Pedagoguery

In 1980, the scientists in charge of Voyager 1 had a choice. They could aim for a close flyby of Saturn's moon Titan, or they could aim for an eventual flyby of Pluto. They chose Titan. What they saw was a featureless orange ball. The moon had a thick atmosphere topped by a haze of hydrocarbons bearing a remarkable resemblance to a dense smog. The Voyager 2 team decided to skip Titan altogether.

While Voyager did not have the instruments to penetrate the haze, it did produce some intriguing data. Titan's atmosphere is primarily nitrogen, making it the only other body in the solar system to have a substantial nitrogen atmosphere. Neptune's moon Triton was later found by Voyager 2 to possess a tenuous nitrogen atmosphere, but it is a mere 1/70,000 the pressure of Earth's, while Titan has an atmosphere just under one and a half times Earth's pressure. Voyager also was able to determine that the surface conditions on Titan allow for methane clouds, rain, and even lakes or oceans.

It wasn't until the arrival of Cassini in the Saturnian system in 2004 that we started getting a clearer picture of the mysterious moon. Launched in 1997, after being delayed by the Challenger disaster. Because of the delay, Cassini had to take a circuitous route to Saturn, making two flybys of Venus, one of Earth, and one of Jupiter to finally make it. Cassini carried the Huygens probe, which was destined to land on the surface of Titan. The data returned by Cassini and Huygens has revolutionized our understanding of Titan, and provided a picture of a moon that is remarkably earthlike in many ways.

When they Huygens probe landed on Titan in 2005, it landed on a sandy surface, with fist sized "rocks" lying all around. The rocks were rounded, as if eroded by running water, and the sand was damp. Of course, the sand wasn't sand, the rocks weren't rocks, and the dampness wasn't water. Instead, the sand appeared to be a granulated form of the atmospheric smog. The rocks were chunks of water ice, and the dampness was liquid methane. Nevertheless, while the materials were different, the overall effect was the same. Titan was the most earthlike body so far explored in the solar system.

Subsequent exploration by Cassini has fleshed out the details more, and has not reduced the similarities. Radar mapping has found huge lakes of liquid ethane and methane at the poles, and huge dune fields at the equator. Radar images of the dune fields show remarkable similarity to dune fields on Earth, such as in the Namib Desert.

In its interior, Titan has a large rock-iron core. The moon may not have gotten hot enough for the iron to separate out from the rock. Above that, is a layer of ice, but not just any ice. It consists of rare, high-pressure phases of ice that are very different from what we encounter on Earth. Above that is a possible ocean of liquid water mixed with ammonia. Above that is a layer of warm, pliable ice, that convects much like Earth's mantle. Finally, you have the crust. The warm mantle can support a form of vulcanism, and there are signs that cryovolcanos have been detected.

However, it is in the hydrological cycle that Titan demonstrates its extremes. On Earth, sunlight is enough to evaporate about one meter of water per year. However, the atmosphere can only hold a couple of centimeters worth of water before it starts to precipitate out. On Titan, however, sunlight can only evaporate about one centimeters of liquid per year, but the atmosphere can hold the equivalent of 10 meters of liquid. The result is long droughts followed by torrential floods, and the landscape shows it.

On Earth, climate is dominated by Hadley circulation. Warm air rises at the equator and heads toward the poles. However, the Earth's rotation shears the circulation, causing it to reach the ground at about 30 degrees of latitude. Since the air at that point is dry, this is where most of the Earth's deserts are located. Due to Titan's very slow rotation and long seasons, the circulation goes from the summer mid-latitudes all the way to the winter pole, making the equatorial parts permanently dry. This also explains why all of Titan's big lakes are near the poles.

Titan's atmosphere bears other resemblances to Earth's. Both posses weather, which takes place in the lower part of the atmosphere, called the troposphere. The temperature in the troposphere gradually declines with increasing altitude. On Earth, this is where nearly all clouds form. On Titan, methane clouds exist here. Because of Titan's lower gravity, and hence the more gradual decline in atmospheric density with height, Titan's troposphere is deeper. Above the troposphere is the stratosphere, where the decline in temperature suddenly reverses. On Earth, this is due to heating by ultraviolet light being absorbed in the ozone layer. On Titan, this is where ethane clouds form, absorbing sunlight. Solar ultraviolet provides the energy to power some complex organic chemistry, producing a thick haze of smog. Above the stratosphere is the mesosphere, where, on Earth, the temperature starts to decline again. In Titan's mesosphere, the temperature is pretty level with altitude, and there is a thin haze layer at this height. Finally, there is the thermosphere. On Earth, this tenuous layer of atmosphere starts to heat up again, until it is nearly 1000 Kelvins. It is so thin, however, that it does not carry much heat. Titan's thermosphere is distinguishable from its mesosphere only by the absence of any haze.

Cassini continues to make passes of the moon, and as a result, oour understanding of this remarkable place continues to grow.

Next time, how stable are the orbits of the planets in our solar system?