Saturn’s Atmosphere is More Varied Than it Looks, With Tropical “Jungles” and Ammonia “Deserts”

Color-composite image of Saturn's varied cloud bands (NASA/JPL/SSI. Composite by Jason Major)
Color-composite Cassini image of Saturn’s varied cloud bands (NASA/JPL/SSI. Composite by Jason Major)

Saturn might look like a placid beige ball in backyard telescopes but in reality it has very dynamic weather patterns and climates, rivaling the storms of Jupiter and the varied climates of Earth, based on long-term microwave observations by the Cassini spacecraft.

(Yes, microwaves are good for much more than heating up your coffee.)

Peering deeply into Saturn’s placid-looking atmosphere with the radar/microwave radiometer aboard NASA’s Cassini spacecraft has revealed a dynamic and in many places turbulent atmosphere that has more similarities with that of its sibling planet Jupiter than previously realized.

The new observations reveal a calm and narrow equatorial belt surrounded by stormy bands like those on Jupiter, although regions at higher latitudes show a different character that is unique to Saturn.

The newly created global microwave maps of Saturn reveal in unprecedented detail a largely stable atmospheric environment with some unexpected variety, including arid, desert-like patches in Saturn’s atmosphere, and regions where humid tropical climes dominate.

For six of the nine years that Cassini has been orbiting Saturn, the microwave radiometer has gathered observations of the whole of Saturn’s atmosphere that could not be made from Earth-based radar systems.

Saturn's "Great Northern Storm" of 2011. It dredged up material from deep within Saturn's atmosphere via powerful convection currents, similar to thunderstorms on Earth.
Saturn’s “Great Northern Storm” of 2011. It dredged up material from deep within Saturn’s atmosphere via powerful convection currents, similar to thunderstorms on Earth. This color-composite shows the scale size of Earth in comparison. (NASA/JPL/SSI/J.Major)

A Big Role for Ammonia
The observations provide a view of Saturn’s ammonia gas distribution never seen before, according to Michael Janssen, the lead author of the new findings, Michael Janssen of NASA’s Jet Propulsion Laboratory, Pasadena, Calif. Ammonia is common in much of Saturn’s largely stable troposphere. But the microwave maps show there are regions where ammonia vapor is depleted due to a greater rate of the gas condensing into liquid. Where these ammonia-depleted patches occur, it’s a sign that beneath them are areas of atmospheric instability. These ammonia-depleted spots also allow clear windows to see into Saturn’s deeper, hotter atmosphere.

Quiet Bands, Turbulent Bands
The maps show the bands between 15 and 55 degrees latitude and minus 15 to minus 32 latitude degrees are relatively quiet, and that there is more turbulence between minus 15 and 15 degrees latitude, except near the equator. The stormy latitude bands are similar to those that have long been observed on Jupiter and are thought to be a consequence of the planet’s fast rotation, although the mechanism is not well understood, said Janssen.

“Storm Alley,” that lively latitudinal belt in the southern hemisphere where long-lived storms break up Saturn’s blandness, is visible in the map. The Great Northern Storm itself is clearly seen in the 2011 map. The depletion of ammonia associated with the storm is shown by a widespread brightness in the atmosphere. Features in Storm Alley and the Great Northern Storm indicate that ammonia is depleted deep in the atmosphere below the ammonia cloud deck.

The researchers found there is little similarity between the north and south. The southern hemisphere was more turbulent until the Great Northern Storm developed.

Cassini’s radar microwave maps are the first ever made of a gas giant planet. While the gas giants of the solar system differ vastly from Earth and the other terrestrial planets in composition and structure, Saturn appears to share with Earth many common atmospheric features. Studies of Saturn’s atmospheric structure and processes provide a new perspective on the commonality and drivers of Earth’s weather and climate systems.

Source: Cassini Science League release by Mary Beth Murrill

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