Cassini Sees Saturn on a Cosmic Dimmer Switch

Feature Nov. 10, 2010

Cassini Sees Saturn on a Cosmic Dimmer Switch


The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-380&cid=release_2010-380


Like a cosmic lightbulb 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 Saturn in the early 1980s. These never-before-seen trends came from a detailed analysis of
long-term 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 fact that Saturn actually emits more than twice the energy it absorbs from the sun has been a
puzzle for many decades now," said Kevin Baines, a Cassini team scientist at NASA's Jet
Propulsion Laboratory, Pasadena, Calif., and a co-author on a new paper about Saturn's energy
output. "What generates that extra energy? This paper represents the first step in that analysis."

The research, reported this week in the Journal of Geophysical Research-Planets, was led by
Liming Li of Cornell University in Ithaca, N.Y. (now at the University of Houston).

"The Cassini CIRS data are very valuable because they give us a nearly complete picture of
Saturn," Li said. "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, far enough 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," Li said. "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. The effective temperature provides a simple way to track the
response of Saturn's atmosphere to the seasonal changes, which is complicated because Saturn's
weather is variable and the atmosphere tends to retain heat. Cassini's observations revealed that
the effective temperature in the northern hemisphere gradually dropped from 2005 to 2008 and
started to warm up again by 2009. In the southern hemisphere, the effective temperature cooled
from 2005 to 2009.

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 the Voyager spacecraft in 1980 and 1981 and 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? One explanation is that cloud patterns at depth could have fluctuated, blocking and
scattering infrared light differently.

"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."

Scientists will be doing that as a next step by comparing the instrument's findings to data
obtained by Cassini's imaging cameras and infrared mapping spectrometer instrument. The
spectrometer, in particular, measures the amount of sunlight reflected by Saturn. Because
scientists know the total amount of solar energy delivered to Saturn, they can derive the amount
of sunlight absorbed by the planet and discern how much heat the planet itself is emitting. These
calculations help scientists tackle what the actual source of that warming might be and whether it
changes.

Better understanding Saturn's internal heat flow "will significantly deepen our understanding of
the weather, internal structure and evolution of Saturn and the other giant planets," Li said.

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 .

#2010-380

Written by Elizabeth Zubritsky and Jia-Rui Cook

Jia-Rui Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jia-rui.c.cook@jpl.nasa.gov

Elizabeth Zubritsky 301-614-3458
Goddard Space Flight Center, Greenbelt, Md.
elizabeth.a.zubritsky@nasa.gov

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