M. Bell

Galileo's First Images of Jupiter and the Galilean Satellites

The first images of Jupiter, Io, Europa, and Ganymede from the Galileo spacecraft reveal new information about Jupiters Great Red Spot (GRS) and the surfaces of the Galilean satellites. Features similar to clusters of thunderstorms were found in the GRS. Nearby wave structures suggest that the GRS may be a shallow atmospheric feature. Changes in surface color and plume distribution indicate differences in resurfacing processes near hot spots on Io. Patchy emissions were seen while Io was in eclipse by Jupiter. The outer margins of prominent linear markings (triple bands) on Europa are diffuse, suggesting that material has been vented from fractures. Numerous small circular craters indicate localized areas of relatively old surface. Pervasive brittle deformation of an ice layer appears to have formed grooves on Ganymede. Dark terrain unexpectedly shows distinctive albedo variations to the limit of resolution.

The landing of the NEAR-Shoemaker spacecraft on asteroid 433 Eros.

The NEAR-Shoemaker spacecraft was designed to provide a comprehensive characterization of the S-type asteroid 433 Eros (refs 1,2,3), an irregularly shaped body with approximate dimensions of 34 × 13 × 13 km. Following the completion of its year-long investigation, the mission was terminated with a controlled descent to its surface, in order to provide extremely high resolution images. Here we report the results of the descent on 12 February 2001, during which 70 images were obtained. The landing area is marked by a paucity of small craters and an abundance of ‘ejecta blocks’. The properties and distribution of ejecta blocks are discussed in a companion paper. The last sequence of images reveals a transition from the blocky surface to a smooth area, which we interpret as a ‘pond’. Properties of the ‘ponds’ are discussed in a second companion paper. The closest image, from an altitude of 129 m, shows the interior of a 100-m-diameter crater at 1-cm resolution.

The Dynamics of Jupiter’s Atmosphere from the Galileo Orbiter Imaging System

Jupiters 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 Jupiters interior to space, and from low latitudes to high latitudes. The processes at work in Jupiters 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 Jupiters 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 Jupiters 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.