Jupiter's atmospheric temperatures: From Voyager IRIS to Cassini CIRS

Retrievals performed on Cassini Composite Infrared Spectrometer data obtained during the distant Jupiter flyby in 2000/2001 have been used to generate global temperature maps of the planet in the troposphere and stratosphere, but to higher latitudes than were shown previously by Flasar et al. [Flasar, F.M., 39 colleagues, 2004a. Nature 427, 132-135; Flasar, F.M., 44 colleagues, 2004b. Space Sci. Rev. 115, 169-297]. Similar retrievals were performed on Voyager 1 IRIS data to provide the first detailed IRIS map of the stratosphere, and high latitudes in the troposphere. Thermal winds were calculated for each data set and show strong vertical shears in the zonal winds at low latitudes, and meridional temperature gradients indicate the presence of circumpolar jets, as well. The temperatures retrieved from the two spacecraft were also compared with yearly ground-based data obtained over the intervening two decades. Tropospheric temperatures reveal gradual changes at low latitudes, with little obvious seasonal or short-term variation [Orton et al., 1994. Science 265, 625-631]. Stratospheric temperatures show much more complicated behavior over short timescales, consistent with quasi-quadrennial oscillations at low latitudes, as suggested in prior analyses of shorter intervals of ground-based data [Orton et al., 1991. Science 252, 537-542; Friedson, A.J., 1999. Icarus 137, 34-55]. A scaling analysis indicates that meridional motions, mechanically forced by wave or eddy convergence, play an important role in modulating the temperatures and winds in the upper troposphere and stratosphere on seasonal and shorter timescales. At latitudes away from the equator, the mechanical forcing can be derived simply from a temporal record of temperature and its vertical derivative. Ground-based observations with improved vertical resolution and/or long-term monitoring from spacecraft are required for this purpose, though the Voyager and Cassini data given indications that the magnitude of the forcing is very small.     

Images of Jupiter from the pioneer 10 and pioneer 11 infrared radiometers: A comparison with visible and 5-μm images

All of the data acquired at Jupiter by the Infrared Radiometers on board Pioneers 10 and 11 are presented in the form of images with geometric control. The images are compared with 5-μm and visible images taken in the same time frame. The association of dark (blue or brown) and light (white or red) areas with warm and cool areas (at 5, 20, and 45 μm) respectively, extends to nearly all features observed on the planet. Where the normal association of light and dark visible markings with the zonal velocity breaks down (e.g., at the latitude of the South Equatorial Belt during the Pioneer encounters), the infrared emission seems to follow the visible cloud structure rather than the zonal velocity structure. Exceptions to the general rule involve 20-μm radiation, which reflects conditions in the altitude range 0.1–0.3 bar. For example, a comparison between Pioneer 10 and 11 images suggests that the South Equatorial Belt became brighter at 20 μm, but remained constant at other wavelengths between the two encounters.     

The atmospheric influence, size and possible asteroidal nature of the July 2009 Jupiter impactor

Near-infrared and mid-infrared observations of the site of the 2009 July 19 impact of an unknown object with Jupiter were obtained within days of the event. The observations were used to assess the properties of a particulate debris field, elevated temperatures, and the extent of ammonia gas redistributed from the troposphere into Jupiter’s stratosphere. The impact strongly influenced the atmosphere in a central region, as well as having weaker effects in a separate field to its west, similar to the Comet Shoemaker-Levy 9 (SL9) impact sites in 1994. Temperatures were elevated by as much as 6 K at pressures of about 50-70 mbar in Jupiter’s lower stratosphere near the center of the impact site, but no changes above the noise level (1 K) were observed in the upper stratosphere at atmospheric pressures less than ∼1 mbar. The impact transported at least ∼2 × 10¹⁵ g of gas from the troposphere to the stratosphere, an amount less than derived for the SL9 C fragment impact. From thermal heating and mass-transport considerations, the diameter of the impactor was roughly in the range of 200-500 m, assuming a mean density of 2.5 g/cm³. Models with temperature perturbations and ammonia redistribution alone are unable to fit the observed thermal emission; non-gray emission from particulate emission is needed. Mid-infrared spectroscopy of material delivered by the impacting body implies that, in addition to a silicate component, it contains a strong signature that is consistent with silica, distinguishing it from SL9, which contained no evidence for silica. Because no comet has a significant abundance of silica, this result is more consistent with a “rocky” or “asteroidal” origin for the impactor than an “icy” or “cometary” one. This is surprising because the only objects generally considered likely to collide with Jupiter and its satellites are Jupiter-Family Comets, whose populations appear to be orders of magnitude larger than the Jupiter-encountering asteroids. Nonetheless, our conclusion that there is good evidence for at least a major asteroidal component of the impactor composition is also consistent both with constraints on the geometry of the impactor and with results of contemporaneous Hubble Space Telescope observations. If the impact was not simply a statistical fluke, then our conclusion that the impactor contained more rocky material than was the case for the desiccated Comet SL9 implies a larger population of Jupiter-crossing asteroidal bodies than previously estimated, an asteroidal component within the Jupiter-Family Comet population, or compositional differentiation within these bodies.