(Sen) - Saturn's jet streams are driven by internal heat, rather than energy from the Sun, scientists have concluded after studying data from NASA's Cassini spacecraft.
The study discovered that condensation of water from Saturn's internal heating leads to temperature differences in the atmosphere. The temperature differences create disturbances known as "eddies" that move air back and forth at the same latitude, and, in turn, accelerate the jet streams like rotating gears driving a conveyor belt.
The research, published in the June issue of the science journal Icarus, was based on years of data collected by Cassini. The paper's lead author was Tony Del Genio of NASA's Goddard Institute for Space Studies and a member of the Cassini imaging team.
A prior study had assumed that the Sun was the source of energy that caused the temperature differences in the atmosphere. This is the case on Earth where storms are created primarily by the Sun's heating rather than condensation.
"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."
The new study was made possible in part because Cassini has been able to make a large number of observations, having been in orbit around Saturn since 2004.
In contrast to Earth's thin atmosphere surrounding a solid and liquid surface, Saturn is a gas giant with a deep atmosphere. Cassini's cameras with near-infrared filters have been peering into this deep atmosphere and captured data from two different altitudes. Images were taken at a high altitude, near the surface where heat from the Sun is strongest, and a lower altitude at the tops of ammonia ice clouds where solar heating is weak but closer to where weather originates. At the lower altitude, deeper down in the planet, water condenses and makes clouds and rain. Cloud tracking software was used to analyse the movement and speeds of the powerful jet streams in hundreds of Cassini images.
Scientists found that the eddies were weak at the higher altitudes where most of the Sun's heating occurs, and stronger deeper in the atmosphere. The research therefore concluded that heating from the Sun was not the cause of the eddies but instead an internal process where internal heat from the planet stirs up water vapor from Saturn's interior. That water vapor condenses as air rises and releases heat as it makes clouds and rain. Condensation heating therefore generates the energy that drives the jet streams.
"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, leader of the Cassini imaging team. "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."
Cassini was launched in 1997 and entered Saturn's orbit in 2004. Cassini continues to provide much information and stunning images of Saturn, its rings and its moons and the mission has been extended to at least 2017.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.
The Cassini imaging operations center (CICLOPS) and team leader (Dr Carolyn Porco) are based at the Space Science Institute in Boulder, Colorado.
NASA's Jet Propulsion Laboratory, Pasadena, California, a division of the California Institute of Technology, manages the overall mission for NASA.