A window on the weather on Titan

It’s been 4 years since NASA’s Cassini spacecraft flew by Titan – the spacecraft vaporized into Saturn’s atmosphere in 2017 – but data collected from the 13-year mission still provides new information about the larger moon. of Saturn. Researchers have long studied flyovers of the Cassini Titan for clues to the moon’s climate and topography, but a new study is strengthening our understanding of the moon’s daily weather.

Over the years, Cassini has revealed that Titan is a planetary body relatively similar to Earth. The moon’s climate goes through seasons that last approximately 7.5 Earth years, and circulation through its atmosphere redistributes heat from the equator to the poles, keeping temperatures relatively uniform and stable. At its surface, liquid natural gas flows through rivers and lakes. It is the only place in the solar system other than Earth that experiences such a flow of liquids on its surface, and researchers have long speculated that these lakes and rivers are fueled by precipitation from methane clouds in the atmosphere of the moon.

“While rain can be predicted by theory, of course there are all kinds of theories,” said Roger Clark, a senior researcher at the Planetary Science Institute not involved in the new research. “One of the theories when we got to Titan was that it would be covered in an ocean of methane and there wouldn’t be a solid surface, so the theories might not have all the data points.” But the new study is a “key data point in the case of active rain.”

“This is the only alien world where we can talk about alien precipitation,” said Rajani Dhingra, NASA postdoctoral researcher at the California Institute of Technology’s Jet Propulsion Laboratory and lead author of the new study.

A shining ephemeral element

The new study builds on earlier studies by Dhingra and colleagues in which the team combed through data from Cassini’s visual and infrared mapping spectrometer and spotted a massive reflective feature on the moon’s surface. . The reflection, which the team called Brilliant Ephemeral Element (BEF) in a 2019 article, was temporary. The team theorized that this was likely the result of sunlight reflecting off a wet surface, similar to how the sun can reflect off wet pavement after rain here on Earth.

If the BEF were the result of rain wetting the surface, this event would also have caused a local change in temperature. The next logical step for Dhingra and his colleagues was to research this temperature change using Cassini’s composite infrared spectrometer, which would further support the idea that the reflective function was the result of a rain event wetting the surface. . But the data on the original BEF was too loud to see any temperature change. So in the new study, published in Geophysical research letters, the team identified another BEF in data from the 121st Cassini flyby on July 25, 2016.

This time, the instruments collected enough spectra both on and off the BEF to identify a temperature drop of about 1.2 Kelvin in the BEF relative to the area around it.

“We were fortunate enough to have this number of spectra to see a noticeable temperature difference in one single day flyover over Titan, so we probably looked at the weather on Titan for the first time,” Dhingra said.

The team suspected that the drop in temperature was due to evaporative cooling and therefore would be temporary. In fact, on the next flyby, the BEF had disappeared. “We don’t know the fate of the rains,” Dhingra said. Did they congregate in a shallow puddle that quickly evaporated, seeped into the ground, or flowed into tributaries that drained into lakes or seas elsewhere? Or did it never reach the surface at all, instead settling as a fog above the surface, only to be blown away by the wind?

Layers of black, blue and orange haze in Titan's atmosphere
Titan’s thick atmosphere, pictured here, is full of methane clouds, which fuel precipitation at the planet’s poles. Credit: NASA

Clark noted that a wet surface, ice, or even clouds can all cause the type of spectral reflections that have been detected on Titan.

These questions will be much easier to answer when NASA’s Dragonfly mission reaches Titan. Dragonfly will launch in 2026 and land on the surface of Titan in 2034. Dhingra is eagerly awaiting his arrival, not least because Titan’s thick atmosphere makes studying precipitation or temperature changes at the surface very tricky from above. .

“I am impressed that we can see something like this in a world that is 10 times farther from the Sun than the Earth, from a mission that was conceived in the 1980s,” Dhingra said. “I can’t understand the science we’re going to be doing with Dragonfly on the surface.”

—Kate Wheeling (@katewheeling), Scientific editor