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New model may predict cyclone activity on other planets.
the last decade, astronomers have observed curious “hotspots” on
Saturn’s poles. In 2008, NASA’s Cassini spacecraft beamed back close-up
images of these hotspots, revealing them to be immense cyclones, each as
wide as the Earth. Scientists estimate that Saturn’s cyclones may whip
up 300 mph winds, and likely have been churning for years.
While cyclones on Earth are fueled by the heat and moisture of the
oceans, no such bodies of water exist on Saturn. What, then, could be
causing such powerful, long-lasting storms?
In a paper published today in the journal Nature Geoscience,
atmospheric scientists at MIT propose a possible mechanism for Saturn’s
polar cyclones: Over time, small, short-lived thunderstorms across the
planet may build up angular momentum, or spin, within the atmosphere —
ultimately stirring up a massive and long-lasting vortex at the poles.
The researchers developed a simple model of Saturn’s atmosphere, and
simulated the effect of multiple small thunderstorms forming across the
planet over time. Eventually, they observed that each thunderstorm
essentially pulls air towards the poles — and together, these many
small, isolated thunderstorms can accumulate enough atmospheric energy
at the poles to generate a much larger and long-lived cyclone.
The team 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, the researchers predicted that
Neptune, which bears similar polar hotspots, should generate transient
polar cyclones that come and go, while Jupiter should have none.
Morgan O’Neill, the paper’s lead author and a former PhD student in
MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS),
says the team’s model may eventually be used to gauge atmospheric
conditions on planets outside the solar system. For instance, if
scientists detect a cyclone-like hotspot on a far-off exoplanet, they
may be able to estimate storm activity and general atmospheric
conditions across the entire planet.
“Before it was observed, we never considered the possibility of a
cyclone on a pole,” says O’Neill, who is now a postdoc at the Weizmann
Institute of Science in Israel.
“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.”
O’Neill’s co-authors are Kerry Emanuel, the Cecil and Ida Green
Professor of Earth, Atmospheric and Planetary Sciences, and Glenn
Flierl, a professor of oceanography in EAPS.
Beta-drifting toward a cyclone
Polar cyclones on Saturn are a puzzling phenomenon, since the planet,
known as a gas giant, lacks an essential ingredient for brewing up such
storms: water on its surface.
“There’s no surface at all — it just gets denser as you get deeper,”
O’Neill says. “If you lack choppy waters or a frictional surface that
allows wind to converge, which is how hurricanes form on Earth, how can
you possibly get something that looks similar on a gas giant?”
The answer, she found, may be something called “beta drift” — a
phenomenon by which a planet’s spin causes small thunderstorms to drift
toward the poles. Beta drift drives the motion of hurricanes on Earth,
without requiring the presence of water. When a storm forms, it spins in
one direction at the surface, and the opposite direction toward the
upper atmosphere, creating a “dipole of vorticity.” (In fact, videos of
hurricanes taken from space actually depict the storm’s spin as opposite
to what’s observed on the ground.)
“The whole atmosphere is kind of being dragged by the planet as the
planet rotates, so all this air has some ambient angular momentum,”
O’Neill explains. “If you converge a bunch of that air at the base of a
thunderstorm, you’re going to get a small cyclone.”
The combination of a planet’s rotation and a circulating storm
generates secondary features called beta gyres that wrap around a storm
and essentially split its dipole in half, tugging the top half toward
the equator, and the bottom half toward the pole.
The team developed a model of Saturn’s atmosphere and ran hundreds of
simulations for hundreds of days each, allowing small thunderstorms to
pop up across the planet. The researchers observed that multiple
thunderstorms experienced beta drift over time, and eventually
accumulated enough atmospheric circulation to create a much larger
cyclone at the poles.
“Each of these storms is beta-drifting a little bit before they
sputter out and die,” O’Neill says. “This mechanism means that little
thunderstorms — fast, abundant, but not very strong 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.”
Next stop: Jupiter
The team also explored conditions in which planets would not form
polar cyclones, even though they may experience thunderstorms. The
researchers found that whether a polar cyclone forms depends on two
parameters: the energy within a planet’s atmosphere, or the total
intensity of its thunderstorms; and the average size of its
thunderstorms, relative to the size of the planet itself. Specifically,
the larger an average thunderstorm compared to a planet’s size, the more
likely a polar cyclone is to develop.
O’Neill applied this relationship to Saturn, Jupiter, and Neptune. In
the case of Saturn, the planet’s atmospheric conditions and storm
activity are within the range that would generate a large polar cyclone.
In contrast, Jupiter is unlikely to host any polar cyclones, as the
ratio of any storm to its overall size would be extremely small. The
dimensions of Neptune suggest that polar cyclones may exist there,
albeit on a fleeting basis.
“Saturn has an intense cyclone at each pole,” says Andrew Ingersoll,
professor of planetary science at Caltech, who was not involved in the
study. “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.”
The researchers are eager to see whether their predictions,
particularly for Jupiter, bear out. Next summer, NASA’s Juno spacecraft
is scheduled to enter into an orbit around Jupiter, kicking off a
one-year mission to map and explore Jupiter’s atmosphere.
“If what we know about Jupiter currently is correct, we predict that
we won’t see these wildly strong cyclones,” O’Neill says. “We’ll find
out next year if our predictions are true.”
This research was funded in part by the National Science Foundation.
Jennifer Chu | MIT News Office
Abby Abazorius Email:email@example.com Phone: 617-253-2709 MIT News Office