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  • 1
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    Dynamic properties of the thermosphere inferred from Pioneer Venus mass spectrometer measurements (1980)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The data obtained by the Pioneer Venus spectrometer experiments indicate that the day-night temperature contrast on Venus is associated with wind velocities of about 200 m/s which transport oxygen, helium, and hydrogen toward the night side. A mass exchange with the mesosphere commensurate with an eddy diffusion coefficient of 3 x 10 to the 7th is required to buffer the horizontal advection so as to reproduce the observed day time bulge in oxygen and the small diurnal variations in helium. The observed time response and magnitude of the day-night density variations require transport processes to be effective over time periods between five and ten days, implying a superrotation rate or prevailing winds in excess of 50 m/s at the equator. Nonlinear mass transport results in wave steepening and contributes to the amplification of the density extrema in hydrogen and helium.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research; 85; Dec. 30
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  • 2
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    High resolution measurements of nightside ion troughs at Venus - Evidence of electrodynamic perturbations (1982)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: The Bennett rf ion mass spectrometer of the Pioneer Venus Orbiter was expressly designed to provide variable temporal resolution for measurements of thermal ion composition and density. The Explore-Adapt mode is used to obtain priority for measuring the most prominent ion species; in the 2/16 configuration, the two dominant ions within the available range of 16 species are selectively sampled at the highest rate of 0.2 sec/sample. The high-resolution measurements are combined with independent observations from the magnetic field, neutral mass spectrometer, and electron temperature experiments in investigating sharply structured troughs in the low-altitude nightside ion concentrations. The results suggest a close correlation between the structure in the ion distributions and the structured configuration of the magnetic field that is draped about the planet. In the regions of the ion depletions, sharp fluctuations in electron temperature and anomalous increases in the density of neutral gases suggest that the ion depletion may be associated with dynamic perturbation in the ion and neutral flows and/or local joule heating.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: (ISSN 0273-1177)
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  • 3
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    Venus nighttime hydrogen bulge (1980)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: The concentration of atomic hydrogen in the Venus thermosphere near 165 km altitude and approximately 18 deg north latitude has been derived from Pioneer Venus in situ measurements of H(+), O(+), O and CO2 concentrations, under the assumption of chemical equilibrium. Altitude profiles of derived H concentration suggest that chemical equilibrium prevails to an altitude of at least 200 km on the dayside and to 165 km on the nightside. Measurements below these limits were made by the ion and neutral mass spectrometers on the orbiter spacecraft between December 1978 and July 1979, while periapsis traversed a complete diurnal cycle. The hydrogen concentration is found to rise sharply at both terminators from a dayside value of approximately 50,000/cu cm, and to exhibit an asymmetric nightside distribution with a peak density in the predawn sector approximately 400 times greater than the dayside value. Analysis suggests that wind-induced diffusion, combined with exospheric return flow, can account for the observed hydrogen behavior. The large day-night temperature contrast enhances advective transport, which produces the large H concentration diurnal variation; the shift of the H concentration nighttime maximum toward dawn is caused by atmospheric superrotation.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters; 7; Nov. 198
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  • 4
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    Variations in ion and neutral composition at Venus - Evidence of solar control of the formation of the predawn bulges in H/+/ and He1 (1982)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: A comparison of ion and neutral composition measurements at Venus for periods of greatly different solar activity provides qualitative evidence of solar control of the day-to-night transport of light ion and neutral species. Concentrations of H(+) and He in the predawn bulge near solar maximum in November, 1979, exhibit a depletion signature correlated with a pronounced modulation in the solar F10.7 and EUV fluxes. This perturbation, not observed in the predawn region during an earlier period of relative quiet solar conditions, is interpreted as resulting from pronounced changes in solar heating and photoionization on the dayside, which in turn modulate the transport of ions and neutrals into the bulge region.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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  • 5
