Composition and thermal profiles of the Jovian upper atmosphere determined by the Voyager Ultraviolet Stellar Occultation Experiment

Abstract

Occultation of the star Regulus—α Leo—by the Jovian atmosphere was monitored by the Voyager 2 spacecraft on July 9, 1979. The absorption recorded in the 910–1200 Å range was caused primarily by the H2‐Lyman and Werner bands. These data provide the first complete measurements of atmospheric density and temperature profiles between 330 and 830 km above the ammonia cloud tops. The molecular hydrogen density at 380 km is found to be 3−1+4 × 1013 cm−3, where the atmospheric temperature is 200±50 K. The thermal gradient above 830 km altitude is found to be approximately 1 K km−1 to reconcile the stellar occultation data with the Voyager 1 solar occultation data for the exosphere. Both experiments were performed in the equatorial region. The observed temperature gradient in the upper atmosphere rules out inertia gravity wave propagation as the primary heating mechanism; the heating must be caused by one or many of a host of other potential sources such as magnetospheric electrons (soft or hard), Joule heating and even solar extreme ultraviolet radiation. The data do not present a strong argument in favor of an earthlike mesopause on Jupiter. The absorption in the 1250–1600 Å range yields volume mixing ratios of methane and ethane of 2.5−2+3 × 10−5 and 2.5−1.5+2.0 × 10−6, respectively, at a height of 325 km above the ammonia cloud tops. An upper limit of 2.5 × 10−6 for the mixing ratio of acetylene has been found at the altitude of 300 km. The Voyager infrared data yield mixing ratios of these hydrocarbons deeper in the stratosphere. A study of the density profiles of the hydrocarbons deduced from the stellar occultation data yields a value of the eddy diffusion coefficient at the homopause to be 1.4−0.7+0.8 × 106 cm² s−1 in the equatorial region which is consistent with the value deduced from the hydrogen Lyman alpha and helium 584 Å emission data. Detailed aeronomical implications of the results are discussed elsewhere (Atreya et al., 1981).

Uranus after Solstice: Results from the 1998 November 6 Occultation

“…This suggests that thermal conduction alone is an unlikely candidate for the responsible cooling mechanism, especially since, as with the radiative cooling argument, we have made a "best case" scenario by ignoring heating sources. However, the UVS thermal profiles underestimated the thermal gradients near 1 µbar on Jupiter by a factor of 3 (Festou et al 1981, Young et al 1997), so we cannot rule out a similar readjustment here. As on Jupiter, one way to constrain the thermal gradients in this region is by measurements of the temperatures near the homopause from H 2 fluorescence (Liu and Dalgarno 1996), in combination with ground-based and UVS occultations (Yelle et al 1996).…”

The atmosphere and ionosphere of Jupiter

“…I0). The current measurements of the hydrocarbon profiles (Voyager data, Festou, et al, 1980), however, would tend to make this shoulder disappear.…”

“…stellar occultation data analysis have been presented by Festou, et al (1980). These data yield temperature and atmospheric density profiles from 160km to about 1750km above the ammonia cloud tops.…”

“…4 Readers wishing to use this figure must consult Atreya, et al (1980) and Festou, et al (1980) for further details. solar maximum, it is difficult, for the reason cited, to make a case for direct correlation of the increased solar activity with the increased temperature.…”

Global ozone monitoring by occultation of stars: an overview of GOMOS measurements on ENVISAT

“…The advent of CCDs (or other multi-pixels detectors), enabling measurement of the whole transmission spectrum at once, increased dramatically the capability of stellar occultation versus the use of filters, or scanning spectrometers. The first multi-pixel occultation (to our knowledge) was inaugurated with the ultraviolet spectrometer UVS placed on board the Voyager 1 and 2 missions: in 1979, an occultation of the star alfa Leonis by the atmosphere of Jupiter was observed from a distance of 14 millions km by Voyager 1 in the full range 110-170 nm allowing one to probe the H 2 and methane vertical profiles (Festou et al, 1981). Since then, this technique was fostered at Service d'Aéronomie, in particular for the study of the atmosphere of the planet Mars with the SPICAM instrument.…”