Saturn's Polar Atmosphere

12 Saturn's Polar Atmosphere K.M. SAYANAGI1, K.H. BAINES2, U.A. DYUDINA3, L.N. FLETCHER4, A. SANCHEZ-LAVEGA´ 5, R.A. WEST2 1Department of Atmospheric and Planetary Sciences, the northern winter solstice in 2002), through the equinox Hampton University, 23 E Tyler St., Hampton, VA 23668, in 2009, and approaching the next solstice in 2017. We con- USA clude the current review by listing unanswered questions and 2Jet Propulsion Laboratory, California Institute of Technol- describing the observations of the polar regions planned for ogy, 4800 Oak Grove Drive, Pasadena, CA 91109, USA the Grand Finale phase of the Cassini mission between 2016 3Division of Geological and Planetary Sciences, California and 2017. Institute of Technology, 1200 California Blvd, Pasadena, CA 91125, USA 4Department of Physics and Astronomy, University of Le- icester, University Road, Leicester, LE1 7RH 5Departamento de F´ısicaAplicada I, E.T.S. Ingenier´ıa,Uni- 12.1 Introduction: views of Saturn's poles from versidad del Pa´ısVasco, Bilbao, Spain the Cassini orbiter Copyright Notice The Cassini mission has explored the Saturnian system in The Chapter, Saturn's Polar Atmosphere, is to be pub- ways that are possible only from an orbiting spacecraft. lished by Cambridge University Press as part of a multi- Cassini is the fourth spacecraft to visit Saturn after Pio- volume work edited by Kevin Baines, Michael Flasar, Nor- neer 11 and Voyagers 1 & 2, and the first spacecraft to orbit bert Krupp, and Thomas Stallard, entitled \Saturn in the the planet. Cassini has been in orbit around Saturn since 21st Century" (`the Volume'). 2004 and accumulated a decade of near-continuous observa- c in the Chapter, K.M. Sayanagi, K.H. Baines, U.A. Dyu- tional data of the planet as of this writing, and will capture dina, L.N. Fletcher, A. Sánchez-Lavega, and R.A. West almost half a Saturn-year by the planned end of the mission c in the Volume, Cambridge University Press in 2017. NB: The copy of the Chapter, as displayed on this website, Among the new data returned from Cassini, its observa- is a draft, pre-publication copy only. The final, published tions of Saturn's polar regions in particular have opened a version of the Chapter will be available to purchase through new era in atmospheric studies of the giant planets. The data Cambridge University Press and other standard distribution returned from the Cassini mission represent a significant ad- channels as part of the wider, edited Volume, once published. vance in two major ways. The first is the long temporal cov- This draft copy is made available for personal use only and erage spanning nearly two solstices that has captured diverse must not be sold or re-distributed. atmospheric phenomena including seasonal changes. In comparison, each of the previous flyby missions captured data Abstract for only several months, which essentially represent tempo- This chapter reviews the state of our knowledge about ral snapshots in the context of planetary climatology on a Saturn's polar atmosphere that has been revealed through planet for which the orbital period is 30 Earth years. Sec- arXiv:1609.09626v2 [astro-ph.EP] 3 Oct 2016 Earth- and space-based observation as well as theoretical ond, Cassini's dynamic orbital trajectory has allowed the and numerical modeling. In particular, the Cassini mis- spacecraft to tour the Saturnian space between the equato- sion to Saturn, which has been in orbit around the ringed rial plane to high above the poles, enabling observations of planet since 2004, has revolutionized our understanding of every latitude of Saturn from a wide range of illumination the planet. The current review updates a previous review by and observation angles. From those various observational ge- Del Genio et al.(2009), written after Cassini's primary mis- ometries, infrared observations coupled with stellar occulta- sion phase that ended in 2008, by focusing on the north polar tion and radio occultation measurements can also provide a region of Saturn and comparing it to the southern high lati- crucial vertical dimension to our understanding of the polar tudes. Two prominent features in the northern high latitudes regions, extending hundreds of kilometers above the visible are the northern hexagon and the north polar vortex; we ex- cloud deck and tropospheric hazes. These advantages have tensively review observational and theoretical investigations enabled detailed studies of both poles of Saturn, revealing to date of both features. We also review the seasonal evolu- surprising new results in the nature of polar winds, vortex tion of the polar regions using the observational data accu- features, aerosol properties and thermal structure, as we de- mulated during the Cassini mission since 2004 (shortly after tail in this review. 