmuir probe measurement technique itself as it applies to the very low densities of the precursor.

Brace, L. H.↗
NASA NTRS1989

Estimation of the rate of volcanism on Venus from reaction rate measurements

Laboratory rate data for the reaction between SO2 and calcite to form anhydrite are presented. If this reaction rate represents the SO2 reaction rate on Venus, then all SO2 in the Venusian atmosphere will disappear in 1.9 Myr unless volcanism replenishes the lost SO2. The required volcanism rate, which depends on the sulfur content of the erupted material, is in the range 0.4-11 cu km of magma erupted per year. The Venus surface composition at the Venera 13, 14, and Vega 2 landing sites implies a volcanism rate of about 1 cu km/yr. This geochemically estimated rate can be used to determine if either (or neither) of two discordant geophysically estimated rates is correct. It also suggests that Venus may be less volcanically active than the earth.

Fegley, Bruce, Jr.↗