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  • 1
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    Escape of Mars atmospheric carbon through time by photochemical means (1993)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: Luhmann et al. recently suggested that sputtering of the Martian atmosphere by re-entering O(+) pickup ions could have provided a significant route of escape for CO2 and its products throughout Mars' history. They estimated that the equivalent of C in an approximately 140-mbar CO2 atmosphere should have been lost this way if the Sun and solar wind evolved according to available models. Another source of escaping C (and O) that is potentially important is the dissociative recombination of ionospheric CO(+) near the exobase. We have evaluated the loss rates due to this process for 'ancient' solar EUV radiation fluxes of 1, 3, and 6 times the present flux in order to calculate the possible cumulative loss over the last 3.5 Gyr.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Mars: Past, Present, and Future. Results from the MSATT Program, Part 1; p 27
    Format: application/pdf
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  • 2
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    Calculated ionization rates, ion densities, and airglow emission rates due to precipitating electrons in the nightside ionosphere of Mars (1992)
    In:  Other Sources
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    Publication Date: 2019-08-14
    Description: The calculations presented in this paper clearly establish that the electron fluxes measured by the HARP instrument, carried on board Phobos 2, could cause significant electron impact ionization and excitation in the nightside atmosphere of Mars, if these electrons actually do precipitate. The calculated peak electron densities were found to be about a factor of 2 larger than the mean observed nightside densities, indicating that if a significant fraction of the measured electrons actually precipitate, they could be the dominant mechanism responsible for maintaining the nightside ionosphere. The calculated zenith column emission rates of the O I 5577-A and 6300-A and CO Cameron band emissions, due to electron impact and dissociative recombination mechanisms, were found to be significant.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 97; A7 J; 10
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  • 3
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    Nitrogen isotopes in the 74001/74002 double-drive tube from Shorty Crater, Apollo 17 (1991)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Trapped N in the black/orange glass at different levels of the 74001/74002 double-drive tube can be used to adress two questions: the isotopic composition of indigenous N exhaled from the lunar interior 3.7 Ga ago; and the nature of extralunar N incident upon the lunar surface during the same epoch. Indigenous lunar N is found to have the same delta-N-15 value as N in terrestrial mantle samples with high He-3/He-4 ratios. This suggests that the lunar interior and the terrestrial mantle acquired their N, and possibly other volatiles, from a common nebular reservoir. The ancient extralunar N reveals evidence for a N-15-enriched component present on the lunar surface 3.7 Ga ago, together with N-15-depleted N. However, the role of these components, for neither of which a solar origin can be excluded, in explaining the long-term increase in N-15/N-14 ratio of regolith N on the moon is not yet clear.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Science Conference; Mar 12, 1990 - Mar 16, 1990; Houston, TX; United States
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  • 4
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    Evolution of isotopic signatures in lunar-regolith nitrogen - Noble gases and nitrogen in grain-size fractions from regolith breccia 79035 (1992)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Ilmenite and pyroxene grain-size separates from 79035 were analyzed for trapped N; the ilmenite was also analyzed for trapped Xe. Ilmenite N contains two or more isotopically distinct trapped components differing in release temperature and therefore plausibly in implantation energy. The isotopically light, higher-temperature (higher-energy?) component has a delta N-15 value equal to -180 percent, significantly above the minimum value observed in bulk 79035, suggesting that parts of 79035 were exposed on the lunar surface earlier than the ilmenite. Using trapped Ar-40/Ar-36 and cosmogenic Ne-21 (Benkert, 1989) a compaction age of about 1 Ga was derived for the ilmenite. This implies a considerably more recent exposure than previously thought, and suggests that the long-term change in delta N-15 of regolith N was more rapid than generally believed. Comparison of these results with those for black/orange glass from 74001/74002 (compaction age 3.7 Ga; delta N-15 in the range -36 to +18 percent (Kerridge et al., 1991) indicates that the long-term trend may have followed a complex evolutionary path. Data for 79035 pyroxenes are consistent with the ilmenite compaction age but suggest a more complex exposure history.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Science Conference; Mar 18, 1991 - Mar 22, 1991; Houston, TX; United States
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  • 5
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    Solar-type xenon - Isotopic abundances in Pesyanoe (1992)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Elemental and isotopic abundances of Ar and Xe were measured in three grain-size separates in the dark phase of the enstatite achondrite Pesyanoe by stepwise heating, using a combination of pyrolysis and combustion steps. The data reveal a low-temperature gas fraction with Ar/Xe ratios close to the solar ratio and isotopically similar to solar-type Ar and Xe observed in lunar samples. The gas released at intermediate temperature steps shows that Ar and Xe are isotopically fractionated, compared to the low-temperature components. Pesyanoe does not have excess Ar-40 and fission Xe, which means its parent body was too small for ion reimplantation. It is concluded that solar-type Xe isotopic abundances can be inferred from the low-temperature component. A comparison of Pesyanoe and lunar data shows that isotopic signatures of solar-wind Xe, as sampled at two different points in solar system space and time, are identical within experimental error.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Science Conference; Mar 18, 1991 - Mar 22, 1991; Houston, TX; United States
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  • 6
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    Solar cycle variations of the electron densities near the ionospheric peak of Venus (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Photochemical equilibrium calculations of electron and ion densities, appropriate for altitudes below about 180 km, were carried out for the Venus dayside ionosphere corresponding to solar cycle maximum and minimum conditions. The results were compared with data from radio occultation measurements. The agreement between the calculations and measurements was, in general, quite good. These comparisons indicate that the most commonly used neutral atmosphere model of Venus (Hedin et al., 1983) predicts densities which are somewhat low near the electron density peak for solar cycle maximum, but provides surprisingly good predictions for solar cycle minimum conditions.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 94; 11997-12
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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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    A comprehensive magnetohydrodynamic model of the Venus ionosphere (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The MHD Venus ionospheric model of Shinegawa and Cravens (1988) is improved here by including the energy equations for ions and electrons in a self-consistent manner. This new model reproduces observed electron density and magnetic field profiles very well, while the basic MHD process of the Venus ionosphere remain virtually unchanged. The results indicate that including energetics does not significantly alter the density and magnetic field profiles. Under unmagnetized conditions, heat fluxes for both ions and electrons must be imposed to reproduce the observed plasma temperature profiles. A heat source for the ions is probably present at higher altitudes in the magnetized ionosphere. Heating processes do not play a significant role in the dynamics at low altitudes.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 11
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