D. M. Hunten

The abundances of constituents of Titan's atmosphere from the GCMS instrument on the Huygens probe.

Saturns largest moon, Titan, remains an enigma, explored only by remote sensing from Earth, and by the Voyager and Cassini spacecraft. The most puzzling aspects include the origin of the molecular nitrogen and methane in its atmosphere, and the mechanism(s) by which methane is maintained in the face of rapid destruction by photolysis. The Huygens probe, launched from the Cassini spacecraft, has made the first direct observations of the satellites surface and lower atmosphere. Here we report direct atmospheric measurements from the Gas Chromatograph Mass Spectrometer (GCMS), including altitude profiles of the constituents, isotopic ratios and trace species (including organic compounds). The primary constituents were confirmed to be nitrogen and methane. Noble gases other than argon were not detected. The argon includes primordial 36Ar, and the radiogenic isotope 40Ar, providing an important constraint on the outgassing history of Titan. Trace organic species, including cyanogen and ethane, were found in surface measurements.

Smoke and Dust Particles of Meteoric Origin in the Mesosphere and Stratosphere

Abstract A height profile of ablated mass from meteors is calculated, assuming an incoming mass of 10−16 g cm−2 s−1 (44 metric tons per day) and the velocity distribution of Southworth and Sekanina, which has a mean of 14.5 km s−1. The profile peaks at 84 km. The fluxes of micrometeorites and residual meteoroids are also calculated. The coagulation of the evaporated silicates into “smoke” particles is then followed by means of a model adapted from a previous study of the stratospheric sulfate layer. Numerous sensitivity tests are made. Features of the results are a sharp cutoff of the particle distribution above 90 km, and a surface area close to 10−9 cm2 cm−3 all the way from 30 to 85 km. Some confirmation is obtained from balloon studies of condensation nuclei, although the various measurements differ greatly. The optical scattering and extinction am shown to be undetectable. Several potential applications are suggested: nucleation of sulfate particles and noctilucent clouds, scavenging of metallic ions...

The composition of the Jovian atmosphere as determined by the Galileo probe mass spectrometer

The Galileo probe mass spectrometer determined the composition of the Jovian atmosphere for species with masses between 2 and 150 amu from 0.5 to 21.1 bars. This paper presents the results of analysis of some of the constituents detected: H2, He, Ne, Ar, Kr, Xe, CH4, NH3, H2O, H2S, C2 and C3 nonmethane hydrocarbons, and possibly PH3 and Cl. 4He/H2 in the Jovian atmosphere was measured to be 0.157 +/- 0.030. 13C/C12 was found to be 0.0108 +/- 0.0005, and D/H and 3He/4He were measured. Ne was depleted, < or = 0.13 times solar, Ar < or = 1.7 solar, Kr < or = 5 solar, and Xe < or = 5 solar. CH4 has a constant mixing ratio of (2.1 +/- 0.4) x 10(-3) (12C, 2.9 solar), where the mixing ratio is relative to H2. Upper limits to the H2O mixing ratio rose from 8 x 10(-7) at pressures <3.8 bars to (5.6 +/- 2.5) x 10(-5) (16O, 0.033 +/- 0.015 solar) at 11.7 bars and, provisionally, about an order of magnitude larger at 18.7 bars. The mixing ratio of H2S was <10(-6) at pressures less than 3.8 bars but rose from about 0.7 x 10(-5) at 8.7 bars to about 7.7 x 10(-5) (32S, 2.5 solar) above 15 bars. Only very large upper limits to the NH3 mixing ratio have been set at present. If PH3 and Cl were present, their mixing ratios also increased with pressure. Species were detected at mass peaks appropriate for C2 and C3 hydrocarbons. It is not yet clear which of these were atmospheric constituents and which were instrumentally generated. These measurements imply (1) fractionation of 4He, (2) a local, altitude-dependent depletion of condensables, probably because the probe entered the descending arm of a circulation cell, (3) that icy planetesimals made significant contributions to the volatile inventory, and (4) a moderate decrease in D/H but no detectable change in (D + 3He)/H in this part of the galaxy during the past 4.6 Gyr.

ULTRAVIOLET SPECTROMETER OBSERVATIONS OF URANUS.

Data from solar and stellar occultations of Uranus indicate a temperature of about 750 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane and acetylene in the lower levels. The ultraviolet spectrum of the sunlit hemisphere is dominated by emissions from atomic and molecular hydrogen, which are kmown as electroglow emissions. The energy source for these emissions is unknown, but the spectrum implies excitation by low-energy electrons (modeled with a 3-electron-volt Maxwellian energy distribution). The major energy sink for the electrons is dissociation of molecular hydrogen, producing hydrogen atoms at a rate of 1029 per second. Approximately half the atoms have energies higher than the escape energy. The high temperature of the atmosphere, the small size of Uranus, and the number density of hydrogen atoms in the thermosphere imply an extensive thermal hydrogen corona that reduces the orbital lifetime of ring particles and biases the size distribution toward larger particles. This corona is augmented by the nonthermal hydrogen atoms associated with the electroglow. An aurora near the magnetic pole in the dark hemisphere arises from excitation of molecular hydrogen at the level where its vertical column abundance is about 1020 per square centimeter with input power comparable to that of the sunlit electroglow (approximately 2x1011 watts). An initial estimate of the acetylene volume mixing ratio, as judged from measurements of the far ultraviolet albedo, is about 2 x 10-7 at a vertical column abundance of molecular hydrogen of 1023 per square centimeter (pressure, approximately 0.3 millibar). Carbon emissions from the Uranian atmosphere were also detected.

