Saturn's planet-size moon, Titan, makes the air in Los Angeles look pristine in comparison. This cold and distant satellite, about the size of the planet Mercury, appears to be suffering one of the worst cases of smog anywhere in the solar system. As planets go, only Venus appears to be in the same league.
Last November, the Voyager 1 craft provided the first close-ups of Titan. This moon holds considerabe interest because it is the only known satellite of Saturn to possess an atmosphere and because of past speculation that life might be possible there. The first Voyager founf Titan's atmosphere to be impenetrably thick. Now, Voyager 2 has followed in the footsteps of its twin and is passing Saturn.It is not swinging as close to Titan as its predecessor, but it is scheduled to take a number of additional pictures of the cloudy moon. And scientists have now had considerable time to analyze and digest the results of the first flyby.
Their conclusions, although hazy, give some idea of what Earth's atmosphere would have been like if it had formed 9 to 10 times farther out in the solar system. Even more important, temperatures at Titan are so cold (almost 300 degrees F. below zero) that the moon appears to have acted as a kind of "deep freeze," preserving an atmosphere similar to that of primordial Earth, where life evolved. In addition, Titan's atmosphere has some unusual characteristics that may help scientists to better understand Earth's climate.
Titan's atmosphere is the only besides Earth that is dominated by nitrogen gas: 82 percent on Titan, as compared with 79 percent on Earth. The thick, gaseous envelope surrounding Venus and the extremely thin one around Mars are made up mostly of carbon dioxide. At its surface, the pressure on Titan is only 60 percent heavier than that on Earth.
Of course, there are many differences. The primary chemical contrasts is that Titan lacks oxygen. It has methane (natural gas) and hydrogen instead. Yet this is very much what a number of scientists believe Earth's early atmosphere was like. This type of atmosphere, it is argued, would have been necessary for the chemical formation of the organic material that laid the groundwork for plant and animal life on Earth. Such life, they believe, is in turn responsible for the oxygen in our atmosphere.
The Voyager instruments have also detected a number of organic materials. These include acetylene, propane, and cyanide. Cyanide is particularly important because it is considered necessary to build the more complex organic molecules such as amino acids, which are the precursors of life.Cyanide had been detected previously in interstellar space, but this was the first time it has been seen in a planetary atmosphere.
"We do not suggest that conditions on Titan are suitable for life," Voyager scientists wrote in a summary of the first encounter; "however, we do note that the chemical processes leading to the formation of organic molecules, precursors of biologically important compounds, appear to have occurred on Titan as well as Earth."
A number of laboratory experiments have been performed attempting to re-create Earth's primordial atmosphere and understand how life might have formed. "The problem with laboratory experiments . . . are that you don't have world enough, or time," explains Tobias Owen of the State University of New York , Stony Brook. Titan represents an entire world in which these processes can be studied.
Of course, there are some problems. One of them is a lack of water in liquid or vapor form, Dr. Owen says. About half of Titan is probably water -- but at the extremely cold temperatures all the water is tightly locked up as rock-hard ice. At least at the present time, water in liquid and vapor form is unknown. As a result, there is no oxygen available to form a number of terrestrial amino acids, he says.
Still, Titan may have oceans. The only difference is that they would be formed of liquid methane rather than water. Methane-vapor clouds and fog as well as methane rain may give the planet's surface an "unexpected familiarity," Owen speculates.
Titan's reddish haze is so thick it blots out the majestic view of Saturn and its rings from the satellite's sky. No one yet knows how much light penetrates to Titan's surface, however. The first Soviet lander to send back a picture of Venus surprised the experts, because it showed the surface to be well illuminated despite its perpetual, cloudy veil.
The atmospheres of Venus and Titan show surprising similarities. Both are extremely thick, keeping the surface temperature very uniform: Venus, uniformly hot, at 900 degrees F.; and Titan, uniformly cold. Scientists believe Titan keeps one face perpetually facing Saturn, just as Venus does to the Sun.
"Titan is a mini-Venus," explains Verner E. Suomi, a University of Wisconsin meteorologist.
Yet there are some significant differences in the two. Besides the drastic difference in temperature and the intensity of sunlight falling on them, Titan rotates much faster than Venus. Also, Titan's atmosphere is though to have a significant seasonal lag. The coldest day on Titan is the first day of spring, and its warmest is the first of autumn. In comparison, Earth's atmosphere has a one- to two-month lag. Examining the atmosphere of other worlds such as Titan will help us understand Earth's climate and how it changes, Dr. Suomi maintains.
"It seems [that] to understand one atmosphere, you have to understand them all," says Dr. Owen.