Engineering topics

Hodges, R. R., Jr.

Publications and source records attributed to Hodges, R. R., Jr..

At least 37 records · Page 2

NASA NTRS1980

Lunar cold traps and their influence on argon-40

In polar areas of the moon the maximum temperatures reached in some permanently shaded areas are well below the temperature required to retain water ice for billions of years, and cold enough to hold other volatiles for shorter periods. Aside from water, the most significant lunar volatiles are the radiogenic gases, of which argon-40 is the most easily detected, both in situ and as retrapped ions in rocks returned from the surface on the moon. Argon-40 escapes from the moon at a surprisingly high rate that is between 3% and 6% of its total production. Its brief lifetime in the lunar exosphere is marked by numerous adsorption/desorption events. Collisions with the lunar surface in cold, permanently shaded areas lead to long term storage, forming reservoirs of trapped gas that may be disturbed occasionally to produce sudden increases in atmospheric argon. It is postulated that this may explain at least part of the time variations in Apollo 17 mass spectrometer measurements of argon that were previously attributed to internal processes associated with the release of radiogenic gases from the moon.

Hodges, R. R., Jr.↗

NASA NTRS1979

Composition and structure of the Venus atmosphere - Results from Pioneer Venus

The composition of the Venusian atmosphere was studied using a mass spectrometer on the Pioneer Venus sounder probe. The single-focusing magnetic-sector spectrometer scanned the mass range from hydrogen through mercury with a dynamic range of six decades. Data taken by the mass spectrometer were compared with those of a gas chromatograph, resulting in slight discrepancies due to the use of a sputter ion pump acting as a sink for entry of rare gases through the inlet leak. A surprisingly large concentration of primordial Ar-36 and Ar-38 was discovered in a ratio of 5 to 1. It was concluded that the large excess of primordial argon was a valid result and that the mixing of HCl in the lower Venusian atmosphere was less than a few parts per million. Arguments against the sun as the source for excess primordial gases on Venus were presented. Concentrations of other elements such as Ne, Kr, He, S, and O were discussed. Although the mass peaks in the spectrum were real, it was not clear whether all of the chemical reactions (i.e., COS production) actually occurred in the atmosphere. Until further analysis can be made, it will be uncertain how the inlet system, which is at atmospheric temperature, affected the results.

Hoffman, J. H.↗

NASA NTRS1979

Venus lower atmospheric composition - Preliminary results from Pioneer Venus

Initial examination of data from the neutral mass spectrometer on the Pioneer Venus sounder probe indicates that the abundances of argon-36, argon-38, and neon-20 in the Venus atmosphere are much higher than those of the corresponding gases in Earth's atmosphere, although the abundance of radiogenic argon-40 is apparently similar for both planets. The lower atmosphere of Venus includes significant concentrations of various gaseous sulfur compounds. The inlet leak to the mass spectrometer was temporarily blocked by an apparently liquid component of the Venus clouds during passage through the dense cloud layer. Analysis of gases released during the evaporation of the droplets shows the presence of water vapor to some compound or compounds of sulfur.

Hoffman, J. H.↗

NASA NTRS1978

Gravitational and radiative effects on the escape of helium from the moon

On the moon, and probably on Mercury and other similar regolith-covered bodies with tenuous atmosphere, the dominant gas is He-4. It arises as the radiogenic product of the decay of uranium and thorium within any planet, but its major source appears to be the alpha particle flux of the solar wind. The moon intercepts solar wind helium at an average rate of 1.1 times 10 to the 24th atom/sec, and loses it at the same rate. Some helium may escape directly as the result of the process of solar wind soil bombardment which may release previously trapped helium at superthermal speeds. Atmospheric models have been calculated with the total helium influx as source. Subsequent comparison of model and measured helium concentrations indicates that the fraction of helium escaping via the atmosphere may range from 20% to 100% of the solar wind influx. Of the escaping atmosphere, most of the helium (about 93%) becomes trapped in earth orbit, while about 5% gets trapped in satellite orbits about the moon. Owing to a 6 month lifetime for helium in solar radiation, the satellite atoms form a lunar corona that exceeds the lunar atmosphere in total abundance by a factor of 4 to 5.

Hodges, R. R., Jr.↗

NASA NTRS1977

Release of radiogenic gases from the moon

The rate of escape of Ar-40 from the moon is calculated from mass-spectrometer data obtained at the Apollo-17 landing site. It is shown that the rate of loss of Ar from the moon varies significantly over periods the order of one lunation and that the average loss rate is about 3 t/a, corresponding to about 6% of the present rate of Ar production by K decay within the moon. These features of the Ar loss-rate data are interpreted as evidence that this gas originates in the partially molten asthenosphere, which in turn requires that early differentiation only affected the outer 600 to 1,000 km of the moon, trapping significant amounts of radioactive materials in the present asthenosphere. The relationship between the venting of Ar and other radiogenic gases in the lunar atmosphere is discussed.

