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Why Saturn's hexagonal pole is now changing color

A new model suggests that Saturn's mysterious polar formation is responding to the slowly changing seasons of the planet's 29-year orbit.

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These two natural color images from NASA's Cassini spacecraft show the changing appearance of Saturn's north pole region between 2012 (above) and 2016 (below).

Space Science Institute/Hampton University/JPL-Caltech/NASA

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The mystery of Saturn's hexagonal storm may finally be solved, thanks to scientists with NASA's Cassini mission.

Since its arrival in 2004, Cassini has gathered data on the strange cloud formation at Saturn's north pole several times, and captured a new twist last month: the usually bluish formation appears to be changing color. A side-by-side comparison of the recent photo and one taken in November 2012 shows that the formation has lightened considerably and now looks more golden.

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The hexagon was first spotted during the Voyager flybys of 1980 and 1981, but since those images were taken at a distance, the storm's existence was not confirmed until the Cassini mission. The cloud formation is unique in the known solar system, and has defied scientists' efforts to fully explain it.

In September of last year, more than three decades after the polar formation's discovery, a group of scientists developed a computer simulation that they say explains the phenomenon better than previous attempts.

"With a very simple model, we have been able to match many of the observed properties of the hexagon," study lead author Raúl Morales-Juberías, a planetary scientist at the New Mexico Institute of Mining and Technology, told Space.com at the time. They modeled an eastward jet in the planet's atmosphere flowing in a curve around Saturn's north pole. Small interactions with other outside currents at cloud level force the clouds into its distinctive shape, with winds below the cloud level keeping the corner of the hexagon sharp. Unlike previous models, Dr. Morales-Juberías's model came close to matching the measured rotation speed of the formation.

Because the flow of gases on a planet's surface are subject to temperature changes, scientists think that the recent color change in the hexagon is related to the changing of the seasons. Since a year on Saturn is equivalent to about 29 Earth years, the slow seasonal change is harder to observe than on planets closer to the sun.

"The hexagon is just a current of air, and weather features out there that share similarities to this are notoriously turbulent and unstable," said Andrew Ingersoll, a Cassini imaging team member at the California Institute of Technology in Pasadena, in 2013. "A hurricane on Earth typically lasts a week, but this has been here for decades – and who knows – maybe centuries."

NASA scientists say the jet stream at the edge of the hexagon may act as a barrier between the interior of the pole and the rest of the planet's atmosphere. As a result, aerosol haze particles, created by reactions between sunlight and atmospheric chemicals, were not produced as readily near Saturn's north pole last winter, which lasted from November 1995 to August 2009. While some of the planet got enough sunlight to create aerosols and spread them over the surface, the hexagonal barrier kept the north pole free of those aerosols while not receiving enough direct sunlight to produce any haze of its own.

Now that Saturn's summer is heating up, scientists hypothesize, the pole is finally receiving enough light to begin producing aerosols again, causing the color change. Atmospheric shifts related to seasonal change might also help deliver aerosols to the pole, as the planet's summer solstice of May 2017 fast approaches.

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The scale of Saturn's hexagon can boggle Earth-based observers. Each of the six sides is about 9,000 miles long, bigger than Earth's diameter.