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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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    Absorption of Solar Radiation by Clouds: Observations Versus Models (1995)
    In:  Other Sources
    Details
    Publication Date: 2011-08-23
    Description: There has been a long history of unexplained anomalous absorption of solar radiation by clouds. Collocated satellite and surface measurements of solar radiation at five geographically diverse locations showed significant solar absorption by clouds, resulting in about 25 watts per square meter more global-mean absorption by the cloudy atmosphere than predicted by theoretical models. It has often been suggested that tropospheric aerosols could increase cloud absorption. But these aerosols are temporally and spatially heterogeneous, whereas the observed cloud absorption is remarkably invariant with respect to season and location. Although its physical cause is unknown, enhanced cloud absorption substantially alters our understanding of the atmosphere's energy budget.
    Keywords: Meteorology and Climatology
    Type: Science; Volume 267; 496-499
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  • 4
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    Cloud Feedback in Atmospheric General Circulation Models: An Update (1996)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Six years ago, we compared the climate sensitivity of 19 atmospheric general circulation models and found a roughly threefold variation among the models; most of this variation was attributed to differences in the models' depictions of cloud feedback. In an update of this comparison, current models showed considerably smaller differences in net cloud feedback, with most producing modest values. There are, however, substantial differences in the feedback components, indicating that the models still have physical disagreements.
    Keywords: Meteorology and Climatology
    Type: Paper-96JD00822 , Journal of Geophysical Research (ISSN 0148-0227); 101; D8; 12,791-12,794
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  • 5
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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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  • 6
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    Absorption of Solar Radiation by Clouds: Interpretations of Satellite, Surface, and Aircraft Measurements (1996)
    In:  Other Sources
    Details
    Publication Date: 2019-08-16
    Description: To investigate the absorption of shortwave radiation by clouds, we have collocated satellite and surface measurements of shortwave radiation at several locations. Considerable effort has been directed toward understanding and minimizing sampling errors caused by the satellite measurements being instantaneous and over a grid that is much larger than the field of view of an upward facing surface pyranometer. The collocated data indicate that clouds absorb considerably more shortwave radiation than is predicted by theoretical models. This is consistent with the finding from both satellite and aircraft measurements that observed clouds are darker than model clouds. In the limit of thick clouds, observed top-of-the-atmosphere albedos do not exceed a value of 0.7, whereas in models the maximum albedo can be 0.8.
    Keywords: Meteorology and Climatology
    Type: Paper-96JD02156 , Journal of Geophysical Research (ISSN 0148-0227); 101; D18; 23,299-23,309
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  • 7
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    The ancient oxygen exosphere of Mars - Implications for atmosphere evolution (1993)
    In:  Other Sources
    Details
    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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  • 8
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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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  • 9
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    Evolutionary impact of sputtering of the Martian atmosphere by O(+) pickup ions (1992)
    In:  Other Sources
    Details
    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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  • 10
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    Comparison of the Seasonal Change in Cloud-Radiative Forcing from Atmospheric General Circulation Models and Satellite Observations (1997)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: We compare seasonal changes in cloud-radiative forcing (CRF) at the top of the atmosphere from 18 atmospheric general circulation models, and observations from the Earth Radiation Budget Experiment (ERBE). To enhance the CRF signal and suppress interannual variability, we consider only zonal mean quantities for which the extreme months (January and July), as well as the northern and southern hemispheres, have been differenced. Since seasonal variations of the shortwave component of CRF are caused by seasonal changes in both cloudiness and solar irradiance, the latter was removed. In the ERBE data, seasonal changes in CRF are driven primarily by changes in cloud amount. The same conclusion applies to the models. The shortwave component of seasonal CRF is a measure of changes in cloud amount at all altitudes, while the longwave component is more a measure of upper level clouds. Thus important insights into seasonal cloud amount variations of the models have been obtained by comparing both components, as generated by the models, with the satellite data. For example, in 10 of the 18 models the seasonal oscillations of zonal cloud patterns extend too far poleward by one latitudinal grid.
    Keywords: Meteorology and Climatology
    Type: Paper-97JD00927 , Journal of Geophysical Research (ISSN 0148-0227); 102; D14; 16,593-16,603
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