Cassini's CIRS Reveals Saturn Is on a "Cosmic Dimmer Switch"

From Science @NASA – Study suggests changes in Saturn’s emitted power are related to varying cloud cover.

Heat emitted from the interior of Saturn (red) shows up in this false-color image of Saturn, made from data taken in 2008 by Cassini's visual and infrared mapping spectrometer. Credit: NASA/JPL/ASI/University of Arizona

Like a cosmic light bulb on a dimmer switch, Saturn emitted gradually less energy each year from 2005 to 2009, according to observations by NASA’s Cassini spacecraft. But unlike an ordinary bulb, Saturn’s southern hemisphere consistently emitted more energy than its northern one. On top of that, energy levels changed with the seasons and differed from the last time a spacecraft visited in the early 1980s. These never-before-seen trends came from an analysis of comprehensive data from the Composite Infrared Spectrometer (CIRS), an instrument built by NASA’s Goddard Space Flight Center in Greenbelt, Md., as well as a comparison with earlier data from NASA’s Voyager spacecraft. When combined with information about the energy coming to Saturn from the sun, the results could help scientists understand the nature of Saturn’s internal heat source.

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.

From one Saturn year to the next, dramatic changes are seen in the emitted power and effective temperature on Saturn. Credit: Adapted from the Journal of Geophysical Research, Vol. 115, E11002, 2010, doi:10.1029/2010JE003631.

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.

More Cassini information is available at http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

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20 thoughts on “Cassini's CIRS Reveals Saturn Is on a "Cosmic Dimmer Switch"

  1. It would be interesting to see the same plot for Earth. Not the surface temperature plots we see all the time, but the effective radiative temperature plots. If the instrument produced a similar coverage map as opposed to a single integrated number, this should lead to actually measuring cloud feedbacks.

  2. With the sun directly over the equator in 2009 as it shifts toward the other pole one year at least will be in balance as one hemi-spere cools and the other warms. Perhaps 80-81 was close to the year of balance and the internal machinations of the planet are totally innocent. The overall cooling would be a quiet sun. This has much to do with global warming and I expect a large research grant cheque soon.[please]

  3. Fasanating to study, some quick question come to mind. This statement : ”
    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.”
    How do they know if it is cooling or warming without knowing how much energy is being absorbed and the radiative balance? This is what popped into my mind then I saw this quote ” says Amy Simon-Miller, who heads the Planetary Systems Laboratory at Goddard and is a co-author on the paper. “ 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.”
    So my question was valid but really, does Mr Miller really think that is all he needs to know to “fully understand what is happening on Saturn”?
    We continue to learn, but the path to humility is equally long.

  4. Has any mechanism been proposed to explain this magnitude of change to Saturn’s interior heat source? Radioactivie heat could not change so rapidly, so it’s out of the frame.
    The large change as shown over a year must admit the possibility that at times in the distant past, Saturn approached zero Kelvin at the cold end and who knows what at the hot end.
    Seems to me to be a classic case of instrument calibration, or subsequent adjustments to instrumented responses. There is abundant literature about adjustments to apparent temperatures.

  5. I’d guess that it’s just the same as on the Earth.
    Solar radiation affecting atmospheric layers differentially and as a consequence moving clouds and haze latitudinally to alter total cloud or haze quantities and overall albedo.
    In the case of Earth ozone reactions are the key but possibly it would be other chemicals around Saturn.

  6. “…during this five-year period, the planet as a whole seemed to be slowly cooling down and emitting less energy.”
    Sounds a bit like what’s happening on earth. Interesting that the sun is in quiet mode and both planets with widely differing atmospheres showed similar trends?
    I wonder if there could be a magnetic connection? Due to a weakened solar magnetic field, Earth’s field has also weakened. Perhaps Saturn’s magnetic moment, which is large at 4.6 X 1018 T m3, is about 580 times larger than that of Earth could also have been effected? Here on Earth their is a correlation between magnetic field and polar temperature, (thanks to Vuk):-
    http://www.vukcevic.talktalk.net/NFC1.htm

  7. “Like a cosmic light bulb on a dimmer switch, Saturn emitted gradually less energy each year from 2005 to 2009, according to observations by NASA’s Cassini spacecraft. ”
    Is that an incandescent bulb or an energy-saving fluorescent one? Come to think of it, my fluorescent bulbs emit less energy every year too. Perhaps they have the same problem as Saturn.

  8. “…during this five-year period, the planet as a whole seemed to be slowly cooling down and emitting less energy.”
    It is strange that the same scientists are saying that because of the increased greenhouse effect the earth is emitting less long wave radiation and is therefore slowly warming up.
    If they are going to make things up before they have done all the science they had better get together and agree the story!

  9. @cal
    Your point it well taken. They clearly state that the reduction in outgoing IR heat is because the planet is cooling: “The emitted energy for each hemisphere rose and fell along with the effective temperature.” Dang. Who’d-a thought.
    I have been told many times on Earth that a reduction in outgoing IR indicates heating through retention by increasing CO2, and further that the heat is missing but hiding somewhere, ‘probably in the deep oceans.’ But the atmosphere is just as transparent to heat as it always was.
    Are planetary scientists educated at different schools than Earth scientists? They make more sense.

  10. Don’t tell this to Hansen -he thinks Earth and Venus are identical twins…
    Therefore nothing else is gong on in the Solar system…

  11. Tenuc says:
    November 11, 2010 at 2:43 am
    Here on Earth their is a correlation between magnetic field and polar temperature, (thanks to Vuk)
    http://www.vukcevic.talktalk.net/NFC1.htm
    Dear Tenuc: We MUST forget our wrong conceptions about the Solar System, The Galaxy, the Universe in general, though this could mean to leave self-conceit aside: Return to the basic teachings, to what masters of old tried to convey to us though we prefer being blind and deaf.
    The EU theorists have made the greatest approach ever, however we keep on rejecting it, like conceited kids, in spite of the fact that all gadgets we enjoy daily belong to that branch of science.
    Our humble contribution (in its latest version):
    http://www.scribd.com/doc/42018959/Unified-Field-Explained-9

  12. Nice to see that reason presides with Saturn, and the understanding of the thermal dynamics when truly applied. Must be due to a lack of tax incentives, (collection over such a distance is real slow). It would be nice as well to see the same perspective used on the understanding of the Earth’s global circulation patterns, and climate.
    Most of what I have done, is to look at the earth as if I were an alien, with the composite satellite data overheard from a distance, to form my opinions on how the Earth’s climate functions.

  13. Pascvaks says:
    November 11, 2010 at 5:06 am
    So little information and so much speculation.

    Ditto Pascvaks, my thoughts exactly!

  14. So that’s why my supposed telescope gadgety thingy can’t dail in Saturn properly. Heh, who would have figured that a NASA sat really was working as it should, but it probably did cost ’em though, but one wonders why.

  15. The shadow cast on the surface by the rings must have quite an effect on temperature differences and hence wind patterns. Those patterns would alter as Saturn orbits the sun.

  16. The differences between Saturn’s northern and southern hemisphere and that fact that Voyager did not see the same asymmetry raises another very important question: does Enceladus’ southern pole ice geyser vary with the big planet’s variation?

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