Astrophile: Icy Titan spawns tropical cyclones

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: Mini-hurricanes of methane rain
Location: North Pole of Saturn's moon Titan

With a maximum surface temperature of -180??C, Saturn's icy moon Titan is no tropical paradise, at least by earthly standards. But it may still have tropical cyclones, and at what sounds like the unlikeliest of places ? near its north pole.

These mini-hurricanes have never been observed anywhere but Earth. If they exist on Titan, that would add to a growing list of features that the distant moon shares with our planet, from lakes, hills, caves and sand dunes to fog, mist, smoggy haze and rain clouds.

Though cyclones - a large family of storms in which winds spiral inward to a low-pressure zone, such as the eye of a hurricane or tornado ? have been glimpsed on Mars and Saturn, a tropical cyclone is a special case that is driven by the heat of evaporation from a warm sea. These storms involve a lot of rain as well as gale-force winds, and have not been glimpsed anywhere but Earth.

As Titan is the only body in the solar system apart from Earth known to have liquid on its surface and, therefore, rain (Titan is so cold that its rain is in the form of liquid methane, not water), Tetsuya Tokano of the University of Cologne in Germany decided to calculate what it would take for Titan to have its own mini-hurricanes.

Methane seas

The first thing that would be required, he says, is the right blend of hydrocarbons in the moon's lakes and seas. "We know ethane is present, and methane probably is," he says. The methane is crucial because it evaporates much more readily, and could deliver the heat needed to drive the storm.

Assuming the methane fraction is large enough, Tokano calculated the heat it would carry and how that would be converted into kinetic energy to power a storm. He reckons that the resulting storm would not be as powerful as hurricanes or typhoons on Earth, but that they could produce surface winds of up to 20 metres per second (72 kilometres per hour). That's 10 times the average wind velocity on Titan: on Earth, it's equivalent to the wind speeds of a midsize tropical storm ? and two-thirds those needed for a full-scale hurricane.

Tokano also looked at where these could storms could form ? and discovered that the 1200-kilometer-long Kraken Mare, and the smaller Ligeia and Punga mares, are the only seas on Titan large enough to support the growth of a tropical cyclone. All three are situated near Titan's North Pole, making a contrast to the tropical cyclones on Earth.

As on Earth, however, any mini-hurricanes on Titan would be seasonal. Tokano says the storms could form in Titan's northern summer, lasting up to 10 days and reaching hundreds of kilometres in diameter, limited by the size of the lakes.

Spectacular storm

It's now spring on northern Titan, and solar warming of the north pole should make the storms possible from 2015 to 2021. (Because Titan is so much further away from the sun than Earth, its year ? and therefore its seasons - are much, much longer.)

That means that when mini-hurricane season next returns to Titan, the Cassini spacecraft, which started orbiting it in 2004, will still be watching. The craft's orbit gives it a better view of Titan's poles than terrestrial telescopes, and its mission is scheduled to continue until 2017.

"It would be spectacular to see this kind of storm over Kraken Mare," says Elizabeth Turtle of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. "This gives us a specific type of storm to look for."

Failure to spot a storm during this period would not tell us much, however, says Tokano, as any of a number of factors might cause Cassini to miss it, or it might just be a slow season.

Because of its similarities to Earth, Titan looks like a good place to hunt for extra-terrestrial life. Though Tokano wouldn't be drawn on how tropical cyclones might feed into this picture, one thing is clear: the frigid moon is certainly living up to its reputation as one of the most intriguing places in our solar system.

Journal reference: Icarus, doi.org/kkx

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