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  • 1
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    The atmosphere of Uranus - Results of radio occultation measurements with Voyager 2 (1987)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: The Uranian atmosphere is investigated on the basis of S-band and X-band occultation observations (including measurements of Doppler frequency perturbations) obtained during the Voyager 2 encounter with Uranus in January 1986. The data are presented in extensive tables and graphs and characterized in detail. The atmosphere is assumed to have an H2/He abundance ratio of about 85/15, but also to contain small amounts of CH4 at above-cloud relative humidity 30 percent, cloud-base relative humidity 78 percent, and below-cloud mixing ratio 2.3 percent by number density. Other parameters estimated include magnetic-field rotation period 17.24 h, 1-bar equatorial radius 25,559 + or - 4 km, polar radius 24,973 + or - 20 km, equatorial acceleration of gravity 8.69 + or - 0.01 m/sec sq, and atmospheric temperature 76 + or - 2 K (assuming 85 + or - 3 percent H2).
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 92; 14987-15
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  • 2
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    The atmosphere of Neptune - Results of radio occultation measurements with the Voyager 2 spacecraft (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: This paper presents the vertical temperature and composition profiles of Neptune's troposphere and stratosphere, covering an altitude of 250 km, obtained from radio tracking data that were acquired during Voyager-2's occultation by Neptune, which began near 62 deg N planetographic latitude and ended near 45 deg S latitude. In the computations, the He/H2 abundance ratio 15/85 was adapted, which is consistent with solar abundance estimates and with recent results from Uranus. It was assumed that aerosols and heavier gases such as CH4, NH3, H2S, and H2O have a negligible effect on the microwave refractivity above the 0.5 bar pressure level.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 17; 1733-173
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  • 3
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    Voyager radio science observations of Neptune and Triton (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Voyager 2 undertook radio science investigations of the Neptune and Triton masses and densities, as well as of their atmospheric and ionospheric vertical structures, the atmospheric composition and low-order gravitational harmonics of Neptune, and ring material characteristics. Upon probing the atmosphere of Neptune to a pressure level of about 500,000 Pa, the effects of a methane cloud region and of ammonia absorption below the cloud have become apparent. The tenuous neutral atmosphere of Triton produced distinct signatures in the occultation data; it is inferred that the Triton atmosphere is controlled by water-pressure equilibrium with surface ices.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science (ISSN 0036-8075); 246; 1466-147
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  • 4
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    Radio occultation studies of the Venus atmosphere with the Magellan spacecraft. 1: Experimental description and performance (1994)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: While primarily designed for radar studies of the Venus surface, the high effective isotropic radiated power (EIRP) from the Magellan spacecraft makes it an ideal transmitter for use in radio occultation measurements of the refractivity and absorptivity of the Venus atmosphere. Such experiments have been conducted involving transmissions at 2.3 GHz and 8.4 GHz (13 cm and 3.6 cm, respectively), during spacecraft ingress. Since the stability of the spacecraft transmitter is critical for accurately determining the Doppler shift and amplitude attenuation created as the ray penetrates the atmosphere, the spacecraft transmitter was locked to a 2.1 GHz uplink from a 70-meter DSN station which also received the signals. Because of the high directivity of the spacecraft antenna, and the significant ray bending in the deep Venus atmosphere, a spacecraft tracking maneuver was designed to keep the spacecraft antenna pointed in the direction of the refracted ray path back to Earth. This tracking maneuver, plus the high EIRP of the Magellan transmitter has yielded 3.6 cm refractivity and absorptivity profiles down to the 35 km altitude and 13 cm profiles down to the altitude of critical refraction (approximately 33 km). The statistical uncertainties in the derived profiles are significantly lower than those previously obtained, resulting in extremely accurate profiles of H2SO4 (g) abundance as discussed in an accompanying paper.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Icarus (ISSN 0019-1035); 110; 1; p. 71-78
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  • 5
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    Mars dust storms - Interannual variability and chaos (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The hypothesis is that the global climate system, consisting of atmospheric dust interacting with the circulation, produces its own interannual variability when forced at the annual frequency. The model has two time-dependent variables representing the amount of atmospheric dust in the northern and southern hemispheres, respectively. Absorption of sunlight by the dust drives a cross-equatorial Hadley cell that brings more dust into the heated hemisphere. The circulation decays when the dust storm covers the globe. Interannual variability manifests itself either as a periodic solution in which the period is a multiple of the Martian year, or as an aperiodic (chaotic) solution that never repeats. Both kinds of solution are found in the model, lending support to the idea that interannual variability is an intrinsic property of the global climate system. The next step is to develop a hierarchy of dust-circulation models capable of being integrated for many years.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; E6; p. 10,951-10,961.
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  • 6
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    Triton - Topside ionosphere and nitrogen escape (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The principal ion in the ionosphere of Triton is N(+). Energetic electrons of magnetospheric origin are the primary source of ionization, with a smaller contribution due to photoionization. To explain the topside plasma scale height, it is postulated that N(+) ions escape from Triton. The loss rate is 3.4 x 10 to the 7th/sq cm per sec or 7.9 x 10 to the 24th ions/sec. Dissociative recombination of N2(+) produces neutral exothermic fragments that can escape from Triton. The rate is estimated to be 8.6 x 10 to the 6th N/sq cm per sec or 2.0 x 10 to the 24th atoms/sec. Implications for the magnetosphere of Neptune and Triton's evolution are discussed.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 17; 1717-172
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  • 7
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    The survival of solar flare tracks in interplanetary dust silicates on deceleration in the earth's atmosphere (1982)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Pulse heating experiments on magnesium-rich olivine and pyroxene, two silicates often found in micrometeorites collected in the stratosphere, show that iron ion tracks remain detectable in the transmission electron microscope for temperature maxima up to approximately 600 C. Assuming thermal emissivities near unity, this implies that micrometeorites with surface densities above 1 mg/sq cm are likely to have their track record erased, and that tracks in reentrant or low-density micrometeorites smaller than 7 microns in size are likely to survive atmospheric entry.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Science Conference; Mar 15, 1982 - Mar 19, 1982; Houston, TX
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  • 8
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    Solar control of the upper atmosphere of Triton (1992)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: If the upper atmosphere and ionosphere of Triton are controlled by precipitation of electrons from Neptune's magnetosphere as previously proposed, Triton could have the only ionosphere in the solar system not controlled by solar radiation. However, a new model of Triton's atmosphere, in which only solar radiation is present, predicts a large column of carbon atoms. With an assumed, but reasonable, rate of charge transfer between N2(+) and C, a peak C(+) abundance results that is close to the peak electron densities measured by Voyager in Triton's ionosphere. These results suggest that Triton's upper atmospheric chemistry may thus be solar-controlled. Measurement of key reaction rate constants, currently unknown or highly uncertain at Triton's low temperatures, would help to clarify the chemical and physical processes occurring in Triton's atmosphere.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science (ISSN 0036-8075); 256; 204-206
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