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  • 1
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    The ancient oxygen exosphere of Mars: Implications for atmosphere evolution (1992)
    In:  CASI
    Details
    Publication Date: 2013-08-29
    Description: This study involves the calculation, by the 2-stream method of Nagy and Cravens, of 'hot' oxygen exosphere density profiles for 'ancient' atmospheres and ionospheres (e.g., different extreme ultraviolet fluxes) and the associated escaping fluxes. We computed the total production rates above different 'nominal' ionopause altitudes (not taking into account the fact that some will reenter the atmosphere). We do not consider the additional neutral escape due to the sputtering process described by Luhmann and Kozyra. The results presented here thus represent conservative estimations of the neutral escape fluxes, but generous estimates of ion loss rates (except that here we do not consider charge exchange and impact ionization ion production processes). Further work along the lines of Luhmann and Kozyra can lead to estimates of sputtering losses over time and the roles played by impact ionization and charge exchange.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Workshop on the Martian Surface and Atmosphere Through Time; p 175
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  • 2
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    History of oxygen and carbon escape from the Martian atmosphere (1992)
    In:  CASI
    Details
    Publication Date: 2013-08-29
    Description: A fraction of the oxygen in the Martian atmosphere continually escapes to space because dissociative recombination of the O2(+) ions in the ionosphere can impart sufficient energy to the product O atoms. In addition, ionization of the extended atomic oxygen corona resulting from the above process adds to escape since the solar wind can carry away O(+) ions born above a few hundred km altitude. A further by-product of this ion-pickup by the solar wind is an additional population of escaping oxygen atoms that are sputtered from the atmosphere near the exobase by pickup ions that are on reentry rather than escaping trajectories. This sputtering process can also remove carbon in the form of intact or dissociated CO2 since all atoms and molecules in the 'target' gas are subject to the collisional energy transfer that characterizes sputtering. We have estimated the present rates of escape of oxygen and carbon due to these mechanisms, as well as the rates at several epochs in the history of the solar system.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Papers Presented to the Workshop on the Evolution of the Martian Atmosphere; p 19-20
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  • 3
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    Comparisons of peak ionosphere pressures at Mars and Venus with incident solar wind dynamic pressure (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Radio occultation measurements of electron density profiles from Mariner 6 and 7, the Mariner 9 extended mission, and the U.S. Viking orbiters, together with model ion and electron temperature profiles, are used to derive thermal pressure profiles in the Mars ionosphere. The comparison of the Mars peak ionosphere pressure with the incident solar and dynamic pressure suggests that at solar maximum the Mars ionosphere, like that of Venus, should generally be sufficient to balance the incident solar wind pressure. At solar minimum, when the ionosphere is weakest and the solar wind dynamic pressure is highest, only the peak pressures at high solar zenith angles (SZAs) at Mars appear to be strong enough to balance the incident solar wind pressure. This is similar to the situation at Venus at solar minimum.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 97; 1017-102
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  • 4
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    The ancient oxygen exosphere of Mars - Implications for atmosphere evolution (1993)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: The paper considers absorption of oxygen (atoms and ions) by the surface as a mechanism for the early Martian atmosphere escape, due to the effect of high EUV flux of the ancient sun. Hot oxygen exosphere densities in ancient atmosphere and ionosphere are calculated for different EUV fluxes and the escape fluxes associated with these exposures. Using these densities, the ion production rate above the ionopause is calculated for different epochs including photoionization, charge exchange, and solar wind electron impact. It is found that, when the inferred high solar EUV fluxes of the past are taken into account, oxygen equivalent to that in several tens of meters of water, planet-wide, should have escaped Martian atmosphere to space over the last 3 Gyr.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; E6; p. 10,915-10,923.
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  • 5
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    Oxygen ionization rates at Mars and Venus - Relative contributions of impact ionization and charge exchange (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: Oxygen ion production rates above the ionopauses of Venus and Mars are calculated for photoionization, charge exchange, and solar wind electron impact ionization processes. The latter two require the use of the Spreiter and Stahara (1980) gas dynamic model to estimate magnetosheath velocities, densities, and temperatures. The results indicate that impact ionization is the dominant mechanism for the production of O(+) ions at both Venus and Mars. This finding might explain both the high ion escape rates measured by Phobos 2 and the greater mass loading rate inferred for Venus from the bow shock positions.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; E2; p. 3311-3318.
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  • 6
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    Evolutionary impact of sputtering of the Martian atmosphere by O(+) pickup ions (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: Calculations of solar wind-induced loss rates for evolving solar and atmospheric conditions like those described by Zhang et al. (1992), but including sputtering of the Martian atmosphere by reentering O(+) pickup ions, are described. The inclusion of the sputter loss increases by about 30 percent the cumulative estimated loss of oxygen to that in about 50 m of water (global surface depth) over the last 3.5 billion years. These ions also sputter CO2 and its fragments in substantial amounts. That integrated loss is equivalent to about 0.14 bar atmospheric CO2 pressure, of the order of some estimates of Mars' early atmospheric inventory.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 19; 21; p. 2151-2154.
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  • 7
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    A post-Pioneer Venus reassessment of the Martian dayside ionosphere as observed by radio occultation methods (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: 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.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 95; 14829-14
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  • 8
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    An observational study of the nightside ionospheres of Mars and Venus with radio occultation methods (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Using nightside electron density profiles obtained with radio occultation data from the Viking orbiters, the nightside ionospheres of Mars and Venus are investigated. It is shown that the Mars nightside ionosphere is generally weaker than the Venus nightside ionosphere, and, when it is present, the peak altitude is higher. Otherwise, there is considerable similarity. In particular, the dependence of peak density on solar zenith angle in the range of the Viking nightside observations (90-130 deg) is found to be similar for both planets.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 95; 17095-17
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  • 9
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    Evidence for periodic reconnection at Uranus? (1988)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The unique orientation of Uranus at the time of the Voyager 2 encounter results in a convection dominated magnetosphere. Plasma and magnetic field data from the tail magnetosheath are presented. Velocity decreases of 5-10 percemt seem to occur with a 17-hour period. At least four repetitions of this decrease are observed, in all cases when flow passes over the dayside polar cap. One possible interpretation of these features is that they are signatures of dayside reconnection. The cause of the velocity decreases would be drag on the reconnected flux tubes which are coupled via Birkeland currents to the ionosphere. The coupling efficiency for power transfer between the solar wind and Uranian magnetosphere implied by these decreases is consistent with previous determinations of this quantity.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 15; 733-736
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