One of the most beautiful space paintings is of Saturn in a blue sky as seen from the snowy surface of Titan by Chesley Bonestell. Ever since Titan was discovered to have a thick methane atmosphere in 1944, space artists have dreamed about it and scientists have wanted to explore it. With the advent of the Space Age, it was not long before Titan became a priority target for a spacecraft.
When Pioneer 10 flew past Saturn in September 1979, the optics were not sharp enough to resolve anything on Titan since it did not fly close enough. Titan appeared nothing more than a small, orange fuzzy ball, but that did not matter because Voyager 1 was well on its way to Saturn at the time and flew only a few thousand miles from Titan in November 1980. If the atmosphere had been clear, the surface could have been seen in fine detail. Unfortunately it was a disappointing flyby. Voyager 1 confirmed that Titan is 3193 miles in diameter making it the second largest moon in the Solar System and is a bone chilling -280ļF. It discovered that Titanís atmosphere is about 500 miles deep, 1.6 times thicker than Earthís, composed mainly of nitrogen with small amounts of methane and a complex mix of hydrocarbons. The ultraviolet radiation from the Sun reacts with the atmosphere to produce a thick, orange smog that completely obscures the surface, which frustrated everyone who wanted to see what the surface looked like. Voyager 2 did not fly as close to Titan during August 1981 in order to preserve its option to fly past Uranus and Neptune later in the decade.
It was speculated that Titan was completely covered by a vast ocean of methane with rain, fog, wind, organic snow, and even thunderstorms! The Hubble Space Telescope was powerful enough to penetrate the thick haze in the infrared during the 1990ís to reveal dark, smooth areas that were thought to be seas and patchy bright areas that could be continents. NASA designed a spacecraft known as Cassini to orbit Saturn, but placed considerable emphasis on Titan. The European Space Agency (ESA) designed the Titan lander called Huygens that was attached to Cassini. It was deployed soon after Cassini arrived at Saturn. There was much speculation and anticipated excitement as to what Huygens would find when it descended through the smog to its scheduled landing site in one of the dark areas, which meant it might make a splash landing into a methane sea. Cassini was launched on October 15, 1997 and the adventure began.
Cassini is a huge, $3.3 billion mission that is as large as a school bus and arrived at Saturn on June 30, 2004. It is still in orbit as of this writing on an extended mission that is hoped will last until at least 2017. Huygens separated from Cassini on Christmas Day, 2004 towards its historic landing on Titan on January 14, 2005. Its goal was to sample Titanís atmosphere during descent while taking panoramic photographs of the landscape below. Huygens plunged through the frigid atmosphere of hydrocarbon haze and detected increasing amounts of methane while descending. Huygens did not emerge from beneath the haze until about 12 miles above the surface. When the surface finally became visible, Huygens revealed a world altered by running liquids complete with deltas and sandbars looking like a vast shoreline complete with low-lying clouds and fog. Huygens landed about one mile offshore in a vast mudflat and transmitted for several hours before its batteries succumbed to the brutal cold of -292ļF. The texture of the soil was like that of moist sand with a thin crust that Huygens punctured when it landed. The soil was very moist with methane close to the surface indicating that it had rained not too long ago, perhaps a few days ago, or maybe a decade ago. The atmosphere grew steadily moister as Huygens descended and when it landed there were several drops of pure methane that had condensed on its warm, metallic exterior.
The stunning single photo of the landing site revealed rock-hard water-ice pebbles and rocks that showed evidence of liquid flowing past them. There was enough evidence that, even though this lakebed was dry now, it was, and probably will be once again, full of liquid. In this intense cold methane can exist as a liquid. The lighting conditions were poor as only one-tenth of one percent of the sunlight reached the surface and the concentration of methane at the surface was enough to create a fine mist that made the surface image slightly fuzzy, but the horizon was visible and the landscape was generally flat. The scenery was bathed in a dim orange light due to the intense scattering by the smog in the thick atmosphere. 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 the number one priority for a multi-billion dollar flagship mission in the coming decades. There are plans and designs already drafted for the spacecraft, which will be unlike any ever flown.
The Titan Saturn System Mission (TSSM) is a joint US/ESA mission that will explore Saturn and its moons Titan and Enceladus. The mission will consist of a US orbiter as big as Cassini and two ESA Titan probes: one a hot air balloon that will float in Titanís clouds and the other a lander that will land in one of the methane seas. It awaits official approval, but one scenario has a launch on September 10, 2020 and will use gravity assist from Earth and Venus several times before arriving at Saturn on October 28, 2029. TSSM will orbit Saturn for two years during which it will encounter Enceladus and its geysers seven times and Titan 16 times. It will deploy the balloon and lander during this phase of the mission before finally going into orbit around Titan. The balloon will hopefully circumnavigate Titan at least once and float about six miles above the surface taking awesome aerial photos of the Earth-like landscape. The lander will splash down onto Kraken Mare, a vast methane sea near the North Pole to become the first floating spacecraft on an extraterrestrial sea. It is expected to last at least three hours on the surface.
The goals for all three spacecraft are to determine the composition of Titanís surface, the distribution of organic compounds, how the methane cycle works, the location of the methane reservoirs, the ages of the surface features, especially the cryovolcanoes and if they are active, the presence of a subsurface ocean and if Titan has a magnetic field. Titanís atmosphere will be studied at all levels including the composition and distribution of compounds, temperature, wind, rainfall, and storms. Something must be replenishing the methane in Titanís atmosphere or it would be gone by now; this is one question that TSSM will hopefully answer. The Titan lake lander portion of TSSM is under consideration as a separate mission. Known as the Titan Mare Explorer (TiME), it could launch as soon as January 17, 2015 and arrive at Titan on June 29, 2022 as the spacecraft would be much lighter to launch and less expensive than TSSM.
Let us imagine we are on a mission to explore Titan. As we approach Titan, we see that it is a featureless hazy orange globe. As we descend, we reach the smog layer at 125 miles above the surface and it is rapidly growing colder. Soon we loose sight of Saturn as the smog thickens. It is a very windy and misty descent as the air pressure increases. At 12 miles above the surface we suddenly break out of the smog and see quite a sight below! Large billowy orange methane lake-effect clouds dump heavy methane rains onshore partially obscuring the coastline of a huge sea below us. We splash down onto a peaceful, dark sea with a flat horizon all around us. The ocean is almost pure methane and nearly a half-mile deep with the bottom covered in tar-like hydrocarbons. It is a frigid Ė300ļF and we can see virga, or streaks of precipitation, hang from the billowy orange clouds overhead. An orange haze covers the entire sky, and beautiful Saturn is lost in the murk. A bright orange glow near the horizon is the setting Sun. It is a feeble light as it is hardly brighter than a moonlit night back on Earth. Such a scene may await the TSSM or TiME lander when it arrives at Kraken Mare.
In a few decades another school bus may arrive at Saturn to travel a world very similar to a frozen Earth at the beginning of its creation. The virtual students aboard that bus, known for now as TSSM, will be eager to learn all that they can from the smoggy, frozen earthlike wonder, Titan.