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 ...

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