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    Comparison of calculated and measured ion densities on the dayside of Venus (1979)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: Data from the Pioneer Venus ion mass spectrometers are compared with model calculations of the ion density distributions appropriate for daytime conditions. The model assumes diffusive equilibrium upper boundary conditions for the major ions (O2(+), O(+), CO2(+), He(+), and H(+)); the agreement between the calculated and measured gross behavior of these ions is reasonably good except for H(+), which may be influenced strongly by convective transport processes. The distributions of five minor ions (C(+), N(+), NO(+), CO(+), and N2(+)) are also calculated for the chemically controlled region (less than approximately 200 km); the agreements are, in general, poor, an indication that the present understanding of the Venus minor ion chemistry is still incomplete.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science; 205; July 6
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  • 6
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    Titan: Evidence for seasonal change - A comparison of Hubble Space Telescope and Voyager images (1992)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A comparison of images of Titan obtained by the HST in August, 1990 with Voyager 1 and 2 images respectively obtained 10 and 9 years earlier has indicated a reversal of the seasonal hemispheric brightness asymmetry near 440 and 550 nm wavelengths; the northern hemisphere is in the more recent observations the brighter of the two, by about 10 percent. Titan's albedo pattern is therefore adequately explained by a seasonal model.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Icarus (ISSN 0019-1035); 97; 1 Ma
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  • 7
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    In situ results on the variation of neutral atmospheric hydrogen at Venus (1985)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The concentrations of neutral hydrogen, n(H), in the Venus atmosphere were derived from an ion and neutral charge exchange relationship involving O(+), H(+), O, and CO2, which were measured by OIMS and ONMS on board the Pioneer Venus for the period 1979-1980 covering three diurnal cycles of Venus. There was a persisting dawn bulge in the diurnal distribution of n(H), which peaked at levels near 5 x 10 to the 7th/cu cm at altitudes below 165 km. Large day-to-day variations of up to a factor of 5 in n(H) were frequently encountered, in addition to some local time variations in the bulge location. Although the appreciable short-term variability in n(H) makes precise assessment of the interannual variations difficult, no distinct evidence for interannual variation in n(H) was found. These results were confirmed by the facts that no significant interannual differences in n(H3) measured directly by ONMS were detected, and that only small (not more than 10 percent, decline) variations in the solar EUV flux were found in the 1979-1980 period.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Advances in Space Research (ISSN 0273-1177); 5; 9, 19
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  • 8
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    Interannual and short-term variations of the Venus nighttime hydrogen bulge (1984)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 89; 10669-10
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  • 9
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    Antarctic polymict eucrite Yamato 792769 and the cratering record on the HED parent body (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Compared to most other Yamato polymict eucrites, Yamato Y792769 eucrite includes fewer and smaller eucritic clasts with homogenized pyroxenes, and its fine-grained matrix is shock-compacted and sintered. In this work, the relationships between the Antarctic eucrite Y792769, monomict eucrites, polymict eucrites, and isotopic ages are investigated, using results of Ar-39/Ar-40 method to date the time of the major thermal event on the Y792769 body and the Rb-Sr and Sm-Nd methods to determine whether relict older ages might have been preserved in some of the breccia materials. The Ar-39/Ar-40 time of the last thermal event which produced the Y792769 texture is 3.99 +/- 0.04 Ga. The complete resetting of the Ar-39/Ar-40 age is consistent with the texture of Y792769 observed in TEM, suggesting that shock compaction converted part of the matrix plagioclase to maskelynite. The Sm-Nd data give an age of 4.23 +/- 0.12 Ga, reflecting partial resetting of the Sm-Nd system during breccia formation. The 3.9 Ga Ar-39/Ar-40 age probably reflects a period of intense meteoroid bompardment which affected the entire inner solar system.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 57; 9; p. 2111-2121.
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  • 10
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    Chemical and isotopic measurements of micrometeoroids by secondary ion mass spectrometry (A0187-2) (1984)
    In:  CASI
    Details
    Publication Date: 2006-02-14
    Description: The objective of this experiment is to measure the chemical and isotopic composition of interplanetary dust particles of mass greater than 10 to the minus 10 power G for most of thermator elements expected to be present.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA. Langley Research Center Long Duration Exposure Facility (LDEF); p 131-134
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