IV.

Atmosphere

Titan’s atmosphere appears as a nearly featureless orange haze. It is 60 percent denser than Earth’s atmosphere and much colder. Nitrogen is the dominant component of Titan’s atmosphere (about 94 percent), with the balance made up of methane (about 5 percent), with small amounts of ethane, hydrogen cyanide, carbon dioxide, water vapor, and other chemicals. Most of these components seem to be well mixed in the atmosphere, but astronomers have been able to detect discrete clouds of methane.

Methane functions as a greenhouse gas on Titan, retaining a small amount of extra heat from the Sun. Without methane, most of the nitrogen atmosphere would likely freeze out over the moon’s surface. Methane also plays a role similar to water in Earth’s atmosphere. A kind of methane drizzle is thought to fall almost constantly. Methane forms clouds and can precipitate out as rain, flowing as liquid on the surface.

Radar on the Cassini probe found large areas that appear to be liquid lakes and seas at the moon’s north pole, currently in winter. These lakes and seas may contain methane and ethane that evaporate during the warmer summer. Other radar images of other regions of Titan appear to show erosion features from flowing liquid in the past. The Huygens probe also found evidence that liquid methane had soaked into the surface where it landed.

The upper atmosphere is exposed to ultraviolet radiation from the Sun and high-energy particles from Saturn’s magnetosphere. Nitrogen and methane at these high altitudes undergo a complex series of chemical reactions that cause large organic molecules to form. These molecules collect into particles that create the orange atmospheric haze that blankets the moon. The particles then settle like snow to the moon’s surface, forming dunes of dusty, organic material. Over time, this process would remove the methane from the atmosphere unless some source replenished it. The origin of the methane in Titan’s atmosphere is not yet known.

Visual and infrared observations by Cassini reported in September 2006 appear to show a vast cloud of ethane surrounding the north pole region of Titan where the lakes and seas were found. Ethane rain or snow may be falling into the lakes. When one of the moon’s poles is in winter shadow, ethane may form deposits of polar ice in the extreme cold.

Titan’s atmosphere is thought to be quite similar to Earth’s early atmosphere, before life began. The formation of complex organic compounds in the moon’s atmosphere may indicate how some of the building blocks of life originated on Earth, as well. Titan’s atmosphere stretches much farther from its surface, however, because of the moon’s low gravity. Cassini has revealed multiple layers of haze higher than 400 km (250 mi) above the moon’s surface. The majority of Earth’s atmosphere lies within 16 km (10 mi) of the surface.

Saturn’s gravity affects Titan’s dense atmosphere, creating tidal forces 400 times more powerful than the tidal forces that the Moon exerts on Earth’s oceans. These tidal forces cause winds that may shape the large fields of dunes that Cassini’s radar detected on the surface.