Saturn's Cosmic Dimmer Switch

In a paper published today by a CIRS-team colleague of mine, Liming Li, we demonstrate how Saturn’s thermal emission seems to vary with time.  Most of this is likely due to Saturn’s seasonal variability (see the earlier entry on Saturn’s Changing Seasons), but there is some evidence that Cassini’s observations of Saturn show a different thermal emission when compared to Voyager’s observations, exactly one Saturnian year ago.  The next step in the analysis is to study Saturn’s absorbed power, as the balance between the power absorbed and emitted is vital to understand the thermal evolution of the planet.  Watch this space!

Liming Li, Barney Conrath , Peter Gierasch , Richard Achterberg , Conor Nixon , Amy Simon-Miller , F. Flasar , Don Banfield , Kevin Baines , Robert West , Andrew Ingersoll , Carolyn C. Porco , Ashwin Vasavada , Anthony Del Genio , Andrei Mamoutkine , Marcia Segura , Gordon Bjoraker , Glenn Orton , Leigh Fletcher , Patrick Irwin , Peter Read , Thierry Fouchet, Saturn’s Emitted Power, J. Geophys. Res., 115, E11002 (http://dx.doi.org/10.1029/2010JE003631)

http://saturn.jpl.nasa.gov/news/cassinifeatures/feature20101110/

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://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini .

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

2010-380

Revival of Jupiter's South Equatorial Belt

On November 9th 2010, Jupiter’s ‘missing’ South Equatorial Belt (SEB) showed signs of a spectacular, violent and eruptive revival, initiating a chain of events that will ultimately revive the typical brown colour of the belt.  A huge plume of bright material erupted in the SEB, prompting us to scramble for time on Gemini, Keck, IRTF and VLT to study this unprecedented opportunity to observe a rare and mysterious phenomenon caused by the planet's winds and cloud chemistry.

The following text has been created by amalgamating the original press releases from JPL, University of Berkeley and the Gemini Observatory.  Text credits go to Robert Sanders (Berkeley) and Priscilla Vega/Jia-Rui Cook (JPL).  Nice write-ups were also on space.com. 

Stripes Are Back in Season on Jupiter

Earlier this year, amateur astronomers noticed that the long-standing stripe, known as the South Equatorial Belt (SEB), just south of Jupiter's equator, had turned white (see the Hubble space telescope image comparison on the right, noting the missing belt). In early November, amateur astronomer Christopher Go of Cebu City in the Philippines observed a prominent bright spot in the unusually whitened belt, piquing the interest of professional and amateur astronomers around the world.  After follow-up observations with NASA's Infrared Telescope Facility (IRTF), the 10-meter Keck telescope and the 8-meter Gemini telescope, all atop Mauna Kea in Hawaii, scientists now believe the stripe is making a comeback.

NASA's Infrared Telescope Facility False obtained this false-color composite image on Nov. 16 (right). The prominent region just to the left of the center, expanded in the insert, shows the region of the South Equatorial Belt outbreak. In the coming weeks, further outbreaks are expected to take place to the west (left) of those seen in this image. The clear atmospheric regions (in red) will begin to fill this latitude band at the same time as the dark brown color typical of this region returns.

Astronomers announced first-glimpse images of the reappearing stripe Nov. 9.  "The reason Jupiter seemed to 'lose' this band — camouflaging itself among the surrounding white bands — is that the usual downwelling winds that are dry and keep the region clear of clouds died down," said Glenn Orton, a research scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. "One of the things we were looking for in the infrared was evidence that the darker material appearing in visible light was actually the start of clearing in the cloud deck, and that is precisely what we saw."

This white cloud deck is made up of white ammonia ice. When the white clouds float at a higher altitude, they obscure the view of the lower brown clouds. Every few decades or so, the South Equatorial Belt turns completely white for perhaps one to three years, an event that has puzzled scientists for decades. This extreme change in appearance has only been seen with the South Equatorial Belt, making it unique to Jupiter and to the entire solar system.

The bright storm that Go observed in the faded belt was quite unusual, said Imke de Pater, UC Berkeley professor of astronomy.  "At infrared wavelengths, images in reflected sunlight show that the spot is a tremendously energetic 'outburst,' a vigorous storm that reaches extreme high altitudes," de Pater said. "The storms are surrounded by darker areas, bluish-grey in the visible, indicative of 'clearings' in the cloud deck."

To confirm the presence of such clearings, the team obtained data at longer wavelengths (5 micron) sensitive to thermal emission from Jupiter’s deep atmosphere. These data confirm that the visibly dark material indeed is being seen through holes in the cloud deck, "perhaps signaling the start of the SEB revival," added Glenn Orton. 

