Floyd Herbert

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.

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.

The signature of hot hydrogen in the atmosphere of the extrasolar planet HD 209458b

About ten per cent of the known extrasolar planets are gas giants that orbit very close to their parent stars. The atmospheres of these ‘hot Jupiters’ are heated by the immense stellar irradiation. In the case of the planet HD 209458b, this energy deposition results in a hydrodynamic state in the upper atmosphere, allowing for sizeable expansion and escape of neutral hydrogen gas. HD 209458b was the first extrasolar planet discovered that transits in front of its parent star. The size of the planet can be measured using the total optical obscuration of the stellar disk during an observed transit, and the structure and composition of the planetary atmosphere can be studied using additional planetary absorption signatures in the stellar spectrum. Here we report the detection of absorption by hot hydrogen in the atmosphere of HD 209458b. Previously, the lower atmosphere and the full extended upper atmosphere of HD 209458b have been observed, whereas here we probe a layer where the escaping gas forms in the upper atmosphere of HD 209458b.

Electromagnetic heating of minor planets in the early solar system

Electromagnetic processes occurring in the primordial solar system are likely to have significantly affected planetary evolution. In particular, electrical coupling of the kinetic energy of a dense T-Tauri-like solar wind into the interior of the smaller planets could have been a major driver of thermal metamorphism. Accordingly a grid of asteroid models of various sizes and solar distances was constructed using dc transverse magnetic induction theory. Plausible parameterizations with no requirement for a high environmental temperature led to complete melting for Vesta (and others with sizes down to 50 km diameter and distance out to 2.8 AU thus approximately reproducing the observed distributions of S objects) with no melting for Pallas and Ceres. Fairly high temperatures were reached in the Pallas model, perhaps implying nonmelting thermal metamorphosis as a cause of its anomalous spectrum (somewhat similar to but distinct from C type). A reversal of this temperature sequence seems implausible, suggesting that the Ceres-Pallas-Vesta dichotomy is a natural outcome of the induction mechanism. Highly localized heating is expected to arise due to an instability in the temperature-controlled current distribution. Localized metamorphosis resulting from this effect may be relevant to the production and evolution of pallasites, the large presumed metal component of S object spectra, and the formation of the lunar magma ocean.

CH4 and haze in Triton's lower atmosphere

Voyager 2 ultraviolet spectrometer occultation observations at Triton have revealed two constituents of the troposphere: CH4 and another absorber visible between 1400 and 1600 A below about 20 km altitude. The CH4appears to be saturated at the surface at both entrance and exit occultation sites. The density scale height and wavelength dependence of the long-wavelength absorber are consistent with Rayleigh scattering in N2. However, the inferred N2 column abundance is inconsistent with the Voyager radio science (RSS) occultation measurement, and so the N2hypothesis is discarded. The most attractive alternative hypothesis for the identity of the long-wavelength opacity is Rayleigh scattering by an aerosol haze, because the optical depth is noticeably wavelength dependent, both within the EUV range of measurement and over the range between the EUV and visual wavelengths. EUV Rayleigh scattering requires that the haze particles be very small (≤ 0.03 μm) and abundant. These characteristics suggest formation as photochemical smog or condensing N2 with a great abundance of nucleation centers. Condensation onto ions allows rapid formation of very small particles, so possibly ions created by penetrating magnetospheric charged particles (0.1 to several MeV) are nucleating the haze. Small particle size also promotes long residence times against settling, essential for accumulation to significant abundance and suggesting that eddy transport dominates settling. If this is the case, the haze scale height likely approximates the gas scale height, and the implied temperature is around 38 K. This is lower than the 50 K estimated by the RSS and is also too low for the dust devil hypothesis for the visible atmospheric plumes.

