The atmosphere of Titan is the layer of gases surrounding Titan, the largest moon of Saturn. Titan is the only natural satellite in the Solar System with an atmosphere denser than Earth's and is one of two moons with an atmosphere substantial enough to drive weather, the other being Triton.
Titan's lower atmosphere is composed primarily of nitrogen (94.2%), methane (5.65%), and hydrogen (0.099%). It also contains trace amounts of various hydrocarbons and other gases, including ethane, acetylene, hydrogen cyanide, carbon dioxide, and argon. The atmosphere's distinctive orange color is caused by complex organic compounds known as tholins.
The surface pressure on Titan is about 50% higher than on Earth, at . This pressure is above the triple point of methane, allowing liquid methane to exist on the surface alongside gaseous methane in the atmosphere.
Overview
Observations by the Voyager space probes showed that Titan's atmosphere is denser than Earth's, with a surface pressure about 1.48 times that of Earth. Titan's atmosphere has an estimated mass of , nearly twice that of Earth's atmosphere. Opaque haze layers block most visible light from the Sun and other sources, obscuring the surface. Titan's combination of high atmospheric density and low gravity means that, in theory, a human could fly through the atmosphere with wings attached to their arms. Its low gravity also allows the atmosphere to extend much farther from the surface than Earth's. Even at an altitude of , the Cassini spacecraft had to account for atmospheric drag to maintain a stable trajectory.
Titan's atmosphere is opaque at many wavelengths, making it difficult to obtain a complete reflectance spectrum of the surface from outside the atmosphere. The first direct images of Titan's surface were obtained after Cassini–Huygens arrived at Saturn in 2004. During its descent, the Huygens probe could not determine the direction of the Sun. Although it was able to take images from the surface, the Huygens team compared the task to "taking pictures of an asphalt parking lot at dusk".
Observation and exploration The presence of a substantial atmosphere on Titan was first suspected by Spanish astronomer Josep Comas i Solà, who observed distinct limb darkening on Titan in 1903 from the Fabra Observatory in Barcelona. Dutch astronomer Gerard Kuiper confirmed the observation in 1944 using spectroscopy and estimated a methane partial pressure of about . Observations in the 1970s showed that Kuiper had substantially underestimated the amount of methane: it was about 10 times more abundant than he had estimated, while the surface pressure was at least twice as high. The high surface pressure indicated that methane could account for only a small fraction of Titan's atmosphere.Coustenis, pp. 13–15 In 1980, Voyager 1 made the first detailed observations of Titan's atmosphere and measured a surface pressure of about , approximately 1.48 times that of Earth's.Coustenis, p. 22
The joint NASA–ESA Cassini–Huygens mission provided extensive observations of Titan and the Saturn system after entering orbit around Saturn on July 1, 2004. Isotopic measurements indicated that the nitrogen in Titan's atmosphere originated from material associated with the Oort cloud, such as comets, rather than from the material that formed Saturn. The mission also found evidence of complex organic compounds on Titan, including polycyclic aromatic hydrocarbons, propylene, and methane.
NASA's Dragonfly mission is planned to land a rotorcraft on Titan in 2034. The mission will study Titan's habitability and prebiotic chemistry at multiple locations.Dragonfly: Exploring Titan's Prebiotic Organic Chemistry and Habitability (PDF). E. P. Turtle, J. W. Barnes, M. G. Trainer, R. D. Lorenz, S. M. MacKenzie, K. E. Hibbard, D. Adams, P. Bedini, J. W. Langelaan, K. Zacny, and the Dragonfly Team. Lunar and Planetary Science Conference 2017. It will also investigate geological processes and measure the composition of Titan's surface and atmosphere.Langelaan J. W. et al. (2017) Proc. Aerospace Conf. IEEE
Vertical structure
Titan's vertical atmospheric structure is similar to Earth's, with a troposphere, stratosphere, mesosphere, and thermosphere. However, Titan's lower surface gravity produces a more extended atmosphere, with scale heights of compared with on Earth. Data from Voyager and Huygens, together with radiative-convective models, have provided a detailed understanding of Titan's atmospheric structure.
The troposphere is the lowest atmospheric layer and contains most of Titan's weather. Methane condenses at high altitudes, causing its abundance to increase below the tropopause, which is at an altitude of about . Methane reaches a nearly constant abundance of 4.9% between about and the surface. Methane rain, haze precipitation, and varying cloud layers occur in the troposphere.
The stratosphere consists of about 98.4% nitrogen, making Titan the only body in the Solar System besides Earth with a dense, nitrogen-rich atmosphere. The remaining 1.6% consists primarily of methane (1.4%) and hydrogen (0.1–0.2%). The main tholin haze layer lies in the stratosphere at an altitude of about . The haze produces a strong temperature inversion because of its high ratio of shortwave to infrared opacity.
The mesosphere contains a detached haze layer at an altitude of about . Temperatures at this altitude are similar to those in the thermosphere, partly because of cooling associated with hydrogen cyanide (HCN) emission lines.
In the thermosphere, particle production begins through processes that produce heavy ions and particles. Cassini closest approach to Titan's atmosphere occurred in this region.
Titan's ionosphere is more complex than Earth's. Its main ionosphere lies at an altitude of about , with an additional layer of charged particles at about . This structure partially divides Titan's atmosphere into two radio-resonating regions. Cassini–Huygens detected extremely low-frequency (ELF) waves on Titan, but their source remains uncertain because no lightning activity has been detected. Major sources of Titan's ionosphere include solar irradiance, electrons and ions from Saturn's magnetosphere that travel along magnetic field lines (, , ), and galactic cosmic rays.
