The atmosphere of Titan is known as the only fully developed atmosphere that exists on a natural satellite in our solar system.
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The presence of a significant atmosphere was first suspected by Spanish astronomer Josep Comas Solà, who observed distinct limb darkening on Titan in 1903,[1] and confirmed by Gerard P. Kuiper in 1944 using a spectroscopic technique that yielded an estimate of an atmospheric partial pressure of methane of the order of 100 millibars (10 kPa).[2] In 1981, Voyager 1 made the first detailed observations of Titan's atmosphere, revealing that its surface pressure was in fact thicker than Earth's, at 1.5 bars.[3] The high surface pressure found on Titan meant that methane could only form a small fraction of Titan's atmosphere.[4]
Observations from the Voyager space probes have shown that the Titanian atmosphere is denser than Earth's, with a surface pressure about 1.45 times that of Earth's. Titan's atmosphere is about 1.19 times as massive as Earth's overall,[5] or about 7.3 times more massive on a per surface area basis. It supports opaque haze layers that block most visible light from the Sun and other sources and renders Titan's surface features obscure. The atmosphere is so thick and the gravity so low that humans could fly through it by flapping "wings" attached to their arms.[6] Titan's lower gravity means that its atmosphere is far more extended than Earth's; even at a distance of 975 km, the Cassini spacecraft had to make adjustments to maintain a stable orbit against atmospheric drag.[7] The atmosphere of Titan is opaque at many wavelengths and a complete reflectance spectrum of the surface is impossible to acquire from the outside.[8] It was not until the arrival of the Cassini–Huygens mission in 2004 that the first direct images of Titan's surface were obtained. The Huygens probe was unable to detect the direction of the Sun during its descent, and although it was able to take images from the surface, the Huygens team likened the process to "taking pictures of an asphalt parking lot at dusk".[9]
The atmospheric composition in the stratosphere is 98.4% nitrogen—the only dense, nitrogen-rich atmosphere in the Solar System aside from the Earth's—with the remaining 1.6% composed of mostly of methane (1.4%) and hydrogen (0.1–0.2%).[10] Because methane condenses out of Titan's atmosphere at high altitudes, its abundance increases as one descends below the tropopause at an altitude of 32 km, leveling off at a value of 4.9% between 8 km and the surface.[10][11] There are trace amounts of other hydrocarbons, such ethane, diacetylene, methylacetylene, acetylene and propane, and of other gases, such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, argon and helium.[11] The orange color as seen from space must be produced by other more complex chemicals in small quantities, possibly tholins, tar-like organic precipitates.[12] The hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of methane by the Sun's ultraviolet light, producing a thick orange smog.[13] Titan has no magnetic field, although studies in 2008 showed that Titan retains remnants of Saturn's magnetic field on the brief occasions when it passes outside Saturn's magnetosphere and is directly exposed to the solar wind.[14] This may ionize and carry away some molecules from the top of the atmosphere. In November 2007, scientists uncovered evidence of negative ions with roughly 10 000 times the mass of hydrogen in Titan's ionosphere, which are believed to fall into the lower regions to form the orange haze which obscures Titan's surface. Their structure is not currently known, but they are believed to be tholins, and may form the basis for the formation of more complex molecules, such as polycyclic aromatic hydrocarbons.[15]
Energy from the Sun should have converted all traces of methane in Titan's atmosphere into more complex hydrocarbons within 50 million years — a short time compared to the age of the Solar System. This suggests that methane must be somehow replenished by a reservoir on or within Titan itself. That Titan's atmosphere contains over a thousand times more methane than carbon monoxide would appear to rule out significant contributions from cometary impacts, since comets are composed of more carbon monoxide than methane. That Titan might have accreted an atmosphere from the early Saturnian nebula at the time of formation also seems unlikely; in such a case, it ought to have atmospheric abundances similar to the solar nebula, including hydrogen and neon.[16] Many astronomers have suggested that the ultimate origin for the methane in Titan's atmosphere is from within Titan itself, released via eruptions from cryovolcanoes.[17][18][19] A possible biological origin for the methane has not been discounted (see Life on Titan).
