TITAN'S WINDS SPEED UP A THIRD IN 27 DAYS

Titan, the largest moon of Saturn, has a thick atmosphere — 1.45 bar at the surface — made of about 98% nitrogen and 2% methane, plus a whole chemistry of nitriles and hydrocarbons. It turns slowly, once every 15.9 Earth days. Yet its atmosphere superrotates: winds circle the entire moon much faster than the solid body spins.

These winds were first inferred indirectly, then confirmed below 150 km by Doppler measurements from the Cassini–Huygens mission. They mix chemicals between day and night, help trap molecules in the winter polar vortex, and are thought to shape Titan’s organic dunes. But measuring them near the equator is hard: the method based on Cassini temperature maps breaks down there.

A surprise from 2016–2017

Earlier observations with ALMA, the Atacama Large Millimeter/submillimeter Array in Chile, had found something unexpected: between August 2016 and May 2017, the equatorial wind near 1,000 km altitude dropped by 47%, from 373 to 196 metres per second. Climate models of Titan, whose top stops around 500 km, do not produce such swings.

Martin Cordiner of NASA’s Goddard Space Flight Center and colleagues from Yale, Bristol, Caltech, the Space Telescope Science Institute and other institutions set out to watch a lower layer of the atmosphere over a longer time.

Reading the wind in a molecule

The team observed acetonitrile (CH₃CN), a molecule in Titan’s atmosphere, on five new dates between June 2022 and May 2023, and re-analysed two archival dates from 2016 and 2017. On one limb of the moon, the gas comes towards us; on the other, it moves away. The tiny shift in the frequency of its radio lines — the Doppler effect — gives the wind speed.

The measurement is most sensitive around 250 km altitude, over a range of 200 to 490 km — the upper stratosphere and lower mesosphere. The seven dates span almost a full Titan season, from late southern autumn to late southern winter.

Fast, and fickle

The fastest winds always sit within about 10 degrees of the equator, up to 240 ± 26 m/s in August 2016.
From 2016 to June 2022, the low-latitude wind slowed steadily by 46%, to 130 m/s.
Then it fluctuated between about 130 and 170 m/s, with jumps of roughly 40 m/s.
Between 12 April and 9 May 2023 — only 27 Earth days, less than two Titan days — it rose from 120 to 159 m/s, a 33% increase. Over the same weeks, the high-latitude winds stayed steady.

The authors call this the first detection of strong, ongoing changes in Titan’s equatorial winds on timescales of about one month or less.

In 2022–2023 the wind profile also showed two lobes: a near-equatorial band and a second band between 40 and 60 degrees south — the winter jet that models predict and that Cassini had seen.

What the models miss

Two state-of-the-art general circulation models, TitanWRF and TAM, reproduce the slow seasonal trend: the long decline, then a more moderate rise. At high latitudes, their wind speeds come within about 30% of the observations.

But the models place the fastest winds at high winter latitudes, around 40 to 50 degrees south, and their equatorial winds are up to twice too slow, the gap being largest around the solstice. Neither captures the abrupt month-scale jumps, at least in their ten-year averages. Both models give similar results even though only TAM includes methane clouds and rain, which suggests that moist processes matter little at these altitudes.

Waves in a slow-spinning sky

The authors suggest that the sudden changes come from atmospheric instabilities. In simulations, waves called Rossby–Kelvin instabilities carry angular momentum towards the equator and drive superrotation; a temporary weakening could explain a sudden slowdown. Around the solstice, the data also hint at angular momentum moving downwards, from higher to lower altitudes.

Their recommendation for modellers: raise the top of Titan models from about 500 to about 1,000 km, refine their vertical grid, and keep watching with ALMA. A strongly variable equatorial jet above a slowly turning moon is, they note, a test case for other superrotating atmospheres, in the Solar System and beyond.