Titan's Hazy Sunsets Shed Light on Alien Atmospheres

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In an effort to better understand the atmospheres of worlds orbiting other stars, NASA’s Cassini mission is using sunsets through the hazy atmosphere of Saturn’s moon Titan to create a solar system ‘exoplanet analog.’

When analyzing the starlight refracting through a distant exoplanet’s atmosphere, astronomers can decipher the composition of that ‘exo’-atmosphere. However, there are many unknowns and ambiguities that need to be ironed out before an accurate gauge of that atmosphere can be determined. So by observing how sunlight shines through Titan’s high-altitude haze, astronomers are learning how they might better analyze the atmospheres of alien worlds light-years distant.

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“It turns out there’s a lot you can learn from looking at a sunset,” said Tyler Robinson, NASA Postdoctoral Research Fellow at NASA’s Ames Research Center in Moffett Field, Calif.

During a sunset on Earth, light from the sun is refracted by the gas and dust in our atmosphere, affecting the wavelength of light we see. Our atmosphere will act like a prism, refracting the light, separating it into the component colors of its spectrum.

Although it is a new field of study, astronomers are developing techniques to detect the refraction effect as exoplanets pass in front of their host stars, thereby detecting their atmospheres as some of the starlight passes through, refracting before being blocked. The information that this refracted light carries, although very slight, can help us learn about exoplanetary atmospheres.

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Throughout its mission in Saturn orbit, Cassini has observed many ‘transit’ events as Titan blocks our sun from view. Now researchers have analyzed data from four Cassini-Titan transit events from 2006 to 2011, providing “results that include the complex effects due to hazes, which can now be compared to exoplanet models and observations,” according to a NASA news release.

One significant finding is that the analysis of refracted light through Titan’s uppermost atmospheric haze — which is located between 90 to 190 miles (150 to 300 kilometers) above the moon’s surface — can reveal some information about atmospheric composition. But Titan’s lower and more dynamic atmosphere blocks the sunlight, preventing a spectra from being recorded.

But on analyzing the sunlight that gets refracted by the high-altitude haze, they found that the haze more strongly affects shorter (bluer) wavelengths. Studies of exoplanet haze have assumed that high-altitude atmospheric gases affect all wavelengths of starlight in the same way. This Titan study has therefore added critical detail to our understanding about how hazy exoplanetary atmospheres may refract light, improving our deduction of atmospheric composition.

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“People had dreamed up rules for how planets would behave when seen in transit, but Titan didn’t get the memo,” said Mark Marley, a co-author of the study, which has been published in the Proceedings of the National Academy of Sciences, also from NASA Ames. “It looks nothing like some of the previous suggestions, and it’s because of the haze.”

This study proves that, once again, Cassini has not only boosted our knowledge of Saturn and its system of moons, but it is now helping us understand the dynamics of atmospheres on planets orbiting other stars.