Saturn’s moon Titan is one of the most scientifically interesting spots in the solar system. The second-largest moon after Jupiter’s Gannymede and bigger than the planet Mercury, it’s shrouded beneath a thick, smoggy atmosphere rich in methane creating a greenhouse effect and constantly unloads complex hydrocarbons that rain down on the surface.

Now, scientists have figured out just how long Titan has had its signature hazy atmosphere.

Discovered in 1655 by Dutch astronomer Christopher Huygens, it wasn’t until 2004 that astronomers managed to peer through the dense atmosphere to uncover the moon underneath. The Cassini spacecraft in orbit around Saturn aimed its instruments at Titan and in 2005 the small Huygens probe actually landed and gathered data from its surface.

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Titan’s atmosphere is heavy. In fact, it’s about 60 percent heavier than Earth’s; standing on Titan’s surface would feel like standing under 20 feet of water. It’s mostly composed of nitrogen with just a touch of methane. The methane on Titan acts a lot like water on Earth — it creates a greenhouse effect to keep the moon’s temperature steady at about -180 degrees Celsius (-292 degrees Fahrenheit).

The moon has a methane cycle similar to the water cycle on Earth with liquid methane and ethane raining down and forming lakes on its surface. Understanding Titan’s methane cycle could help scientists understand Earth’s water cycles, and shed some light on how these two worlds are so similar yet so different — you’d definitely not like to get caught out in the methane rain!

But what’s really interesting about Titan’s atmosphere is the short life span of methane. Made of one carbon atom joined to four hydrogen atoms, methane breaks down readily when exposed to direct sunlight and is converted into more complex molecules and particles. The reaction generates the hydrocarbons that rain down and form dunes of organic material.

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Scientists know how long it takes for methane to break down, and by measuring the concentration of heavier methane isotopes its possible to figure out how long Titan has been enveloped in its organic atmosphere.

Isotopes of a molecule are versions with different weight. In the case of carbon, carbon-13 is rarer and heavier than its more common sibling carbon-12. Methane is occasionally made with the heavier carbon-13 making it a heavier atom than methane made with carbon-12. The lighter methane is broken down the UV light faster, meaning the relative concentration of methane made with carbon-13 increases over time.

By modeling how the concentration of heavy methane has changed over time, scientists have been able to determine just how long Titan’s “chemical factory” atmosphere has been running.

The baseline age for Titan’s atmosphere going into the study was 1.6 billion years, about a third of the age of the moon itself. If it turned out that methane escapes from the top of the atmosphere, it would be younger. But if evidence suggested methane is replenished with fresh isotopes from the surface, through a theoretical subsurface ocean of water and ammonia for example, then the atmosphere would be much older. A buildup of methane on the surface lakes and in the atmosphere would be another sign of an older atmosphere.

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Scientists were able to measure the type and concentration of methane isotopes using Cassini’s Ion and Neutral Mass Spectrometer and the Huygens Gas Chromatograph Mass Spectrometer. Both identify and count molecules based on mass and by comparing the amount of heavy methane on the surface and in the atmosphere, scientists can understand the impact of escape on the molecule in the atmosphere.

The data show no obvious buildup of methane, so it’s safe to conclude that Titan’s atmosphere is only about a billion years old, much younger than the moon. Previous work suggests that the atmosphere was born from a methane eruption somewhere between 350 million and 1.35 billion years ago, which supports the latest finding on its age.

This result is the work of Conor Nixon of the University of Maryland, College Park and Kathleen Mandt of the Southwest Research Institute, San Antonio. Both scientists’ results will appear in the April 20 issue of the Astrophysical Journal.

Image: Saturn’s hazy moon Titan. Credit: NASA/JPL-Caltech/Space Science Institute