The white band wasn't the only change on the big, gaseous planet. At the same time, Jupiter's Great Red Spot became a darker red color. Orton said the color of the spot — a giant storm on Jupiter that is three times the size of Earth and a century or more old — will likely brighten a bit again as the South Equatorial Belt makes its comeback.

The South Equatorial Belt underwent a slight brightening, known as a "fade," just as NASA's New Horizons spacecraft was flying by on its way to Pluto in 2007. Then there was a rapid “revival” of its usual dark color three to four months later. The last full fade and revival was a double-header event, starting with a fade in 1989, revival in 1990, then another fade and revival in 1993. Similar events have been captured visually and photographically back to the early 20th century, and they are likely to be a long-term phenomenon in Jupiter’s atmosphere.

This false-color image, taken Nov. 11 by the Keck telescope (left), shows sunlight reflected off Jupiter's upper cloud deck — the same clouds that are seen in visible light. The bright spot in the South Equatorial Belt is the outbreak where winds are lofting particles to high altitudes. Click here for larger downloadable image and detailed caption and credits. (Credit: UC Berkeley, University of Toronto, University of San Carlos, Philippines)

Scientists are particularly interested in this event because it’s the first time they've been able to use modern instruments to determine the details of the chemical and dynamical changes of this phenomenon.  "These observations may help to unravel the mystery of why this transition occurs, and may allow us to understand the longevity of Jupiter's belt/zone structure," added Leigh Fletcher, a scientist at Oxford University in England.

The event also signifies another close collaboration between professional and amateur astronomers. The amateurs, located worldwide, are often well equipped with instrumentation and are able to track the rapid developments of planets in the solar system. These amateurs are collaborating with professionals to further study the changes that are of great value to scientists and researchers everywhere.

"I was fortunate to catch the outburst," Go said. "I had a meeting that evening, and it went late. I caught the outburst just in time as it was rising. Had I imaged earlier, I would not have caught it."

Go witnessed the disappearance of the stripe earlier this year, and in 2007 he was the first to catch the stripe's return. "I was able to catch it early this time around because I knew exactly what to look for," he said.

Since the discovery of the first spot, there have been several more outbreaks of varying strengths. The SEB revival is happening fast, with violent eruptions, de Pater said.  Observing this event carefully may help to refine the scientific questions that will be posed by NASA’s Juno spacecraft, due to arrive at Jupiter in 2016, and a larger mission to orbit Jupiter and explore its satellite Europa after 2020.  More observations at near- and mid-infrared wavelengths are planned for the coming weeks.

Image Captions:

Keck Images:  False color images of Jupiter and the SEB outbreak taken with the 10-meter W.M. Keck telescope in Hawaii on UT November 11, 2010, just 2 days after the initial discovery of the "outbreak". This false color composite is constructed from images taken in narrow-band filters centered at 1.21 micron (green), 1.58 micron (red), and 1.65 micron (blue). At 1.21 and 1.58 micron we see sunlight reflected off Jupiter's upper cloud deck - the same clouds that are seen in visible light. The narrow band image at 1.65 micron shows sunlight reflected back from hazes just above these clouds. The bright "spot" in the SEB is the outbreak where winds are lofting particles to high altitudes. Image Credit: James Graham (University of California, Berkeley and UofToronto/Dunlap Institute), Shelley Wright, Imke de Pater, Michael Wong (University of California, Berkeley). Christopher Go (University of San Carlos, Philippines) sharpened the images slightly using the RegiStax software, developed by Cor Berrevoets.

IRTF Images:  False color images of Jupiter and the SEB outbreak taken with the 3-meter NASA Infrared Telescope Facility in Hawaii on UT November 16, 2010. This image is a false color composite at three wavelengths that probe diagnostic altitudes in Jupiter's atmosphere: 1.58 microns (blue) sensitive to sunlight reflected from Jupiter's main cloud deck - also detected in visible light; 4.00 micron (green) detects sunlight reflected from higher-altitude particles well above the main deck; and 4.85 (red) micron detects the thermal emission arising from the tops of Jupiter's clouds, with the hottest emissions coming from the deepest atmosphere, and signifying regions with minimal overlying cloud cover. The prominent region just to the left of the center, and expanded in the insert, shows the region of the South Equatorial Belt (SEB) outbreak. The initial outbreak is identified at the upper right, with a second outbreak to the lower left. Between them, in red, is a region of clear atmosphere, probably the result of downwelling from the easternmost plume. In the coming weeks, further outbreaks are expected to take place to the west (left) of those seen in this image. The clear atmospheric regions (in red) will begin to fill this latitude band at the same time as the dark brown color typical of this region returns. Image credit: Glenn Orton, Padma Yanamandra-Fisher, Gregorio Villar (Jet Propulsion Laboratory, California Institute of Technology), David Griep (Institute for Astronomy, University of Hawaii), Leigh Fletcher (University of Oxford), Imke de Pater and Michael Wong (University of California, Berkeley).