Saturn’s Seasonal Atmosphere: Cassini CIRS contrasts to ground-based observations

Abstract

Thermal-infrared imaging observations spanning more than three decades from the VISIR instrument on the VLT, COMICS on Subaru and archived observations from NASA IRTF are used to characterise Saturn’s seasonal changes. Radiative transfer modelling (using NEMESIS [8]) provides the northern hemisphere temperature progression of the atmosphere over 15 years (2005-2020), both during and beyond the Cassini mission. Comparisons with a record of temperature variability from Cassini/CIRS measurements [10] establish the ability to replicate ground-based observations with CIRS temperature and composition inputs to NEMESIS. Imaging observations taken one Saturn year apart (1989-2018) show the limited extent of the interannual variability of Saturn’s northern hemisphere climate. Further comparisons from VISIR observations in 2017 and COMICS in 2020 show the seasonal progression of Saturn since the demise of Cassini. We characterize the seasonal atmospheric temperature progression of Saturn over the course of a full Saturnian year for the first time.

Introduction

With the culmination of Cassini's unprecedented 13-year exploration of the Saturn system in September 2017, and with no future missions currently scheduled to visit the ringed world, the requirement to build on Cassini's discoveries now falls upon Earth-based observatories. Mid-infrared observations have been used to characterise features such as the extreme temperatures within an enormous storm system in 2011 [1,6], the cyclic variations in temperatures and winds associated with the 'Quasi-Periodic Oscillation' (QPO) in the equatorial stratosphere [2] and the onset of a seasonal warm polar vortex over the northern summer pole [3].

Saturn's axial tilt of 27º subjects its atmosphere to seasonal shifts in insolation [4], the effects of which are most significant at the gas giant's poles. The north pole emerged from northern spring equinox in 2009 (planetocentric solar longitude L_s=0º), and northern summer solstice in May 2017 (L_s=90º), providing Earth-based observers with their best visibility of the north polar region since 1987, with its warm central cyclone and long-lived hexagonal wave [5,6].

Studying these interconnected phenomena within Saturn's atmosphere (particularly those that evolve with time in a cyclic fashion) requires regular temporal sampling throughout Saturn's long 29.5-year orbit. We present here a showcase of research from the wealth of archived observations from VLT/VISIR and Subaru/COMICS since 2005, as well as infrared imagery obtained from NASA/IRTF throughout the 1980s.

1.1 Temperature progression

Methane (CH₄) is used to determine stratospheric temperatures due to its even distribution across the planet. Figure 1 shows the changes in the stratospheric and tropospheric conditions as seen by VISIR over 3 years; these images are representative of a range of filters between 7-20 µm. We probe changes in the atmospheric 2D temperature brightness distribution across the planet disc in VISIR and COMICS observations taken from April 2005 (L_s=303.6º) to July 2020 (L_s=124.6º); thereby discerning the spatial variability as well as temporal (figure 2). VISIR observations concurrent with the Cassini/CIRS observations are used to cross-check the time-series from Cassini, which can be extended beyond the end-of-mission with the newer VISIR and COMICS observations. These profiles provide a new measure of long-term temperature variability in the context of an established model.

1.2 Interannual Variability

VISIR imaging from September 2017 have provided a unique opportunity, as they were acquired nearly one Saturn year apart from the 1989 observations of Gezari et al, (1989) [7], which were the first images of Saturn in the mid-IR to be taken with a 2D detector, rather than raster scanning. Examining the differences in brightness temperatures and composition indicate limited interannual variation for Saturn’s northern hemisphere (Figure 3). This also provides unique insight into the timescale of Saturn’s equatorial stratospheric oscillation which will be contrasted with a previously suggested biennial cycle [2]. The seasonal temperature progression measured in Section 1.1 also enables us to place this interannual variability in a wider context and provides further opportunity for insightful comparison with the comparatively shorter-term temperature variability.

Figure 1: VISIR observations from P95-102 sensing the troposphere (right) and stratosphere (left).  Polar warming is evident in the stratosphere; but is considerably smaller than that seen during southern summer and in the historical record of the 1980s.  The warm polar hexagon is seen at the north pole, the first such observation from the ground. Work to remove residual striping is ongoing and has been successfully applied to the 2017 images. 2015-16 images have been published by Fletcher et al., 2017 [2].

 

 

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(b)

Figure 2: (a) The latitudinal brightness temperature progression of COMICS and VISIR ground-based observations from 2005 (dark blue) to 2017 (dark red), (b) the latitudinal brightness temperature progression of CIRS observations (from Fletcher et al, 2009) from 2005 (dark blue) to 2017 (dark red). 

Figure 3: A comparison of a latitudinal brightness temperature profile sampled from a 12.4µm observation (of Gezari et al. 1989) in red and a latitudinal bright temperature profile from a synthetic image generated using the temperatures and composition from CIRS observations in 2017 shown in black. The contrast between these profiles shows the degree of interannual variability at 12.4µm.

Acknowledgements

This research is funded by a European Research Council consolidated grant under the European Union’s Horizon 2020 research and innovation program, grant agreement 723890. We would like to thank co-author Mael Es-Sayeh for his significant contribution to this research.

References

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[2] Fletcher et al., 2017, Nature Astronomy, 1, p765-770

[3] Fletcher et al. 2015, Icarus. 251, 131-153

[4] Fletcher et al., 2015, https://arxiv.org/abs/1510.05690

[5] Fletcher et al., 2008, Science. 319, 79-81

[6] Fouchet et al., 2016, Icarus, 277, p196-214

[7] Gezari et al., 1989, Nature, 342, 777–780

[8] Irwin et al. 2008, JQSRT 109:1136-1150

[9] Orton et al., 2008, Nature 453, p198

[10] Fletcher et al. 2009, Icarus 200, 154-175