On December 25, 2004, the Huygens probe was released from the Cassini spacecraft and flew in a ballistic trajectory to Titan. After a 20 day flight, the probe on January 14, 2005 entered Titan's atmosphere and successfully landed, becoming the first spacecraft to land on Titan.
The probe carried with it six instruments that gathered information about Titan's atmosphere and surface. The six instruments carried by the probe measured the chemical composition and temperature of the atmosphere and the size and composition of the atmospheric aerosols, they imaged the clouds and determined the visibility of the atmosphere, and they measured the reflectance of Titan's surface. After the landing, the probe's instruments determined the composition and properties of Titan's surface. All data from the probe was relayed to Earth by the Cassini spacecraft.
An image of Titan's surface taken by the Huygens probe as it descends to the surface. River channels are clearly visible, as is a coast line of a dry basin. The fluid responsible for these features is liquid methane, which precipitates from methane clouds in Titan's atmosphere. (Courtesy ESA/NASA/JPL/University of Arizona)
Wind velocities were to have been determined by measuring the Doppler shift of radio signals at the Cassini spacecraft, but an error within the Cassini spacecraft prevented this measurement. Instead, radio signals from Huygens were measured at Earth with a global network of radio telescopes.
After atmospheric friction slowed its descent, Huygens jettison its heat shield, deployed its parachute, and drifted for two and a half hours through the atmosphere, analyzing the atmosphere's composition and structure. At 120 km above the surface, the probe encountered its strongest winds, which flowed in the direction of the moon's west to east rotation; the probe encountered large variations in the wind velocity until it reached an altitude of 60 km, after which the wind velocity decreased slowly. Dropping through the methane haze at an altitude of 30 km, Huygens began photographing Titan's surface, revealing a network of river channels, cut by liquid methane, winding through the highlands, and dumping onto a flat, dry basin. One series of photographs that cover 360° along the horizon shows a light-colored highland rising out of a dark plain. Across the plain run streaks of white, which are thought to be a methane and ethane fog. The winds towards the surface were weak.
A 360° panorama of Titan's surface as seen by the Huygens probe at 8 km altitude as it drifts to the right at 6 to 7 meters per second. The probe's landing spot is the dark region to the right in the image. The white wisps on the plain are thought to be an ethane and methane fog (Courtesy ESA/NASA/JPL/University of Arizona).
Huygens landed in a dry basin of loose soil and round, 19 cm pebbles, impacting the surface at about 25km per hour with what the researchers described as a splat. The surface resembles the bed of a dry river, with indications that fluid methane at times has flowed over the pebbles. The soil is soft like mud. The surface and atmosphere seen by Huygens has a yellowish orange color. Huygens continued to return data from the surface until it lost contact with the Cassini orbiter when Cassini passed over Titan's horizon.
A colored image of Titan's surface after landing. The rocks visible in this image are around 10 cm in size. (Courtesy ESA/NASA/JPL/University of Arizona).
While superficially resembling Earth in appearance, the composition of Titan's surface is quite different: Titan has a surface of water-ice, frozen by Titan's below -170° C temperatures. The action of liquid methane on the water-ice soil and pebbles is similar to the action of water on earth's silicate rocks. The process of liquid methane flowing over the plain and then evaporating has broken down the surface to fine granules that is similar to sand. Dark organic compounds appear to have precipitated from the atmosphere and to have been deposited by flowing methane at the bottoms of the river channels and collection basins.
These results confirm earlier speculations that Titan has a weather cycle based on liquid and gaseous methane. Methane clouds form in Titan's atmosphere, and a methane rain falls to the surface. After flowing through the river network to river basins, the methane evaporates back into the atmosphere. At the time of the landing, the river beds and basin were dry.
Observations by Huygens suggests that Titan is geologically active. It had been thought that Titan has a core of liquid water and ammonia. The detection by Huygens of the isotope argon-40 in Titan's atmosphere suggests that volcanic activity is taking place on Titan. These volcanoes would spew water and ammonia from Titan's interior.