A decade ago, Titan was known as a hidden, hazy world. Findings made by NASA's Cassini mission and the European Space Agency's Huygens probe have unveiled Titan as an "alien Earth," providing scientists with a unique world to explore.
A sampling of the top discoveries at Titan includes:
Lakes and Seas Titan has been revealed to be a world with lakes and seas, made up of liquid methane and ethane. It is believed that these bodies of hydrocarbons are replenished by methane and ethane rainfall from clouds in the moon's atmosphere. Titan is the only other place in the solar system known to have an Earth-like cycle of liquids flowing across its surface.
Active Meteorology and Surface Processes Liquid methane drizzles onto Titan's surface. Just like clouds on Earth, clouds on Titan form through a cycle of evaporation and condensation, with methane vapor rising from the surface, forming clouds and falling back down as precipitation. Huygens data suggest the presence of layered methane clouds in Titan's troposphere, at altitudes between about 5 and 20 miles (8 and 30 kilometers). Titan's "hydrological" cycle causes visible changes on the moon's surface.
Organic Sand Seas Seas of sand dunes, like those in Earth's Arabian desert, are observed in the dark equatorial regions of Titan. Scientists believe the sand is not made of silicates as on Earth, but of solid water ice coated with hydrocarbons that fall from the atmosphere. Images show Titan's dunes are gigantic, reaching, on average, 0.6 to 1.2 miles (1 to 2 kilometers) wide, hundreds of miles (kilometers) long and around 300 feet (100 meters) high.
The location of the Huygens probe's resting place, a soft, sandy riverbed, was only confirmed after some time by the detection of two dark, longitudinal sand dunes, about 20 miles (30 kilometers) north of the landing site. The elusive landforms were visible in images from both Cassini radar and the probe.
First Determination of Depth for an Extraterrestrial Sea Ligeia Mare, Titan's second-largest sea, was revealed to be about 560 feet (170 meters) deep. This represents the first time scientists have been able to determine the depth of a body of open liquid on the surface of another world. This was possible, in part, because the liquid turned out to be mostly clear methane, allowing the radar signal to pass through it easily.
River Channels and Ice Cobbles Images taken during the Huygens probe's descent revealed river channels and flood plains. The probe's cameras unveiled a plateau with a large number of dark channels cut into it, forming drainage networks that bore many similarities to those on Earth. The narrow channels converged into broad rivers, which drained into a broad, dark, lowland region. Earth-like river rocks, composed of water ice, were also observed at the Huygens probe landing site. Radar evidence from Cassini suggests that flash flooding has sculpted streambeds on Titan with these rounded cobbles of water ice, which likely originated in water-ice bedrock in higher terrain.
The Collapse of the Detached Haze The massive atmosphere of Titan is shrouded in thick layers of photochemical smog. One of the "detached" layers has fallen in altitude from over 310 miles to only 240 miles (about 500 kilometers to only 380 kilometers) between 2006 and 2010. The changing altitudes indicate that Titan's smog layers are coupled to a seasonal climate cycle.
Rich Chemistry in the Atmosphere, including Propylene The Huygens probe made the first direct measurements of Titan's lower atmosphere. Data returned by the probe included altitude profiles of the gaseous constituents, isotopic ratios and trace gases (including organic compounds). Huygens also directly sampled aerosols in the atmosphere and confirmed that carbon and nitrogen are their major constituents. Cassini detected propylene, a chemical used to make household plastic, in Titan's atmosphere. This is the first definitive detection of the plastic ingredients on any moon or planet, other than Earth. Other chemicals observed indicate a rich and complex chemistry originating from methane and nitrogen and evolving into complex molecules, eventually forming the smog that surrounds the icy moon.
Argon-40 Isotope in the Atmosphere Huygens' detection of Argon-40, an isotope or type of the element argon, in Titan's atmosphere indicates that the interior of Titan is still active. This is unusual in a moon and one of the first clues of subsurface liquid water on Titan. The presence of the Huygens probe on Titan's surface was essential in detecting this substance, as it is mostly concentrated toward the bottom of the atmosphere (due to its relatively heavier weight compared to the lighter molecules comprising the atmosphere).
Liquid Water Subsurface Ocean Cassini's numerous gravity measurements of Titan revealed that this moon is hiding an internal, liquid water/ammonia ocean underneath its surface. Huygens also detected radio signals during its descent that strongly suggested the presence of an ocean 35 to 50 miles (55 to 80 kilometers) below the moon's surface. The discovery of a global ocean of liquid water adds Titan to the handful of worlds in our solar system that could potentially contain habitable environments.
The Cassini Solstice Mission is a joint United States and European endeavor. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.