In 1944, Gerard Kuiper discovered the existence of methane absorption bands in Titan's spectrum. This was the first confirmation that Titan has an atmosphere. The fact that Titan's atmosphere had a reddish tint led scientists to believe that organic molecules may have been created by atmospheric chemistry driven by ultraviolet sunlight and/or the interaction with Saturn's magnetospheric plasma.
Since Titan is the only solid body in the solar system beyond Mars with a substantial atmosphere, at 200 km thick, it was a very important target for the Voyager missions. These missions, which took place 1980 and 1981, gave scientists the first close look at Titan. Voyager 1 came as close as 4000 km as it passed by. The missions resulted in an abundance of data about Titan. Voyager images revealed an opaque atmosphere with thin, high hazes. It was discovered that there were seasonal variations which caused a significant difference in brightness between the northern and southern hemispheres and polar hoods.
Voyager 1 was sent on a trajectory that took it through Titan's Earth and Sun occultation zones, in other words it went through Titan's shadow. The passage through Earth's occultation zone was done so radio signals from the spacecraft would pass through Titan's atmosphere on their way to Earth, while the passage through the Sun's occultation zone was done so the atmosphere could be analyzed by looking through it towards the Sun. This revealed that the dominant atmospheric constituent is nitrogen.
On Titan, despite its smaller size, the surface pressure is one and a half bars, which is 50 percent greater than Earth's. The surface temperature was found to be 94 kelvins, indicating that there is little greenhouse warming. The temperature profile in Titan's atmosphere has a shape similar to that of Earth: warming at the surface, cooling with increasing altitude up to the tropopause at 42 km (70 K), then increasing again in the stratosphere.
It was found that photochemical smog caused the opacity of Titan's atmosphere. Voyager's infrared spectrometer detected many minor constituents generated primarily by photochemistry of methane, which produces hydrocarbons such as ethane, acetylene, and propane. Methane, the atmospheric gas detected from Earth, represents several percent of the composition of the atmosphere. Methane also interacts with nitrogen atoms to form nitriles such as hydrogen cyanide.
As water is to Earth, ethane may be to Titan. Voyager indicates that the atmosphere is supersaturated with methane. Although methane can form rain and snow, there is a lack of dust particles on which the methane would be able to condense. Conditions on Titan are too cold for water vapor to exist. Since this is true, the methane in the atmosphere has no water vapor to react with to form carbon dioxide. If this was possible, Titan would have a carbon dioxide and nitrogen rich atmosphere loosely resembling those of Venus, Mars, and prebiotic Earth. Methane and ethane may be able to condense in Titan's atmosphere, which leads to the possibility of ethane rain or snow. If this is true, enough ethane may have fallen to create a liquid layer up to 1 km deep. There also may be methane rain in the lower levels of the atmosphere.
Titan appears to have winds. The temperature difference from the equator to 60 degrees latitude may be as much as 15 kelvins, which suggests that Titan might have jet streams similar to those in Earth's stratosphere. Wind speeds in Titan's stratosphere may reach 100 meters per second. The occultation of the star 28 Sgr observed from Earth in 1989 confirmed this theoretical analysis by detecting the shape of Titan's atmospheric bulge, which is influenced by high altitude winds. The atmosphere should be much calmer in the troposphere, where the temperature as a function of latitude varies by only a few degrees.
The effect of Saturn's magnetosphere on Titan's atmosphere
Saturn's magnetosphere carries a mixture of particles, including electrons, various species of ions and neutral atoms and molecules, several populations of very energetic charged particles and charged dust grains. These particles react with the electric and magnetic fields throughout the magnetoshpere.
The ionized gases in the magnetosphere contain charged particles and are called plasma. Large currents in the plasma are caused by organized motions of the charged particles. The plasma's fluctuating fields can scatter the charged particles in a manner similar to collisions in a neutral gas and cause a mixing of all the magnetospheric components.
Since Titan resides in Saturn's magnetosphere, its atmosphere interacts with it. The plasma flow from the magnetosphere hits Titan's backside causing the plasma flow to slow down and wrap itself around Titan. This incoming flow is important because "the plasma is a significant source of atmospheric ionization that triggers the creation of organic molecules in Titan's atmosphere" (Spilker). This, of course, provides a possible explanation of the formation of hydrocarbons in Titan's atmosphere.
The significance of argon in Titan's atmosphere
Since argon and nitrogen condense at the same temperatures, the ratio of argon to nitrogen in Titan's atmosphere should be the same as the ratio in the solar nebula if the nitrogen came from the solar nebula. If this is true, and the nitrogen is not from an outside source, such as comets, or formed from ammonia, then Titan could be a good indicator of "original" planetary atmospheres.
However, argon isn't easy to detect since it is a noble gas. It is believed that argon is required in the atmosphere at the 1 percent level to match the molecular weight detected by radio occultation data. Since argon may be present at the one percent level, it does not match the solar nebula ratio of 6 percent. Thus, it is may be unlikely that nitrogen in Titan's atmosphere came from the solar nebula. The discovery of argon would help us answer the following questions:
- What is the source of molecular nitrogen, the primary constituent of Titan's atmosphere? (Spilker)
- Is it primordial (accumulated ad Titan formed) or was it originally accreted as ammonia, which subsequently broke down to form nitrogen and hydrogen? (Spilker)
- Did the nitrogen come from comets? (Spilker)
Until the Cassini-Huygens mission is complete, we will not know the answers to these questions.
|
