Glenn S. Orton

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.

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.

Infrared polar brightening on Jupiter: III. Spectrometry from the Voyager 1 IRIS experiment

Spectra from the Voyager 1 IRIS experiment confirm the existence of enhanced infrared emission near Jupiters north magnetic pole in March 1979. The spectral characteristics of the enhanced emission are consistent with a Planck source function. A temperature-pressure profile is derived for the region near the north magnetic pole, from which quantitative abundance estimates of minor species are made. Some species previously detected on Jupiter, including CH3D, C2H2 and C2H6, have been observed again near the pole. Newly discovered species, not previously observed on Jupiter, include C2H4, C3H4, and C6H6. All of these species except CH3D appear to have enhanced abundances at the north polar region with respect to midlatitudes. Upper limits are determined for C4H2 and C3H8. The quantitative results are compared with model calculations based on ultraviolet results from the IUE satellite. The plausibility of the C6H6 identification in discussed in terms of the literature on C2H2 polymerization. The relation of C6H6 to cuprene is also discussed.

Optical properties of NH3 ice from the far infrared to the near ultraviolet.

The optical constants (nr,ni) for solid ammonia in the cubic phase from 50 to 71,000 cm−1 (0.14–200 μm) are displayed in both graphical and tabular form. The refractive indices nr were obtained from previously published spectra of the absorption index ni by means of the Kramers-Kronig dispersion relation. Mie scattering parameters in the same spectral range are graphically illustrated for particle sizes from 1 to 100 μm. An application of these results to the atmosphere of the planet Jupiter is also presented.

An intense stratospheric jet on Jupiter.

The Earths equatorial stratosphere shows oscillations in which the east–west winds reverse direction and the temperatures change cyclically with a period of about two years. This phenomenon, called the quasi-biennial oscillation, also affects the dynamics of the mid- and high-latitude stratosphere and weather in the lower atmosphere. Ground-based observations have suggested that similar temperature oscillations (with a 4–5-yr cycle) occur on Jupiter, but these data suffer from poor vertical resolution and Jupiters stratospheric wind velocities have not yet been determined. Here we report maps of temperatures and winds with high spatial resolution, obtained from spacecraft measurements of infrared spectra of Jupiters stratosphere. We find an intense, high-altitude equatorial jet with a speed of ∼140 m s-1, whose spatial structure resembles that of a quasi-quadrennial oscillation. Wave activity in the stratosphere also appears analogous to that occurring on Earth. A strong interaction between Jupiter and its plasma environment produces hot spots in its upper atmosphere and stratosphere near its poles, and the temperature maps define the penetration of the hot spots into the stratosphere.

Temperature and Composition of Saturn's Polar Hot Spots and Hexagon

Saturns poles exhibit an unexpected symmetry in hot, cyclonic polar vortices, despite huge seasonal differences in solar flux. The cores of both vortices are depleted in phosphine gas, probably resulting from subsidence of air into the troposphere. The warm cores are present throughout the upper troposphere and stratosphere at both poles. The thermal structure associated with the marked hexagonal polar jet at 77°N has been observed for the first time. Both the warm cyclonic belt at 79°N and the cold anticyclonic zone at 75°N exhibit the hexagonal structure.

Evolution and persistence of 5‐μm hot spots at the Galileo probe entry latitude

We present a study on the longitudinal locations, morphology, and evolution of the 5-μm hot spots at 6.5°N latitude (planetocentric) from an extensive Infrared Telescope Facility-National Science Foundation Camera (IRTF-NSFCAM) data set spanning more than 3 years, which includes the date of the Galileo probe entry. A probabilistic analysis of the data shows that within periods of several months to even more than a year, there are eight or nine longitudinal areas with high likelihood of containing a 5-μm hot spot. These areas drift together with respect to System III at a rate which changes only slowly in time, and they are quasi evenly spaced, suggesting a wave feature. A spectral analysis of the radiance data reveals that planetary wavenumbers 8, 9, and 10 are predominant in the data, 10 having more spectral power in several time periods when the speed was 103.5–102.5 m/s, while planetary wavenumber 8 has much more power when the speed is (99.5±0.5) m/s. By using the Galileo probe zonal wind speed [Atkinson et al., 1997] at the level of the main cloud that is opaque to the radiation at 5 μm (∼2 bar), our drift corrections imply a westward phase speed for the proposed wave. The wavenumbers and phase speeds are found to be consistent with an equatorial Rossby wave, and the dispersive properties of this wave can account for the observed simultaneous changes in the dominant wavenumber and drift speed. We take advantage of this interpretation to infer properties of the vertical structure at 6.5°N.

