Scientists, using images collected over several years by NASA’s Cassini spacecraft, have discovered that the heat from within Saturn powers turbulent jet streams, regions on the planet where winds blow faster than in other places.
These jet streams churn east and west across Saturn. Scientists have been trying to understand for years the mechanism that drives these wavy structures in Saturn’s atmosphere and the source from which the jets derive their energy.
Condensation of water from Saturn’s internal heating led to temperature differences in the atmosphere. The temperature differences created eddies, or disturbances that move air back and forth at the same latitude, and those eddies, in turn, accelerated the jet streams like rotating gears driving a conveyor belt.
A competing theory had assumed that the energy for the temperature differences came from the Sun. That is how it works in the Earth’s atmosphere.
“We know the atmospheres of planets such as Saturn and Jupiter can get their energy from only two places: the sun or the internal heating. The challenge has been coming up with ways to use the data so that we can tell the difference,” said Tony Del Genio of NASA’s Goddard Institute for Space Studies, a member of the Cassini imaging team and lead author of a paper appearing in the June issue of the journal Icarus.
The new study was possible in part because Cassini has been in orbit around Saturn long enough to obtain the large number of observations required to see subtle patterns emerge from the day-to-day variations in weather.
“Understanding what drives the meteorology on Saturn, and in general on gaseous planets, has been one of our cardinal goals since the inception of the Cassini mission,” said Carolyn Porco of the Space Science Institute in Boulder. “It is very gratifying to see that we’re finally coming to understand those atmospheric processes that make Earth similar to, and also different from, other planets.”
Rather than having a thin atmosphere and solid-and-liquid surface like Earth, Saturn is a gas giant whose deep atmosphere is layered with multiple cloud decks at high altitudes. A series of jet streams slice across the face of Saturn visible to the human eye and also at altitudes detectable to the near-infrared filters of Cassini’s cameras. While most blow eastward, some blow westward. Jet streams occur on Saturn in places where the temperature varies significantly from one latitude to another.
Thanks to the filters on Cassini’s cameras, which can see near-infrared light reflected to space, scientists now have observed the Saturn jet stream process for the first time at two different, low altitudes. One filtered view shows the upper part of the troposphere, a high layer of the atmosphere where Cassini sees thick, high-altitude hazes and where heating by the sun is strong. Views through another filter capture images deeper down, at the tops of ammonia ice clouds, where solar heating is weak but closer to where weather originates. This is where water condenses and makes clouds and rain.
“With our improved tracking algorithm, we’ve been able to extract nearly 120,000 wind vectors from 560 images, giving us an unprecedented picture of Saturn’s wind flow at two independent altitudes on a global scale,” said co-author and imaging team associate John Barbara, also at the Goddard Institute for Space Studies.
The team’s findings provide an observational test for existing models that scientists use to study the mechanisms that power the jet streams.
Bibliographic information: Del Genio AD, Barbara JM. 2012. Constraints on Saturn’s tropospheric general circulation from Cassini ISS images. Icarus 219 (2), June 2012, Pages 689–700; doi: 10.1016/j.icarus.2012.03.035