Engineering topics
Publications and source records attributed to Barth, C. A..
An ultraviolet spectrometer aboard the Apollo 17 orbiting spacecraft attempted to measure ultraviolet emissions from the lunar atmosphere. The only emissions observed were from a transient atmosphere introduced by the lunar landing engine. The absence of atomic hydrogen implies that solar wind protons are converted to hydrogen molecules at the lunar surface.
The original scientific objectives and the achieved results are discussed. The upper atmosphere was found to have a variable temperature, with a mean exospheric temperature of 325 K; composed predominantley of CO2. Measurements of hydrogen and ozone are analyzed.
Selected results are presented for an analysis of Mariner 9 1304-A data from the first 100 passes. Based on a comparison of limb data, the exospheric temperature appears lower on several Mariner 9 passes than when Mariner 6 and 7 encountered the planet in 1969. Temperatures of 300 and 350 K are considered in the atmospheric modeling. At 300 K, derived values of the atomic oxygen concentration are typically between 0.5 and 1% of the total density at 135 km, based on fitting theoretical intensities to limb data from several passes. Structure in the limb data below 200 km suggests the possibility that approximately 0.2 kR of the observed approximately 0.8 kR near 150 km is due to dissociative excitation of CO2. There is variability in limb profiles, thus indicating changes in the O distribution with time. The variability does not appear to follow a recognizable pattern. Interpretation of the 1304-A disk data shows a correlation in intensity with the 10.7-cm solar flux and larger O concentrations in the afternoon than in the morning. The correlation in intensity with the 10.7-cm flux is probably due largely to variation in the solar 1304-A fluxes.
It is possible to model the ultraviolet terminator observations of Mariner 9 either with a homogeneous atmosphere or with a scattering layer in addition to a homogeneous atmosphere. The characteristic shape of the twilight intensity variation is dependent on the latitude of observation. Observations north of 55N require a homogeneous model. Twilight measurements south of 55N indicate that a scattering layer may be a general Martian occurrence, at least in the evening. The day-to-day surveillance of the Martian terminator during the first 216 Mariner 9 revolutions indicates haze formation in the evening, and its continued presence until early morning is a daily occurrence in mid-latitudes up to 50N.
Selected results are presented for a considerable quantity of Mariner 9 1304-A data for the first 100 revolutions from both the bright limb and disk of Mars. The limb data suggest that the exospheric temperature is less than it was in 1969 when Mariners 6 and 7 encountered the planet. Similar (O) concentrations (0.5 to 1%) are derived for a temperature of about 300 K. Structure in the limb profiles below 200 km suggests the possibility that about 0.2 kR of the observed approximately 0.8 kR near 150 km is due to dissociative excitation of CO2. Significant differences in selected limb profiles suggest that local as well as random variations in (O) occur.
On Mars, the Mariner observations show a twenty-fold variation in the amount of ozone, depending on the presence or absence of another minor constituent, water vapor, in the atmosphere. In the evolution of earth's primitive atmosphere, the formation of an ozone layer may have played an important role in the prebiotic chemistry that took place on the surface. The seasonal formation and disappearance of ozone in the contemporary Martian atmosphere may be of consequence in any prebiotic chemistry that may be occurring there.
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Ozone is observed to be present in the polar regions of Mars and to have a seasonal variation. In the summer, the amount present in the polar atmosphere is less than 3 micrometer-atmospheres. In the fall, ozone increases in amount and is found in association with the formation of the polar hood. In winter, the maximum amount of ozone is present, 57 micrometer-atmospheres over the polar hood and 16 over the polar cap. In spring, the amount over the polar cap decreases monotonically until by the beginning of summer the ozone disappears. Ozone is not observed in the equatorial region during any season.
The Mariner 9 Ultraviolet Spectrometer has observed the 2550 A ozone spectral absorption feature on Mars. This absorption was previously detected in the south polar region by Mariner 7 in 1969. Mariner 9 did not observe ozone at any time in the equatorial region, nor at the south polar cap during its summer season. However, ozone was found in the north polar region beginning at a latitude of 45 deg N and extending northward. Ozone later appeared in the southern hemisphere southward of 50 deg S as the Mars autumnal equinox approached. The presence of ozone on Mars seems to be coupled to the water vapor content of its atmosphere.
The ultraviolet spectrometer (UVS) onboard the Apollo 17 orbiting spacecraft was used to measure emissions from the lunar atmosphere. The UVS and calibration are discussed along with the lunar atmosphere observations and lunar albedo measurements.
Reflectance properties of Mars were measured in a 100-A band (centered at 3050 A) by the UV spectrometer on board Mariner 9 during the period from Nov. 14, 1971 to Mar. 1, 1972. A topographic map of Mars based on the scattering of UV light from the Mars atmosphere (after clearing of the dust storm) is illustrated. The UV light which is Rayleigh-scattered by the Mars molecular atmosphere (with allowance for uniform turbidity) is proportional to surface pressure regardless of the atmospheric temperature structure. Comparison with Mariner 9 radio occultation measurements determines the fraction of total reflectance that is due to atmospheric scattering.
Mariner 9 ultraviolet spectrometer observations show the Mars airglow consists principally of emissions that arise from the interaction of solar ultraviolet radiation with carbon dioxide, the principal constituent of the Mars atmosphere. Two minor constituents, atomic hydrogen and atomic oxygen, also produce airglow emissions. The airglow measurements show that ionized carbon dioxide is only a minor constituent of the ionosphere. Using the airglow measurements of atomic oxygen, it is possible to infer that the major ion is ionized molecular oxygen. The escape rate of atomic hydrogen measured by Mariner 9 is approximately the same as that measured two years earlier by Mariner 6 and 7. If the current escape rate has been operating for 4.5 billion years and if water vapor is the ultimate source, an amount of oxygen has been generated that is far in excess of that observed at present. Mariner 9 observations of Mars Lyman alpha emission over a period of 120 days show variations of 20%.
Mariner 9 observations of the limb intensity profile of the CO Cameron bands in the Martian airglow demonstrate the validity of a postulated relationship between the equivalent subsolar zenith intensity and the 10.7-cm solar radio flux. Comparison of averaged limb intensities of the CO2+ doublet and the Cameron bands on four favorable occasions is consistent with the intensities being directly proportional in a ratio of 0.24 to 1.
On the basis of data provided by the IR interferometer spectrometer on the Mariner 9 spacecraft, it is suggested that the composition of the Martian dust corresponds approximately to that of rocks of intermediate silicon dioxide content. The large dynamical pole flattening obtained from satellite observations has been confirmed by the Mariner 9 data.
The UV airglow spectrum of Mars has been measured from an orbiting spacecraft during a 30-day period in November and December 1971. Significant variations in the scale height of the CO Cameron band airglow have been observed during a period of variable solar activity. The atomic oxygen and hydrogen airglow lines are present during all the observations. Measurements of the reflectance of the lower atmosphere of Mars show the spectral characteristics of particle scattering and a magnitude that is about 50% of that measured during the Mariner 6 and 7 experiments in 1969.
Photoelectric spectra have been obtained for a number of early-type stars in the 1100- to 2000-A region with the Mariner 9 UV spectrometer. The resonance lines of H I, Si IV, and C IV are easily identified, as are features due to C II, C III, Si III, Fe II N IV. The absolute energy distribution derived from the data lie about 20% below those of OAO-2 in the 1200- to 2000-A region.
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