The dynamics of Jupiter's atmosphere from the Galileo orbiter imaging system

Abstract

Jupiter's atmosphere displays a rich variety of dynamical phenomena. There are alternating east-west jets in both hemispheres and a strong eastward current astride the equator. The jets are remarkably steady in spite of vigorous turbulence in the shear zones between them, and in some cases even on the cores of the jets. There are large ovals, the largest of which is the Great Red Spot, that are imbedded in the jet system. There are certain regions that appear to be almost devoid of large cloud particles. Surprisingly, these cloud-free regions are often located near large plume-like structures that appear to be sources of cloud particles, perhaps active convective systems. All this activity constitutes a fluid mechanical engine that transfers heat from Jupiter's interior to space, and from low latitudes to high latitudes. The processes at work in Jupiter's atmosphere and interior are not well understood. Modeling efforts are limited because the physical system is not well defined at levels beneath the uppermost clouds. The Galileo probe provided important new information about deeper layers. In addition, the Galileo orbiter carries instruments with better capability than ever before to discriminate between different heights and give a three dimensional description of Jovian atmospheric dynamics. Early imaging results from the first orbit of the Galileo spacecraft are described by Belton et al. (1996). The present paper describes further results, through the third orbit. The Galileo orbiter camera system gives the highest spatial resolution of all the Galileo remote sensing instruments, and also has excellent capability for vertical discrimination by using spectral differences in atmospheric transmissivity. Figure 1 shows the temperature and pressure structure of the atmospheric region where Jupiter's observed dynamical activity is seated. Dotted lines indicate the tropospheric frost point temperatures at different pressure levels for water, ammonia, and methane, under the assumption that oxygen, nitrogen, and carbon are present in Jupiter's atmosphere in approximately twice the abundance as in the Sun (see Niemann et al., 1996 for Galileo probe measurements). The intersection of the frost point with the actual temperature profile gives the expected level of cloud formation. In reality, meteorological redistribution of condensing gases will lead to a complicated, time variable cloud system. A major objective of the Jupiter atmosphere remote sensing experiments is to determine the cloud distribution, both laterally and in height. Another objective is to track the motions of clouds, and for the first time, establish the vertical position of small scale cloud structures. This in turn will give a three dimensional determination of the wind field.

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