Titan - Saturn's Largest Moon - from NASA's Cassini Spacecraft Poster
Titan - Saturn's Largest Moon - from NASA's Cassini Spacecraft Poster
The Arrival of the Dynamic Duo
Until Cassini arrived at Saturn with a powerful suite of science instruments, the surface of Titan was hidden beneath a thick atmosphere.
NASA's Cassini spacecraft would eventually complete more than 100 targeted flybys of Titan, sending European Space Agency’s Huygens probe to land on the mysterious, alien world—the first landing on a surface in the outer solar system. As it descended for two and a half hours, Huygens took measurements of Titan’s atmospheric composition and pictures of its surface. The hardy probe not only survived the descent and landing but continued to transmit data for more than an hour on Titan's frigid surface until its batteries were drained.
Since that historic moment in 2005, scientists worldwide have pored over volumes of data about Titan, sent to Earth by Huygens and Cassini. The information gathered by the dynamic spacecraft duo has revealed many details of a surprisingly Earth-like world and raised fascinating new questions for future study.
We now know that Titan has lakes and seas composed of liquid methane and ethane near its poles, with vast, arid regions of hydrocarbon-rich dunes girdling its equator. And deep below the surface, Titan harbors a large internal ocean.
Seasons on Titan
Titan is the only other place in the solar system with an Earth-like cycle of liquids flowing across its surface as the planet cycles through its seasons. Each Titan season lasts about 7.5 Earth years. Since 2011, Cassini has caught glimpses of the transition from fall to winter at Titan’s south pole—the first time anyone has seen the onset of a Titan winter—and watched as summer came to the north. “We're monitoring the weather on Titan, watching for predicted methane rainstorms at the north pole,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory.
Deserts and Seas
Rippling sand dunes, like those in Earth's Arabian desert, can be seen in the dark equatorial regions of Titan. Scientists think 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 icy 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 variety of features on Titan’s surface has surprised and delighted scientists and the public alike. “I am intrigued by how many features on Titan’s surface are remarkably Earth-like,” Spilker said, “including hydrocarbon rivers, lakes and seas, and equatorial dunes, with liquid methane playing the role on Titan that water plays on Earth.”
An Unusual Atmosphere
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 followed up Huygens' measurements from space, detecting other chemicals, including propylene (a chemical used to make household plastic) and poisonous hydrogen cyanide, in Titan's atmosphere. The various chemicals observed indicate a rich and complex chemistry originating from methane and nitrogen and evolving into complex molecules, eventually forming the smog surrounding the icy moon. It is believed that methane and ethane rain falls from clouds in Titan's atmosphere, but the ultimate source of the methane is still unclear. “The most interesting question is why is there still lots of methane in the atmosphere of Titan. Where's it coming from?” said Jonathan Lunine, a Cassini interdisciplinary scientist from Cornell University.
Researchers found that Titan's ice shell, which overlies a very salty ocean, varies in thickness around the moon, suggesting the crust is in the process of becoming rigid.
Underground Ocean
Cassini's numerous gravity measurements of Titan revealed that this moon hides an internal, liquid water and ammonia ocean beneath its surface. Huygens also measured 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.
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