English: These natural color views (and corresponding animated movie sequences) from NASA's Cassini spacecraft compare the appearance of Saturn's north-polar region in June 2013 and April 2017.

In both views, Saturn's polar hexagon dominates the scene. The comparison shows how clearly the color of the region changed in the interval between the two views, which represents the latter half of Saturn's northern hemisphere spring.

In 2013, the entire interior of the hexagon appeared blue. By 2017, most of the hexagon's interior was covered in yellowish haze, and only the center of the polar vortex retained the blue color. The seasonal arrival of the sun's ultraviolet light triggers the formation of photochemical aerosols, leading to haze formation. The general yellowing of the polar region is believed to be caused by smog particles produced by increasing solar radiation shining on the polar region as Saturn approached the northern summer solstice on May 24, 2017.

Scientists are considering several ideas to explain why the center of the polar vortex remains blue while the rest of the polar region has turned yellow. One idea is that, because the atmosphere in the vortex's interior is the last place in the northern hemisphere to be exposed to spring and summer sunlight, smog particles have not yet changed the color of the region. A second explanation hypothesizes that the polar vortex may have an internal circulation similar to hurricanes on Earth. If the Saturnian polar vortex indeed has an analogous structure to terrestrial hurricanes, the circulation should be downward in the eye of the vortex. The downward circulation should keep the atmosphere clear of the photochemical smog particles, and may explain the blue color.

This pair of images compliments a previously released pair from 2012 and 2016 (see PIA21049).

Images captured with Cassini's wide-angle camera using red, green and blue spectral filters were combined to create these natural-color views.

The 2013 view (left in the combined view), was captured on June 25, 2013, when the spacecraft was about 430,000 miles (700,000 kilometers) away from Saturn. The original versions of these images, as sent by the spacecraft, have a size of 512 by 512 pixels and an image scale of about 52 miles (80 kilometers) per pixel; the images have been mapped in polar stereographic projection to the resolution of approximately 16 miles (25 kilometers) per pixel. The second and third frames in the animation were taken approximately 130 and 260 minutes after the first image.

The 2017 sequence (right in the combined view) was captured on April 25, 2017, just before Cassini made its first dive between Saturn and its rings. During the imaging sequence, the spacecraft's distance from the center of the planet changed from 450,000 miles (725,000 kilometers) to 143,000 miles (230,000 kilometers). The original versions of these images, as sent by the spacecraft, have a size of 512 by 512 pixels. The resolution of the original images changed from about 52 miles (80 kilometers) per pixel at the beginning to about 9 miles (14 kilometers) per pixel at the end. The images have been mapped in polar stereographic projection to the resolution of approximately 16 miles (25 kilometers) per pixel. The average interval between the frames in the movie sequence is 230 minutes.

The Cassini mission is a cooperative project of NASA, ESA (the European Space Agency) and the Italian Space Agency. The Jet Propulsion Laboratory, a division of Caltech in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.

For more information about the Cassini-Huygens mission visit https://saturn.jpl.nasa.gov and https://www.nasa.gov/cassini. The Cassini imaging team homepage is at https://ciclops.org.