Hodges, R. R., Jr.↗

NASA NTRS1977

Formation of the lunar helium corona and atmosphere

Helium is one of the dominant gases of the lunar atmosphere. Its presence is easily identified in data from the mass spectrometer at the Apollo 17 landing site. The major part of these data was obtained in lunar nighttime, where helium concentration reaches the maximum of its diurnal cyclic variation. The large night to day concentration ratio agrees with the basic theory of exospheric lateral transport reported by Hodges and Johnson (1968). A reasonable fraction of atmospheric helium atoms has a velocity in excess of the gravitational escape velocity. The result is a short average lifetime and a tenuous helium atmosphere. A description is presented of an investigation which shows that the atmosphere of the moon has two distinct components including low energy atoms, which are gravitationally bound in trajectories that intersect the lunar surface, and higher energy atoms, which are trapped in satellite orbits. The total helium abundance in the lunar corona is shown to be about 1.3 times 10 to the 30th power atoms.

Hodges, R. R., Jr.↗

NASA NTRS1976

Investigation of the daytime lunar atmosphere for lunar synthesis program

Synthesis studies of the daytime lunar atmoshere were directed toward improved understanding of fundamental lunar atmospheric dynamics and the relationship of the detectable atmosphere to physical processes of the lunar surface and interior. The primary source of data is the Apollo 17 lunar surface mass spectrometer. The Ar40 is radiogenic and its escape rate from the lunar atmosphere requires release of a significant fraction (about 8%) of the argon produced from the decay of K40 within the moon. Furthermore the process of argon release from the solid moon is time varying and related to seismic activity. Most of the helium on the moon is due to release of implanted solar wind alpha particles from the regolith.

Hodges, R. R., Jr.↗

NASA NTRS1976

The escape of solar-wind carbon from the moon

About 8 tons of solar-wind carbon is implanted in the lunar soil each year. This inflow is adequate to supply the present regolith carbon abundance (about 100 ppm) in 1-m thickness of soil per billion years. However, the present meteor flux is only capable of mixing to a depth of 10 cm over a billion-year period. As a result, soil mixing cannot act to assimilate the total influx of carbon in the soil. It is necessary that most of the carbon influx be balanced by escape. Model atmospheres for three candidate gases, CH4, CO, and CO2, have been computed. Each model is sized to provide the needed escape rate.

Hodges, R. R., Jr.↗

NASA NTRS1975

Formation of the lunar atmosphere

Measurements of Ar-40 and helium made by the Apollo 17 lunar surface mass-spectrometer are used in the synthesis of atmospheric supply and loss mechanisms. The argon data indicate that about 8% of the Ar-40 produced in the moon due to decay of K-40 is released into the atmosphere and subsequently lost. Variability of the atmospheric abundance of argon requires that the source be localized, probably in an unfractionated, partially molten core. If so, the radiogenic helium released with the argon amounts to 10% of the atmospheric helium supply. The total rate of helium escape from the moon accounts for only 60% of the solar wind particle influx. This seems to require a nonthermal escape mechanism for trapped solar-wind gases, probably involving weathering of exposed soil-grain surfaces by solar-wind protons.

Hodges, R. R., Jr.↗

NASA NTRS1975

Molecular gas species in the lunar atmosphere

Evidence is presented from the data obtained by the Apollo 17 lunar mass spectrometer which indicates the presence of methane and perhaps very small amounts of ammonia and carbon dioxide in the lunar atmosphere. This evidence is based on predawn enhancement of the concentrations of the mass peaks at the parent position for these molecular gas compounds. Methane is shown to be the most abundant molecular gas, although its exceedingly low concentration (1000 mol/cu cm) is slightly less than that of Ar-36. Several reasons are considered for the very low concentration of methane in the lunar atmosphere.

Hoffman, J. H.↗

NASA NTRS1975

Diurnal variations of atomic hydrogen - Observations and calculations

Theoretical calculations of the diurnal variation of atomic hydrogen in the thermosphere are presented that simultaneously evaluate the effects of thermosphere rotation, thermal escape, charge exchange with O(+) ions, charge exchange with hot H(+) ions, transport due to winds, and ballistic fluxes in a consistent manner. The first five effects cause a significant departure from the zero net ballistic flux condition. A thermospheric wind with a peak velocity of 50 m per sec at exobase altitude and a total charge exchange flux varying from 0 to 0.2 billion per sq cm per sec are found to be consistent with the observations.

Tinsley, B. A.↗

NASA NTRS1975

Implications of atmospheric Ar-40 escape on the interior structure of the moon

Radiogenic Ar-40 escapes from the lunar atmosphere at a rate of about 2 x 10 to the 21st atoms/sec. This amounts to 8% of the rate of argon production in the entire moon by potassium decay. A curious feature of the argon escape rate is a variability with time scale of several months. It is shown that the variation in argon loss correlates with high-frequency lunar teleseismic events. The only apparent region of the moon which could possibly supply the amount of argon needed for escape via a plausible temporal mechanism is a semimolten asthenosphere which may be entirely primitive unfractionated lunar material, or an Fe-FeS core that is enriched in potassium. A core that is devoid of potassium is not compatible with the atmospheric argon measurements.

