Near-Infrared Spectroscopy and Spectral Mapping of Jupiter and the Galilean Satellites: Results from Galileo's Initial Orbit

Abstract

The Near Infrared Mapping Spectrometer performed spectral studies of Jupiter and the Galilean satellites during the June 1996 perijove pass of the Galileo spacecraft. Spectra for a 5-micrometer hot spot on Jupiter are consistent with the absence of a significant water cloud above 8 bars and with a depletion of water compared to that predicted for solar composition, corroborating results from the Galileo probe. Great Red Spot (GRS) spectral images show that parts of this feature extend upward to 240 millibars, although considerable altitude-dependent structure is found within it. A ring of dense clouds surrounds the GRS and is lower than it by 3 to 7 kilometers. Spectra of Callisto and Ganymede reveal a feature at 4. 25 micrometers, attributed to the presence of hydrated minerals or possibly carbon dioxide on their surfaces. Spectra of Europa's high latitudes imply that fine-grained water frost overlies larger grains. Several active volcanic regions were found on Io, with temperatures of 420 to 620 kelvin and projected areas of 5 to 70 square kilometers.

An experimental estimate of Europa's “ocean” composition independent of Galileo orbital remote sensing

“…Figure 4 shows the 0.7 to 2.5/am spectrum of our leach evaporite in linear combination with an equal amount of H20 ice. It is compared with spectra of lineae, within dark terrain and those outside dark terrain [Carlson et al, 1996]. Note that differences between our evaporite spectrum and the lineae are not notably greater than differences among the lineae themselves.…”

Distributions of CO2 and SO2 on the surface of Callisto

“…These combined spatial and spectral resolutions of NIMS have made many recent discoveries possible. Among them, absorptions at 4.05 and 4.25 pm were attributed to the presence of SO 2 and CO 2, respectively [Carlson et al, 1996;McCord et al, 1997McCord et al, , 1998]. These components, likely trapped in stabilizing host materials, have been observed on the surface of Callisto in both bright, ice-rich and dark, ice-poor areas.…”

The solar reflected component in Jupiter's 5‐μm spectra from NIMS/Galileo observations

“…[Carlson et al, 1996]. It includes the atmospheric parameters and the abundances from the probe measurements [Niemann et at., 1996].…”

“…In relatively cloud free regions, like the 5-/•m hot spots discovered by Westphal [1969], atmospheric sounding reaches tropospheric levels as deep as 5-8 bars [Roos-Serote et al, this issue], permitting the detection of minor species, such as PH3, Gert4, ASH3, H20 , in the troposphere. Brightness temperatures as high as 280 K have been measured with Galileo near-infrared mapping spectrometer (NIMS) in some spectra [Carlson et al, 1996]. Ground-based imaging at 5/•m [Terrile and Westphal, 1977;Orton et al, 1996] shows high contrast images, where cold areas appear as very black, featureless regions, with little contrast, and where hot areas, mostly confined to the Jovian belts, exhibit the highest brightness temperature within the north equatorial belt (NEB) hot spots.…”