By combining data from the Keck II telescope and the joint NASA/European Space Agency/Canadian Space Agency James Webb Space Telescope, a group of scientists observed cloud convection in the northern hemisphere of Saturn’s moon Titan — the first time such atmospheric activity has been observed in the region. A majority of Titan’s methane lakes and seas are located in the northern hemisphere, meaning that they are likely constantly replenished by methane and ethane rain from clouds in that hemisphere.
What’s more, Webb’s data was also used to detect a carbon-containing molecule within Titan’s atmosphere. The molecule may provide planetary scientists with insight into the chemical processes that drive weather and other atmospheric phenomena on Titan.
Titan has been of substantial interest to planetary scientists for decades. The moon is remarkably similar to Earth, featuring a thick atmosphere with weather and a surface littered with small lakes and large seas. However, Earth and Titan differ drastically in the elemental composition of their atmospheres and surfaces. While the cycling of water drives Earth’s weather, Titan’s weather is driven by a methane cycle, meaning that Titan’s lakes, seas, and clouds are filled with methane, not water.
“Titan is the only other place in our Solar System that has weather like Earth, in the sense that it has clouds and rainfall onto a surface,” said lead author Conor Nixon of NASA’s Goddard Space Flight Center in Maryland.
Comparison of Webb and Keck’s imagery of Titan from July 2023. (Credit: NASA/ESA/CSA/STScI/Keck Observatory)
Titan was observed numerous times by Webb at L2 and the ground-based W.M. Keck telescopes in Hawaii, specifically in November 2022 and July 2023. The telescopes identified clouds in the mid and high northern latitudes of the moon and observed these clouds rising to higher atmospheric altitudes over time. While clouds have been observed on Titan before, the observations provided the first evidence for cloud convection in Titan’s northern hemisphere, which was experiencing summer at the time.
As mentioned, most of Titan’s lakes and seas are located within the moon’s northern hemisphere. The evaporation of methane from the lakes likely results in the formation of the clouds seen in the Webb and Keck observations. The subsequent precipitation of methane from the clouds replenishes the liquid methane within the bodies of water.
Nixon et al. utilized the telescopes’ different sensitivities to infrared light to determine the altitude of the clouds. Due to its lower gravity, only about 14% that of Earth’s, Titan’s lower atmospheric layers expand. For example, Titan’s troposphere, or the lowest layer of its atmosphere, extends to approximately 45 km altitude, whereas Earth’s troposphere extends to only 12 km altitude. Due to the extended depth of Titan’s atmospheric layers, the team was able to use different infrared filters on Webb and Keck to investigate the lower layers of the moon’s atmosphere and measure the specific altitudes of the clouds.
The team found that, over several days, the clouds would continue to rise in the atmosphere to higher and higher altitudes. Interestingly, though, no precipitation was observed.
“Webb’s observations were taken at the end of Titan’s northern summer, which is a season that we were unable to observe with the Cassini-Huygens mission. Together with ground-based observations, Webb is giving us precious new insights into Titan’s atmosphere that we hope to be able to investigate much closer-up in the future with a possible ESA mission to visit the Saturn system,” said co-author Thomas Cornet of the European Space Agency (ESA).
From a planetary science perspective, Titan’s similarities to Earth make the moon very intriguing. However, these similarities also greatly interest astrobiologists, who view the moon’s complex organic and carbon-containing chemistry as a potential home for some life forms. Studying Titan’s organic molecules and carbon chemistry may provide astrobiologists with insight into the processes that create and sustain life.
With Titan being a methane world, the hydrocarbon is present in the moon’s atmosphere, surface, lakes, seas, and more. Methane located within the moon’s atmosphere is split apart by sunlight (or electrons from Saturn’s magnetic field) and recombined into other molecules that create compounds like ethane and other complex carbon molecules. However, many of Titan’s chemical processes have remained a mystery to astrobiologists.
That is, until Webb’s latest observations of the moon. Webb’s observations revealed the presence of the methyl radical CH3 molecule. The molecule forms when methane is broken apart, and its detection by Webb will allow scientists to observe chemical reactions on Titan for the first time.
Webb’s imagery of Titan from July 2023. (Credit: NASA/ESA/CSA/STScI/ Keck Observatory)
“For the first time, we can see the chemical cake while it’s rising in the oven, instead of just the starting ingredients of flour and sugar, and then the final, iced cake,” said co-author Stefanie Milam of Goddard.
However, Titan’s dominant methane and hydrocarbon chemistry will have long-term effects on the moon. As discussed, methane in the atmosphere is broken apart, recombining into new compounds that ultimately reach the moon’s surface. Meanwhile, the leftover hydrogen within the atmosphere will begin to escape into space. The result of these chemical processes is that Titan’s generous supply of methane and hydrocarbons will be depleted over time. The only way this supply will not deplete is if there is a source to replenish the lost compounds.
This very process occurred on Mars long ago, when water molecules within its atmosphere were broken apart and the resulting hydrogen was ejected into space. Without a source to replenish the lost water, Mars ultimately became the barren, dry, and desert-like planet it is now.
“On Titan, methane is a consumable. It’s possible that it is being constantly resupplied and fizzing out of the crust and interior over billions of years. If not, eventually it will all be gone and Titan will become a mostly airless world of dust and dunes,” Nixon explained.
While future observations of Titan are planned with a variety of telescopes, scientists will soon be able to finally observe and study the planet up close with NASA’s upcoming Dragonfly rotorcraft mission. Dragonfly will launch in July 2028 and land on Titan in 2034 within the moon’s Shangri-La dune fields.
Nixon et al.’s study was recently published in the journal Nature Astronomy.
(Lead image: Image of Titan taken by Cassini. Credit: NASA/JPL-Caltech/SSI/CICLOPS/Kevin M. Gill)
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