The findings were reported November 9 in the Journal of Geophysical Research-Planets by Liming Li of Cornell University in Ithaca, N.Y. (now at the University of Houston), and colleagues from several institutions, including Goddard and NASA's Jet Propulsion Laboratory in Pasadena Calif., which manages the Cassini mission. "The Cassini CIRS data are very valuable because they give us a nearly complete picture of Saturn," says Li. "This is the only single data set that provides so much information about this planet, and it's the first time that anybody has been able to study the power emitted by one of the giant planets in such detail."
The planets in our solar system lose energy in the form of heat radiation in wavelengths that are invisible to the human eye. The CIRS instrument picks up wavelengths in the thermal infrared region, which is beyond red light, where the wavelengths correspond to heat emission.
"In planetary science, we tend to think of planets as losing power evenly in all directions and at a steady rate," says Li. "Now we know Saturn is not doing that." (Power is the amount of energy emitted per unit of time.)
Instead, Saturn's flow of outgoing energy was lopsided, with its southern hemisphere giving off about one-sixth more energy than the northern one, Li explains. This effect matched Saturn's seasons: during those five Earth years, it was summer in the southern hemisphere and winter in the northern one. (A season on Saturn lasts about seven Earth years.) Like Earth, Saturn has these seasons because the planet is tilted on its axis, so one hemisphere receives more energy from the sun and experiences summer while the other receives less energy and is shrouded in winter. Saturn’s equinox, when the sun was directly over the equator, occurred in August 2009.
In the study, Saturn's seasons looked Earth-like in another way: in each hemisphere, its effective temperature, which characterizes its thermal emission to space, started to warm up or cool down as a change of season approached. Because Saturn's weather is variable and the atmosphere tends to retain heat (called heat inertia), the temperature changes in complicated ways throughout the atmosphere. "The effective temperature provides us a simple way to track the response of Saturn's atmosphere, as a system, to the seasonal changes," says Li. Cassini's observations in the northern hemisphere revealed that the effective temperature gradually dropped from 2005 to 2008 and then started to warm up again by 2009. In Saturn's southern hemisphere, the effective temperature cooled from 2005 to 2009, as the equinox started to approach.
The emitted energy for each hemisphere rose and fell along with the effective temperature. Even so, during this five-year period, the planet as a whole seemed to be slowly cooling down and emitting less energy.
To find out if similar changes were happening one Saturn year ago, the researchers looked at data collected by Voyager in 1980 and 1981. Like Cassini CIRS, Voyager recorded fluctuations in the energy emitted by the planet and in the effective temperature. But Voyager did not see the imbalance between the southern and northern hemispheres; instead, the two regions were much more consistent with each other.
Why wouldn't Voyager have seen the same summer-versus-winter difference between the two hemispheres? The amount of energy coming from the sun (called solar radiance), which drives weather and atmospheric temperatures, could have fluctuated from one Saturn year to the next. The patterns in Saturn's cloud cover and haze could have, too.
"It's reasonable to think that the changes in Saturn's emitted power are related to cloud cover," says Amy Simon-Miller, who heads the Planetary Systems Laboratory at Goddard and is a co-author on the paper. "As the amount of cloud cover changes, the amount of radiation escaping into space also changes. This might vary during a single season and from one Saturn year to another. But to fully understand what is happening on Saturn, we will need the other half of the picture: the amount of power being absorbed by the planet."
Li is finishing an analysis of the solar energy that came to Saturn, based on data sets collected by two other Cassini instruments, the imaging science subsystem and the visual and infrared mapping spectrometer. He agrees that this information is crucial because Saturn, like its fellow giant planets Jupiter and Neptune, is thought to have its own source of internal energy. (The fourth giant planet, Uranus, does not seem to have an internal source.) By studying the changes in Saturn's outgoing energy along with the changes in incoming solar energy, scientists can learn about the nature of the planet's internal energy source and whether it, too, changes over time.
"The differences between Saturn's northern and southern hemisphere and that fact that Voyager did not see the same asymmetry raise a very important question: does Saturn's internal heat vary with time?" says Li. "The answer will significantly deepen our understanding of the weather, internal structure and evolution of Saturn and the other giant planets."
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency, and the Italian Space Agency. NASA's Jet Propulsion Laboratory, Pasadena, Calif., a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The composite infrared spectrometer team is based at NASA Goddard, where the instrument was built.
