Titan was discovered in 1655 by the Dutch astronomer Christiaan Huygens. For centuries, it was thought to be the largest moon in the Solar System. By the early 1940s Titan was found to have a thick atmosphere that made it appear larger than it actually was. Jupiter’s moon Ganymede is the largest at 3270 miles, but Titan, in second place, is still a respectable 3200 miles across. It is larger than Mercury and would qualify as a planet if it orbited the Sun on its own.

Titan was such a unique world and big mystery in the 1970s that it was a priority for encounter by Voyager 1. In fact, scientists sacrificed the flyby of Pluto so they could study Titan up close. The close flyby of Titan meant that Voyager 1 would arrive at Saturn too soon and too fast to be redirected to Pluto. If Voyager 1 failed at Titan, Voyager 2 was on standby and would sacrifice its planned Uranus and Neptune flybys to pass close to Titan. Pluto was unreachable and in the wrong direction for Voyager 2 to reach it from Neptune. Titan clearly was a world worth studying and no scientist wanted to sacrifice a chance to study Titan up close. The composition of Titan’s atmosphere was unknown. It was theorized to be mainly methane, but others thought nitrogen was the primary gas. There was speculation as to how cloudy Titan might be. Was Voyager 1 going to pass a moon of clear skies, patchy clouds, haze, or be completely socked in with clouds like Venus? A few scientists even speculated that conditions on Titan were favorable for methane lakes, or even a global ocean.

Voyager 1 flew within 4033 miles of Titan on November 12, 1980. The view disappointed everyone; Titan was a featureless, smoggy orange ball with an atmosphere 125 miles thick and a haze layer extending another 240-310 miles above that. The atmosphere turned out to be 95% nitrogen with methane making up most of the rest. The surface temperature was found to be a brutal -290ºF. The atmosphere was substantial with a pressure 1.6 times Earth’s. All of these conditions were perfect for the existence of liquid methane and ethane on the surface that could foster an evaporation cycle like Earth, which meant it could rain from storm clouds. The downpours could produce methane rivers that flow across the surface and carve canyons and valleys. The thick smog that shrouds Titan makes it impossible to see the surface and neither of the Voyagers was equipped to see through the murk. Exploring Saturn and Titan became such a high priority that the $3.3 billion Cassini orbiter spacecraft was developed, which included the Huygens Titan lander developed by the European Space Agency (ESA) that had a camera and could float if it landed in liquid. Cassini was launched on October 15, 1997 with the Huygens probe attached to its side and slipped into orbit around Saturn on June 30, 2004. Huygens was deployed from Cassini later that year and successfully landed on Titan on January 14, 2005.

Huygens made a dramatic descent through Titan’s thick atmosphere and emerged from beneath the haze about twelve miles above its surface. The probe photographed amazing scenery of what looked like meandering rivers draining into what appeared to be a dark lake. Patches of low clouds or fog was seen along the shoreline. Huygens photographed two different types of rivers. One had several branches flowing into one large channel that meandered before emptying into the lake. The other was nearly perfectly straight as if it were following a crack with other fractures joining the main channel at right angles. Huygens landed on a moist lakebed where it was humid and cold at -290ºF as expected. Huygens was still warm enough from the descent that droplets of methane were seen forming on it. The one photo scene from the surface resembled a dry riverbed with rounded rocks and flow patterns, which indicated that liquid methane flowed recently, perhaps a few years or even a few days ago. The moist lake bed was flat with low hills seen on the distant horizon all bathed in a dim, eerie orange light due to the thick smog. Huygens survived about three hours and took several photos of the same scene, but saw no changes.

Cassini continues to explore Titan with about 100 flybys so far, flying as close as 590 miles above the surface. It has imaged about 50% of Titan in visible light, infrared, and radar wavelengths. Titan’s thick, hazy atmosphere limits the use of cameras and infrared imagers but the radar mapper can penetrate the haze and see objects as small as 1150 feet across. Cassini has revealed that Titan is only somewhat like Earth. Titan’s crust is made of water ice, which is as solid as granite in the intense cold. Liquid methane does not dissolve water easily so most of it flows over the frozen water like water flowing over bedrock. Erosion is slow but the liquid methane will pick up pebbles and transport them downstream as sediment. This process carves out valleys, channels, and canyons.

Cassini has revealed hundreds of dark patches on Titan, mainly in the north polar regions, which were later confirmed to be lakes. Several are filled with liquid while others appear to be dried up. Some lakes appear to fill ancient craters and calderas of long-extinct volcanoes. Cassini revealed that part of the shoreline of Ontario Lacus, a methane lake as large as Lake Ontario at 146 miles across and the largest lake in Titan’s southern hemisphere, has receded by about six miles since it was discovered. Several smaller, new lakes appeared after white clouds dissipated after an apparent cloud burst, and then years later the lakes dried up. A big mystery is the stability of the rivers and shorelines. On Earth plants help keep these stable, but there is no life on Titan, so it is not clear what keeps them stable.

Titan has several lakes or seas in the northern polar region that are larger than Lake Superior and may exceed 500 feet deep. These seas appear dark on radar images indicating they are extremely smooth, too smooth to be a solid, but perfect if a liquid. Proof finally came on July 8, 2009 when an infrared imager aboard Cassini caught a glint off one of Titan’s giant northern seas, Kraken Mare. These seas and lakes are probably fed by heavy thunderstorms that may dump as much as 100 inches of methane rain in only a few hours. The worst downpour on Earth dumped 12 inches of rain in one hour. Titan is far from the Sun so the process of convection is slow allowing the convective clouds to build up much greater heights and hold more moisture than any on Earth. There has yet to be any evidence for lightning on Titan, but it is possible. Cassini will monitor the weather until its mission ends on September 15, 2017.

It will be necessary to send another spacecraft to either orbit Titan or land on its surface, preferably to land in a lake. There were plans to send a probe splashing down onto Kraken Mare in 2023 but was turned down in favor of a Mars geophysical mission called Insight. Now with budget cuts worse than ever, it may be decades before NASA has another chance to visit Titan. With Cassini’s demise, the rivers and lakes of Titan will slip back to obscurity, behind the thick smoggy haze, not to be seen again until we are willing and able to return.