Uncovering the Mysteries of Saturn

As the spectacularly successful Cassini mission nears the end of its 12-year exploration of Saturn, strange and wondrous discoveries continue to be made. Some of them are truly breathtaking – and unique to the awe-inspiring gas giant.

The final phase of the spacecraft’s sojourn has been dubbed the Cassini Solstice mission, which is focused on uncovering more information about Saturn’s complex and mysterious atmosphere. Scientists are especially keen to gain a better understanding of the variations in atmospheric conditions that cause the bands and striations they observe.

NASA’s Cassini spacecraft captured this view of Saturn on July 16, 2016, from a distance of about 1 million miles. (Image credit: NASA/JPL-Caltech/Space Science Institute)

One area of particular interest to the Cassini team is the singular formation at the northern pole of the planet. This cosmic marvel, unique in the Solar System, is a beautifully sculpted, hexagonally shaped polar cap. Since it was first glimpsed in Voyager fly-bys in 1980 and 1981, scientists have wondered exactly what it was and what function it serves.

A 35+-Year Storm

Not until Cassini was able to transmit close-up views of the hexagon were scientists able to theorize how it was formed. Apparently what we are observing is an almost unbelievably huge planetary storm, one that has been going on for at least 35 years, and possibly much longer.

The dimensions of this storm are truly mind-boggling:

  • The hexagon is approximately 20,000 miles wide.
  • Each side of the hexagon is about 9,000 miles long – larger than the diameter of Earth (nearly 8,000 miles).
  • It has a total surface area of 192 million square miles, almost the same size as Earth (197 million square miles).
  • Thermal images show that it reaches about 60 miles down into Saturn’s atmosphere.
  • The storm’s jet stream flows east at a speed of approximately 220 mph.

What makes it hexagonal? Scientists have attempted to recreate the unusual storm pattern in a laboratory setting, and the most successful replication indicates that large differences in the speed of Saturn’s winds are responsible. Scientific modeling showed that even small variations in the wind speed or direction would create this striking six-sided shape.

These images, taken by Cassini's wide-angle camera, show the changing appearance of Saturn’s north polar region. (Credit: NASA/JPL-Caltech/Space Science Institute/Hampton University)

Blue Turns to Gold

One amazing feature of Saturn’s hexagonal storm is its changing color. Comparing images of the storm taken in 2012 to those snapped in July 2016, the difference is startling: The storm area, which was previously a hazy blue, is now golden brown.

What can account for this difference? There are several theories, but the most likely cause is seasonal change. Like Earth, Saturn is tilted on its axis, resulting in four seasons that each last about seven years. And, as it happens, the planet’s summer solstice is right around the corner – May 2017.

Saturn’s rings cast dark shadows on the planet as the winter season approaches. (Image credit: NASA/JPL-Caltech/Space Science Institute)

The changing colors appear to be the result of Saturn’s northern pole getting more direct exposure to the Sun. When there was less direct sunlight (in autumn and winter), the photochemicals in its atmosphere were less likely to cross the threshold of the storm’s jetstream. Now that there is more exposure to sunlight, there are more photochemicals in the storm, and they turn brown and gold in the atmosphere.

Eye in the Sky

Our knowledge of the Saturnian system has been advanced immeasurably by the Cassini mission. As it nears its conclusion, scientists recognize the critical imperative to avoid the contamination of the potentially habitable moons. Consequently, mission controllers have devised a fitting “grand finale” for Cassini.

Artist’s concept of NASA’s Cassini spacecraft at Saturn. (Image credit: NASA/JPL-Caltech/Space Science Institute)

Cassini is a collaboration among NASA, the European Space Agency, and the Italian Space Agency.

On September 15, 2017, the spacecraft will be ordered to dive into Saturn itself. It will keep sending signals to Earth for as long as possible. As it descends, it will transmit useful data on the makeup of the planet’s atmosphere and on weather conditions deep inside Saturn’s clouds. Finally, the intrepid interplanetary adventurer Cassini will expire, having advanced our understanding and appreciation of Saturn until the very end.