Evidence Of Clouds Bubbling Up Over Titan’s Northern Hemisphere
Maunakea, Hawaiʻi – Astronomers using W. M. Keck Observatory on Maunakea, Hawaiʻi Island have, for the first time, found evidence for cloud convection in the northern hemisphere of Titan, Saturn’s moon.
Titan is an intriguing world cloaked in a yellowish, smoggy haze. Similar to Earth, the atmosphere is mostly nitrogen and has weather, including clouds and rain. Unlike Earth, whose weather is driven by evaporating and condensing water, frigid Titan has a methane cycle.
“With Keck Observatory’s excellent imaging capability, we were able to see methane clouds evolving and changing close to Titan’s north pole over multiple days, in the region where large seas and lakes of methane were discovered by the Cassini spacecraft,” said Conor Nixon, research scientist with NASA Goddard Space Flight Center and lead author of the study. “This enables us to better understand Titan’s climate cycle, how the methane clouds may generate rain and replenish methane evaporated from the lakes.”
From the Earth’s surface, observations of astronomical objects are blocked at some wavelengths by gases in our atmosphere. However, due to the high altitude and relatively stable atmosphere of Maunakea along with adaptive optics on the advanced camera NIRC2 (Near-infrared Camera, Second Generation), the team was able to conduct monitoring of Titan’s clouds to produce highly detailed images. The results were published today in the journal Nature Astronomy.
“Since the different filters on the Keck NIRC 2 camera see to different depths in Titan’s atmosphere, we were able to see on the last night of observing (July 14th) that the clouds had moved upwards in altitude, like a convective cell on the Earth,” explained Nixon.
Time series of JWST NIRCam and Keck II NIRC2 Titan cloud observations for June and July 2023. The content of each row and the image processing follow that of Fig. 4a. Note that the NIRC2 Br γ image was not available for the 13 July 2023 observation. See Fig. 6 for the altitudes probed in the different filters — Nature
The team observed Titan in November 2022 and July 2023 using both Keck Observatory and the James Webb Space Telescope. Those observations not only showed clouds in the mid and high northern latitudes on Titan — the hemisphere where it is currently summer — but also showed those clouds apparently rising to higher altitudes over time. While previous studies have observed cloud convection at southern latitudes, this is the first time evidence for such convection has been seen in the north. This is significant because most of Titan’s lakes and seas are located in its northern hemisphere and evaporation from lakes is a major potential methane source. Their total area is similar to that of the Great Lakes in North America.
On Earth the lowest layer of the atmosphere, or troposphere, extends up to an altitude of about 7 miles (12 kilometers). However, on Titan, whose lower gravity allows the atmospheric layers to expand, the troposphere extends up to about 27 miles (45 kilometers). Keck Observatory and Webb used different infrared filters to probe to different depths in Titan’s atmosphere, allowing astronomers to estimate the altitudes of the clouds. The science team observed clouds that appeared to move to higher altitudes over a period of days, although they were not able to directly see any precipitation occurring.
The Twilight Zone program at Keck Observatory was key in enabling Nixon and his team to monitor how the weather on Titan changes over time. This program is run jointly by several University of California, California Institute of Technology and NASA researchers in collaboration with the Keck Observatory to observe several bright targets during periods when the sky is too bright for standard astronomical observations.
Bands of CO2, CH3D, C2H6 and CO resolved by JWST NIRSpec. a, Average of 36 NIRSpec spectra of Titan centred at 14° N in the 4.20–5.00 μm spectral region (red line). The spectrum is compared with a synthetic spectrum including reflected solar radiation, thermal emission and non-LTE emission from CO2 (blue dashed line) and with a synthetic spectrum without the thermal emission or the CO2 non-LTE emission (green dashed line). CH3D emission and absorption from the 2ν6 band near 4.32 μm almost cancel each other. The vertical dotted lines indicate the location of the individual CO2 lines. b, The observed spectrum (red line) compared with a synthetic spectrum including reflected solar radiation, thermal emission and non-LTE emission from CO only (blue dashed line). c, Contributions of the CO fundamental, first hot, isotopic 13CO(1 → 0) and C18O(1 → 0), and second hot bands to the CO emission. — Nature
Titan’s Weather
On Titan, methane plays a similar role to water on Earth when it comes to weather. It evaporates from the surface and rises into the atmosphere, where it condenses to form methane clouds. Occasionally it falls as a chilly, oily rain onto a solid surface where water ice is hard as rocks.
“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,” explained Nixon.
Titan is an object of high astrobiological interest due to its complex organic (carbon-containing) chemistry. Organic molecules form the basis of all life on Earth, and studying them on a world like Titan may help scientists understand the processes that led to the origin of life on Earth.
Titan’s Chemistry
The basic ingredient that drives much of Titan’s chemistry is methane, or CH4. Methane in Titan’s atmosphere gets split apart by sunlight or energetic electrons from Saturn’s magnetosphere, and then recombines with other molecules to make substances like ethane (C2H6) along with more complex carbon-bearing molecules.
Webb’s data provided a key missing piece for the understanding of the chemical processes: a definitive detection of the methyl radical CH3. This molecule (called “radical” because it has a “free” electron that is not in a chemical bond) forms when methane is broken apart. Detecting this substance means that scientists can see chemistry in action on Titan for the first time, rather than just the starting ingredients and the end products.
The Future of Titan’s Atmosphere
This hydrocarbon chemistry has long-term implications for the future of Titan. When methane is broken apart in the upper atmosphere, some of it recombines to make other molecules that eventually end up on Titan’s surface in one chemical form or another, while some hydrogen escapes from the atmosphere. As a result, methane will be depleted over time, unless there is some source to replenish it.
A similar process occurred on Mars, where water molecules were broken up and the resulting hydrogen lost to space. The result was the dry, desert planet we see today.
“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,” said Nixon. “We will be excited to follow up on these observations in coming months and years to see how the weather patterns change, especially in the period after the equinox in May 2025 when dramatic changes are predicted.”
Paper: https://www.nature.com/articles/s41550-025-02537-3
Astrobiology, Astrochemistry,