For more information visit http://www.nasa.gov/mission_pages/cassini/whycassini/dimmer-switch.html
- Fermi Telescope Caps First Year With Glimpse of Space-Time [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Antarctic Airborne Science Mission Nears Mid-Point [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- NASA Awards Education Research Grants to Minority Universities [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- New Celestial Map Gives Directions for GPS [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- NASA Invites Reporters to Tranquility Node Ceremony at Kennedy [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Happy Halloween [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- NASA's Fermi Telescope Detects Gamma-Ray From "Star Factories" in Other Galaxies [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- NASA Announces Advisory Council Chairs and Committee Structure [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- NASA and X Prize Announce Winners of Lunar Lander Challenge [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- MESSENGER Spacecraft Reveals More Hidden Territory on Mercury [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Successful Flight Through Enceladus Plume [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Education Secretary Hosts DC Students for Talk with Space Station [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Operation Ice Bridge Studies Antarctic Sea Ice [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- NASA West Point Welcomes Home One of Their Heroes [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- NASA Spitzer Observes a Chaotic Planetary System [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- X-38 Crew Return Vehicle Finds New Home [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Frost-Covered Phoenix Lander Seen in Winter Images [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Have some faith He is doing his best [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- The Cross [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Spring Bloom in New Zealand Waters [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- NASA Hubble image showcases star birth in M83, the Southern Pinwheel [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Take Me Out to the Ballpark - On Mars! [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Poisk Poised for Live NASA TV Space Station Docking [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Ceremony Reset for ESA Handover of Tranquility to NASA [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- NASA Reproduces a Building Block of Life in Laboratory [Last Updated On: November 8th, 2009] [Originally Added On: November 8th, 2009]
- Now Online: Aeronautics Goes E-Book [Last Updated On: December 12th, 2009] [Originally Added On: December 12th, 2009]
- Water on the Moon, Drought on Earth: NASA Experts Available for Radio And Podcast Interviews During Major Science Meeting [Last Updated On: December 12th, 2009] [Originally Added On: December 12th, 2009]
- Launch of NASA's Wise Spacecraft Delayed Until Dec. 14 [Last Updated On: December 12th, 2009] [Originally Added On: December 12th, 2009]
- Connecticut Students Set for Cosmic Conversation with Space Station Commander [Last Updated On: December 12th, 2009] [Originally Added On: December 12th, 2009]
- Magnetic Dance of Titan and Saturn To Be Main Attraction during Flyby [Last Updated On: December 12th, 2009] [Originally Added On: December 12th, 2009]
- Hubble's Deepest View of Universe Unveils Never-Before-Seen Galaxies [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Earth's Moon [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- How do you Make a Helicopter Safer to Fly? You Crash One. [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Physicist Earns Title as Kennedy's Best [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Hubble Unveils Never-Before-Seen Galaxies [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Saturn's Mysterious Hexagon Emerges from Winter Darkness [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Fermi Sees Brightest-Ever Blazar Flare [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Magnetic Power Revealed in Gamma-Ray Burst Jet [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Just 5 Questions: Aerosols [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Hometown Heroes 2009: Astronaut & Terrible Towel Return to Pittsburgh [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Garver Honors Four for Saving the Life of a Fifth at NASA Langley [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Administrator Bolden Speaks at AAIA-WIA Luncheon [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Best (Meteor) Shower of 2009 - No Towel Required [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- NASA Making Government More Accessible With Cutting-Edge Use Of New Media [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Scarce Water, Our Quiet Sun and Space Rocks Among NASA News Highlights at American Geophysical Union Meeting [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Press Credentials Deadlines Set for Next Space Shuttle Flight [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Reddish Dust and Ice Migration Darken Saturn's Moon Iapetus [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Galaxy Collision Switches on Black Hole [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- I’m watching the launch of NASA’s WISE spacecraft [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- SOFIA Aloft [Last Updated On: December 13th, 2009] [Originally Added On: December 13th, 2009]
- Guide to the International Space Station Laboratory Racks Interactive [Last Updated On: December 14th, 2009] [Originally Added On: December 14th, 2009]
- Freezing WISE's Hydrogen [Last Updated On: December 14th, 2009] [Originally Added On: December 14th, 2009]
- Local High School Wins Invention Challenge [Last Updated On: December 14th, 2009] [Originally Added On: December 14th, 2009]
- WISE Ready to Soar Into Space [Last Updated On: December 14th, 2009] [Originally Added On: December 14th, 2009]
- NASA Data Reveal Major Groundwater Loss in California's Heartland [Last Updated On: December 15th, 2009] [Originally Added On: December 15th, 2009]
- NASA Looks for Safer Icing Forecast For Pilots [Last Updated On: December 15th, 2009] [Originally Added On: December 15th, 2009]
- Challenges of Living and Working Aboard the Space Station: NASA Astronaut Nicole Stott Available for TV Interviews [Last Updated On: December 15th, 2009] [Originally Added On: December 15th, 2009]
- NASA Astronaut, Food Scientist Available for Interviews about Holiday Feasts in Space [Last Updated On: December 15th, 2009] [Originally Added On: December 15th, 2009]
- NASA Launches Web Site for Teenagers That Want More Class [Last Updated On: December 15th, 2009] [Originally Added On: December 15th, 2009]
- A Unique Geography -- and Soot and Dust -- Conspire Against Himalayan Glaciers [Last Updated On: December 15th, 2009] [Originally Added On: December 15th, 2009]
- New Study Turns Up the Heat on Soot's Role in Himalayan Warming [Last Updated On: December 15th, 2009] [Originally Added On: December 15th, 2009]
- NASA's WISE Eye on the Universe Begins All-Sky Survey Mission [Last Updated On: December 15th, 2009] [Originally Added On: December 15th, 2009]
- NASA Offers Sound Clips for Radio, Online Newscasters [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- NASA Gets Up-Close Look at Far Corner of the Globe [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- Orion Launch Abort System Attitude Control Motor Test-fired [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- Hubble Finds Smallest Kuiper Belt Object Ever Seen [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- The Dark Side of Carbon [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- R97UYEA6HD8W [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- NASA's AIM Satellite and Models are Unlocking the Secrets of Mysterious "Night-Shining" Clouds [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- Classroom Learning Takes Off with NASA-Funded Education Projects [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- NASA Buys Additional Space Shuttle Reusable Solid Rocket Motors [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- Searching for New Vaccines and Studying Butterflies in Space; NASA Offers TV Interviews about Latest Space Station Science Research [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- NASA Partners with Saudi Arabia on Moon and Asteroid Research [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- New Results from a Terra-ific Decade in Orbit [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- Deposits in Martian Trough Point to Complex Hydrological Past [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- NASA Outlines Recent Greenhouse Gas Research [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- Unexpected Wheel-Test Results [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- Solar Storms and Radiation Exposure on Commercial Flights [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- Global Digital Elevation Model [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]
- Hubble's Festive View of a Grand Star-Forming Region [Last Updated On: December 17th, 2009] [Originally Added On: December 17th, 2009]