Hydrogen Escape Rate Research Articles

Solar Cycle Variations of the Hydrogen Escape Rate and the CO Mixing Ratio on Mars

A one-dimensional photochemical model of the Martian atmosphere has been developed to study variations in the atmospheric composition with solar activity. The low sensitivity of O and CO number densities near the ionospheric peak to solar activity suggests that eddy mixing on Mars is proportional to the solar activity index (K(z) = (F10.7 cm/30) × 1013n-1/2). Due to the higher escape rate and the stronger eddy diffusion at solar maximum the H2 densities above the ionospheric peak are smaller by a factor of 6 than those at the solar minimum. The solar cycle variation of CO+2 by a factor of 2.2 cannot compensate for the variation of H2, and the atomic hydrogen escape flux at solar minimum is larger by a factor of 1.7 than that at solar maximum (the variation of the H number density is a factor of 30). The total hydrogen (H + 2H2) escape flux is stable (though not diffusion limited) and equal to 2.5 × 108 cm-2 sec-1. If eddy diffusion does not depend on solar activity, then the H escape is stable while the total escape flux varies by a factor of 2. The main reason for the hydrogen escape rate being close to the diffusion limit in the early models is that the rate coefficient of the H2 dissociation reaction was larger in the early models by a factor of 3 than the current value. Solar cycle variation of the lower and middle atmosphere is also studied. The CO mixing ratio in the lower and middle atmosphere would vary by a factor of 4 if the species lifetime were a few months. A mechanism of this variation is analyzed. However, since the real lifetime is 6 years the variation is only a factor of 1.35, with a time lag of 2 years.

Triton's atmosphere: A source of N and H for Neptune's magnetosphere

Mass loading of the Neptunian magnetosphere occurs primarily by thermal escape of H, H2, and N from Triton's upper atmosphere. The global escape rate of hydrogen is ∼ 7 × 1025s−1, determined by the global average methane photolysis rate, whereas the escape rate of nitrogen for our preferred model is ∼ 3.4 × 1025s−1, and is controlled by the global and orbital average energy deposition rate due to precipitating magnetospheric electrons. The escape rate of H+ and N+ is <4% of the neutral escape rate and implies that mass loading of the Neptunian magnetosphere is not localized to Triton's corona. The ratio of hydrogen to nitrogen escape rates for our preferred model is ∼2:1, comparable to the [H+]/[N+] abundance ratio inferred for Neptune's magnetosphere.

Comparison of hydrogen gas embrittlement of austenitic and ferritic stainless steels

Hydrogen-induced slow crack growth (SCG) was compared in austenitic and ferritic stainless steels at 0 to 125 °Cand 11 to 216 kPa of hydrogen gas. No SCG was observed for AISI 310, while AISI 301 was more susceptible to hydrogen embrittlement and had higher cracking velocity than AL 29-4-2 under the same test conditions. The kinetics of crack propagation was modeled in terms of the hydrogen transport in these alloys. This is a function of temperature, microstructure, and stress state in the embrittlement region. The relatively high cracking velocity of AISI 301 was shown to be controlled by the fast transport of hydrogen through the stress-induced α′ martensite at the crack tip and low escape rate of hydrogen through the γ phase in the surrounding region. Faster accumulation rates of hydrogen in the embrittlement region were expected for AISI 301, which led to higher cracking velocities. The mechanism of hydrogen-induced SCG was discussed based upon the concept of hydrogen-enhanced plasticity.

