Darrell F. Strobel

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.

Clouds, aerosols, and photochemistry in the Jovian atmosphere

Abstract In this paper we review current ideas about the composition, horizontal and vertical distribution, and microphysical properties of clouds and aerosols in Jupiters upper troposphere and stratosphere. We also discuss several key photochemical species, their relation to aerosol formation, and their implications for transport processes. We treat photochemistry in the context of comparative planetology and point out important similarities and differences among the outer planet atmospheres. Our approach emphasizes observational data of relevance to cloud properties, and to this end we assemble a wide assortment of ground-based and spacecraft observations. We challenge some widely held views about the distribution of clouds in the troposphere and present a rationale for alternative interpretations.

The Far-Ultraviolet Oxygen Airglow of Europa and Ganymede

Far-UV spectra of Europa and Ganymede, acquired by the Hubble Space Telescope Goddard High Resolution Spectrograph, indicate that, in addition to faintly reflected sunlight, both satellites emit O I 1304 A and O I 1356 A airglow radiation. The observed brightnesses of the reflected solar C II 1335 A feature indicate that the disk-averaged albedos of Europa and Ganymede are about 1.5% and 2.6%, respectively. Airglow emissions from both satellites are characterized by the flux ratio F(1356 A)/F(1304 A) of roughly 1-2, diagnostic of dissociative electron impact excitation of O2. Inferred O2 vertical column densities are in the range (2.4-14) × 1014 cm-2 for Europa and (1-10) × 1014 cm-2 for Ganymede. The observed double-peaked profile of Ganymedes O I 1356 A feature indicates a nonuniform spatial emission distribution that suggests two distinct and spatially-confined emission regions, consistent with the satellites north and south poles.

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.

Transient Water Vapor at Europa’s South Pole

Europas Plumes Jupiters moon Europa has a subsurface ocean and a relatively young icy surface. Roth et al. (p. 171, published online 12 December 2013; see the Perspective by Spencer) analyzed spectral images taken by the Hubble Space Telescope that show ultraviolet emissions from the moons atmosphere, and report a statistically significant emission signal extending above the satellites southern hemisphere. This emission is consistent with two 200-km-high plumes of water vapor. Tidal stresses likely play a role in opening and closing fractures at the surface. Hubble Space Telescope images of Jupiter’s moon Europa reveal emission consistent with transient water vapor plumes. [Also see Perspective by Spencer] In November and December 2012, the Hubble Space Telescope (HST) imaged Europa’s ultraviolet emissions in the search for vapor plume activity. We report statistically significant coincident surpluses of hydrogen Lyman-α and oxygen OI 130.4-nanometer emissions above the southern hemisphere in December 2012. These emissions were persistently found in the same area over the 7 hours of the observation, suggesting atmospheric inhomogeneity; they are consistent with two 200-km-high plumes of water vapor with line-of-sight column densities of about 1020 per square meter. Nondetection in November 2012 and in previous HST images from 1999 suggests varying plume activity that might depend on changing surface stresses based on Europa’s orbital phases. The plume was present when Europa was near apocenter and was not detected close to its pericenter, in agreement with tidal modeling predictions.

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.

Interaction of the Jovian magnetosphere with Europa: Constraints on the neutral atmosphere

A three-dimensional plasma model was developed to understand the sources and sinks that maintain Europas neutral atmosphere and to study the interaction of the Jovian magnetosphere with this atmosphere and the formation of an ionosphere. The model includes self-consistently the feedback of the plasma action on the atmosphere through mass balance. Suprathermal torus ions with a contribution from thermal ions sputter O 2 from the water ice surface, and thermal torus ions remove the O 2 atmosphere by sputtering. For an oxygen column density of 5 × 10 18 m -2 the calculated intensities of the oxygen lines OI 130.4 nm and 135.6 nm produced by electron impact dissociation agree with observations by the Hubble Space Telescope [Hall et al., 1995]. Mass balance is also consistent with this column density, with a net atmospheric mass loss of 50 kg s -1 . For a given neutral atmosphere and magnetospheric conditions, the electrodynamic model computes self-consistently plasma density, plasma velocity, electron temperature of the thermal and the suprathermal population, electric current and electric field in the vicinity of Europa, with the assumption of a constant homogeneous Jovian magnetic field. Europas ionosphere is created by electron impact ionization where the coupling of the ionosphere with the energy reservoir of the plasma torus by electron heat conduction supplies the energy to maintain ionization. The calculated distribution of electron densities with a maximum value of nearly 10 4 cm -3 is in general agreement with densities derived by Kliore et al. [1997] from the Galileo spacecraft radio occultations. The Alfvenic current system closed by the ionospheric Hall and Pedersen conductivities carries a total current of 7 × 10 5 A in each Alfven wing.

Three‐dimensional plasma simulation of Io's interaction with the Io plasma torus: Asymmetric plasma flow

A three-dimensional, stationary, two-fluid plasma model for electrons and one ion species was developed to understand the local interaction of Ios atmosphere with the Io plasma torus and the formation of Ios ionosphere. Our model calculates, self-consistently, the plasma density, the velocity and the temperatures of the ions and electrons, and the electric field for a given neutral atmosphere and imposed Io plasma torus conditions but assumes for the magnetic field the constant homogeneous Jovian field. With only photoionization in a pure SO2 atmosphere it is impossible to correctly model the plasma measurements by the Galileo spacecraft. With collisional ionization and photoionization the observations can be successfully modeled when the neutral atmospheric column density is Ncol = 6 × 1020 m−2 and the atmospheric scale height is H = 100 km. The energy reservoir of the Io plasma torus provides via electron heat conduction the necessary thermal energy for the maintenance of the collisional ionization process and thus the formation of Ios ionosphere. Anisotropic conductivity is shown numerically as well as analytically to be essential to understand the convection patterns and current systems across Io. The electric field is very greatly reduced, because the ionospheric conductances far exceed the Alfven conductance ΣA, and also strongly twisted owing to the Hall effect. We find that the electric field is twisted by an analytic angle tan Θtwist = Σ2/(Σ1 + 2ΣA) from the anti-Jupiter direction toward the direction of corotation for constant values of the Pedersen and Hall conductances Σ1 and Σ2 within a circle encompassing Ios ionosphere. Because the electron velocity is approximately equal to the E × B drift velocity, the electron flow trajectories are twisted by the same angle toward Jupiter, with E and B the electric and magnetic fields, respectively. Since Σ1 ∼ Σ2, the electron flow is strongly asymmetric during convection across Io, and the magnitude of this effect is directly due to the Hall conductivity. In contrast, the ions are diverted slightly away from Jupiter when passing Io. Large electric currents flow in Ios ionosphere owing to these substantially different flow patterns for electrons and ions, and our calculations predict that a total electric current of 5 million A was carried in each Alfven wing during the Galileo flyby. We also find a total Joule heating rate dissipated in Ios ionosphere of P = 4.2 × 1011 W.

Chemistry and evolution of Titan's atmosphere☆

Abstract The chemistry and evolution of Titans atmosphere is reviewed in the light of the scientific findings from the Voyager mission. It is argued that the present N2 atmosphere may be Titans 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 Titans exosphere may dissociate enough N2 into hot, escaping N atoms to remove ∼0.2 of Titans present atmosphere over geologic time. The energy dissipation of magnetospheric electrons exceeds solar e.u.v. energy deposition in Titans atmosphere by an order of magnitude and is the principal driver of nitrogen photochemistry. The environmental conditions in Titans upper atmosphere are favorable to building up complex molecules, particularly in the north polar cap region.