Temperature, N2, and N density profiles of Triton's atmosphere: Observations and model

Improved analysis of the Voyager Ultraviolet Spectrometer (UVS) observations of the solar occultation by Triton yields the isothermal temperature and N2 number densities in the altitude range 475–675 km: T = 102 ± 3 K, [N2]= (4 ± 0.4) × 108 cm−3 at 575 km. A distinct steplike absorption feature at 850 A is due to the atomic nitrogen ionization continuum. It allows a measurement of N number densities in the range from 170 to 570 km, which correspond to diffusive equilibrium above 300 km with [N] = (1±0.25) ×108 cm−3 at 400 km and T = 100±7 K. Deviations from diffusive equilibrium become important below 300 km, and [N] = (5 ± 2.5) × 108 cm−3 at 200 km. The exobase altitude is 870 km, and the total escape rate of atomic nitrogen is (1±0.3) × 1025 s−1. The main condensible product of methane chemistry is ethylene, C2H4, with a peak number density of 6 × 106 cm−3 near 25 km. The striking similarity of the thermospheric properties at both occultation sites despite substantial differences in latitudes, seasons, local time, and incoming flux of magnetospheric electrons implies very effective winds and suggests that one-dimensional modeling is applicable. Temporal variations of the temperature profile should be rather small in spite of strong variations of the electron flux. The presence of CO in the atmosphere as suggested by recent measurements of the CO absorption bands in the surface ice results in cooling by the rotational lines, deactivation and radiation of vibrational excitation of N2, and heating by quenching of N(2D). Heating efficiencies of electron precipitation and solar EUV radiation in the ranges of the N2 continuum and bands are calculated equal to 0.2–0.29, 0.24, and 0.19, respectively. The column integrated cooling by the CO lines is weaker than the thermospheric heating and cannot form a mesosphere. A CO mixing ratio of up to 10−2 is consistent with thermal balance calculations. Sixteen optimized versions of the model are considered with profiles of T and [N2] which agree with the measurements. Profiles of N, H2, and H densities are calculated. The calculated profile of atomic nitrogen and its escape rate are in excellent agreement with the measured values. Mixing ratios of H2 and H at 400 km are equal to 240 and 23 ppm, respectively. The total escape rate of hydrogen atoms (H + 2H2) is determined by photolysis of methane by H Lyman α radiation, does not depend on the ionospheric processes which transform H2 to H, and is equal to 2.3 × 1026 s−1. The ratio of H to N escape rates on Triton, 20, does not correspond to the measured ratio H+/N+∼ 2–3 in Neptunes magnetosphere and should be taken into account in its modeling. The recommended model for Tritons atmosphere is based on the temperature profile calculated for a CO mixing ratio of 10−3, a ratio of the average to the maximum electron fluxes of 0.162, and includes data on temperature, N2, N, CH4, H2, and H number densities.

Primordial Electrical Induction Heating of Asteroids

Abstract Recently observed systematic trends of asteroidal composition with respect to size and heliocentric distance are interpreted according to the hypothesis that primordial asteroidal thermal evolution was driven by electromagnetic induction in a dense pre-main-sequence solar wind. It is found that the spatial distribution of induction heating corresponds well with that inferred from the present-day distribution of asteroidal compositional types when reasonable proto-Solar-System characteristics are assumed. From this match is deduced a rough constraint on the induction heating epochs duration (≈10 5 years) and the temperature-dependent electrical conductivity of the protoasteroidal material (about the same as that of the C2 meteorite Murcheson).

Radial profiles of ion density and parallel temperature in the Io plasma torus during the Voyager 1 encounter

We have analyzed a moderately large set of Voyager 1 ultraviolet spectrometer preencounter scans of the Io warm plasma torus in order to deduce its three-dimensional spatial structure. The density distributions of the five ions O+, O++, S+, S++, and S+++ have been determined in order to deduce their scale heights and thus the ion kinetic temperature parallel to the magnetic field. We find that the parallel temperature of the ions decreases with increasing Jovicentric distance. This variation is opposite to the profile of perpendicular ion temperature derived from in situ plasma measurements but consistent with an adiabatic thermal profile. To reconcile these measurements, we propose that rapid radial transport is adiabatically cooling the torus plasma and mixing it with hot plasma from the middle magnetosphere. We also derive a plasma ionization profile that increases with Jovicentric distance, consistent with in situ measurements of the electron temperature, which also rises with distance. From these observations we conclude that (1) torus plasma transport and the associated adiabatic cooling is faster than ion heating in the outer torus; (2) outer torus electrons are heated by a process that neglibly heats the ions; and (3) this electron heating process is likely to be collisional thermalization with inwardly transported hot plasma from the middle magnetosphere, as postulated by other authors. We also propose a picture of torus dynamics that is consistent both with these conclusions and with the in situ measurements.