Characteristics of the Galileo probe entry site from Earth‐based remote sensing observations

A reassessment of ground-based observations confirms to better than a 98% confidence level that the Galileo probe entered a 5-μm hot spot, a region of unusual clarity and dryness, some 900±300 km north of its southern boundary. Cloud conditions at that point were similar to those in the center of this region, some 600 km further north. At the time of the probe entry, the region was evolving to a slightly larger size and even thinner cloud conditions, as evidenced by its rapidly brightening appearance at 4.78 μm. The low reflectivity of the region in red light is highly anticorrelated with 4.78-μm thermal emission, but this correlation breaks down in the blue. In general, the reflectivity of most hot spots is remarkably uniform, although the 4.78-μm thermal emission is highly variable. A cloud structure most consistent with both the observed reflected sunlight and thermal emission properties consists of two layers: (1) a cloud layer above the 450-mbar level extending up to the 150-mbar level that probably consists of submicron sized particles and (2) a tropospheric cloud that is probably below the 1-bar level, possibly ammonia hydrosulfide, with low optical thickness in the infrared. A population of particles larger than ∼3 μm, clearly present at the NH3 ice cloud level outside hot spots, is absent inside them. The NH3 gas abundance near 300–400 mbar pressure does not appear to be unusually depleted in hot spots. Zonal structures in the tropospheric temperature field near the probe entry site were not correlated with the location of 5-μm hot spots but moved at speeds closer to the internal rotation rate of the planet. The properties of the tropospheric thermal waves at the probe entry latitude show little correlation to the properties of the 5-μm hot spot waves. Temperatures at the probe entry site derived from remote sensing are warmer than the Atmospheric Structure Instrument (ASI) experiment results near the tropopause, probably because the low-temperature ASI features are confined to regions smaller than the ∼6000-km resolution characteristic of the remote sensing.

Submillimeter and millimeter observations of jupiter

We report narrowband photometry of the Jovian disk in 10 passbands covering the range from 0.35 to 3.3 mm wavelength. Absolute calibration was referenced to Mars. The derived brightness temperature spectrum is analyzed in the context of existing contraints on the atmospheric temperature structure and composition from ground-based studies at shorter wavelengths and from various spacecraft measurements. Our results for wavelengths between 0.35 and 0.45 mm suggest that the radiances can be matched by models which include NH3 ice particles which are between 30 and 100 μm in size, regardless of the scale height characterizing the cloud. It is difficult however, to model the relatively cool observations longward of 0.7 mm unless additional absorbers are assumed in the atmosphere or a different NH3 lineshape is assumed. If the absolute calibration scale were increased by 5%, the results would be fit by a clear atmosphere (or a small particle cloud) model, with no need to invoke additional absorption in the Jovian atmosphere.

Cloud structure and atmospheric composition of Jupiter retrieved from Galileo near‐infrared mapping spectrometer real‐time spectra

The first four complete spectra recorded by the near infrared mapping spectrometer (NIMS) instrument on the Galileo spacecraft in 1996 have been analyzed. These spectra remain the only ones which have been obtained at maximum resolution over the entire NIMS wavelength range of 0.7–5.2 μm. The spectra cover the edge of a “warm” spot at location 5°N, 85°W. We have analyzed the spectra first with reflecting layer models and then with full multiple scattering models using the method of correlated-k. We find that there is strong evidence for three different cloud layers composed of a haze consistent with 0.5-μm radius tholins at 0.2 bar, a cloud of 0.75-μm NH3 particles at about 0.7 bar, and a two-component NH4SH cloud at about 1.4 bars with both 50.0- and 0.45-μm particles, the former being responsible for the main 5-μm cloud opacity. The NH3 relative humidity above the cloud tops is found to decrease slightly as the 5-μm brightness increases, with a mean value of approximately 14%. We also find that the mean volume mixing ratio of ammonia above the middle (NH4SH) cloud deck is (1.7±0.1) × 10−4 and shows a similar, though less discernible decrease with increasing 5-μm brightness. The deep volume mixing ratios of deuterated methane and phosphine are found to be constant and we estimate their mean values to be (4.9±0.2) × 10−7 and (7.7±0.2) × 10−7, respectively. The fractional scale height of phosphine above the 1 bar level is found to be 27.1±1.4% and shows a slight decrease with increasing 5-μm brightness. The relative humidity of water vapor is found to be approximately 7%, but while this and all the previous observations are consistent with the assumption that “hot spots” are regions of downwelling, desiccated air, we find that the water vapor relative humidity increases as the 5-μm brightness increases.