Titan's Atmosphere and Climate

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. ???, XXXX, DOI:10.1002/, Titan's Atmosphere and Climate S. M. Hörst1 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA. arXiv:1702.08611v1 [astro-ph.EP] 28 Feb 2017 D R A F T March 1, 2017, 1:19am D R A F T X - 2 HORST:¨ TITAN'S ATMOSPHERE Key Points. ◦ Titan has the most complex atmospheric chemistry in the solar system. ◦ Titan's atmosphere and surface share a unique connection. ◦ Titan provides the opportunity to test our understanding of many planetary processes Abstract. Titan is the only moon with a substantial atmosphere, the only other thick N2 atmosphere besides Earth's, the site of extraordinarily complex atmospheric chemistry that far surpasses any other solar system atmosphere, and the only other solar system body with stable liquid currently on its surface. The connection between Titan's surface and atmosphere is also unique in our Solar system; atmospheric chemistry produces materials that are deposited on the surface and subsequently altered by surface-atmosphere interactions such as aeolian and fluvial processes resulting in the formation of extensive dune fields and expansive lakes and seas. Titan's atmosphere is favorable for organic haze formation, which combined with the presence of some oxygen bearing molecules indicates that Titan's atmosphere may produce molecules of prebiotic interest. The combination of organics and liquid, in the form of water in a sub- surface ocean and methane/ethane in the surface lakes and seas, means that Titan may be the ideal place in the solar system to test ideas about habitability, prebiotic chemistry, and the ubiquity and diversity of life in the Uni- verse. The Cassini-Huygens mission to the Saturn system has provided a wealth of new information allowing for study of Titan as a complex system. Here I review our current understanding of Titan's atmosphere and climate forged D R A F T March 1, 2017, 1:19am D R A F T HORST:¨ TITAN'S ATMOSPHERE X - 3 from the powerful combination of Earth-based observations, remote sensing and in situ spacecraft measurements, laboratory experiments, and models. I conclude with some of our remaining unanswered questions as the incredible era of exploration with Cassini-Huygens comes to an end. D R A F T March 1, 2017, 1:19am D R A F T X - 4 HORST:¨ TITAN'S ATMOSPHERE 1. Introduction Titan is unique in our solar system: it is the only moon with a substantial atmosphere, the only other thick N2 atmosphere besides that of Earth, the site of extraordinarily complex atmospheric chemistry that far surpasses any other solar system atmosphere, and the only other solar system body that currently possesses stable liquid on its surface. Titan's mildly reducing atmosphere is favorable for organic haze formation and the presence of some oxygen bearing molecules suggests that molecules of prebiotic interest may form in its atmosphere [Hörstet al., 2012]. The combination of liquid and organics means that Titan may be the ideal place in the solar system to test ideas about habitability, prebiotic chemistry, and the ubiquity and diversity of life in the Universe [Lunine, 2009]. The possible existence of an atmosphere around Titan was first suggested in 1908 in a paper titled \Observations des satellites principaux de Jupiter et de Titan" by José Comas Solà.He believed he observed limb darkening, which is indicative of an atmosphere [Comas Solà, 1908]. Gerard Kuiper's discovery of methane (CH4) around Titan provided conclusive evidence that Titan possesses an atmosphere [Kuiper, 1944]. During the 1970s, analyses of ground-based infrared spectra began to provide con- straints on the temperature structure [Morrison et al., 1972; Danielson et al., 1973] and composition [Trafton, 1972a, b; Gillett et al., 1973; Gillett, 1975] of Titan's atmosphere. The detection of a greenhouse effect [Morrison et al., 1972] coupled with detailed investigations of the spectral features of CH4 [Trafton, 1972b; Lutz et al., 1976] led to the conclusion that CH4 might be only a minor constituent in Titan's atmosphere and that other absorbers, such as C2H6 or some type of dust particle, might be present [Daniel- D R A F T March 1, 2017, 1:19am D R A F T HORST:¨ TITAN'S ATMOSPHERE X - 5 son et al., 1973]. The presence of N2 in Titan's atmosphere was first suggested by Lewis [1971] based on the idea that Titan accreted NH3 during formation, which later photolyzed to form N2. Photochemical models predicted significant abundances of C2H2 and C2H6 [Strobel, 1974; Allen et al., 1980], although only a few models included N2 and nitrogen chemistry [Atreya et al., 1978]. The encounter of Pioneer 11 with Titan in 1979 confirmed the presence of an aerosol absorber in the atmosphere [Tomasko, 1980] and provided a