Titan (moon)
- This page is about the moon of Saturn. For other meanings, see Titan (disambiguation).
Titan (tye'-tun, Greek Τιτάνας) is the largest moon of Saturn and the second largest moon in the solar systemGanymede. It was discovered on March 25 1655 by the Dutch astronomer Christiaan HuygensSolar System to be discovered after the Galilean moons of Jupiter. Titan is the only moon in our solar system to have a dense atmosphereCassini-Huygens mission, and new information about it is continuously accumulating.
| Missing image Titan_Visible.jpg Titan in natural color | |||||||
| Discovery | |||||||
|---|---|---|---|---|---|---|---|
| Discoverer | Christiaan Huygens | ||||||
| Date | March 25, 1655 | ||||||
| Orbital characteristics | |||||||
| Semimajor axis | 1,221,931 km | ||||||
| Eccentricity | 0.028880 [4] | ||||||
| Orbital period | 15.94542 d | ||||||
| Inclination | 0.34854° (to Saturn's equator) | ||||||
| Is a satellite of | Saturn | ||||||
| Physical characteristics | |||||||
| Mean diameter | 5150 km | ||||||
| Surface area | 83×106km2 | ||||||
| Mass | 1.345×1023 kg | ||||||
| Mean density | 1.88 g/cm3 | ||||||
| Equatorial surface gravity | 1.35 m/s2, or .14 gee | ||||||
| Rotation period | (synchronous) | ||||||
| Axial tilt | zero | ||||||
| Albedo | 0.21 | ||||||
| Surface temp. |
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| Atmospheric characteristics | |||||||
| Pressure | 160 kPa | ||||||
| Nitrogen | 95 percent | ||||||
| Methane | 5 percent | ||||||
| Contents |
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Name
Huygens named his discovery simply Saturni Luna (Latin for "Saturn's moon", which can also be written Luna Saturni) (De Saturni Luna observatio nova, 1656; XV). Later, Jean-Dominique Cassini named the four moons he discovered (Tethys, Dione, Rhea and Iapetus) Lodicea Sidera ("the stars of Louis") to honour king Louis XIV. Astronomers fell into the habit of referring to them as Saturn I through Saturn V. Other epithets used were the "Huygenian satellite of Saturn" (or "Huyghenian"), or the "sixth satellite of Saturn" (Saturn VI, still in use) (in order of distance from Saturn, once Mimas and Enceladus were also discovered in 1789).
The name "Titan" and the names of all seven satellites of Saturn then known come from John Herschel (son of William Herschel, discoverer of Mimas and Enceladus) in his 1847 publication Results of Astronomical Observations made at the Cape of Good HopeTitans, sisters and brothers of Cronos (the Greek Saturn), be used.
Visibility from Earth
Titan has a magnitude between +7.9 and +8.7 and reaches an angular distance of about 20 Saturn radii from Saturn. It can be observed through small telescopes (diameter greater than 5cm) or strong binoculars. It subtends a disk of 0.8" in diameter.
Physical characteristics
Titan is larger than the planet MercuryGanymedePluto.
Titan is similar in bulk properties to Ganymede, Callisto, Triton, and (probably) Pluto. Titan is about half water ice and half rocky material. It is probably differentiated into several layers with a 3400 km rocky center surrounded by several layers composed of different crystal forms of ice. Its interior may still be hot. Though similar in composition to Rhea and the rest of Saturn's moons, it is denser due to gravitational compression.
Atmosphere
Titan_multi_spectral_overlay.jpg
Titan is the only known moon with a fully developed atmosphere that consists of more than just trace gases. The presence of a significant atmosphere was first discovered 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. Since that time, observations from Voyager space probes have shown that Titan's atmosphere is denser than Earth's, with a surface pressure more than one and a half times that of our planet and supports an opaque cloud layer that obscures Titan's surface features. It is thought that Titan may possess bodies of liquid ethane. Recent radar measurements from Earth suggest that there is no large-scale ocean of ethane on Titan, but it may still be present in smaller lakes.
The atmosphere is 95% nitrogen — the only dense nitrogen-rich atmosphere in the solar system aside from our own — with significant traces of various hydrocarbons making up much of the remainder (including methane, ethane, diacetylene, methylacetylene, cyanoacetylene, acetylene, propane, along with carbon dioxide, carbon monoxide, cyanogen, hydrogen cyanide, and helium). These 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. Titan has no magnetic field and sometimes orbits outside Saturn's magnetosphere, directly exposing it to the solar wind. This may ionize and carry away some molecules from the top of the atmosphere.
At the surface, Titan's temperature is about -179.15 °C. At this temperature water ice does not sublimate, effecting a nearly water-vaporless atmosphere. Scattered variable clouds punctuate an overall haze in Titan's atmosphere. These clouds are probably composed of methane, ethane or other simple organics. Other more complex chemicals in small quantities must produce the orange color as seen from space.
The thick atmosphere blocks most sunlight from reaching Titan's surface. 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, scientists say the process was like photographing asphalt at dusk tholin, but this has not been confirmed. The presence of argon 40 was also discovered in the atmosphere, evidence of cryovolcanism producing a "lava" of water ice and ammonia Cassini flyby photographed bright, high clouds at Titan's south pole, but they do not appear to be methane, as had been expected. This discovery has baffled scientists, and studies are currently underway to determine the composition of the clouds and decide whether our understanding of Titan's atmosphere needs to be revised Venus, with an atmosphere that rotates much faster than its surface.
Surface features
Overall topography
Titan_globe.jpg
At present, maps of Titan's surface show broad regions of bright and dark terrain, including a large, highly reflective area about the size of Australia identified in infra-red images from the Hubble Space Telescope and the Cassini spacecraft. This region has been officially named Xanadu; it is not certain what kind of terrain it represents. There are similarly-sized dark areas elsewhere on the moon, observed by Hubble, the Keck telescopes, and the Very Large Telescope, which some speculated may be methane or ethane seas, though Cassini observations seem to indicate otherwise. Cassini has taken higher-resolution pictures of all these features, and has also spotted some enigmatic linear markings, which some scientists have suggested may indicate tectonic activity, as well as regions of bright material cross-cut by dark lineaments within the dark terrain.
In order to understand Titan's surface features better, the Cassini spacecraft is currently using radar altimetry and synthetic aperture radar imaging to map portions of Titan during its close fly-bys of the moon. The first images have revealed a complex, diverse geology with both rough and smooth areas. There are features that seem volcanic in origin, which probably disgorge water mixed with ammonia. There are also streaky features that appear to be caused by windblown particles. The few objects that seem to be impact craters appeared to have been filled in, perhaps by raining hydrocarbons. The area mapped so far appears to be fairly smooth with no height variation greater than 50 metres radar images of Titan's complex surface