Saturn’s giant storms leave footprints of ammonia in the lower atmosphere that persist for centuries after the storm blew itself out.
Every decade or two, Saturn generates a truly formidable storm. Winds roar at up to a thousand miles an hour, storm cloud trails wrap around the whole planet, and hail made of ammonia shakes downward. The storm finally abated after more than six months, but its impact on Saturn’s atmosphere will last much longer, according to a recent study by a University of Michigan planetary scientist. Cheng Li and colleagues. They recently discovered that Saturn’s giant storms carry ammonia vapor deep into the planet’s atmosphere, where it can linger for centuries, like a fingerprint marking the passage of a storm.
Lee and colleagues published their research in the journal Science advances.
How giant storms leave footprints in the sky
Saturn’s upper cloud consists mostly of ammonia (a nitrogen atom bonded to three hydrogen atoms), floating in an atmosphere of hydrogen. At deeper, warmer altitudes, the clouds are mostly water. But in data from the Very Large Array radio telescope, Li and his colleagues detected radio emissions from ammonia patches trapped in the lower layers of Saturn’s atmosphere, where they normally shouldn’t be.
Ammonia was out of place drifting at 43 degrees north latitude, where NASA’s Cassini spacecraft (RIP) saw a giant, planet-spanning storm in 2010 (Saturn’s winds blow mostly east and west, which is why giant storms tend to be long and narrow—and why strange features in the atmosphere tend to stay at the same latitude that spawned them).
The storm lasted just over six months, and Lee and his colleagues say it carried ammonia from high-altitude clouds into the lower layers of the atmosphere — where it’s now trapped.
Using VLA data, Lee and his colleagues discovered several other swaths of misplaced ammonia in Saturn’s lower atmosphere. It mostly coincided with the latitudes of the five other massive storms that astronomers had observed orbiting Saturn since 1876. This means that some of the ammonia clouds had been trapped in Saturn’s lower atmosphere for nearly 120 years.
One speck of ammonia that Lee and his colleagues spotted doesn’t match any of the storms on record, so it may be more than 150 years old: a cloudy imprint of a storm never seen by human eyes.
Mushballs are the strangest and most amazing precipitation
Part of the explanation may be what Lee and his colleagues call pancake balls: little balls of snow with a liquid mixture of ammonia and water inside them (like Gushers, except for the flavors and poison, too). Balls of fungus fall from storm clouds into the upper echelons of Saturn’s superstorms, carrying ammonia deeper into the atmosphere.
In the aftermath of a storm, the upper atmosphere remains warm for a while; It’s dry too, after I dropped a lot of ammonia in my volley of mushroom balls. For a while, the upper layers of the atmosphere are much warmer than the layers below, and this warm, dry air acts as a blanket, keeping the ammonia clouds trapped in the layer below.
Even a storm as massive and powerful as Saturn’s nodal storms cannot leave footprints in clouds forever. Eventually, the turbulence of Saturn’s atmosphere will cause the storm’s hot air to mix with cooler air from other latitudes, and the trapping banks of ammonia clouds will be free to rise. Finally another storm will come.
Where do we go from here?
When Lee and his colleagues studied the tracts of trapped ammonia left behind by giant storms on Saturn, they noticed that each “footprint” tended to split in two. One patch of ammonia is drifting north from the storm’s latitude, while the other is drifting south. This is one of the things they want to study in more detail with future observations.
It’s hard to predict the weather here on Earth, let alone on an alien world as turbulent as Saturn, but history suggests another giant storm should break out and wrap around the planet in the next 10 to 20 years. When that happens, Li and his colleagues hope astronomers and planetary scientists will study how the storm develops and what happens to Saturn’s atmosphere in its aftermath.
Meanwhile, the team plans to look at Saturn’s southern hemisphere in 2025. The VLA data that Li and colleagues used in their latest study only cover the planet’s northern hemisphere, because right now, the tilt of Saturn’s axis means the southern hemisphere is terribly hidden. Behind the planet’s iconic rings. By 2025, Saturn will be at a different point in its orbit, and telescopes on Earth will be able to see the southern hemisphere.
All of the giant storms astronomers have observed so far have occurred in the planet’s northern hemisphere, so Lee and his colleagues expect that if they’re right, they shouldn’t see any trapped patches of ammonia in the south.
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