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  • LUNAR AND PLANETARY EXPLORATION  (7)
  • 1990-1994  (7)
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  • 1
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    Electron impact excitation and dissociation of N2 via the b 1Pi(u) state (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Electron impact excitation of the b 1Pi(u) state in N2 plays a prominent role in the dissociation of the molecule and thus in the production of atomic nitrogen in planetary atmospheres. Electron impact excitation cross sections combined with electron-impact-induced fluorescence measurements can yield the corresponding dissociation cross sections. Serious discrepancies exist among excitation cross sections reported in the literature. To clarify the situation, these cross sections were measured at two impact energies using electron energy loss spectroscopy. The new results are in agreement with recent values deduced from optical measurements and fall midway between previous results which are too high or low by factors of 2.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 17
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  • 2
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    A reanalysis of Voyager UVS observations of Titan (1992)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Data from the Voyager 1 Ultraviolet Spectrometer (UVS) encounter at Titan were reanalyzed to reveal new spatial and spectral information. The observations, data reduction and spectrum analysis are discussed. The following conclusions were drawn: Titan's airglow is driven by a combination of mechanisms including both EUV deposition and magnetospheric particle precipitation; the emissions are distributed throughout a range of altitudes in the atmosphere; enhanced nightside emissions of the Helium 58.4 nm line may be the result of a thermospheric diurnal temperature variation; the CD2 spectrum, with enhanced Ly(alpha) and N-2 64.5 and 67.0 nm emission, implies that some of the high altitude excitation is due to impact dissociative excitation of N2 by electrons with E greater than 150 eV; the CD2 spectrum is consistent with the existence of a high altitude bright spot in the dayside disk with dimensions from 400 to 900 km.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: ESA, Symposium on Titan; p 67-74
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  • 3
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    Detection of the hydroxyl radical in the Saturn magnetosphere (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: The detection of neutral OH molecules near the orbit of Tethys is reported. The findings suggest that neutral OH is one of the dominant species in Saturn's inner magnetosphere, implying a source rate for H2O 20 times greater than current theoretical estimates. One possible explanation is that the micrometeorite erosion rates of the inner satellites are significantly higher than expected.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Nature (ISSN 0028-0836); 363; 6427; p. 329-331.
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  • 4
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    The abundance of O(2+) in the Jovian magnetosphere (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: From a synthesis of data from the Plasma-Science and Ultraviolet-Science instruments on Voyager 1 a radial profile is presented of O(2+) abundance between 4.9 and 42 Jovian radii. A sharp rise is noted in O(2+) mixing ratio near 7.5 Jovian radii, coincident with a sharp rise in effective electron temperature at the outer boundary of the Io plasma torus. Beyond 8.5 Jovian radii the O(2+) mixing ratio is found to be roughly constant which indicates freezing of the ionization prevailing at the outer edge of the hot torus.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 19; 79-82
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  • 5
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    The distribution of atomic hydrogen in the magnetosphere of Saturn (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-08-27
    Description: Three sets of previously unpublished Voyager ultraviolet spectrometer observations of the Saturn system conducted by Voyager 1 and 2 are presented. Voyager 1 observations during the postencounter period provided a map of the distribution looking down on the equatorial plane. The reduced data show a nonuniform distribution in local time with a preponderance of emission on the duskside. The emission extends radially inward to the top of the Saturn atmosphere with stronger signals appearing close to the planet, strongly suggesting that the principal source is the sunlit Saturn atmosphere. In the subsolar sector of the magnetosphere no excess emission in the vicinity of Titan's orbit is detectable. Voyager 1 and 2 preencounter observations also show H Ly-alpha emissions increasing monotonically toward the planet but with a distinctive dawnside excess. It is concluded that the preliminary results reporting a toroidal hydrogen distribution with a cavity inside 8 RS are invalid because of limited spatial resolution and poor statistics.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 97; A4 A
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  • 6
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    Source processes for the alkali metals in the atmosphere of Mercury (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: A large (fivefold) increase in Mercury's potassium (K) column abundance on October 14, 1987, has been reported by Sprague et al. (1990), who attributed the enhancement to diffusion through the surface in the Caloris Basin, from depths of order 10 km. The postulated source rate is much larger than any previously estimated diffusion rate, and, if true, certainly affects consideration of the origin of other atmospheric species. However, Killen et al. (1991) have pointed out that the claim is not supported by the published observations of K or sodium as a whole. Sprague et al. (1991) have responded by further hypothesizing the existence of several other sources of gas diffusing out of the regolith, all of which are time variable. In any case, the Sprague et al. data indicate large variations in abundance, and it is important to understand the cause. With this issue in mind, the available abundance estimates for correlation with possible controlling physical parameters has ben examined. A significant correlation between the average zenith K column abundance and indices of solar activity has been found.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 18; 1659-166
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  • 7
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    Limits to the lunar atmosphere (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: Apollo UV spectrometer experiment set limits on the density of oxygen of less than 500/cu cm, and the Apollo Lunar Atmospheric Composition Experiment data imply a value less than 50/cu cm above the subsolar point. These limits are surprisingly small relative to the measured value for sodium. A simple consideration of sources and sinks predicts significantly greater densities of oxygen. It is possible but doubtful that the Apollo measurements occurred during an epoch in which source rates were small. A preferential loss process for oxygen on the darkside of the moon is considered in which ionization by electron capture in surface collisions leads to escape through acceleration in the local electric field. Cold trapping in permanently shadowed regions as a net sink is considered and discounted, but the episodic nature of cometary insertion may allow formation of ice layers which act as a stabilized source of OH. On the basis of an assumed meteoroid impact source, a possible emission brightness of 50 R in the OH(A - X)(0,0) band above the lunar bright limb is predicted.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 1351-136
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