Titan, Saturn's largest moon, has exerted its own pull on Earth's scientists, independent of their fascination with its mother planet. Scientists and science-fiction writers alike have long speculated that Titan's methane-rich atmosphere might be similar to Earth's 4 billion years ago, before life appeared here.
One of the most exciting days of the Cassini-Huygens mission occurred on Jan. 14, 2005, when the Huygens probe became the first spacecraft to land on Titan. Mission scientists burst into cheers when the Green Bank radio telescope in West Virginia—1 of 17 radio telescopes around the world listening for signals from Huygens—picked up a faint but unmistakable crackling sound. The signal indicated that the probe had awakened from a 20-day “sleep” following its launch from Cassini and had come within about 1,300 kilometers (800 miles) of Titan's surface.
At that point, one exciting event happened after another. Within three minutes, Titan's dense atmosphere slowed the descending craft's speed from about 20,000 to 1,400 kilometers (12,300 to 900 miles) per hour. A small parachute deployed, further slowing the probe and pulling its main chute from its storage pack. At 120 kilometers (75 miles), another small parachute replaced the main chute for the final descent.
At an altitude of about 170 kilometers (100 miles), the probe's heat shield popped off, and its instruments began collecting and transmitting data. Then, as the probe descended for the next 2 hours and 30 minutes, Titan revealed some of its secrets.
During its descent, Huygens's instruments recorded sounds, measured wind speeds, and analyzed Titan's atmosphere. Observations by the Voyager spacecraft had revealed that nitrogen makes up most of Titan's atmosphere, as it does Earth's. In 1944, however, American astronomer Gerard Kuiper had discovered, using a spectrometer, that Titan had an atmosphere that included methane. Methane is a type of hydrocarbon, a compound of carbon and hydrogen. On Earth, hydrocarbons are found in large quantities in coal, natural gas, and petroleum. In the moon's upper atmosphere, ultraviolet sunlight breaks apart the methane molecules, the way ultraviolet light destroys ozone molecules in Earth's upper atmosphere. The fragments of methane then recombine to form propane, acetylene, benzene, and the other molecules that make up Titan's smoggy atmosphere.
Methane gas in Titan's atmosphere provides the building blocks to produce complex organic molecules that might form life. However, because of the intense cold (-178 °C [-290 °F]) at the surface, all water is frozen as hard as a rock and the chemical reactions that might have occurred on Earth and resulted in the development of life couldn't happen there.
The haziness of Titan's atmosphere surprised us. We had expected the probe to drop out of the haze when it reached an altitude of from 50 to 70 kilometers (30 to 45 miles). But the haze did not thin enough to give the probe's cameras a relatively clear view of the landscape until about 30 kilometers (20 miles) above the surface. At 20 kilometers (12 miles), Huygens found methane clouds. In fact, the Huygens probe discovered that the haze continues all the way to the surface.
As the probe descended, it took hundreds of pictures. Titan's surface, Huygens discovered, has intricate geologic features that scientists believe were shaped by physical processes similar to those that have shaped—and continue to shape—Earth. These include erosion caused by wind and, particularly, flowing liquids. On Titan, liquid methane plays the role that water does on Earth, scientists believe. The methane evaporates, condenses, forms clouds, and rains down, creating streams and rivers. The atmosphere, however, would run out of methane unless it was continually being replenished. These observations indicate that Titan's surface has been modified and changed over its history.
Finally, Huygens landed with what NASA called “a splat”—into Titanian mud. The first instrument to hit the surface was a long, sticklike device called a penetrometer, which was attached to the bottom of the probe. This device measured the force of the probe's impact and the properties of the material on the surface. Data from the penetrometer indicated something rigid—solid crust or a pebble—at the surface and clay or wet sand just below the surface.
Heat given off by Huygens warmed the landing surface, and the probe's spectrometer measured a sudden increase in methane gas boiling out of the surface. This event reinforced the idea that methane forms clouds and produces rain that erodes the surface. Much to scientists' surprise, Cassini's instruments have not detected any lakes or oceans of liquid hydrocarbons. Before Cassini's arrival at Titan, models predicted that liquid methane and ethane should have been accumulating on Titan's surface for a long time, creating large oceans and lakes. Titan's rivers and lakes appeared dry at the Huygens landing site, but methane rain may have occurred not long before the landing.
Where did all of this liquid go? Did it even ever exist? What are the details about the unusual bright spot on Titan that scientists spotted in May 2005? These are some of the questions that scientists hope to answer after reviewing the Cassini-Huygens data. These unanswered questions provide even more reason to be intrigued with the mysteries that lie behind Saturn, its dazzling rings, and its numerous moons.