There is a moon orbiting Saturn that is different from all others in the Solar System. It is the 2nd largest moon in the Solar System at 3270 miles in diameter. As different as it is, Titan is the closest world to being Earth-like. It was not until 1944 that Titan started to reveal how unusual it was.
Technology was advancing in the 1930ís and 1940ís to where the spectrum of light from objects could be interpreted to determine their chemical composition. When scientists pointed their instruments towards Titan in 1944 it was discovered that it had an atmosphere and it contained methane. This was surprising since all of the other moons were turning out to be airless worlds. It was not long after this that the famous space artist, Chesley Bonestell, painted Saturn in a blue sky above the snow-covered landscape of Titan. The more Titan was studied in the decades that followed, the more interesting it became. Once the Space Age began, the urge to send a spacecraft past Titan was strong. By the late 1960ís it was determined that it was possible to use Jupiterís powerful gravity to slingshot a spacecraft past Saturn and Titan in a reasonable amount of time. It was also speculated that lakes of liquid methane could exist on its surface due to a nearly constant surface temperature around -300ļF. The atmosphere was also determined to be thick enough to support liquid on the surface and possibly even clouds. In fact it was feared that Titan may be too cloudy for the surface to be seen.
Voyager 1 flew past Saturn and Titan on November 12, 1980, but in spite of a very close flyby of Titan, nothing could be seen except a globe totally enveloped in an orange haze, which turned out to be a photochemical smog from the reaction of the Sunís ultraviolet radiation with Titanís atmosphere. The atmosphere was thick and heavy and was actually 1.6 times as thick as Earthís. The lack of detecting any surface detail disappointed everyone. In the mid-1990ís, the Hubble Space Telescope, which has special instruments and filters to peer through haze, revealed that the surface of Titan had large expanses of dark and bright areas. It was thought that the dark areas were the suspected lakes of liquid methane. A $3.3 billion dollar orbiter mission to Saturn had been commisioned in the late 1980ís so the timing of the Hubble Space Telescope discovery could not have been better.
Cassini is a NASA mission that was launched on October 15, 1997 and arrived at Saturn on June 30, 2004. This orbiter mission is going into the history books as one of the most successful ever. It deployed a probe made by the European Space Agency (ESA) know as Huygens, which landed on Titan on January 14, 2005. During its descent it revealed a world that truly looked Earth-like. It passed beneath the thick haze layer at 12 miles above the surface. Far below were tributaries, deltas, and a shoreline. Small streams appeared to flow into larger rivers that flowed into deltas and then into a vast, dark plain that had patches of methane fog. The humidity increased as Huygens descended and it measured the nitrogen and methane levels along with the wind speed and direction. It looked like Huygens would splashdown in a dark methane sea, but it turned out to be a vast mudflat. It was a moist environment there as methane condensed on the warm surface of the spacecraft. The scenery was bathed in a dim orange light due to the thick atmosphere and smog (it was dark because of the smog and being nearly one billion miles from the Sun). Daylight on Titan was no brighter than twilight on Earth. The single color photo from the surface showed a vast, muddy plain with a nearly flat horizon. It appeared slightly hazy as expected and was a frigid -292ļF. The small rocks near the spacecraft had flow features around them, which indicated that liquid methane may have flowed not long before, perhaps even days before the landing. The dark river and lakebeds were probably full of dark deposits of organic compounds that precipitated onto Titan as snow or rain and quickly washed off the higher peaks. Small islands and features that look like sandbars in the dry lakebeds were strong evidence for flowing liquid. Titan is very much like a primordial Earth in a deep freeze and this makes it a high-priority for a follow-up multi-billion dollar mission in the coming decades. Huygens only lasted about three hours in the cold, but it was a historic moment.
Cassini continues to orbits Saturn and frequently flies within 590 miles of Titan using all its instruments to pierce the hazy veil. It found clouds that come and go and a huge north polar cloud cap that probably rained methane onto the surface. Near and beneath the cloud cap Cassini discovered huge, very dark, smooth areas that could only be lakes due to the smoothness being almost like a mirror. These were different than the dark areas that the Hubble Space Telescope saw, which turned out to be mud flats and dark sand dunes that cover a large area of Titan. Some of the lakes are larger than Lake Superior and rival Earthís seas. It was almost certain that the dark areas were liquid as smaller dark areas would appear after clouds dissipated, apparently raining out methane onto the surface. If further proof was needed for liquid, it finally came in 2009 when Cassini caught a glint of sunlight reflecting from the surface through the haze with its infrared detectors. It could only be liquid and it was another historic moment for a mission already full of discovery and surprises. To add to the discovery, Cassini measured the changes in a large lake in Titanís southern hemisphere that is as large as Lake Ontario and found that it is slowly evaporating. It is probably too deep for it to fully evaporate and methane may soon begin to rain onto it and fill it as winter approaches.
Cassini continues to orbit Saturn to this very day and will do so until September 15, 2017 when it will run low on fuel and be de-orbited to burn up in Saturnís atmosphere. This will be done to avoid the risk of it crashing into Titan, or Enceladus, thereby contaminating them with any earthly microbes that may have survived. A future mission to Saturn and Titan was to be a lake lander called the Titan Mare Explorer, or TiME, which would have floated on one of the larger lakes and measured the depth, composition, wave height, and environment. It was up for consideration as a new Discovery mission this August that was to launch in 2016 and would have arrived at Titan in 2023. Unfortunately, it lost out to a new mission to explore the interior of Mars, which means probably forever losing the chance to explore an alien sea in our lifetimes. Unfavorable orbital mechanics and budget constraints will make it nearly impossible to return to Saturn and Titan for decades to come. Therefore it will be Cassini that will be the grand discoverer of Titanís secrets that will keep scientists busy and everyone else in awe for the rest of our lives.