Titan Sheds Light on Alien Atmospheres

Saturn’s largest moon, Titan, played a cameo as an exoplanet, allowing astronomers to better understand how a thick layer of haze or clouds might affect their observations of more distant alien worlds.

This celestial circle of light is produced by the glow of sunlight scattered through the periphery of Titan's atmosphere as the Sun is occulted by Titan. Image Credit: NASA / JPL / Space Science Institute

This celestial circle of light is produced by the glow of sunlight scattered through the periphery of Titan's atmosphere as it passes in front of the Sun.
Image Credit: NASA / JPL / Space Science Institute

Exoplanet portraits show an array of extremes. We often hear stories of uncanny weather, where glass falls from the sky and winds gust faster than the speed of sound, peppering the atmosphere with shock waves. But these weather reports are educated guesses, pulled from measurements of light output and chemical makeup.

Although these guesses are crucial to truly understanding alien worlds — providing the next step in moving beyond merely counting exoplanets to characterizing them — the research is relatively young.

So astronomers are turning their eyes a little closer to home to guide their ventures further afield. A team led by Tyler Robinson (NASA Ames Research Center) has observed Saturn’s hazy moon Titan with the Cassini spacecraft in order to shed light on future exoplanet observations. The results, soon to be published in the Proceedings of the National Academy of Sciences, explicitly show how a thick, hazy atmosphere affects observations.

Sniffing Out Exoplanet Atmospheres

How do astronomers detect a distant exoplanet’s atmosphere? The most convenient method for current telescopes is to watch for a planet to pass in front of its star. The star’s light will pass through the planet’s atmosphere during its transit, and abundant water vapor, gaseous metals such as sodium and potassium, or even crystals that form in clouds can absorb or scatter the star’s light. The atmosphere will imprint its chemical fingerprints on the starlight that reaches Earth.

Astronomers can gain insight into the atmospheres of these alien worlds by collecting this transmission spectrum, or the spectrum of starlight that passes through the planet’s atmosphere.

One of the best-studied small exoplanets, GJ 1214b, is well known in exoplanet circles, however, for its lack of spectral features. Its transmission spectrum doesn’t show the expected absorption lines, instead it’s almost featureless. Astronomers have blamed a thick, high-altitude layer of clouds or haze, which would absorb and scatter light equally across a wide wavelength range.

Titan’s Exoplanet Cameo

“So we set out to discover what a hazy, well-studied world would look like in transit. Titan fits the bill,” says Robinson. “We wanted to understand under what circumstances hazes can prevent us from characterizing and understanding an atmosphere. The flip side is that we also wanted to know the circumstances under which you can detect gas features despite a haze.”

While we can never see Saturn, or Titan, pass in front of the Sun as viewed from Earth, NASA’s Cassini spacecraft can. The team used Cassini’s infrared eyes to watch the Sun appear to pass behind Titan and light up its atmosphere.

“As Cassini moved in its orbit, Titan passed directly between the Sun and Cassini,” says coauthor Jonathan Fortney (University of California, Santa Cruz). “This allowed us to measure how transparent Titan’s atmosphere is, and most importantly how that changes with [altitude].” The team measured the moon’s transmission spectrum between 0.8 and 5 microns.

The animation below shows a simulated sunset through Titan’s atmosphere at four different infrared wavelengths. Just as the rising moon appears red because our atmosphere scatters the blue colors of light, the Sun’s disk will “disappear” more quickly at wavelengths where Titan’s haze or specific molecules, like methane, absorb or scatter more light.

Model of the Sun setting through Titan’s atmosphere at different wavelengths. The small dot on the left indicates the Sun’s altitude. From left to right, the large discs show how sunlight is absorbed at 1.0, 2.9, 3.4, and 4.5 microns. The haze in Titan’s atmosphere absorbs light most strongly at 1.0 microns, and more weakly at longer wavelengths. In addition to the haze, methane absorbs light at 3.4 microns, and carbon monoxide contributes at 4.5 microns. Tyler Robinson

Model of the Sun setting through Titan’s atmosphere at different wavelengths. From left to right, the large discs show how sunlight is absorbed at 1.0, 2.9, 3.4, and 4.5 microns. The haze in Titan’s atmosphere absorbs light most strongly at 1.0 microns, methane at 3.4 microns, and carbon monoxide at 4.5 microns.
Tyler Robinson

Surprisingly, the Robinson and colleagues did not see the featureless spectrum they might have expected. Instead, Titan’s haze extinguishes bluer light much better than redder light. The results are more complex than previously thought and theoretical astronomers, who assume a featureless spectrum for ease, will have to incorporate more complex physics in order to accurately reproduce these results.

Model tweaks will be particularly important for GJ 1214b, where the case of a Titan-like, high-altitude haze runs into trouble. To produce a featureless transmission spectrum, the exoplanet’s atmosphere would need larger haze particles than those detected on Titan, and such large particles would be difficult to keep aloft.

Still, the Titan observations point to future prospects for the study of exoplanet atmospheres.

“Our data tell an optimistic, but cautionary tale,” says Robinson. When observed in blue light, Titan’s gas absorption features are all but lost in the haze, but at redder wavelengths, where the haze is less opaque, Titan shows strong features due to methane. The results lend hope to our ability to one day detect similar features in exoplanet transit spectra.

So it appears that the James Webb Space Telescope — slated to launch in October 2018 — may have better luck peering through the clouds at redder colors.

Reference:

T. Robinson et al. “Titan Solar Occultation Observations Reveal Transit Spectra of a Hazy World,” to be published in the Proceedings of the National Academy of Sciences.


Find out more about weird weather on alien worlds in our May 2014 issue, where exoplanet expert Jonathan Fortney reports on the cutting edge of exoplanet studies.

COMMENT