Emily LakdawallaJan 09, 2014

Polar vortices across the solar system

If you live in the U.S. you've heard a lot about the "polar vortex" that's making lives miserable for nearly everybody on the continent this week. (Everybody, that is, except for southern Californians. Ahem.) I guess a comment was made somewhere that the notion of a polar vortex was a recent invention -- I did not see that original comment, but I saw the response to it by all the atmospheric scientists I follow on Twitter. I was particularly amused by a retweet of television  meteorologist Al Roker's response: he went to his bookshelf and pulled out the 1959 American Meteorological Society Glossary of Meteorology and showed that there was such a thing as a polar vortex being discussed even in those ancient times.

If you can't read the text, it says:

polar vortex -- (Also called polar cyclone, polar low, circumpolar whirl.) The large-scale cyclonic circulation in the middle and upper troposphere centered generally in the polar regions. Specifically, the vortex has two centers in the mean, one near Baffin Island and another over northeast Siberia. The associated cyclonic wind system cromprises the westerlies of middle latitudes.

Now, I am not an atmospheric scientist. But one thing I have learned well about atmospheric science is that what happens in Earth's atmosphere, usually happens in other atmospheres. Indeed, there is a polar vortex in every atmosphere -- it's just what happens when there is an atmosphere over a spinning ball. So here's a little tour of polar vortices in the solar system.

Mercury has no atmosphere -- it has an "exosphere", which essentially means that although there are molecules moving about in the space above the solid surface, there are too few of them for collisions to take place. Without collisions, you have no wind, no circulation.

Next out is Venus. Venus has wonderful polar vortices -- in fact, there is usually a dipole, a double vortex at each pole. That's what Pioneer Venus saw, and it's also what Venus Express usually sees.

Venus' south pole
Venus' south pole A view of Venus in the thermal infrared, at a wavelength of 5 microns. The brightest part of the image is the uppermost atmosphere reflecting solar radiation on the dayside of Venus. On the nightside, subtle cloud features are visible, especially near the south pole, as thermal radiation is emitted from Venus' upper atmosphere at an altitude of around 60 kilometers. This image was captured on April 12, 2006 from a distance of 210,000 kilometers.Image: ESA / VIRTIS / INAF-IASF / Obs. de Paris-LESIA

Sometimes, though, the vortex changes shape, becoming a single vortex, or other, more complex shapes. I wrote about this variety in 2010; here's one movie of Venus' polar vortex.

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Venus' southern vortex A movie of the complicated swirling motions of Venus' south polar vortex as seen at a near-infrared wavelength of 3.8 microns, acquired by the Venus Express VIRTIS camera. The set of images show the temperature of the clouds' top. Darker regions correspond to higher temperature and thus lower altitude. The temperature contrast leads to an apparent three-dimensional effect of the images.Video: ESA / VIRTIS / INAF-IASF / Obs. de Paris-LESIA

The Moon, like Mercury, only has an exosphere. So the next circulating-air-world is Mars. Mars has a polar vortex, too. Here's one photo from Mars Express' VMC.

Polar vortex on Mars from Mars Express VMC
Polar vortex on Mars from Mars Express VMC Vortex clouds – cloud systems being blown in a circle around the poles - have long been known on planets of the solar system, including Earth and Mars. Venus Express has confirmed a double vortex formation at Venus. This VMC images shows the circular pattern of clouds circling the Martian North Pole (which is just in the darkness). This gives a good impression of how powerful this vortex is.Image: ESA

That's the best photo I could find in searching around the Internet, but I knew that there have been cameras at Mars to monitor its weather for decades, the wide-angle camera on Mars Global Surveyor and its successor, MARCI on Mars Reconnaissance Orbiter. But those data sets are not easy to use. Thankfully, MARCI scientist Bruce Cantor was able to help me out with this lovely image from MARCI of the Martian polar vortex. He was careful to point out that the vortex is not circular; it's oval-shaped. Just like on Venus!

Mars polar vortex from MARCI
Mars polar vortex from MARCI This is a MARCI color mosaic of the north polar region of Mars from 40-90 degrees North, 0-360 degrees West, generated from data taken on October 22, 2012 (Ls = 192.8), early-autumn in the northern hemisphere. The black star shape in the center is the boundary of the terminator ("polar night"). The whitish areas are water-ice clouds. The diffuse ice clouds in the north polar region are part of the north polar hood that is a steady feature at this time of the Martian year. The polar hood has an egg-shaped structure. The clouds of the hood help denote the southern boundary of the polar vortex, which extends as far south as 49 degrees latitude.Image: NASA / JPL / MSSS

The oval shape of Mars' north polar vortex is referred to as a "wave-2 type" structure. What does that mean? If you projected the photo I show above in a cylindrical projection, the north polar vortex would look like a wavy horizontal band, with two peaks and two valleys.

Moving outward, none of the asteroids retains an atmosphere, so next we go to the outer planets. I'm going to let just one planet stand in for all of them -- Saturn. I wrote at length about the cool shape of the north polar atmospheric circulation. Saturn's north polar vortex doesn't have a wave-2 type structure; it has a wave-6 type structure. That makes it look like a hexagon. Here's another old post, on how other integer wavenumber vortical structures have been made in the laboratory. But Cassini has recently revealed the action at the center of that hexagon -- a wonderful swirl-upon-swirl of storms.

Saturn's north polar vortex (an animation)
Saturn's north polar vortex (an animation) Cassini took 14 images of Saturn's north polar vortex on 27 November 2012 over a period of many hours as the planet rotated beneath it. The 14 images have been processed to remove the geometric effects of Cassini's oblique viewpoint and of Saturn's rotation, holding the outer bright ring of white clouds fixed. With these motions removed, you can see individual vortices rotating and shearing, and the central clouds rotating faster than the outer ones.Image: NASA / JPL-Caltech / SSI / Kevin McAbee

And finally, there's Titan, which has developed a very interesting-looking south-polar vortex since the passage through equinox. Here's an animation that I hadn't seen before. In it, you can see that Titan's polar vortex has the same oval (wave-2) shape that Mars' does, except that it's much smaller. I'm really surprised by its fast motion; remember that Titan rotates at the same speed that it orbits Saturn, once every 16 days. The atmosphere moves much faster than the moon itself does.

Titan's south polar vortex in motion
Titan's south polar vortex in motion Cassini took 13 snapshots of Titan every 2 hours between 15 December 2012 20:52 UTC and 16 December 2012-12-16 UTC to catch its south polar vortex in motion. The view is looking primarily at the moon's south pole.Image: NASA / JPL-Caltech / SSI / Gordan Ugarkovic

Where else are there atmospheres in the solar system, apart from the other three giant planets? One notable type of atmosphere-bearing world I haven't mentioned yet lies in the Kuiper belt. Pluto has an atmosphere, and so does Neptune's moon Triton, for that matter. Will New Horizons see a polar vortex when it passes by next year? Maybe! I'm looking forward to finding out.

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