• Saturn's north polar vortex.

    Saturn's north polar vortex.

    Image courtesy of Caltech/Space Science Institute

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Small thunderstorms may add up to massive cyclones on Saturn

Saturn's north polar vortex and rings.

New model may predict cyclone activity on other planets.


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For 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.


Topics: Astrophysics, Climate, Earth and atmospheric sciences, Environment, Physics, Research, space, Space, astronomy and planetary science, School of Science, NASA

Comments

"A Theory of the Relativistic Fermionic
Spinrevorbital" as published at
website: http://www.academicjournals.or...
gives a prior theory of the dynamics determined by this model whereby
many smaller thunderstorms of weaker internal momenta (reactants) undergo
transformations forming a more powerful cyclone of stronger momenta (product).
For free copy click "full text PDF". On page 2 the Laws
of Ferrochemistry are given. The "Second Law of Ferrochemistry
involves Little Rule 1 and involves that the coupling relationships between
systems of physicochemical reactions and itself internally and/or the minimum
needed external magnetic field and/or external spin0 revolution-orbital
(spinrevorbital) matter, energy, momentum, density and/or acceleration to alter
dynamics and kinetics of the system of physicochemical reaction are such that
the greater the energy of the physicochemical reactants in space time then the
easier and inherent the internal coupling of the spinrevoritals of the multiple
reactants and/or the smaller the minimum needed external magnetic field and/or
spinrevorbitals' energy and momentum density in surrounding space time to
couple with the physicochemical reactions and alter the course (dynamics) and
rates (kinetics) of the physicochemical reactions." By this Rule 1
[of Law 2] the stronger thunderstorms in Saturn can couple to weaker rotation
field even Saturn's rotation about its axis. The Earth's thunderstorms
are weaker and couple less so relative to Saturns' thunderstorms. Thereby the
thunderstorms on earth do not separate their top and bottoms to the same extent
for bottom to go to poles and top of storm to migrate to equator as they do on
Saturn. But such on earth is seen as the positive part (top) of thunderstorm
often lags the bottom negative biased part of thunderstorms on earth!
Moreover I give the Law for why the large cyclone forms on Saturn but not
on Neptune or Jupiter as in Little Rule 2 for the Third Law of Ferrochemistry
involving Little Rule 2. "The Third Law of Ferrochemistry involves
Little Rule 2 (2000) and considers that for systems of small particle densities
and high internal magnetic fields in the presence (internally or externally) of
sufficiently strong magnetic field and/or sufficiently large energy spin
revolutionary orbital (spinrevorbital) energy, matter, momenta, density,
acceleration and momenta beyond the coupling strength by Law 2 then the
physicochemical reaction dynamics is either altered such that the spinrevorbital
momenta of the products are larger than spinrevorbital momenta of reactants in
the slow rotational limit of the activating conditions or the physicochemical
reaction dynamics is altered such that the spinrevorbital momenta of the
products are smaller than the spinrevorbital momenta of the reactants in the
fast rotational limit of the activating conditions. " By this Third
Law for Little Rule 2 the thunderstorms of Saturn (reactants) have smaller
momenta and transform to the polar cyclone (product) of larger momentum in the
slow rotational limit of Saturn's relatively slow planetary rotation! The
Laws of Ferrochemistry and the Little Rules as outlined here by RBL give a
solid foundation for these simulations of Saturn! I will not state Law 4
of Little Rule 3 but it applies more to conditions on the earth and it gives a
basis for why the thunderstorms on earth do not separate and form stronger
polar vortex at the poles of the earth. During the winter the drop in
temperatures may allow transient vortex to form near the earth's poles which
rapidly reverses to drift toward the equator of the earth to explain the recent
polar vortex and extreme winter cold in USA during winter of 2013 relative to a
summer thunderstorm of higher temperature splitting to have its bottom drift
north to the north pole! But back to the article In the next paragraph I
even give the boundary conditions for the Rule 2 as applies to Saturn (with
identical to the size and energy restriction given by the MIT researchers).
I state "systems of fewer atoms tend to behave by Rule 2 over long
time and they behave by Rule 3 over shorter times. Systems of larger
energy tend to behave by Rule 2 over longer times and they behave by Rule 3
over shorter times." "Rule 2 applies to parts of larger systems,
higher energies and orbitals in smaller systems." Such parts of
larger systems, higher energies (for larger thunderstorm size to planetary
size) is identical to the conditions of large size of the storm relative to
size of the planet and higher energy of the storm for Rule 2 on Saturn.
But the weaker energy storms on Jupiter of smaller size relative to the
whole planet Jupiter more appropriately are describe by Rule 3 which
"applies to while structures , lower energies and orbits! This is
very powerful and beautiful, broad, deep and consistent. Although in the
paper I do not directly point and illustrate these laws to Saturn and Jupiter.
I do allude to novel magnetogravity driven chemistry by such effects in
Saturn and Jupiter on page 14. On page 14 by these dynamics I describe
the atmospheres of Saturn and Jupiter as liquid crystalline! In the whole
frame of this paper I discover a way to unify gravity and magnetism and general
theory and quantum mechanics. By my unification the liquid crystallinity
of the atmosphere of Saturn emerges. I note the high pressures and low
temperatures involve a transformation of heat to pressures to electric to
gravity in the Saturn and Jupiter for a strong gravitational force with
magnetism to bind atoms! I note here that such magneto-gravitational
chemical bonding in Saturn manifest the crystallinity of its atmosphere as the
hexagonal shape of its polar cyclone! This is very powerful and beautiful
and I thank GOD for the vision! RBL

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