On March 25, 1655, Dutch astronomer Christiaan Huygens, using a telescope he built himself, observed a small bright dot suspiciously close to the planet Saturn. Huygens correctly surmised that it might be a moon of that planet, and confirmed as much by following it in its orbit over the next few days.

We now know this distant moon, Titan, to be strangely one of the most Earthlike and most interesting worlds in the solar system. "Huygens would be pleased," says Ralph Lorenz, a planetary scientist and Titan authority from the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md.

"He held the view that the universe was full of other planets and that many might have life, and for that matter, their own astronomers just like himself. But he recognized that other worlds would be different too, that while there might be rain on a moon of Saturn, it must be rain of another material because Saturn, so far from the sun, would be too cold for water to be a liquid."

In fact, we now know that Titan has clouds, rain and occasional rivers of methane - more familiar to us on Earth as natural gas. Another component of natural gas, ethane, is slightly less volatile, and is thought to be a significant component of large seas on Titan.

These seas, which have tides caused by Saturn's gravity, have been mapped out using radar and near-infrared observations on the NASA-European Space Agency (ESA)-Agenzia Spaziale Italiana (ASI) Cassini spacecraft, which has been flying past Titan typically every few weeks since it arrived in 2004.

A year on Titan is 29.5 Earth years long, so the seasons have changed slowly as Cassini has been observing - and with northern spring underway, sunlight is now falling on the seas. "This will let us learn more about the seas using optical methods," Lorenz says, "and, since the summer warmth will stir stronger winds, perhaps we'll observe waves on the surface of the seas, which have so far been as flat as a mill pond."

The second-largest of these seas, named Ligeia Mare, is the destination of the proposed Titan Mare Explorer (TiME) mission. TiME, one of three candidates to be NASA's next Discovery Program mission, would perform the first direct inspection of an ocean environment beyond Earth by landing in, and floating on, Ligeia Mare.

The TiME capsule would launch in 2016 and reach Titan in 2023, parachuting onto the large sea. For 96 days the capsule would study the composition and behavior of the sea and its interaction with Titan's weather and climate. TiME would also seek evidence of the complex organic chemistry that may be active on Titan today and that may be similar to processes that led to the development of life on the early Earth.

"In some ways, Titan is so like the Earth," says Ellen Stofan, TiME principal investigator from Proxemy Research Inc. in Gaithersburg, Md.

"On any given day it could be raining, or you could stand along the shore of a sea and even see small waves on the surface. There are all kinds of organic compounds that are falling out of the atmosphere into the sea - we'd love to learn more about the chemical reactions that take place. They will not be life as we know it, which is not viable in Titan's seas. But there will be chemistry in the seas that may give us insight into how organic systems progress toward life."

If NASA selects TiME, Stofan would lead the mission as principal investigator and APL would manage it. Lockheed Martin in Denver would build the TiME capsule, with scientific instruments provided by APL, Goddard Space Flight Center in Greenbelt, Md., and Malin Space Science Systems in San Diego.