Reanalysis of Voyager 2 UVS Occultations at Uranus: Hydrocarbon Mixing Ratios in the Equatorial Stratosphere

Abstract Application of noise filtering and inversion techniques to single-channel UVS lightcurves obtained during the Voyager 2 solar occultation at Uranus has yielded tighter constraints on the structure and composition of the upper equatorial stratosphere at the time of the encounter. Specifically, atmospheric pressure and temperature profiles in the altitude region bracketted by total number densities 2 × 10 15 cm −3 and 5 × 10 16 cm −3 have been derived, based on the observed H 2 Rayleigh scattering opacity profiles ( wavelengths > 153 nm ) with an assumed helium mixing ration of 0.15: at the density level 3.3 (±0.2) × 10 16 cm −3 , the pressure is 0.60 (±0.01) mbar with a temperature 133 (±8) K. Lightcurves obtained at shorter wavelengths reveal the mixing ratio of accetylene to be increasing (0.7 × 10 −8 –1.5 × 10 −8 ) with increasing pressure in the pressure interval 0.10−0.30 mbar. Ethane is also present at these pressures with a mixing ratio of roughly 10 −8 ; if methane is present at mixing ratios in excess of roughly 3 × 10 −7 , then the ethane estimate may need to be halved. Comparison with photochemical models indicates values of 10 3 -10 -4 cm 2 sec −1 for the eddy mixing coefficient at the methane homopause, depending on the manner in which eddy mixing is assumed to vary with atmospheric number density. It has not been possible to obtain meaningful results from the stellar occultations, which are characterized by poor signal-to-noise ratios.

The Uranian aurora and its relationship to the magnetosphere

About 32 h of Voyager Ultraviolet Spectrometer (UVS) observations of Uranus H2 band airglow emission (875 ≤ λ ≤ 1115 A) have been analyzed using the singular value decomposition (SVD) approach to inversion, producing an intensity map showing aurora at both magnetic poles. An H Lyman α aurora may also be present but is difficult to separate from scattered solar and local interstellar medium components. SVD analysis of variance shows that the intensity estimate is significantly larger than the error estimate over both Uranographic poles and part of the equatorial region, fortuitously including both magnetic polar regions. The Goddard Space Flight Center Q3 magnetic field model correctly predicts that the aurora should be larger in area and emit more power at the weaker N magnetic pole than at the stronger S magnetic pole. However, the auroral emissions are quite localized in magnetic longitude and so do not form complete auroral ovals. The brightest auroral emission at each magnetic pole is confined to a range of ≈ 90° of magnetic longitude centered on the magnetotail direction, at moderate magnetic L parameter (5 ≤ L ≤ 10), but some emission at each pole is distributed over a range of more than 180° of longitude. The S polar auroral intensity maximum is coincident with the source of the broadband bursty and broadband smooth Uranian kilometric radio emission (UKR), while the N polar auroral intensity maximum may coincide with the dayside UKR source. The N and S auroral intensity maxima also lie at the conjugate magnetic footprints of the maximum intensities of whistler-mode plasma wave emission and 22- to 35-keV electron fluxes observed by Voyager. The magnetic longitudes of the aurora are completely inconsistent with the “windshield wiper” effect for either ions or electrons, indicating that some other effect, such as rapid depletion of the population of precipitating particles or highly localized strong pitch-angle diffusion, may be acting to localize emission. The low apparent L of the precipitating particles indicates that their energies may be ≤ 10 keV. Hence magnetospheric convection is likely to be important, and thus particles exciting the aurora may not remain on constant L shells. The precipitating particles may be a relatively low-energy population at high L that is heated to aurora-exciting energy by adiabatic compression during convection to low L. We estimate that the total auroral power output at H Lyman α and shorter wavelengths is about 3 × 109 to 7 × 109 W, requiring about 10 times that much power for excitation.