Saturn's seasons are made in the shade

Saturn's atmosphere is affected by seasonal changes just like Earth's is, but perhaps even more so, because the poles spend nearly 15 Earth-years in winter darkness and the next 15 years in sunlight. On top of that, the cooling shadow of the rings causes differences in heating and sunlight-driven chemistry between the shaded and sunlit parts of Saturn's atmosphere.

Cassini's years in orbit around Saturn have produced the first long-term space-based measurements of seasonal change on a gas-giant planet. In particular, Cassini's composite infrared spectrometer has recorded seasonal changes in Saturn's atmosphere, witnessing rapid responses in the atmospheric temperatures and hazes as the equinox approached last August. At that time, the incoming sunlight hit the rings edge-on, reducing the ring shadow to just a thin line across the planet's middle. Mission scientists have been struck by how quickly the atmosphere changed in response to the shifting ring shadow.

Even though the ring-shaded portion of the atmosphere is cooler than the rest, the planet seems to quickly adjust the disequilibrium by filling the cool patches with warmer air. "Because the shadowed region of the atmosphere moves over time, it makes for interesting atmospheric dynamics in the regions newly emerging from ring shadow in the springtime," said researcher Leigh Fletcher of NASA's Jet Propulsion Laboratory and the University of Oxford. "Cassini found that Saturn's atmosphere changes drastically depending on what season you're in. When the shadow is gone, the atmosphere responds quickly by warming up."

As equinox approached last August, Saturn's northern spring hemisphere began to change rapidly, Fletcher said. The stratosphere in Saturn's northern mid-latitudes warmed faster than anywhere else on the planet as it emerged from the shadow of the rings. Between Cassini's arrival in 2004 and the spring equinox in 2009, that area warmed by 6 to 8 Kelvin. At the same time, the blue shades seen in Saturn's northern hemisphere at the start of the mission were rapidly replaced by the more familiar yellow-ochre hues, demonstrating a close relation between season, temperatures and cloud colors. Saturn's warm south polar stratosphere was also seen to cool over the course of the mission, and Cassini scientists expect the development of an analogous warm north polar stratosphere as spring progresses.

This figure shows shifting stratospheric temperatures obtained by Cassini between 2005 (left) to 2008 (right). The difference between the temperatures is shown in the middle, with red indicating warming in the stratosphere and blue indicating cooling.

The most warming was observed in the northern mid-latitudes, in regions emerging from the shadow of Saturn's rings. The most cooling was observed in a collar around the south pole, as direct sunlight shifted away from that region. The changes were about 6 to 8 Kelvin.

The temperature changes show the effect of changing seasons on the giant planet. Cassini arrived at Saturn in 2004, when the planet was in the middle of a balmy southern summer with its south pole oriented towards the sun. The north pole was shrouded in winter darkness. Around August 2009, Cassini observed the equinox, when the sun shone over Saturn's equator, lighting the rings edge-on only and warming both hemispheres the same amount. That was the start of southern autumn and northern spring.

The black line in the figure represents Saturn's equator and the plane of its rings. The annotated version includes color-coded temperature scales at the bottom.

Cassini's composite infrared spectrometer obtained data on the changing temperatures from October 2004 to July 2009. The figures on the left and right were assembled from two subsets of the data, providing complete global coverage from pole to pole in 2005 and 2008. The data represent mean temperatures by latitude and were not dependent on longitude. The graphic uses some artistic interpretation in spreading out the latitudinal data.

Credit: NASA/JPL/GSFC/Oxford University

Observing how the stew of gases in Saturn's atmosphere circulate, heat, and cool contributes to our basic understanding of planetary atmospheres, including Earth's, Fletcher said. "This sort of study places Earth's own seasonal variations into the context of seasonal changes throughout our solar system."

"In some ways, Saturn is a simpler system than Earth -- no biogenic influences on the climate, no surfaces or continents to interrupt the flow of the atmosphere from one place to the next," he said. "But in other ways, Saturn's atmosphere is complicated, with dynamics, wave activity and chemistry that scientists are still studying and trying to understand." Fletcher points to the example of a recently discovered long-lived equatorial wave pattern at Saturn that ripples back and forth within Saturn's upper atmosphere. In this region, temperatures switch from one altitude to the next in a candy cane-like, striped, hot-cold pattern. These varying temperatures force the wind in the region to keep changing direction from east to west, jumping back and forth. As a result, the entire region oscillates like a wave.

Cassini's mission was recently extended through 2017. This will be a particularly exciting time if the northern hemisphere continues to respond to the changing season as fast as it has in the past couple of years, said Fletcher. "We could start to see the development of a warm north polar stratosphere, mirroring the one observed in the south. We'll almost certainly see a 'flip' in Saturn's temperature field, and by the end of Cassini's lifetime, the northern hemisphere will be warmer than the south, almost the complete reverse of conditions at the start of the mission."

By the end of its mission, Cassini will have a record of Saturn's seasons for at least portions of spring, summer, autumn and winter, making Saturn the best-studied gas giant planet. "Cassini is the first planetary spacecraft in history to provide that opportunity," said Fletcher. "To really understand a planet's weather and environment, you have to watch how things change and evolve with time."

Source: Jet Propulsion Laboratory
	More on • Saturn • Cassini