Each of the planets and moons in the Solar System is so different from Earth that it makes our planet seem unique. Orbiting the beautiful ringed planet Saturn nearly one billion miles away is a world frozen in time that appears to represent primordial Earth. Previously unknown to scientists was how incredibly similar to Earth this small world, Titan, really is. When the truth was discovered in 2005 that Titan is as dynamic as the Earth, it stunned and amazed everyone.
It was found in 1944 that Titan was surrounded by a substantial atmosphere of methane, perhaps as thick as Earthís. A classic painting of Saturn floating in a blue sky above the snowy landscape of Titan by Chesley Bonestell became an inspiration to all who were interested in space exploration and in the amazing universe even though, as we learned more, the painting proved to be incorrect. Titan is the largest moon of Saturn at 3200 miles in diameter and is larger than Pluto and Mercury. As the Space Age began, it was clear that missions to the planets would include close flybys of Saturn and Titan. Voyager 1 flew within a few thousand miles of Titan during November 1980 only to find it veiled in a thick photochemical smog of hydrocarbons. The atmosphere was determined to be composed primarily of nitrogen (as is Earthís) with small amounts of methane, but the surface remained hidden. Clouds seen deep within the haze led to the speculation that Titan was at just the right temperature at -280ļ F to allow methane to exist as a liquid and that perhaps Titan had a vast global ocean of liquid methane or even ethane. Further studies from Earth with the Hubble Space Telescope in infrared light revealed mysterious dark and light regions. Almost immediately it was thought that the dark areas might be seas and the bright areas might be hills or mountains covered with methane snow. It became apparent that a probe was needed to fully study Titan. Hence, the Cassini mission was born in the late 1980ís that would orbit Saturn and drop a probe into Titanís atmosphere.
Cassini, a huge $3.3 billion mission launched on October 15, 1997, was as large as a school bus and bristled with scientific instruments that included a European-built Titan probe called Huygens. Its goal was to plunge through and sample Titanís atmosphere during descent while taking panoramic photographs of the landscape below. It was not designed as a lander and therefore was not expected to survive a rough landing, but it was designed to float if it should land in liquid. Cassini successfully entered into orbit around Saturn on June 30, 2004 and began its exciting adventure of discovery. Nearly four-dozen flybys of Titan were planned for Cassini while orbiting Saturn through at least the end of its primary mission on July 1, 2008. Several flybys have already approached to within 600 miles of Titanís surface. Cassini has been able to pierce the haze in the infrared to see finer detail and has discovered patchy, white clouds of methane near the South Pole, but it has seen very few craters. This suggests that Titan must be dynamic as something is destroying most of the craters. It was very tempting to interpret the vast dark surface areas as seas of methane, but Cassini saw no evidence of liquid. As fate would have it, the Huygens probe was expected to land in one of the largest of the dark areas. If it were liquid, Huygens would be capable of seeing it before impact, and since it was capable of floating, it could determine the depth and composition of the ocean as well as detect waves by the amount it bobbed when it landed. Huygens separated from Cassini on Christmas Day 2004 and began its historic encounter with Titan on January 14, 2005.
Huygens plunged through the frigid atmosphere of hydrocarbon haze and detected increasing amounts of methane while descending. It was buffeted by strong winds early in its descent, but it grew calmer towards the surface where it encountered a shift in wind direction moments before landing. The haze layer was thick and Huygens did not emerge from beneath it until about 12 miles above the surface. When the surface finally became visible, Huygens made a dramatic discovery: Titan is a world altered by running liquid perhaps to this very day! Huge, dark, branching streams and rivers flowed into a vast dark lakebed complete with deltas and sandbars. Smaller channels showed evidence of eruptive springs. As Huygens descended closer to the surface, the landscape appeared to be like a vast shoreline complete with low-lying cloudbanks and possibly fog. It grew steadily moister as Huygens approached a hard landing, but a veer in the wind at the final moment allowed it to land gently. The landing was so gentle that Huygens actually transmitted for several hours before its batteries succumbed to the brutal cold of -292ļ F. The lighting conditions were poor as only one-tenth of one percent of the sunlight reached the surface and the haze added to the difficulty of obtaining sharp pictures, but several were sharp enough to forever alter the theory of Titanís evolution.
Based on the descent profile, Huygens landed about one mile offshore in a vast mudflat. The texture of the soil is like that of moist sand with a thin crust that Huygens punctured when it landed. The soil is very moist with methane close to the surface indicating that it rained not too long ago, maybe only a day ago, but perhaps as long as a year ago. The stunning single photo of the landing site reveals rock-hard water-ice pebbles and rocks that show evidence of liquid flowing past them. There is enough evidence that, even though this lakebed is dry now, it was and probably will once again be full of liquid. In this intense cold only methane can exist as a liquid. The dark river and lakebeds are 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 are strong evidence for flowing liquid. The concentration of methane at the surface was enough to create a fine mist that made the surface image slightly fuzzy, but the horizon is visible and the landscape is generally flat. The scenery is bathed in a dim orange light due to the intense scattering of light by the smog in the thick atmosphere. The atmospheric pressure at the surface is 1.6 times that of Earthís.
An unusual dark spot as large as Lake Ontario, located near the South Pole, has the appearance of being an actual lake full of liquid with a smooth shoreline. This feature is very close to a cluster of clouds that may be raining methane that flows into the lake. Several other smooth dark spots, some as large as the Earthís largest seas, dot the north polar regions and have been confirmed to be filled with liquid methane, which makes Titan the only other world in the Solar System besides Earth to have liquid on its surface. These seas and lakes contain bays, peninsulas, islands, and river channels. A mysterious red spot was detected by Cassini and might be a region of active volcanism while another feature looks like a real volcano. Cassini has also seen extensive channels, dune fields, and smooth shorelines. At this stage in Titanís existence all of the methane should be gone, but something is replenishing it. The discovery of a huge polar cloud cap over Titanís North Pole, which is currently emerging from winter, may be raining methane onto the surface and filling the seas and lakes. With Huygens now eternally silent, it is up to Cassini to try to unravel the mysteries. It is capable of piercing the haze with radar and will attempt to map Titan as much as possible during the mission, which has been extended to at least July 1, 2010.
Titan beckons us to return and explore it with future missions. In the decades ahead, balloons may be deployed to photograph the landscape from aloft and to study weather patterns to determine how methane circulates through the atmosphere. There might even be a chance of encountering storm clouds, lightning or volcanic activity as the balloons would be able to travel long distances. Flowing methane rivers, waves, and ripples on the lakes caused by winds could be visible. Rovers are capable of exploring the surface, but the intense cold and dim light will be a big challenge in trying to keep them alive long enough to roam and return useful science. The challenges are daunting, but Titan resembles the early Earth at the time when life was just gaining a foothold on our then barren planet. Solving Titanís biggest mysteries could provide answers to our very own beginnings on Earth.