The ionotail of Venus: Its configuration and evidence for ion escape

The Pioneer Venus Orbiter measurements of the plasma and magnetic environment of the near tail of Venus show that the ionosphere becomes increasingly filamentary with increasing altitude, apparently forming cometlike tail rays that extend several thousand kilometers behind the planet. We call this region the ionotail of Venus. The tail rays are envisioned as plumes of high‐beta plasma of ionospheric origin that are surrounded by regions of low‐density, low‐beta plasma. The ionotail appears to be in quasi‐equilibrium, with the plasma pressure in the rays approximately balanced by the magnetic pressure of the region surrounding the rays. The magnetic field in this region is approximately sunaligned as we assume are the tail rays. Magnetic field reversals observed in the tail ray boundaries suggest the presence of strong current sheets there. Unlike the lower ionosphere whose major ion is thermal O+, a detailed study of tail ray plasma between 2000 and 2500 km altitude shows that the major ions are superthermal O+, with energies in the range of 9‐16 eV. The electrons are much cooler, with energies of about 1 eV. A minor, more energetic ion component, having energies exceeding 40 eV is also observed within the tail rays and occasionally between the rays as well. These Pioneer Venus Orbiter measurements reveal an ionotail that is highly dynamic, a region in which solar wind induced magnetic fields configure the ionospheric structures and accelerate the ions beyond the planetary escape velocity. We estimate a total planetary O+ escape rate of 5 × 1025 ions/s, and we infer an H+ escape rate of about half that value, about a factor of 2 below the hydrogen escape rate due to H+ charge exchange with the hydrogen exosphere of Venus.

The implication for the presence of a magnetosphere on Uranus in the relationship of EUV and radio emission

Two apparent contradictory sets of facts exist concerning the presence of a magnetosphere at Uranus. The Voyager planetary radio astronomy experiment has not detected a Uranus signal at a range <0.7 AU, whereas IUE satellite observations show relatively strong emission indicating the presence of substantial particle excitation of the atmosphere. The character of the EUV emission implies the presence of an ionosphere, and mass‐loading of the extended system comparable to that of Saturn. If the ingredient for production of an active magnetosphere is present, the non detection of radio emission then suggests that Uranus has a very weak or non existent magnetosphere. The apparent paradox of an excited atmosphere in the absence of an active magnetosphere may possibly be explained in terms of the peculiar characteristics of the excited sunlit equatorial exospheres of Jupiter and Saturn. We suggest that 1) the observed Uranus EUV emission may be a similar phenomenon to those observed in the sub‐solar equatorial regions of Jupiter and Saturn, which appear to be disconnected from auroral or magnetospheric activity, and 2) the Uranus intrinsic magnetic field is probably weak or nonexistent because of the availability of substantial mass for producing an active magnetosphere as derived from the nature of the EUV emission. We predict a substantial escape rate of atomic hydrogen (∼2 × 1028 S−1).

Chemistry and evolution of Titan's atmosphere

Abstract The chemistry and evolution of Titan's atmosphere is reviewed in the light of the scientific findings from the Voyager mission. It is argued that the present N2 atmosphere may be Titan's initial atmosphere rather than photochemically derived from an original NH3 atmosphere. The escape rate of hydrogen from Titan is controlled by photochemical production from hydrocarbons. CH4 is irreversibly converted to less hydrogen rich hydrocarbons, which over geologic time accumulate on the surface to a layer thickness of ∼0.5 km. Magnetospheric electrons interacting with Titan's exosphere may dissociate enough N2 into hot, escaping N atoms to remove ∼0.2 of Titan's present atmosphere over geologic time. The energy dissipation of magnetospheric electrons exceeds solar e.u.v. energy deposition in Titan's atmosphere by an order of magnitude and is the principal driver of nitrogen photochemistry. The environmental conditions in Titan's upper atmosphere are favorable to building up complex molecules, particularly in the north polar cap region.

Chemistry and evolution of Titan's atmosphere

The chemistry and evolution of Titan's atmosphere is reviewed in the light of the scientific findings from the Voyager mission. It is argued that the present N 2 atmosphere may be Titan's initial atmosphere rather than photochemically derived from an original NH 3 atmosphere. The escape rate of hydrogen from Titan is controlled by photochemical production from hydrocarbons. CH 4 is irreversibly converted to less hydrogen rich hydrocarbons, which over geologic time accumulate on the surface to a layer thickness of ∼0.5 km. Magnetospheric electrons interacting with Titan's exosphere may dissociate enough N 2 into hot, escaping N atoms to remove ∼0.2 of Titan's present atmosphere over geologic time. The energy dissipation of magnetospheric electrons exceeds solar e.u.v. energy deposition in Titan's atmosphere by an order of magnitude and is the principal driver of nitrogen photochemistry. The environmental conditions in Titan's upper atmosphere are favorable to building up complex molecules, particularly in the north polar cap region.

