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A polar expedition to Saturn: why is the ringed planet crowned by a distinct hexagonal cloud pattern?

Saturn's rotational axis is tilted by almost 27[degrees] to the plane of its orbit, giving the ringed planet a full array of seasons. While seasons on Earth last only 3 months, Saturn's endure for more than 7 years, owing to its 29V2-year-long orbital period.

Northern summer solstice on Saturn will occur in May 2017, when the planet's north pole and the northern face of its rings will be tipped toward the Sun--and toward Earth--to their maximum extent. While the increasingly apparent opening of the rings is undoubtedly the principal attraction, this favorable viewing geometry also affords an opportunity to observe colorful features at high northern latitudes on the planet's globe.

Saturn displays alternating dusky belts and bright zones that resemble those of Jupiter but are comparatively muted in contrast and bland in appearance. In many respects, Saturn's atmosphere is similar in structure and composition to Jupiter's, with a deck of water clouds at the bottom, ammonium hydrosulfide (N[H.sub.4]SH) clouds in the middle, and a deck of frozen ammonia (N[H.sub.3]) clouds at the top--the one we see--discolored by traces of nitrogen, sulfur, and phosphorous compounds. The upper troposphere, which lies above this layered sandwich, consists of colorless hydrogen and helium with a trace of methane (C[H.sub.4]) and a murky haze of photochemical smog.

Two factors account for the dramatic difference in the appearance of these two gas giants. Clouds form deeper down at the cooler temperatures that prevail at Saturn's greater distance from the Sun. This thermal effect is greatly reinforced by the planet's weaker gravity. (Although only slightly smaller in diameter than Jupiter, Saturn is only 30% as massive.) Jupiter's atmosphere is pulled much more powerfully toward the center of the planet, so the pressure gradient in the Jovian atmosphere is almost three times steeper.

Viewing Saturn's cloud deck through the planet's hazy, distended upper troposphere washes out details and subdues the saturation of colors. Saturn's dark belts appear pale greyish brown rather than the reddish sepia characteristic of Jupiter's belts, while the ringed planet's tropical and temperate zones feature a delicate palette of straw yellow, butterscotch, and saffron hues.

Saturn's dusky polar regions, however, often display assorted cooler colors reminiscent of the frigid ice giants Uranus and Neptune. Ranging from olive green to teal, aquamarine, and azure blue, they can be detected by a keen-eyed observer through a telescope of only 6 to 8 inches in aperture. Yellow (Wratten 12 or 15), orange (Wratten 21), and red (Wratten 23A or 25) biters greatly enhance this distinction.

As long ago as 1806 William Herschel noted that Saturn's poles appear to darken as they slowly tilt sunward. Decades of observations amassed by the British Astronomical Association's Saturn Section show a close correlation between solar altitude and the darkness of the polar regions, suggesting that Saturn's atmosphere responds quickly to the increasing input of sunlight.

The Sun's intensity at Saturn is only V90 of that on Earth, and the ringed planet radiates about twice as much internal heat as it receives from the Sun. So warming temperatures are not responsible for intensifying the polar colors. Instead, the greenish and bluish hues are widely attributed to Rayleigh scattering by hydrocarbon aerosols produced by solar ultraviolet radiation and charged particles, augmented by the selective absorption of light from the red end of the spectrum by methane. But the chemistry of Saturn's polar hazes is surprisingly complex--for example, polarimetry data suggest that the haze particles are shaped like microscopic grape clusters--and in truth the exact cause for the distinct hues remains uncertain.

A Curious Aspect

Saturn's colorful northern polar region is the site of a persistent hexagonal cloud feature centered at latitude 76[degrees] north. Curiously, it has no counterpart at the planet's south pole. First noticed in 1988 on old Voyager spacecraft images, it was still present and seemingly unaltered when the Cassini spacecraft imaged Saturn's north pole in 2006. The Voyager images revealed that clouds along the hexagon are propelled by berce jet-stream winds that exceed 400 kilometers per hour.

Physicists at the University of Oxford recently recreated the appearance of this mysterious feature by placing a cylinder of water on a slowly spinning table to simulate Saturn's atmosphere spinning with the planet's rotation. Inside this vessel they placed a small ring that rotated more rapidly than the cylinder to mimic an atmospheric jet stream. The researchers used fluorescent green dye to make the currents visible. They soon noticed that the greater the difference in rotation rate, the less circular the turbulent boundary became. Stable vortices of similar size formed on the slower, outer side of the fluid interface, and these interacted with each other to space themselves out evenly around the perimeter. By varying the rate at which the inner ring spun, the investigators were able to generate a variety of polygons, but a hexagonal form was the most common and stable. You can watch a video of the simulation at http://is.gd/saturn_hexagon.

Is that what's happening on Saturn? Perhaps, though Cassini has not observed any differential rotation between the hexagon's interior and exterior. Also, why is there no comparable feature surrounding the planet's south pole?

In any case, the north-polar hexagon is huge, spanning 25,000 km (twice Earth's diameter), and it subtends more than 3 arcseconds for earthbound observers--four times the apparent diameter of Titan, Saturn's largest satellite. Although well-equipped webcam imagers are routinely capturing stunning images of this feature, discerning its straight-sided outline poses a challenging test for visual observers. But don't let that dissuade you from trying. Saturn reached opposition in late May, so now is an excellent time to see the ringed planet at its best--hexagon or no.

The Moon * July 2015

Phases

FULL MOON
July 2,2:20 UT

LAST QUARTER
July 8, 20:24 UT

NEW MOON
July 16,1:24 UT

FIRST QUARTER
July 24,4:04 UT

FULL MOON
July 31,10:43 UT

Librations
Bailly (crater)     July 4
Plutarch (crater)   July 18
Cabeus (crater)     July 26
Montes Rook         July 31

Distances

Perigee         July 5,[19.sup.h] UT
228,101 miles   diam. 32'7"

Apogee          July 21, [11.sup.h] UT
251,553 miles   diam. 29' 14"


Thomas Dobbins has observed rare phenomena on many bodies within our solar system, both real and illusory.
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Title Annotation:OBSERVING: Exploring the Solar System
Author:Dobbins, Thomas
Publication:Sky & Telescope
Date:Jul 1, 2015
Words:1055
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