Temperature
Titan receives approximately 1% of the solar radiation received by Earth. Its average surface temperature is about . At this temperature, water ice has very low vapor pressure, resulting in little water vapor in Titan's atmosphere. Methane nevertheless produces a significant greenhouse effect, raising the surface temperature above Titan's equilibrium temperature.
Titan's atmospheric haze also produces an anti-greenhouse effect by reflecting incoming sunlight back into space, reducing the solar energy reaching the surface and lowering its temperature. The anti-greenhouse effect lowers Titan's surface temperature by about 9 K, while the greenhouse effect raises it by about 21 K. Together, these effects produce a surface temperature about 12 K higher than Titan's estimated effective temperature of in the absence of an atmosphere.
Composition and chemistry Titan's lower atmosphere is composed primarily of nitrogen (94.2%), methane (5.65%), and hydrogen (0.099%). It also contains trace amounts of hydrocarbons, including ethane, diacetylene, methylacetylene, acetylene, propane, and polycyclic aromatic hydrocarbons (PAHs), as well as other gases such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, acetonitrile, argon, and helium. Isotopic studies of nitrogen ratios also suggest that acetonitrile may be present in greater quantities than hydrogen cyanide and cyanoacetylene.
Photochemistry and tholins
Titan's atmospheric chemistry is driven largely by photochemistry. Ultraviolet radiation from the Sun breaks apart methane in the upper atmosphere, initiating reactions that produce hydrocarbons and a thick orange haze of complex organic compounds known as tholins. The table below summarizes the main production and loss mechanisms of the most abundant photochemically produced molecules. +Chemistry in Titan's atmosphereMoleculeProductionLossHydrogenMethane photolysisEscapeCarbon monoxideO + CH3->H2CO + HH2CO + h\nu->CO + H2/2HCO + OH->CO2 + HEthane2CH3 + M->C2H6 + MCondensationAcetyleneC2H + CH4->C2H2 + CH3C2H2 + h\nu->C2H + HCondensationPropaneCH3 + C2H5 + M->C3H8 + MCondensationEthyleneCH + CH4->C2H4 + HCH2 + CH3->C2H4 + HC2H4 + h\nu->C2H2 + H2/2HHydrogen cyanideN + CH3->H2CN + HH2CN + H->HCN + H2CondensationCarbon dioxideCO + OH->CO2 + HCondensationMethylacetyleneCH + C2H4->CH3CCH + HCH3CCH + h\nu->C3H3 + HH + CH3CCH->C3H5DiacetyleneC2H + C2H2->C4H2 + HC4H2 + h\nu->C4H + H
Magnetic field Titan's intrinsic magnetic field is negligible and may be nonexistent. However, studies in 2008 showed that Titan can retain remnants of Saturn's magnetic field during brief periods when it passes outside Saturn's magnetosphere and becomes directly exposed to the solar wind. This interaction can ionize molecules in Titan's upper atmosphere and carry some of them away. One such event occurred when a coronal mass ejection struck Saturn's magnetosphere, exposing Titan to shocked solar wind in the magnetosheath. This increased particle precipitation and produced unusually high electron densities in Titan's ionosphere.
Titan orbits Saturn at a distance of about 20.3 Saturn radii, placing it within Saturn's magnetosphere much of the time. However, Saturn's rotational period of 10.7 hours is much shorter than Titan's orbital period of 15.95 days. This difference produces a relative speed of about between Saturn's magnetized plasma and Titan. Rather than shielding Titan's atmosphere from the solar wind, this interaction may enhance chemical reactions that contribute to atmospheric loss.
Chemistry of the ionosphere In November 2007, scientists found evidence of negative ions in Titan's ionosphere with masses up to about 13,800 times that of hydrogen. These ions are thought to descend into the lower atmosphere, where they contribute to the formation of the orange haze that obscures Titan's surface.
Smaller negative ions have been identified as linear carbon-chain anions, while larger ions show evidence of more complex structures, possibly derived from benzene. These ions may play an important role in the formation of complex organic molecules, including tholins, polycyclic aromatic hydrocarbons, cyanopolyynes, and their derivatives. Similar negative ions have been shown to promote the formation of larger organic molecules in molecular clouds outside the Solar System. This similarity suggests that Titan's negative ions may provide insight into organic chemistry in other astronomical environments.
Circulation and dynamics
Simulations of Titan's global atmospheric circulation, based on wind-speed measurements recorded by the Huygens probe during its descent, indicate the presence of a large Hadley cell. During the descent, Titan's southern hemisphere was experiencing summer. Warm air rose in the south and moved northward at high altitudes, while cooler air descended in the northern hemisphere and returned southward near the surface. Titan's slow rotation allows this circulation to extend from pole to pole. Models indicate that the circulation reverses approximately every 12 years, with a transition lasting about three years, over Titan's 29.5-year orbit around the Sun.
This circulation produces a global low-pressure zone comparable to Earth's Intertropical Convergence Zone (ITCZ). Unlike Earth's ITCZ, which is largely confined to tropical latitudes by the distribution of continents and oceans, Titan's low-pressure zone shifts between the poles with the seasons. Its migration is associated with the movement of methane-bearing clouds and rainfall.