Helium in Jupiter's atmosphere: Results from the Galileo probe Helium Interferometer Experiment

On December 7, 1995, the NASA Galileo probe provided the first in situ measurements of the helium abundance in the atmosphere of Jupiter. Our Jamin interferometer measured precisely the refractive index of the Jovian atmosphere in the pressure region from 2 to 12 bars. From these measurements, we derive the atmospheric helium mole fraction to be 0.1359±0.0027. The corresponding helium mass fraction matches closely, but accidentally, the current helium abundance of the atmosphere of the Sun. However, both the Jovian and the solar value fall somewhat below the protosolar value. This suggests that in both Jupiter and the Sun processes are active which separate helium from hydrogen.

Production and Escape of Terrestrial Hydrogen

Abstract The agronomy of hydrogen and its compounds is discussed in a simplified model intended to represent a diurnal and global average. Vertical transport by eddy and molecular diffusion is included for the major components H2O, H and H2. The flow at 100 km is found to be dominated by H2, which is converted to H in the thermosphere and escapes. The flux is insensitive to the details of the chemistry and to large variations in the assumed eddy coefficients. Slightly above the homopause at 100 km, the H2 flux is close to its “limiting” value, which is proportional to the H2 mixing ratio. This mixing ratio, in terms of total H atoms, is slightly less than that of H2O, H2 and CH4 at 30 km. Any variation of the latter therefore varies the escape flux in proportion. The model reproduces the observed escape flux fairly well, along with other observations of OH and H in the mesosphere. Reduction of the input mixing ratio by a factor of 2 would improve the agreement. It is suggested that the H2O mixing ratio at...

The sodium and potassium atmosphere of the moon and its interaction with the surface

Abstract Observations of lunar atmospheric sodium and potassium from May 1988 to July 1991 are reported and analyzed. Densities at 80° north and south are less than equatorial ones by a factor of 2–3. For our observations, which do not reach above 800 km from the limb, the apparent scale heights for the intensity are 119–611 km for Na, and 85 and 154 km for K; most of these are much larger than would be expected for atoms thermalized to the surface temperature. However, the intensity drops off with increasing radius at a much greater rate than would be observed for an atmosphere that is mostly escaping. We interpret our data using both single- and two-component analyses. We amplify an earlier suggestion that source atoms are quickly redistributed into thermal and suprathermal populations by “competing release mechanisms” acting at the surface. The suprathermal distributions are produced by solar radiation releasing atoms adsorbed on the surface ( photodesorption ). We present reasons why the energy distribution seems to mimic a Maxwellian. The competing release mechanisms explain an obvious trend of decreasing apparent scale heights toward the subsolar point, where the density in the lowest 100 km appears to be dominated by thermally desorbed atoms. The suprathermal component is expected to appear at greater altitudes, but the early subsolar data do not extend high enough to reveal it. Six of the data sets are tentatively resolved into thermal and suprathermal components. The variation with latitude is naturally explained if a larger fraction of the atoms at large solar zenith angles are adsorbed to the surface, rather than being visible in the atmosphere. Migration to the dark side may also play a role. It is shown that at most a very small fraction of the observed atoms below a few hundred km altitude can be on escape trajectories. We apply these ideas to the budget of atomic oxygen. We suggest that the inventory of oxygen atoms is greatly reduced because they stick to the surface with high efficiency similar to that of the alkalis, and subsequently recombine with each other or with partially reduced oxides of such atoms as Mg, Fe, Al, and Si.

Conversion of para and ortho hydrogen in the Jovian planets

A mechanism is proposed which partially equilibrates the para and ortho rotational levels of molecular hydrogen in the atmospheres of Jupiter, Saturn, and Uranus. Catalytic reactions between the free-radical surface sites of aerosol particles and hydrogen molecules yield significant equilibration near 1 bar pressure, if the efficiency of conversion per collision is between 10−8 and 10−10 and the effective eddy mixing coefficient is 104 cm2/sec. At lower pressures the ortho-para ratio retains the value at the top of the cloud layer, except for a very small effect from conversion in the thermosphere. The influence of conversion on the specific heat and adiabatic lapse rate is also investigated. The effect is found to be generally small, though it can rise to 10% inside the aerosol layer.

Origin and character of the lunar and mercurian atmospheres

Abstract This article presents a description of the lunar and mercurian atmospheres and the outstanding, unanswered questions. In that spirit we will not repeat the information found in the reviews of the lunar (Morgan and Stern, 1991) and mercurian (Hunten et al. , 1988) atmospheres, which provide a historical context of the discoveries and initial measurements. These atmospheres are tenuous and transient; different components have lifetimes of hours to weeks. The sources that maintain them are of considerable interest, but direct information on sources is difficult to obtain. We therefore begin by describing the atmospheres and their behavior, and go on to consider the loss processes. This information is then used in the discussion of the most likely sources.

Venus upper atmosphere neutral gas composition - First observations of the diurnal variations

Measurements of the composition, temperature, and diurnal variations of the major neutral constituents in the thermosphere of Venus are being made with a quadrupole mass spectrometer on the Pioneer Venus orbiter. Concentrations of carbon dioxide, carbon monoxide, molecular nitrogen, atomic oxygen, and helium are presented, in addition to an empirical model of the data. The concentrations of the heavy gases, carbon dioxide, carbon monoxide, and molecular nitrogen, rapidly decrease from the evening terminator toward the nightside; the concentration of atomic oxygen remains nearly constant and the helium concentration increases, an indication of a nightside bulge. The kinetic temperature inferred from scale heights drops rapidly from 230 K at the terminator to 130 K at a solar zenith angle of 120�, and to 112 K at the antisolar point.