Semi-annual oscillations in Saturn’s low-latitude stratospheric temperatures

Abstract

Observations of oscillations of temperature and wind in planetary atmospheres provide a means of generalizing models for atmospheric dynamics in a diverse set of planets in the Solar System and elsewhere. An equatorial oscillation similar to one in the Earth's atmosphere 1,2 has been discovered in Jupiter 3-6 . Here we report the existence of similar oscillations in Saturn's atmosphere, from an analysis of over two decades of spatially resolved observations of its 7.8-mm methane and 12.2-mm ethane stratospheric emissions, where we compare zonal-mean stratospheric brightness temperatures at planetographic latitudes of 3.66 and 15.56 in both the northern and the southern hemispheres. These results support the interpretation of vertical and meridional variability of temperatures in Saturn's stratosphere 7 as a manifestation of a wave phenomenon similar to that on the Earth and in Jupiter. The period of this oscillation is 14.8 6 1.2 terrestrial years, roughly half of Saturn's year, suggesting the influence of seasonal forcing, as is the case with the Earth's semi-annual oscillation 1 .These conclusions are based on a sequence of filtered mid-infrared maps or images of Saturn, through narrow-to medium-band spectral filters that are sensitive to upwelling radiance emerging from Saturn's stratosphere. As in our study of Jupiter 6 , we preferred to use the emission of stratospheric methane at wavelengths of around 7.8 mm to detect the stratospheric temperature field near the 20-mbar pressure level in the atmosphere, because methane is expected to be well mixed in Saturn's stratosphere. Thus, all variations in the thermal radiance must be attributed to variations in temperature, rather than in the methane abundance. However, because 7.8-mm methane emission is much fainter for Saturn than it is for Jupiter, most of our earliest observations with lengthy raster scans consist only of observations of much brighter stratospheric emission from ethane at wavelengths of around 12.2 mm (see the Supplementary Information), because only these images had sufficient signal-to-noise ratios to be useful. Figure 1 shows examples of 7.8-mm methane emission observed from the NASA Infrared Telescope Facility (IRTF) in two different phases of the oscillation. Details of the observations are given in the Supplementary Information.The angular resolution of scans and images at the IRTF was limited by diffraction to no better than 0.7 arcsec (at latitude 4u) for 7.8-mm methane emission and 1.1 arcsec (at latitude 7u) for 12.2-mm ethane emission, with some additional blurring arising from seeing (that is, distortion due to terrestrial atmospheric turbulence). (Here and below, latitude values without an explicit attribution refer to either the northern or the southern hemisphere.) It is possible to resolve differences between emission at planetographic latitudes of 3.6u and 15.5u (planetocentric latitudes of 3.0u and 13.0u) in all the images used in this study, which is a requirement for this investigation. We ignored regions of the planet that ...

Saturn Atmospheric Structure and Dynamics

“…Long-term ground-based infrared observations of 7.8-µm and 12.2-µm stratospheric emission revealed a semiannual oscillation (14.8 years) in Saturn's low latitude stratospheric temperatures, suggesting a link to seasonal forcing (Orton et al 2008;section 7.3.7.2). The response of Saturn's temperature to seasonally varying radiative forcing was calculated for the upper troposphere by Bézard et al (1984) and for the stratosphere by Bézard and Gautier (1985).…”

“…They found a general equator-to-south-pole temperature gradient, with the south pole ~10K warmer than the equator and the gradient slightly stronger at lower pressures. Orton and Yanamandra-Fisher (2005) made high spatial resolution thermal infrared images of Saturn in the stratospheric methane and upper tropospheric hydrogen bands during early southern summer, allowing them to determine temperatures at 3 mbar and 100 mbar. The 3 mbar stratospheric temperatures show a ~15K temperature gradient from the equator to the south pole, with a sharp temperature increase at 70°S.…”

Music of the stratospheres

“…But enzymes have evolved to accelerate only biological reactions, under the narrow set of conditions that are compatible with life. Two papers from the same group, one in this issue (Röthlisberger et al 1 , page 190) and another in Science (Jiang et al 2 ), show how these limitations can be overcome. They describe a method for designing enzymes that catalyse unnatural reactions, and demonstrate its use for two different chemical transformations.…”

“…Studies by Orton et al 1 and Fouchet et al 2 (pages 196 and 200 of this issue) reveal that Saturn's equatorial stratosphere exhibits a 15-year oscillation in wind direction and temperature that is remarkably similar in structure to 2-year and 4-year oscillations in the stratospheres of Earth and Jupiter, respectively. Considering that a stratosphere is like a complicated sound box attached to the elaborate musical instrument that is the troposphere, this discovery is like listening to notes played on three random orchestral instruments and finding that they all are from the string section -and perhaps all from the same instrument.…”

“…Orton et al 1 There are two terrestrial candidates as analogues to Saturn's 15-year signal, the quasibiennial oscillation 3 (QBO) and the semiannual oscillation 4 . The QBO has a mean period of 28 months, during which time the direction of the jet stream at a given altitude in Earth's equatorial stratosphere switches from eastward to westward and back again, alternating with time, just as is now seen on Saturn.…”