lower limit of 1.37 g/cm2 on Titan's density [Smith, 1980]. The encounters of Voyager 1 and 2 with Titan in the early 1980s confirmed that Titan possesses a substantial N2 atmosphere, with a surface pressure 1.5 times that of Earth and a surface temperature of 94 K [Broadfoot et al., 1981; Lindal et al., 1983; Hunten et al., 1984]. Infrared spectra taken by the Voyager spacecraft revealed the presence of a variety of organic molecules (C2H2,C2H4,C2H6,C3H8, CH3C2H, C4H2, etc.) [Hanel et al., 1981; Kunde et al., 1981] and provided the first glimpse of the complexity of the chemical and physical processes occurring in Titan's atmosphere. Despite providing an enormous amount of new information about Titan's atmosphere, Titan's thick photochemical haze and abundant methane prevented the instruments car- ried by Pioneer 11 and the Voyager spacecraft from seeing Titan's surface. Images beamed back to Earth as the spacecraft sped through the Saturnian system showed only a fea- tureless orange ball, providing no hint of the incredible landscape below. It remained shrouded for another 23 years until the arrival of the Cassini-Huygens mission in 2004. The arrival of the Cassini-Huygens mission to the Saturn system ushered in a new era in the study of Titan. Carrying a variety of instruments capable of remote sensing and in situ investigations of Titan's atmosphere and surface, the Cassini Orbiter and the Huygens D R A F T March 1, 2017, 1:19am D R A F T X - 6 HORST:¨ TITAN'S ATMOSPHERE Probe have provided a wealth of new information about Titan and have finally allowed humankind to see the surface. Perhaps more so than anywhere else in the solar system, Titan's atmosphere and surface are intimately linked. As discussed in detail below, the organic material transported to form Titan's extensive sand seas was formed initially by chemical and physical processes in the atmosphere, the liquids that carve out dendritic channels like those seen at the Huygens landing site cycle between the atmosphere and the surface (in the case of methane) or are produced by chemistry in the atmosphere (in the case of ethane, propane, hydrogen cyanide, etc). Long term climate cycles result in the observed asymmetry of lakes and seas on the surface [Aharonson et al., 2009]. Cryovolcanism may resupply methane and other gases to the atmosphere. Here I review our current understanding of Titan's atmosphere and climate forged from the powerful combination of Earth-based observations, remote sensing and in situ spacecraft measurements, laboratory experiments, and models. I conclude with a discussion of some of our remaining unanswered questions as the incredible era of exploration with Cassini-Huygens comes to an end. 2. Titan's atmospheric structure and composition The two main constituents of Titan's atmosphere are molecular nitrogen (N2) and methane (CH4). Titan receives about 1% of the solar flux that reaches Earth. Of the flux incident at the top of Titan's atmosphere, only 10% reaches the surface (compared to 57% for Earth) (see e.g., Griffith et al. [2012a]; Read et al. [2015]). Titan is therefore much colder than Earth, with an effective temperature of ∼82 K. The combination of the greenhouse effect provided by CH4 and collision induced absorption (N2-N2,N2-CH4,N2- H2) and the anti-greenhouse from the stratospheric haze layer [McKay et al., 1991] results D R A F T March 1, 2017, 1:19am D R A F T HORST:¨ TITAN'S ATMOSPHERE X - 7 in a surface temperature of approximately 94 K [Lindal et al., 1983; Fulchignoni et al., 2005; Schinder et al., 2011]. The pressure at Titan's surface is ∼1.5 bar [Lindal et al., 1983; Fulchignoni et al., 2005] resulting in surface conditions that are near the triple point of methane, much like water on Earth, which allows for liquid methane on the surface and gaseous methane in the atmosphere. The fluvial and aeolian features on the surface, discussed in Section 7 indicate that Titan has an active \hydrological" cycle that is, in many ways, both very similar to and very different from that of Earth. In addition to the features observed on the surface, large storms occasionally erupt in Titan's atmosphere and are presumably responsible for many of the fluvial features. 2.1. Temperature structure The vertical temperature structure in Titan's atmosphere (see Figure 1) is most anal- ogous to that of Earth, with well defined tropo-, strato-, meso-, and thermospheres (see e.g., Smith et al. [1982b]; Lindal et al. [1983]; Hubbard et al. [1990]; Sicardy et al. [1990]; Fulchignoni et al. [2005]). Although Titan's atmosphere is colder than Earth's, Titan's atmosphere is more extended, with scale heights of 15 to 50 km compared to 5 to 8 km on Earth due to Titan's lower gravity [Flasar et al., 2014].

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