R. Mehlman

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

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 Europas 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.

The distribution of sulfur dioxide and other infrared absorbers on the surface of Io

The Galileo Near Infrared Mapping Spectrometer was used to investigate the distribution and properties of sulfur dioxide over the surface of Io, and qualitative results for the anti-Jove hemisphere are presented here. SO2, existing as a frost, is found almost everywhere, but with spatially variable concentration. The exceptions are volcanic hot spots, where high surface temperatures promote rapid vaporization and can produce SO2-free areas. The pervasive frost, if fully covering the cold surface, has characteristic grain sizes of 30 to 100 µm, or greater. Regions of greater sulfur dioxide concentrations are found. The equatorial Colchis Regio area exhibits extensive snowfields with large particles (250 to 500 µm diameter, or greater) beneath smaller particles. A weak feature at 3.15 µm is observed and is perhaps due to hydroxides, hydrates, or water. A broad absorption in the 1 µm region, which could be caused by iron-containing minerals, shows a concentration in Ios southern polar region, with an absence in the Pele plume deposition ring.

Hot spots on Io: Initial results from Galileo's near infrared mapping spectrometer

The Near-Infrared Mapping Spectrometer on Galileo has monitored the volcanic activity on Io since June 28, 1996. This paper presents preliminary analysis of NIMS thermal data for the first four orbits of the Galileo mission. NIMS has detected 18 new hot spots and 12 others which were previously known to be active. The distribution of the hot spots on Ios surface may not be random, as hot spots surround the two bright, SO2-rich regions of Bosphorus Regio and Colchis Regio. Most hot spots seem to be persistently active from orbit to orbit and 10 of those detected were active in 1979 during the Voyager encounters. We report the distribution of hot spot temperatures and find that they are consistent with silicate volcanism.

Analysis of Jupiter north equatorial belt hot spots in the 4–5 μm range from Galileo/near-infrared mapping spectrometer observations: Measurements of cloud opacity, water, and ammonia

This paper presents the analysis of hot spot observations in the Jovian North Equatorial Belt obtained with the near-infrared mapping spectrometer (NIMS) instrument on the Galileo spacecraft. The data were acquired during the closest approach sequences between June 1996 and April 1997. We focus on the spectral window between 4.5 and 5.2 μm determining the cloud opacity above 2 bar, the water vapor relative humidity, and the ammonia abundance between 4 and 8 bar. We find a linear relationship between the cloud opacity and the continuum level of the spectrum. For a given radiance level of an individual spectrum, significant variations in the water vapor relative humidity are seen. However, no clear evidence for a relationship between the cloud opacity and the water relative humidity is seen. A cloud structure similar to that measured by the Galileo entry probe, with no significant cloud opacity below 2 bar, is adequate. The air in the hot spots is found to be overall dry, consistent with the probe measurements. None of the considered spectra show water vapor relative humidities exceeding 10%. Significant spatial variations of the water vapor relative humidity are found, and the distribution over the observed hot spot regions is complex. Because of a low sensitivity of the NIMS spectra to ammonia, uncertainties in the derived ammonia abundance are much higher than for water. There is, however, a possible trend in all the observed hot spots toward more ammonia inside than outside the hot spots at the sounded pressure levels.

Galileo infrared imaging spectrometry measurements at the Moon

Imaging spectrometer observations were made of the surface of the Moon during the December 1990 flyby of the Earth-Moon system by the Galileo spacecraft. This article documents this data set and presents analyses of some of the data. The near infrared mapping spectrometer (NIMS) investigation obtained 17 separate mosaics of the Moon in 408 spectral channels between about 0.7 and 5.2 μm. The instrument was originally designed to operate in orbit about Jupiter and therefore saturates at many spectral channels for most measurement situations at 1 AU. However, sufficient measurements were made of the Moon to verify the proper operation of the instrument and to demonstrate its capabilities. Analysis of these data show that the NIMS worked as expected and produced measurements consistent with previous ground-based telescopic studies. These are the first imaging spectrometer measurements of this type from space for the Moon, and they illustrate several major points concerning this type of observation and about the NIMS capabilities specifically. Of major importance are the difference between framing and scanning instruments and the effects of the spacecraft and the scan platform on the performance of such an experiment. The science return of subsequent NIMS and other investigation measurements will be significantly enhanced by the experience and results gained.