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Mariner 6 and 7 ultraviolet spectrometers that flew by Mars in 1969 observed the Lyman‐α dayglow of atomic hydrogen. Data in the altitude range 200 to 24,000 km are analyzed to determine the structure of the Martian exosphere. The classical evaporative theory is applied to calculate a hydrogen density distribution. A spherical model of the dayglow Lyman‐α emission, using radiative transfer theory, is used to produce theoretical intensities for comparison with the data in order to determine temperature and density in the exosphere. It is found that the exospheric temperature is 350°±100°K and that the number density at 250 km is 3±1×104 cm−3. The existence of a Lyman‐α corona implies a primary source of hydrogen on Mars, probably the photodissociation of water vapor.

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Four Lyman a airglow measurements of the limb and disc of Mars, made by ultraviolet spectrometers on Mariner 6 and 7 in 1969 and Mariner 9 in 1971, are analyzed to determine the amount and distribution of atomic hydrogen above 80 km. The variation of atomic hydrogen with altitude is calculated by using time-independent chemical diffusion models from 80 to 250 km, and tin exospheric model is used above 250 km. By employing radiative transfer theory that includes effects of pure absorption and accounts for temperature variations in the atmosphere a spherical model of the airglow Lyman a emission is used to produce theoretical intensities for comparison with the data. It is found that (1) the exospheric temperature and distribution in 1971 are consistent with those determined in 1969, (2) the vertical optical depth above 80 km was 2.2 in 1969 and 5 in 1971, and (3) the derived atomic hydrogen distribution from 80 to 250 km requires a source of atomic hydrogen above 80 km. Comparison of observed profiles with chemical diffusion models implies a large downward flow of atomic hydrogen at 80 km coupled with a large upward flow of molecular hydrogen. Analysis of measured Lyman a radiation produced by resonance scattering of the 1216-A (12S-22P) solar Lyman a line by planetary atomic hydrogen H is a method of determining the distribution of atomic hydrogen in a planetaryatmosphere. Lyman a airglow measurements, together with appropriate radiative transfer and density models, yield the density distribution and temperature of the upper atmosphere. Mariner 6 and 7 ultraviolet spectrometers that flew by Mars in 1969 observed its Lyman a airglow [Barth et al., 1969] and have provided a unique set of data. These are the only Lyman a data available that contain extended bright limb measurements as well as disc measurements of a planetary atmosphere. Bright limb measurements are sensitive to temperature and density at the base of the exosphere or critical level and give reliable estimates of both the amount of hydrogen in the exosphere and the temperature at the critical level [Anderson and Hord, 1971]. Disc measurements are less sensitive to temperature and density at the critical level but depend more strongly on the distribution of H below the critical level. Disc data from Mariner 6 and 7 are analyzed separately and in conjunction with the limb data to determine the amount and distribution of H from the critical level down to the altitude where pure absorption of Lyman a photons by carbon dioxide. CO2 becomes important. Information gained from the Mariner 6 and 7 analysis is applied to Lyman a data from the Mariner 9 ultraviolet spectrometer to determine the density distribution of atomic hydrogen above 80 km on Mars in 1971. Radiative transfer theory required for this analysis was discussed by Strickland and Anderson [1973] and Anderson and Hord [1971]. Theoretical distributions of atomic hydrogen from 80 to 25,000 km are obtained by solution of the coupled equations of continuity and diffusion below 250 km (the base of the ...

The H Balmer α nightglow is investigated by using Monte Carlo models of asymmetric geocoronal atomic hydrogen distributions as input to a radiative transfer model of solar Lyman β radiation in the thermosphere and exosphere. The radiative transfer model includes all orders of scattering, temperature variation with altitude and solar zenith angle, and anisotropic velocity distributions. The influences of multiple scattering of Lyman β radiation and of observing geometry on the H Balmer α intensity and effective temperature are evaluated in detail. Morning and evening hydrogen distributions for minimum, medium, and maximum solar activity are used in calculations of nightglow emission rates and line profiles. For each of the hydrogen models the H Balmer α intensity and effective temperature are displayed as a function of solar depression angle, observation zenith angle, and azimuth relative to the sun, to make possible detailed comparison to observations. Except for rather unique observing conditions, multiple scattering effects cannot be ignored for the determination of either intensity or temperature. Observed effective temperatures which are significantly less than the exobase temperature may be due either to low‐altitude thermospheric hydrogen or to high‐altitude exospheric hydrogen and depend on the observing conditions and the level of solar activity. Previous (selected) observations of the H Balmer α nightglow and effective temperatures are compared to models which most closely represent the solar conditions of the observations. Results of the theoretical investigation of H Balmer α emissions and of model‐data comparisons indicate that predicted morning‐evening concentration variations yield small intensity variations for solar depression angles greater than 20°. The variations with solar depression angle of observed intensities and temperatures both show discrepancies in comparison with model results. A substantial disagreement occurs in absolute intensity: the long‐standing result that the observations are a factor of 2 higher than model results is reaffirmed with recent observational data and improved estimates of the solar H Lyman β flux. Observing schemes are discussed which lead to first approximations to hydrogen content independent of solar flux and to the high‐altitude atomic hydrogen distribution.