Saturn's Hidden Turbulence Revealed

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. This aspect of Cassini's wide-ranging studies is part of NASA's effort to determine Saturn's composition and atmospheric dynamics.

Cassini's closeness to its target is an advantage in gaining an understanding of the Saturn's inner workings, and Cassini's radar microwave maps are the first ever made of a gas giant planet. The results provide much higher-resolution mapping than any ground-based measurements. The four gas giant planets are considered miniature versions of the sun, so knowing more about their interiors can provide a better understanding of the composition and dynamics of our star.

Obtaining these measurements called for acrobatic data-gathering movements for the Cassini spacecraft that are different than the steady horizontal left-to-right scanning approach used for Titan mapping. To capture data from the larger target of Saturn, spacecraft operators rocked Cassini up and down so the fixed instrument could cover the north-south axis of the map coverage. This was accomplished through control of Cassini's reaction wheels, accelerating and decelerating the spacecraft around one of its axes at the fastest possible rate. The rotation of Saturn beneath the Cassini spacecraft produced the sweep of coverage around the planet.

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.

One More Saturn Map Planned
The Cassini team plans to gather microwave radar data for one more global equatorial map in 2015, allowing a comparison of Saturn's atmospheric variability over a 10-year period.

Comparable but much more thorough microwave measurements of Jupiter's atmosphere will be carried out by the microwave radiometer experiment on board NASA's Juno spacecraft, due to reach Jupiter in 2016.

The composition and atmospheric dynamics of Saturn and the other gas giants shed light on the how the planets of the solar system formed out of the solar nebula.

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.

This Cassini Science League entry is an overview of science papers authored, or co-authored, by at least one Cassini scientist. The information above was derived from or informed by the following publications:

1) "Saturn's Thermal Emission at 2.2-cm Wavelength as Imaged by the Cassini RADAR Radiometer," Michael A. Janssen, Andrew P. Ingersoll, Michael D. Allison, Samuel Gulkis, Anne E. Laraia, Kevin H. Baines, Scott G. Edgington, Yanhua Z. Anderson, Kathleen Kelleher, and Fabiano A. Oyafuso, Icarus. Available online June 21, 2013.
2) "Analysis of Saturn's Thermal Emission at 2.2-cm Wavelength: Spatial Distribution of Ammonia Vapor," A.L. Laraia, A.P. Ingersoll, M.A. Janssen, S. Gulkis, F. Oyafuso, and M. Allison, Icarus. Available online June 27, 2013.

- Mary Beth Murrill, Cassini Science Communication Coordinator