T. M. Donahue

Extreme Ultraviolet Observations from Voyager 1 Encounter with Jupiter

Observations of the optical extreme ultraviolet spectrum of the Jupiter planetary system during the Voyager 1 encounter have revealed previously undetected physical processes of significant proportions. Bright emission lines of S III, S IV, and O III indicating an electron temperature of 105 K have been identified in preliminary analyses of the Io plasma torus spectrum. Strong auroral atomic and molecular hydrogen emissions have been observed in the polar regions of Jupiter near magnetic field lines that map the torus into the atmosphere of Jupiter. The observed resonance scattering of solar hydrogen Lyman α by the atmosphere of Jupiter and the solar occultation experiment suggest a hot thermosphere (≥ 1000 K) wvith a large atomic hydrogen abundance. A stellar occultation by Ganymede indicates that its atmosphere is at most an exosphere.

Ultraviolet Spectrometer Observations of Neptune and Triton

Results from the occultation of the sun by Neptune imply a temperature of 750 � 150 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane, acetylene, and ethane at lower levels. The ultraviolet spectrum of the sunlit atmosphere of Neptune resembles the spectra of the Jupiter, Saturn, and Uranus atmospheres in that it is dominated by the emissions of H Lyman α (340 � 20 rayleighs) and molecular hydrogen. The extreme ultraviolet emissions in the range from 800 to 1100 angstroms at the four planets visited by Voyager scale approximately as the inverse square of their heliocentric distances. Weak auroral emissions have been tentatively identified on the night side of Neptune. Airglow and occultation observations of Tritons atmosphere show that it is composed mainly of molecular nitrogen, with a trace of methane near the surface. The temperature of Tritons upper atmosphere is 95 � 5 kelvins, and the surface pressure is roughly 14 microbars.

Stability of the Martian Atmosphere

A detailed chemical dynamic model is presented for a moist martian atmosphere. Recombination of carbon dioxide is catalyzed by trace amounts of water. The abundances of carbon monoxide and molecular oxygen should vary in response to changes in atmospheric water and atmospheric mixing.

The Galileo probe mass spectrometer: composition of Jupiter's atmosphere.

The composition of the jovian atmosphere from 0.5 to 21 bars along the descent trajectory was determined by a quadrupole mass spectrometer on the Galileo probe. The mixing ratio of He (helium) to H2 (hydrogen), 0.156, is close to the solar ratio. The abundances of methane, water, argon, neon, and hydrogen sulfide were measured; krypton and xenon were detected. As measured in the jovian atmosphere, the amount of carbon is 2.9 times the solar abundance relative to H2, the amount of sulfur is greater than the solar abundance, and the amount of oxygen is much less than the solar abundance. The neon abundance compared with that of hydrogen is about an order of magnitude less than the solar abundance. Isotopic ratios of carbon and the noble gases are consistent with solar values. The measured ratio of deuterium to hydrogen (D/H) of (5 ± 2) × 10−5 indicates that this ratio is greater in solar-system hydrogen than in local interstellar hydrogen, and the 3He/4He ratio of (1.1 ± 0.2) × 10−4 provides a new value for protosolar (solar nebula) helium isotopes. Together, the D/H and 3He/4He ratios are consistent with conversion in the sun of protosolar deuterium to present-day 3He.

Extreme ultraviolet observations from the Voyager 2 encounter with Saturn

Combined analysis of helium (584 angstroms) airglow and the atmospheric occultations of the star δ Scorpii imply a vertical mixing parameter in Saturns upper atmosphere of K (eddy diffusion coefficient) ∼ 8 x 107 square centimeters per second, an order of magnitude more vigorous than mixing in Jupiters upper atmosphere. Atmospheric H2 band absorption of starlight yields a preliminary temperature of 400 K in the exosphere and a temperature near the homopause of ∼ 200 K. The energy source for the mid-latitude H2 band emission still remains a puzzle. Certain auroral emissions can be fully explained in terms of electron impact on H2, and auroral morphology suggests a link between the aurora and the Saturn kilometric radiation. Absolute optical depths have been determined for the entire C ring andparts of the A and B rings. A new eccentric ringlet has been detected in the C ring. The extreme ultraviolet reflectance of the rings is fairly uniform at 3.5 to 5 percent. Collisions may control the distribution of H in Titans H torus, which has a total vertical extent of ∼ 14 Saturn radii normal to the orbit plane.

