NASA Cassini sunny Hydrocarbon seas on Titan

This near-infrared, colour mosaic from NASA's Cassini spacecraft shows the sun glinting off of Titan's north polar seas.

While Cassini has captured, separately, views of the polar seas and the sun glinting off of them in the past, this is the first time both have been seen together in the same view.

The sunglint, also called a specular reflection, is the bright area near the 11 o'clock position at upper left.

This mirror-like reflection, known as the specular point, is in the south of Titan's largest sea, Kraken Mare, just north of an island archipelago separating two separate parts of the sea.

This particular sunglint was so bright as to saturate the detector of Cassini's Visual and Infrared Mapping Spectrometer (VIMS) instrument, which captures the view.

It is also the sunglint seen with the highest observation elevation so far, the sun was a full 40 degrees above the horizon as seen from Kraken Mare at this time, much higher than the 22 degrees seen in PIA18433.

Because it was so bright, this glint was visible through the haze at much lower wavelengths than before, down to 1.3 microns.

The southern portion of Kraken Mare (the area surrounding the specular feature toward upper left) displays a "bathtub ring," a bright margin of evaporate deposits, which indicates that the sea was larger at some point in the past and has become smaller due to evaporation.

The deposits are material left behind after the methane & ethane liquid evaporates, somewhat akin to the saline crust on a salt flat.

The highest resolution data from this flyby, the area seen immediately to the right of the sunglint, cover the labyrinth of channels that connect Kraken Mare to another large sea, Ligeia Mare.

Ligeia Mare itself is partially covered in its northern reaches by a bright, arrow-shaped complex of clouds.

The clouds are made of liquid methane droplets, and could be actively refilling the lakes with rainfall.

The view was acquired during Cassini's August 21, 2014, flyby of Titan, also referred to as "T104" by the Cassini team.

The view contains real colour information, although it is not the natural color the human eye would see.

Here, red in the image corresponds to 5.0 microns, green to 2.0 microns, and blue to 1.3 microns.

These wavelengths correspond to atmospheric windows through which Titan's surface is visible. The unaided human eye would see nothing but haze.

Credit: NASA/JPL-Caltech/University of Arizona/University of Idaho

NASA Cassini: Sunsets on Titan reveal the complexity of hazy exoplanets - Video

Using data collected by Cassini's Visual and Infrared Mapping Spectrometer (VIMS) while observing Titan's sunsets, researchers created simulated spectra of Titan as if it were a planet transiting across the face of a distant star. 

The research helps scientists to better understand observations of exoplanets with hazy atmospheres. 

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. 

JPL manages the mission for NASA's Science Mission Directorate, Washington. 

The California Institute of Technology (CalTech) in Pasadena manages JPL for NASA. The VIMS team is based at the University of Arizona in Tucson. 

Credit: NASA/JPL-Caltech

Scientists working with data from NASA's Cassini-Huygens mission have developed a new way to understand the atmospheres of exoplanets by using Saturn's smog-enshrouded moon Titan as a stand-in.

The new technique shows the dramatic influence that hazy skies could have on our ability to learn about these alien worlds orbiting distant stars.

Tyler Robinson

The work was performed by a team of researchers led by Tyler Robinson, a NASA Postdoctoral Research Fellow at NASA's Ames Research Center in Moffett Field, California.

The findings were published May 26 in the Proceedings of the National Academy of Sciences.

"It turns out there's a lot you can learn from looking at a sunset," Robinson said.

Light from sunsets, stars and planets can be separated into its component colors to create spectra, as prisms do with sunlight, in order to obtain hidden information.

Despite the staggering distances to other planetary systems, in recent years researchers have begun to develop techniques for collecting spectra of exoplanets.

When one of these worlds transits, or passes in front of its host star as seen from Earth, some of the star's light travels through the exoplanet's atmosphere, where it is changed in subtle, but measurable, ways.

This process imprints information about the planet that can be collected by telescopes. The resulting spectra are a record of that imprint.

Spectra enable scientists to tease out details about what exoplanets are like, such as aspects of the temperature, composition and structure of their atmospheres.

Tyler Robinson and his colleagues exploited a similarity between exoplanet transits and sunsets witnessed by the Cassini spacecraft at Titan.

These observations, called solar occultations, effectively allowed the scientists to observe Titan as a transiting exoplanet without having to leave the solar system. In the process, Titan's sunsets revealed just how dramatic the effects of hazes can be.

Multiple worlds in our own solar system, including Titan, are blanketed by clouds and high-altitude hazes. Scientists expect that many exoplanets would be similarly obscured.

Clouds and hazes create a variety of complicated effects that researchers must work to disentangle from the signature of these alien atmospheres, and thus present a major obstacle for understanding transit observations.

Due to the complexity and computing power required to address hazes, models used to understand exoplanet spectra usually simplify their effects.

"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."

With Titan as their example, Robinson and colleagues found that hazes high above some transiting exoplanets might strictly limit what their spectra can reveal to planet transit observers.

The observations might be able to glean information only from a planet's upper atmosphere. On Titan, that corresponds to about 90 to 190 miles (150 to 300 kilometers) above the moon's surface, high above the bulk of its dense and complex atmosphere.

An additional finding from the study is that Titan's hazes more strongly affect shorter wavelengths, or bluer, colors of light.

Studies of exoplanet spectra have commonly assumed that hazes would affect all colours of light in similar ways. Studying sunsets through Titan's hazes has revealed that this is not the case.

Mark Marley

"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's technique applies equally well to similar observations taken from orbit around any world, not just Titan.

This means that researchers could study the atmospheres of planets like Mars and Saturn in the context of exoplanet atmospheres as well.

Curt Niebur

"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.

More information: Titan solar-occultation observations reveal transit spectra of a hazy world, PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1403473111