Hodges, R. R., Jr.↗

NASA NTRS1974

Model atmospheres for Mercury based on a lunar analogy

Similarities in daytime spectral reflectivities and nighttime infrared emission from Mercury and the moon are shown to imply that the atmosphere of Mercury must be tenuous, like that of the moon. The theory of formation, transport, and loss in the lunar atmosphere is applied to Mercury. Models of the Hermian atmosphere at perihelion and aphelion are presented, based on the solar wind as the dominant source of gases. Only the noncondensable species - hydrogen, helium and neon - are considered. Of these, helium is the most abundant atmospheric gas, with maximum concentration of about 40,000,000 per cu cm at the nighttime surface. The maximum concentration of H2 is 6,000,000 per cu cm, and that of neon is 700,000 per cu cm.

Hodges, R. R., Jr.↗

NASA NTRS1974

Measurements of solar wind helium in the lunar atmosphere

Measurements of lunar atmospheric helium during 1973 from the Apollo 17 surface mass spectrometer are presented. The average helium abundance is shown to be about 70% of the theoretical model value, suggesting that the solar wind helium flux in 1973, during sunspot minimum, was substantially less than the expected average flux. Large-amplitude transients in the helium data indicate rapid response of the lunar atmosphere to changes in solar wind. The atmospheric helium abundance is shown to be correlated with the geomagnetic index Kp.

Hodges, R. R., Jr.↗

NASA NTRS1974

The lunar atmosphere

In contrast to the earth's atmosphere, the atmosphere of the moon is exceedingly tenuous and appears to consist mainly of noble gases. The solar wind impinges on the lunar surface, supplying detectable amounts of helium, neon and Ar-36. Influxes of solar wind protons and carbon and nitrogen ions are significant, but atmospheric gases containing these elements have not been positively identified. Radiogenic Ar-40 and Rn-222 produced within the moon have been detected. The present rate of effusion of argon from the moon accounts for about 0.4% of the total production of Ar-40 due to decay of K-40 if the average abundance of potassium in the moon is 1000 ppm. Lack of weathering processes in the regolith suggests that most of the atmospheric Ar-40 originates deep in the lunar interior, perhaps in a partially molten core. If so, other gases may be vented along with the argon.

Hodges, R. R., Jr.↗

NASA NTRS1974

Episodic release of Ar-40 from the interior of the moon

Measurements of lunar atmosphere made by the mass spectrometer at the Apollo 17 landing site during the first 9 lunations of 1973 show an apparently cyclical variation of radiogenic Ar-40, with maximum to minimum abundance ratio of about 2. There seems to be a 6-7 month periodicity in the oscillation, but the limited data base makes a fixed oscillatory pattern uncertain. The significance of the variation of atmospheric argon lies in the implication that its source is episodic in nature, ranging in strength from about 1% of the total rate of production of Ar-40 in the moon to near zero. This requires a presently active mechanism for transient venting of gas from deep within the moon.

Hodges, R. R., Jr.↗

NASA NTRS1973

Helium and hydrogen in the lunar atmosphere

Of the gas species supplied by the solar wind, only helium and hydrogen are light enough to be lost from the moon by Jeans' thermal escape mechanism. To study the behavior of helium and hydrogen, a Monte Carlo technique has been used, in which random ballistic trajectories of individual molecules are traced over a spherical moon. In the computation, a particle is 'created' on the sunlit surface, and the locations of its subsequent encounters with the surface are recorded until it escapes. Global distributions of helium and hydrogen concentrations have been computed, based on the hypothesis that the release of neutral gases from the lunar surface is confined to daytime and is correlated with the solar wind influx. The resulting helium model is in good agreement with the measurements from the Apollo 17 lunar surface mass spectrometer.

Hodges, R. R., Jr.↗

NASA NTRS1973

Differential equation of exospheric lateral transport and its application to terrestrial hydrogen

The differential equation description of exospheric lateral transport of Hodges and Johnson is reformulated to extend its utility to light gases. Accuracy of the revised equation is established by applying it to terrestrial hydrogen. The resulting global distributions for several static exobase models are shown to be essentially the same as those that have been computed by Quessette using an integral equation approach. The present theory is subsequently used to elucidate the effects of nonzero lateral flow, exobase rotation, and diurnal tidal winds on the hydrogen distribution. Finally it is shown that the differential equation of exospheric transport is analogous to a diffusion equation. Hence it is practical to consider exospheric transport as a continuation of thermospheric diffusion, a concept that alleviates the need for an artificial exobase dividing thermosphere and exosphere.

Hodges, R. R., Jr.↗