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.

GALILEO PROBE MEASUREMENTS OF D/H AND 3HE/4HE IN JUPITER'S ATMOSPHERE

The Galileo Probe Mass Spectrometer measurements in the atmosphere of Jupiter give D/H = (2.6 ± 0.7) × 10-5 3He/4He = (1.66 ± 0.05) × 10-4These ratios supercede earlier results by Niemann et al. (1996) and are based on a reevaluation of the instrument response at high count rates and a more detailed study of the contributions of different species to the mass peak at 3 amu. The D/H ratio is consistent with Voyager and ground based data and recent spectroscopic and solar wind (SW) values obtained from the Infrared Spectroscopic Observatory (ISO) and Ulysses. The 3He/4He ratio is higher than that found in meteoritic gases (1.5 ± 0.3) × 10-4. The Galileo result for D/H when compared with that for hydrogen in the local interstellar medium (1.6 ± 0.12) × 10-5 implies a small decrease in D/H in this part of the universe during the past 4.55 billion years. Thus, it tends to support small values of primordial D/H - in the range of several times 10-5 rather than several times 10-4. These results are also quite consistent with no change in (D+3He)/H during the past 4.55 billion years in this part of our galaxy.

Evolution of a Nitrogen Atmosphere on Titan

Photochemical calculations indicate that if NH3 outgassed from Titan it should have been converted to a dense N2 atmosphere during the lifetime of the satellite. A crucial step in the process involves a gas phase reaction of N2H4 with H. The most favorable conditions for this step would be the intermediate production of a CH4-H2 greenhouse capable of raising the gas temperature to 150�K. Subsequently about 20 bars of N2 could have evolved. The pressure-induced opacity of 20 bars of N2 should suffice to explain the recently measured 200�K surface temperature. Unlike the situation on Jupiter, NH3 is not recycled on Titan by reactions involving N2 or N2H4. This may explain the failure of recent attempts to detect NH3 in the upper atmosphere of Titan.

Noble gas abundance and isotope ratios in the atmosphere of Jupiter from the Galileo Probe Mass Spectrometer

The Galileo Probe Mass Spectrometer provided the first data on the noble gas mixing and isotope ratios in the Jovian atmosphere. These measurements and the comparison with solar values constrain models of Jupiters formation. Significant refinements to the initially reported abundances of argon, krypton, and xenon have been enabled through post-encounter laboratory calibrations using a refurbished engineering unit mass spectrometer nearly identical to the flight unit. The abundances relative to hydrogen for argon, krypton, and xenon are respectively 2.5±0.5, 2.7±0.5, and 2.6±0.5 times the solar ratios. The mixing ratios of He and Ne found in these studies are consistent with previously reported values of 0.8 and 0.1 times solar respectively. The Jovian 36Ar/38Ar ratio is 5.6±0.25 and the 20Ne/22Ne ratio is 13±2, consistent with the solar values of 5.77 and 13.81, respectively, that are derived from lunar mineral grain analysis. The distribution of xenon isotopes at Jupiter also resembles the solar distribution.

Jupiter - Structure and composition of the upper atmosphere

The Voyager ultraviolet stellar occultation data yield a temperature of 200 + or - 50 K at about 400 km, and the solar occultation data give 1100 + or - 200 K at 1450 km above the ammonia cloud tops. The temperature gradient between 400 and 1450 km is approximately 1 K/km. The mesospheric temperature structure gives no strong indication of an earth-like mesopause. The heating of the upper atmosphere appears to result from a combination of magnetospheric charged particle precipitation, ion drag, inertia gravity waves, and solar EUV. The volume mixing ratios of CH4 and C2H6 at 325 km are measured to be 2.5(+3, -2) x 10 to the -5th and 2.5(+2.0, -1.5) x 10 to the -6th, respectively, which are lower than in the stratosphere. The C2H2 volume mixing ratio is not greater than 5 x 10 to the -6th at 300 km. The homopause value of the equatorial eddy diffusion coefficient is found to be 1-2 x 10 to the -6th sq cm/s.