There is also a pattern of air circulation found flowing in the direction of Titan's rotation, from west to east. Observations by Cassini of the atmosphere made in 2004 also suggest that Titan is a "super rotator", like Venus, with an atmosphere that rotates much faster than its surface.[20]
Titan's ionosphere is also more complex than Earth's, with the main ionosphere at an altitude of 1,200 km but with an additional layer of charged particles at 63 km. This splits Titan's atmosphere to some extent into two separate radio-resonating chambers. The source of natural Extremely Low Frequency (ELF) waves on Titan, as detected by Cassini-Huygens, is unclear as there does not appear to be extensive lightning activity. Titan's internal magnetic field is negligible, and perhaps even nonexistent.[21] Its orbital distance of 20.3 Saturn radii does place it within Saturn's magnetosphere occasionally. However, the difference between Saturn's rotational period (10.7 hours) and Titan's orbital period (15.95 days) causes a relative speed of about 100 km/s between the Saturn's magnetized plasma and Titan.[21] That can actually intensify reactions causing atmospheric loss, instead of guarding the atmosphere from the solar wind.[22]
The persistence of a dense atmosphere on Titan has been enigmatic as the atmospheres of the structurally similar satellites of Jupiter, Ganymede and Callisto, are negligible. While the disparity is still poorly understood, data from recent missions have provided basic constraints on the evolution of Titan's atmosphere.
Roughly speaking, at the distance of Saturn, solar insolation and solar wind flux are sufficiently low that elements and compounds that are volatile on the terrestrial planets tend to accumulate in all three phases.[23] Titan's surface temperature is also quite low, about 94 kelvins (K).[24][25] Consequently, the mass fractions of substances that can become atmospheric constituents are much larger on Titan than on Earth. In fact, current interpretations suggest that only about 50% of Titan's mass is silicates,[26] with the rest consisting primarily of various H2O (water) ices and NH3-H2O (ammonia hydrates). NH3, which may be the original source of Titan's atmospheric N2 (dinitrogen), may constitute as much as 8% of the NH3-H2O mass.[27] As Tobie et al. illustrate in Figure 1,[27] Titan is most likely differentiated into layers, where the liquid water layer beneath ice Ih may be rich in NH3.
Tentative constraints are available, with the current loss mostly due to low gravity[28] and solar wind[29] aided by photolysis. The loss of Titan's early atmosphere can be estimated with the 14N/15N isotopic ratio, as the lighter 14N is preferentially lost from the upper atmosphere under photolysis and heating. Since Titan's original 14N/15N ratio is poorly constrained, the early atmosphere may have had more N2 by factors ranging from 1.5 to 100 with certainty only in the lower factor.[28] Since N2 is the primary component (98%) of Titan's atmosphere,[30] the isotopic ratio suggests that much of the atmosphere has been lost over geologic time. Nevertheless, atmospheric pressure on its surface remains nearly 1.5 times that of Earth as it began with a proportionally greater volatile budget than Earth or Mars.[25] It is possible that most of the atmospheric loss was within 50 million years of accretion, from a highly energetic escape of light atoms carrying away a large portion of the atmosphere (hydrodynamic blow off event).[29] Such an event could be driven by heating and photolysis effects of the early Sun's higher output of X-ray and ultraviolet (XUV) photons.
Since Callisto and Ganymede are structurally similar to Titan, it is unclear why their atmospheres are insignificant relative to Titan's. Nevertheless, the origin of Titan's N2 via geologically ancient photolysis of accreted and degassed NH3, as opposed to degassing of N2 from accretionary clathrates, may be the key to a correct inference. Had N2 been released from clathrates, 36Ar and 38Ar that are inert primordial isotopes of the solar system should also be present in the atmosphere, but neither has been detected in significant quantities.[31] The insignificant concentration of 36Ar and 38Ar also indicates that the ~40 K temperature required to trap them and N2 in clathrates did not exist in the Saturnian subnebula. Instead, the temperature may have been higher than 75 K, limiting even the accumulation of NH3 as hydrates.[32] Temperatures would have been even higher in the Jovian subnebula due to the greater gravitational potential energy release, mass, and proximity to the Sun, greatly reducing the NH3 inventory accreted by Callisto and Ganymede. The resulting N2 atmospheres may have been too thin to survive the atmospheric erosion effects that Titan has withstood.[32]
An alternative explanation is that cometary impacts release more energy on Callisto and Ganymede than they do at Titan due to the higher gravitational field of Jupiter. That could erode the atmospheres of Callisto and Ganymede, while the cometary material would actually build Titan's atmosphere. However, the 2H/1H (i.e., D/H) ratio of Titan's atmosphere is 2.3±0.5×10−4,[31] nearly 1.5 times lower than that of comets.[30] The difference suggests that cometary material is unlikely to be the major contributor to Titan's atmosphere.
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