Hazy Titan Sunsets Help In Research Of Exoplanet Atmospheres

Lawrence LeBlond for redOrbit.com – Your Universe Online

By studying a hazy sunset on Titan via the Cassini spacecraft, scientists are learning new ways to better understand the atmospheres of exoplanets orbiting distant stars. Scientists developed a new technique based on the data provided by NASA’s Cassini mission that shows how hazy skies could impact the ability to study alien worlds.

“It turns out there’s a lot you can learn from looking at a sunset,” Tyler Robinson, a NASA Postdoctoral Research Fellow at NASA’s Ames Research Center in Moffett Field, California, said in a statement. Robinson led a team of researchers on the work, publishing a paper in the Proceedings of the National Academy of Sciences.

Robinson explained that in order to obtain information masked by hazy atmospheres scientists can separate the light from sunsets, stars and planets into its component colors to create spectra, similar to how prisms refract sunlight. In recent years, researchers began developing techniques for collecting spectra from distant planets. When an exoplanet transits its host star, as seen from Earth, some of the light passes through the exoplanet’s atmosphere, where it is changed in subtle, yet measurable, ways that can be collected by telescopes. The resulting spectra are a record of that imprint.

Scientists can learn many details about exoplanets through spectra, such as temperature, composition and structure of the atmosphere.

Using Cassini’s observation of Titan sunsets, the research team was able to understand exoplanet transits a little better. The Titan observations, known as solar occultations, allowed Robinson and his colleagues to effectively observe Titan as a transiting exoplanet. Sunsets on Titan reveal just how dramatic the effects of atmospheric haze can be.

Scientists believe that many exoplanets may be obscured by high-altitude haze and clouds, just as many are in our own solar system. Such features can create a number of obstacles that researchers must work out in order to determine signatures of alien atmospheres, as well as presenting a major obstacle for understanding transit observations. Models developed to understand exoplanet spectra usually simplify the effects of haze, because of its complex nature.

“Previously, it was unclear exactly how hazes were affecting observations of transiting exoplanets,” said Robinson. “So we turned to Titan, a hazy world in our own solar system that has been extensively studied by Cassini.”

The team analyzed four Titan sunsets between 2006 and 2011 using Cassini’s visual and infared mapping spectrometer instrument. The results of their analyses provided results that include the complex effects of haze, which the team said can now be compared to exoplanet models.

Using Titan as an example, the team found that exoplanet haze may strictly limit what their spectra can reveal about transit observations. Because of this, observers may only be able to collect information about the upper atmosphere. On Titan, this is about 90 to 190 miles above the surface, much higher than the bulk of the moon’s dense and complex atmosphere.

They also found that the haze on Titan more strongly affects shorter wavelengths, or bluer colors of light. Scientists have assumed that hazes on exoplanets would affect all colors of light in similar ways. But the Titan sunset studies showed that this is not the case.

“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 at NASA Ames. “It looks nothing like some of the previous suggestions, and it’s because of the haze.”

The team said the technique applies to any world, including those in our own solar system, such as Mars and Saturn.

“It’s rewarding to see that Cassini’s study of the solar system is helping us to better understand other solar systems as well,” said Curt Niebur, Cassini program scientist at NASA Headquarters in Washington.

The Cassini-Huygens mission is a joint venture between NASA, ESA and the Italian Space Agency.