Jet streams on Saturn driven by internal heat, not the sun

The North American jet stream that flows from west to east across the continent is a major contributor to weather, bringing cold air down from the north or sealing off southern regions from winter temperatures. It also provides an assist to eastbound aircraft. This jet stream and others on the Earth’s surface are driven by atmospheric heating by the sun, and astronomers had assumed that similar jet streams on Saturn had the same origin. But new data compiled from imaging by NASA’s Cassini probe, which has been orbiting the planet since July 1, 2004, show that Jupiter’s jet streams have a different power source -- the planet’s own internal heat.

Saturn is a gas giant with a deep atmosphere that is layered with multiple cloud decks at high altitudes. Jet streams exist at several layers, some of them visible to the naked eye and others hidden deep beneath the upper clouds. Cassini’s cameras can capture near-infrared light reflected into space by the lower layers, however, providing our first glimpse into atmospheric dynamics at lower levels where sunlight does not penetrate to any significant extent. A team headed by Tony Del Genio of NASA’s Goddard Institute for Space Studies in New York used cloud tracking software to analyze the movements and speeds of clouds in 560 images collected between 2005 and 2012, extracting nearly 120,000 wind vectors.

The team reported in the journal Icarus that the jet streams are linked to eddies in Saturn’s atmosphere. These eddies are weak in the upper atmosphere, where previous research has shown that heating is due primarily to the sun, and are much stronger at lower levels. The researchers could thus deduce that it is planetary heat that is driving them, not the sun’s heat. Internal warmth at the lowest levels of the planet evaporates water, which rises higher into the upper layers of the atmosphere before condensing to produce clouds and rain. That condensation releases heat, which produces the eddies that drive the jet streams.

[For the record, 10:40 p.m., June 27: The original version of this story incorrectly said Jupiter, not Saturn.]

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