Titan (astronomy) - astronomy.

Titan (astronomy) - astronomy. I INTRODUCTION Titan (astronomy), largest moon of the planet Saturn and the only moon in the solar system with a dense atmosphere. Scientists think Titan may provide important clues about conditions on the primitive Earth, and even how life began. Titan is spherical, measuring about 5,150 km (about 3,200 mi) in diameter, larger than the planet Mercury. It is the second largest moon in the solar system, after Jupiter's moon Ganymede. Saturn's first moon to be discovered, Titan was found in 1655 by Dutch astronomer Christiaan Huygens. A space probe named for Huygens landed on the surface of Titan in 2005. The moon is named after the Titans, a family of giants in Greek mythology. II ORBIT Titan orbits Saturn at a distance of about 1.22 million km (about 759,000 mi), completing an orbit about once every 16 Earth days. Like Earth's Moon, Titan is tidally locked, meaning that Titan rotates once in the same amount of time that it completes an orbit around Saturn. This rate of rotation keeps one side of Titan facing toward the planet at all times. Titan's orbit parallels Saturn's equator and is nearly circular. Because Titan orbits Saturn in the plane of the planet's equator, it shares Saturn's 26.7° tilt to the Sun. As a result, Titan experiences seasons in the course of its year, which is about 30 Earth-years long, in the same way Saturn does. Like Saturn, each hemisphere of Titan experiences seasons that last for many Earth years. Winter began for Titan's north pole in 2002 while its south pole started summer. The north pole will experience summer again in 2017. III OBSERVATION AND EXPLORATION Titan appears as a featureless orange ball when observed with optical telescopes. The National Aeronautics and Space Administration (NASA) Voyager probes took pictures of Titan as they flew past Saturn in 1980 and 1981, but they were unable to see surface details because of the moon's hazy atmosphere. The Hubble Space Telescope and the Infrared Space Observatory (ISO) satellite of the European Space Agency (ESA) looked at Titan in the infrared range but could only make out minimal surface and atmospheric details. Most of our knowledge of Titan comes from the Cassini spacecraft and the Huygens probe that Cassini carried to Titan. The Cassini-Huygens mission was a joint effort between NASA, the ESA, and Italy's space agency. Cassini began studying Saturn and its moons when it arrived in 2004. Huygens parachuted to Titan's surface early in 2005. The Cassini probe has made many fly-bys of the moon as the probe orbits Saturn. It has been able to produce images of the moon's surface using a camera with a special infrared filter that can penetrate the haze. Radar from Cassini has also mapped the surface in even greater detail. Other instruments on Cassini have studied the moon's atmosphere. The Huygens probe descended through the moon's thick atmosphere, and discovered strong winds. Below the haze it was able to take panoramic photographs of the surface landforms. It also transmitted a photograph of the surface after it landed, showing rounded cobbles made of ice. IV ATMOSPHERE Titan's atmosphere appears as a nearly featureless orange haze. It is 60 percent denser than Earth's atmosphere and much colder. Nitrogen is the dominant component of Titan's atmosphere (about 94 percent), with the balance made up of methane (about 5 percent), with small amounts of ethane, hydrogen cyanide, carbon dioxide, water vapor, and other chemicals. Most of these components seem to be well mixed in the atmosphere, but astronomers have been able to detect discrete clouds of methane. Methane functions as a greenhouse gas on Titan, retaining a small amount of extra heat from the Sun. Without methane, most of the nitrogen atmosphere would likely freeze out over the moon's surface. Methane also plays a role similar to water in Earth's atmosphere. A kind of methane drizzle is thought to fall almost constantly. Methane forms clouds and can precipitate out as rain, flowing as liquid on the surface. Radar on the Cassini probe found large areas that appear to be liquid lakes and seas at the moon's north pole, currently in winter. These lakes and seas may contain methane and ethane that evaporate during the warmer summer. Other radar images of other regions of Titan appear to show erosion features from flowing liquid in the past. The Huygens probe also found evidence that liquid methane had soaked into the surface where it landed. The upper atmosphere is exposed to ultraviolet radiation from the Sun and high-energy particles from Saturn's magnetosphere. Nitrogen and methane at these high altitudes undergo a complex series of chemical reactions that cause large organic molecules to form. These molecules collect into particles that create the orange atmospheric haze that blankets the moon. The particles then settle like snow to the moon's surface, forming dunes of dusty, organic material. Over time, this process would remove the methane from the atmosphere unless some source replenished it. The origin of the methane in Titan's atmosphere is not yet known. Visual and infrared observations by Cassini reported in September 2006 appear to show a vast cloud of ethane surrounding the north pole region of Titan where the lakes and seas were found. Ethane rain or snow may be falling into the lakes. When one of the moon's poles is in winter shadow, ethane may form deposits of polar ice in the extreme cold. Titan's atmosphere is thought to be quite similar to Earth's early atmosphere, before life began. The formation of complex organic compounds in the moon's atmosphere may indicate how some of the building blocks of life originated on Earth, as well. Titan's atmosphere stretches much farther from its surface, however, because of the moon's low gravity. Cassini has revealed multiple layers of haze higher than 400 km (250 mi) above the moon's surface. The majority of Earth's atmosphere lies within 16 km (10 mi) of the surface. Saturn's gravity affects Titan's dense atmosphere, creating tidal forces 400 times more powerful than the tidal forces that the Moon exerts on Earth's oceans. These tidal forces cause winds that may shape the large fields of dunes that Cassini's radar detected on the surface. V SURFACE The Cassini and Huygens probes have revealed a geologically young surface on Titan, with few visible impact craters. The features seen so far appear to be formed from dirty water ice overlain mainly by organic (carbon-containing) deposits. The moon's surface temperature is a frigid -178°C (-289°F). At such temperatures water ice probably plays a role very similar to silicate rocks on Earth and makes up the moon's crust. The organic deposits form highlands and dunes, and may coat flat plains. In some cases the dunes are as high as 100 m (330 ft) and stretch for as long as 1,500 km (930 mi), running parallel to each other like those in the Sahara desert on Earth. The terrain is rugged and shows evidence of erosion from methane rain and from liquid seeping from underground. The rocklike ice chunks on the surface, photographed by Huygens, have a rounded shape that could result from their tumbling in flash floods. Scientists have also identified possible cold volcanoes that may spew a mix of ammonia and water ice. Titan is Saturn's densest moon. Planetary scientists theorize that Titan has a rocky core about 3,400 km (about 2,100 mi) in diameter, surrounded by ice. Some researchers think an underground ocean of liquid water may surround the ice. Above this possible ocean is the moon's crust, made of solid ice. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.