Atomic hydrogen escape rate due to charge exchange with hot plasmaspheric ions

Data on ion and electron temperatures and concentrations to several thousand kilometers of altitude were obtained from the Atmosphere Explorer C satellite for 1974 and to 850 km from Arecibo incoherent scatter radar measurements. These data were used to normalize diffusive equilibrium profiles. From these profiles and by using the neutral atmospheric model of Jacchia (1971) and a new hydrogen model, the charge-exchange-induced neutral hydrogen escape fluxes for equatorial and middle latitudes were calculated. The data confirm earlier estimates that the charge exchange loss is more important than Jeans escape for the earth. It is also found that inside the plasmapause this charge exchange process with hot plasmapheric ions is the major production and loss process for the satellite population in the hydrogen geocorona.

Mesospheric Hydrogen Related to Exospheric Escape Mechanisms

Abstract The thermal escape rate of hydrogen inferred from exospheric density measurements is too low by a factor of at least 2 to accord with the mixing ratio of H2O, H2 and CH4 observed in the stratosphere and mesosphere. The effect on the mixing ratio of adding hydrogen fluxes to support the polar wind by lateral flow and to supply a loss to charge transfer with fast protons is investigated. It is shown that these additional mechanisms can make up the deficit. The exobase hydrogen density must adjust locally to supply hydrogen to the three separate escape mechanisms at the rate demanded by the mixing ratio in the lower atmosphere and the exospheric temperature.

Volatile element influx on Venus from cometary impacts

Several different reasonably concordant methods of estimating the flux of carbonaceous chondrite and cometary material (both meteoric dust and comet nuclei) through the inner solar system are shown to imply that such sources may have a dominant effect on the present abundances of several important constituents of the atmosphere of Venus. In particular, the entire supply of hydrogen compounds on Venus may owe its origin to infall of such material. The escape rate of atomic hydrogen may be in approximate balance with the influx rate of hydrogen in the forms of bound meteoritic water and cometary ices. I suggest that the atmospheric inventories of H, S, Cl, F and possibly N on Venus are provided by infall, and need not be endogenous to Venus.

Mariner 9 ultraviolet spectrometer experiment: Mars airglow spectroscopy and variations in Lyman alpha

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%.

The Primitive Earth: Thermal Models of the Upper Atmosphere for a Methane-Dominated Environment

Abstract In discerning between alternate hypotheses pertaining to the dominant composition of the primitiveenvironment of the earth, the presence of free hydrogen in the atmosphere is of prime importance. Anatmosphere composed of CH4 and NH3, in contrast to a N2 and CO2 atmosphere, depends upon the availability of free hydrogen. However, the gravitational escape rate of hydrogen based on the current exospherictemperature maximum (~2000K) indicates that an atmosphere composed of CH4 and NH would havepersisted for only l0-l06 years, even with substantial outgassing of hydrogen from the crust. Therefore, anecessary prerequisite to resolving the question of the stability and durability of a primitive CH4-NHsatmosphere is a knowledge of the exospheric temperature of this early environment. Under chemical equilibrium in a primitive CH4-NHs atmosphere, CH4 would be the predominant constituent in the lower atmosphere. Ammonia shields water vapor from photodissociation, while absorbingthe majority of its energy in...

Escape Rate of Hydrogen from a Carbon Dioxide Atmosphere

view Abstract Citations (12) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Escape Rate of Hydrogen from a Carbon Dioxide Atmosphere Chamberlain, Joseph W. Abstract The departure from the classical rate of escape (given by the Jeans formula) of atomic hydrogen from an atmosphere composed mainly of CO2 has been calculated with a Monte Carlo program. The results, having possible interest for the Mars and Venus atmospheres, supplement earlier similar calculations for atomic hydrogen in atomic oxygen and helium in oxygen. As expected from general considerations, the deviation from the classical result is greater for hydrogen in CO2 than when the masses are more nearly equal, but the correction factor is still not a major one Publication: The Astrophysical Journal Pub Date: February 1969 DOI: 10.1086/149905 Bibcode: 1969ApJ...155..711C full text sources ADS |