Huge polar hurricanes on Saturn are a puzzling phenomenon, since the gaseous planet lacks an essential ingredient for brewing up such hurricanes – bodies of water on its surface. According to new research led by Dr Morgan O’Neill from Massachusetts Institute of Technology, the answer may be something called ‘beta drift’ – a phenomenon by which a planet’s spin causes small thunderstorms to drift toward the poles.
Cassini spacecraft has recently provided scientists the first close-up, visible-light views of a behemoth hurricane swirling around Saturn’s north pole. The hurricane’s eye is about 1,250 miles (2,000 kilometers) wide, 20 times larger than the average hurricane eye on Earth. Image credit: NASA / JPL-Caltech / SSI.
“Before it was observed, we never considered the possibility of a cyclone on a pole. Only recently did Cassini give us this huge wealth of observations that made it possible, and only recently have we had to think about why polar cyclones occur,” Dr O’Neill said.
In their study, Dr O’Neill and co-authors developed a model of Saturn’s atmosphere, and simulated the effect of multiple small thunderstorms forming across the planet over time.
Eventually, they observed that multiple thunderstorms experienced beta drift over time, and eventually accumulated enough atmospheric circulation to generate a much larger and long-lived hurricane at the poles.
“Each of these storms is beta-drifting a little bit before they sputter out and die. This mechanism means that little thunderstorms over a long period of time can actually accumulate so much angular momentum right on the pole, that you get a permanent, wildly strong cyclone,” Dr O’Neill said.
The scientists found that whether a cyclone develops depends on two parameters: the size of the planet relative to the size of an average thunderstorm on it, and how much storm-induced energy is in its atmosphere.
Given these two parameters, they predicted that Neptune, which bears similar polar hotspots, should generate transient polar cyclones that come and go, while Jupiter should have none.
“Saturn has an intense cyclone at each pole. The model successfully accounts for that. Jupiter doesn’t seem to have polar cyclones like Saturn’s, but Jupiter isn’t tipped over as much as Saturn, so we don’t get a good view of the poles. Thus the apparent absence of polar cyclones on Jupiter is still a mystery,” said Prof Andrew Ingersoll of the California Institute of Technology, who was not involved in the study.
The team’s model, described in the journal Nature Geoscience, may eventually be used to gauge atmospheric conditions on extrasolar planets.
For instance, if scientists detect a cyclone-like hotspot on a distant exoplanet, they may be able to estimate storm activity and general atmospheric conditions across the entire planet.
_____
Morgan E O’Neill et al. Polar vortex formation in giant-planet atmospheres due to moist convection. Nature Geoscience, published online June 15, 2015; doi: 10.1038/ngeo2459
