Titan's atmosphere and climate

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 subsurface 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 universe. 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 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.

An Axisymmetric Limit for the Width of the Hadley Cell on Planets With Large Obliquity and Long Seasonality

“…On Titan, observational studies show that the maximum surface temperature stays close to the equator during Titan's year (Jennings et al, 2016); yet cloud observations show a significant seasonal variation (e.g., Turtle et al, 2018). Models of Titan's climate vary depending on their physical aspects (Hörst, 2017), with some models associating polar clouds with the Hadley cell ascending branch (e.g., Schneider et al, 2012) while others locate the ascending branch at midlatitudes (e.g., Lora et al, 2015). The warmest latitude also varies between models (e.g., the difference between dry and moist cases in Newman et al, 2016).…”

Atmospheric Circulation, Chemistry, and Infrared Spectra of Titan-like Exoplanets around Different Stellar Types

“…Our results suggest that overall haze production rates might be similar for a range of stellar spectra, implying that very Titan-like exoplanets could exist around a wide range of host stars. However, it should be noted that we rely on a parameterization of haze production using haze precursors to arrive at our estimated production rates, and that the actual chemical pathways leading to haze formation are as yet poorly understood (see Hörst 2017, for a review). For example, both models (e.g., Lavvas et al 2013) and observations (Coates et al 2007;Crary et al 2009;Liang et al 2007;Wahlund et al 2009;Waite et al 2007) of Titan's atmosphere indicate that ion chemistry plays an important role in haze formation; our photochemical model lacks ion chemistry, and therefore our haze formation picture is incomplete.…”

Deflating Super-puffs: Impact of Photochemical Hazes on the Observed Mass–Radius Relationship of Low-mass Planets

“…In comparison, aerosols that form at low pressures through the actions of photochemistry-hazes-have an advantage since there is no need for lofting. For example, optical transits of Saturn's moon Titan probe altitudes upwards of 300 km (10-100 µbar) above the surface due to opacity from photochemical hazes (Robinson et al 2014), while haze formation occurs at pressures as low as 0.1 nbar at an altitude of 1000 km, which is comparable to the solid body radius of Titan of 2575 km (Hörst 2017). Morley et al (2013Morley et al ( , 2015 showed that a flat spectrum for GJ 1214b can be generated using photochemical hazes if ≥10% of the products of methane and nitrogen photolysis are converted into hazes with particle radius ∼0.1 µm.…”