Not only does Saturn have a mysterious hexagonal shape etched into the bands of cloud above its north pole, it also has a unique magnetic structure. This is suggested by recent results recorded by the NASA Cassini probe that passed over the pole to see a huge active auroral region, much larger and more dynamic than expected. Interestingly, the NASA press release has not linked the strange aurora with the long-lived hexagonal shape in the gas giant’s atmosphere. Could the hexagon be formed by a unique magnetic structure above Saturn? Or could both phenomena be connected in some other way?
Tom Stallard, a scientist working with Cassini data at the University of Leicester, describes the new Cassini infrared observations above the north pole of Saturn:
“We’ve never seen an aurora like this elsewhere. It’s not just a ring of auroras like those we’ve seen at Jupiter or Earth. This aurora covers an enormous area across the pole. Our current ideas on what forms Saturn’s aurora predict that this region should be empty, so finding such a bright aurora here is a fantastic surprise.” – Tom Stallard, NASA press release
So, it would appear we have another piece in the puzzle that is Saturn’s north pole. Recently, Cassini returned high resolution images of the magnificent pattern etched into the clouds around the pole. The (almost) perfect hexagon has been observed since 1980 when the Voyager 1 probe journeyed past the planet, and the same pattern exists 28 years later. So, we are dealing with a long-lived, stable pattern, that remains unaltered even when vast and powerful cyclones buffet its hexagonal boundary.
Last month I investigated what could be causing the pattern, and the only explanation I had was the possibility of some kind of standing wave (a Rossby-like process). As the atmosphere of Saturn is more dense than the Earth’s, perhaps a global wave could endure for decades, possibly longer. However, over at the Universe Today, some readers commented that a pattern like this had been seen before during Danish experiments on spinning fluids. Occasionally a fluid being spun inside a smooth-sided beaker will spontaneously snap into a geometric formation. An explanation for this fluid behaviour has, so far, not been found.
So we already have two possibly explanations for the near-perfect hexagon in Saturn’s atmosphere, one that dominates weather processes here on Earth (Rossby waves) and one that has been observed in the dynamics of spinning fluids in the laboratory.
Now there is another, attractive possibility.
Aurorae are generated when charged solar wind particles interact with the magnetospheres of planets. Ions are channelled to the polar regions where the magnetic field feeds down into the planet’s atmosphere. As the energetic ions interact with the atmospheric gases, light is generated. Depending on the density of the atmospheric elements, light will be emitted when the gases are excited through particle collisions. For example, in the Earth’s atmosphere green and red light in optical wavelengths can be observed when molecular oxygen is excited.
It is unclear what component of Saturn’s atmosphere is glowing in the infrared part of the spectrum, but the interesting thing is that the auroral oval appears to be located directly above the atmospheric hexagonal shape. The auroral oval above Earth’s poles sketch the location of the point at which magnetosheath plasma (i.e. the outer “edge” of the magnetosphere) can access the Earth’s upper atmosphere (the ionosphere). This is basically where energetic particles (originally from the solar wind) trapped inside the layers of the Earth’s magnetic field are “funnelled” downward via the “polar cusp”, so auroral interactions can occur (refer to the diagram above). A similar magnetic configuration is assumed for Saturn.
It seems that scientists working with the Cassini data are surprised by the size of Saturn’s auroral oval, suggesting we do not fully understand the configuration of Saturn’s magnetosphere. The infrared auroral oval is the point at which Saturn’s own polar cusp is located, magnetic fieldlines feeding into Saturn’s atmosphere, blasting solar wind ions into Saturn’s atmospheric gases. Could there be a connection between solar wind processes and the enduring “Saturn hexagon”? This doesn’t really explain why it should be hexagonal in shape, but the coincidence of the mystery auroral oval located directly over the mystery hexagon is too stark to ignore.
NASA details about the aurora image (top):
This image of the northern polar region of Saturn shows both the aurora and underlying atmosphere, seen at two different wavelengths of infrared light as captured by NASA’s Cassini spacecraft.
Energetic particles, crashing into the upper atmosphere cause the aurora, shown in blue, to glow brightly at 4 microns (six times the wavelength visible to the human eye). The image shows both a bright ring, as seen from Earth, as well as an example of bright auroral emission within the polar cap that had been undetected until the advent of Cassini. This aurora, which defies past predictions of what was expected, has been observed to grow even brighter than is shown here. Silhouetted by the glow (cast here to the color red) of the hot interior of Saturn (clearly seen at a wavelength of 5 microns, or seven times the wavelength visible to the human eye) are the clouds and haze that underlie this auroral region. For a similar view of the region beneath the aurora see http://photojournal.jpl.nasa.gov/catalog/PIA09185 .
This image is a composite captured with Cassini’s visual and infrared mapping spectrometer. The aurora image was taken in the near-infrared on Nov. 10, 2006, from a distance of 1,061,000 kilometers (659,000 miles), with a phase angle of 157 degrees and a sub-spacecraft planetocentric latitude of 52 degrees north. The image of the clouds was obtained by Cassini on June 15, 2008, from a distance of 602,000 kilometers (374,000 miles) and a sub-spacecraft planetocentric latitude of 73 degrees north.