1 2 Sayanagi et al. 12.1.1 Context of the current review Neptune's polar environments may be most analogous to Saturn's in terms of seasonal effects because their obliqui- This review of Saturn's poles integrates the most recent ties are similar; Neptune's obliquity is 28.32◦. Indeed, sev- Cassini data, presenting multi-faceted results on cloud mor- eral observations of Neptune's south pole have resulted in phology, thermal structure, aerosol properties, and atmo- comparisons with Saturn (e.g., Hammel et al. 2007; Orton spheric dynamics. As such, this review builds on and ex- et al. 2007). In particular, Neptune's hotspot near the south tends previous work based on the Voyager flyby missions pole, revealed by large ground-based telescopes armed with (e.g., Gehrels and Matthews 1984; in particular, Tomasko adaptive optics (e.g., Luszcz-Cook et al. 2010; Orton et al. et al. 1984 and Ingersoll et al. 1984), and on the first 5 years 2012; Fletcher et al. 2014; de Pater et al. 2014), appears to of the Cassini mission (Dougherty et al. 2009; specifically the be similar to those on Saturn (e.g., Fletcher et al. 2008, and reviews by Del Genio et al. 2009 and West et al. 2009). Re- this chapter). views of Jupiter's atmosphere also provide important points As our knowledge of the poles of Jupiter, Uranus and Nep- of comparison for the current review. The book by Gehrels tune continues to grow, the rich Saturn dataset returned by (1976) contains reviews of Jupiter's polar atmosphere using Cassini, provides the framework for understanding the polar the Pioneer 11 Infrared Radiometer data (Ingersoll et al., environments of all four giant planets. 1976; Orton and Ingersoll, 1976). Another review book on Jupiter by Bagenal et al.(2004) treats the state of the knowledge after the Galileo mission (West et al., 2004; Ingersoll 12.1.2 Summary of Cassini polar datasets et al., 2004; Moses et al., 2004). In addition, Dowling(1995) In the current review, the primary sources of data are the and Vasavada and Showman(2005) provide in-depth reviews remote-sensing instruments onboard the Cassini spacecraft of processes related to jetstream formation and maintenance that cover the wavelengths between the ultraviolet and the in giant planet atmospheres, and form the basis of our review thermal infrared. Here, we provide brief descriptions of the of giant planet polar atmospheric dynamics. A textbook by instruments that have played key roles in the studies pre- Sánchez-Lavega(2011) also provides a thorough introduc- sented in this review. tory treatment of physical and chemical processes relevant There are two infrared instruments that have played key to planetary atmospheres. roles. First, the Composite Infrared Spectrometer (CIRS; The Cassini observations of Saturn's poles represent our Flasar et al. 2004) is a Fourier transform spectrometer that most in-depth polar dataset for a giant planet. The picture provides spatially resolved spectral data at thermal infrared revealed for Saturn's poles provides a key point of compar- wavelengths between 7 µm and 1 mm; for the purpose of ison for Jupiter, Uranus and Neptune, at a critical time observing Saturn's atmosphere, the data returned by the when knowledge of the polar environments of these plan- instrument allow determinations of atmospheric thermal ets is rapidly growing. structure as well as chemical composition. The other in- From Earth, Jupiter's poles are not visible because the frared instrument, the Visible and Infrared Mapping Spec- planet orbits in the ecliptic plane and its rotation axis has trometer (VIMS; Brown et al. 2004), is also an imaging spec- virtually no tilt; consequently, the poles can be viewed ad- trometer and covers the wavelength range between 0.3 and equately only by spacecraft flying over the non-equatorial 5.1 µm. In particular, images captured in its 5-µm channel region of the planet. To date, only Pioneer 11 and Ulysses reveal clouds silhouetted against the background thermal have departed the ecliptic significantly, and only the former emissions; the cloud features that appear in the VIMS 5-µm carried an imaging instrument. In 2016, NASA's Juno space- channel likely reside at the depth of several bars (Baines craft will enter orbits that pass directly over Jupiter's north et al., 2005). The 5-µm images' spatial resolutions are of- and south poles, exploring these environments in detail for ten high enough to track some cloud features to determine the first time. the wind fields (e.g., Choi et al. 2009 and Baines et al. Uranus, with its rotation axis tilted by 98◦, offers a unique 2009). High-resolution images in wavelengths between near- opportunity to observe its poles from Earth. In the images infrared and near-ultraviolet are captured using the Imaging of Uranus returned by Voyager 2, only a small number of Science Subsystem (ISS; Porco et al. 2004). ISS consists of discrete features were seen at latitudes poleward of 45◦S two framing cameras with Charge-Coupled Device (CCD) latitude (Allison et al., 1991; West et al., 1991); however, detectors; the Narrow-Angle Camera (NAC) and the Wide- modern digital image processing analysis by Karkoschka Angle Camera (WAC).

Load more