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The Changing Orientation of Saturn's Rings
The first clear detection of X-rays from the giant, gaseous planet Saturn has been made with NASA's Chandra X-ray Observatory. Chandra's image shows that the X-rays are concentrated near Saturn's equator, a surprising result since Jupiter's X-ray emission is mainly concentrated near the poles. Existing theories cannot easily explain the intensity or distribution of Saturn's X-rays.
Chandra observed Saturn for about 20 hours in April of 2003. The spectrum, or distribution with energy of the X-rays, was found to be very similar to that of X-rays from the Sun.
"This indicates that Saturn's X-ray emission is due to the scattering of solar X-rays by Saturn's atmosphere," said Jan-Uwe Ness, of the University of Hamburg in Germany and lead author of a paper discussing the Saturn results in an upcoming issue of Astronomy & Astrophysics. "It's a puzzle, since the intensity of Saturn's X-rays requires that Saturn reflects X-rays fifty times more efficiently than the Moon."
The observed 90 megawatts of X-ray power from Saturn's equatorial region is roughly consistent with previous observations of the X-radiation from Jupiter's equatorial region. This suggests that both giant, gaseous planets reflect solar X-rays at unexpectedly high rates. Further observations of Jupiter will be needed to test this possibility.
The weak X-radiation from Saturn's south-polar region presents another puzzle (the north pole was blocked by Saturn's rings during this observation). Saturn's magnetic field, like that of Jupiter, is strongest near the poles. X-radiation from Jupiter is brightest at the poles because of auroral activity due to the enhanced interaction of high-energy particles from the Sun with its magnetic field. Since spectacular ultraviolet polar auroras have been observed to occur on Saturn, Ness and colleagues expected that Saturn's south pole might be bright in X-rays. It is not clear whether the auroral mechanism does not produce X-rays on Saturn, or for some reason concentrates the X-rays at the north pole.
"Another interesting result of the observation is that Saturn's rings were not detected in X-rays," noted Scott Wolk of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, a coauthor of the paper. "This requires Saturn's rings to be less efficient at scattering X-rays than the planet itself."
The same team detected X-radiation from Saturn using the European Space Agency's XMM-Newton Observatory. Although these observations could not locate the X-rays on Saturn's disk, the intensity of the observed X-rays was very similar to what was found with Chandra and consistent with a marginal detection of X-rays from Saturn reported in 2000 using the German Roentgensatellite ( ROSAT).
The research team, which used Chandra's ACIS instrument to observed Saturn, also included J. Schmitt (Univ. of Hamburg) as well as Konrad Dennerl and Vadim Burwitz (Max Planck Institute, Garching Germany). NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA's Office of Space Science, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.
University of California at Berkeley Press Release, December 18, 2002Mauna Kea, Hawaii - Teams of astronomers at the California Institute of Technology and the University of California, Berkeley, have discovered methane clouds near the south pole of Saturn's largest moon, Titan, resolving a fierce debate about whether clouds exist amid the haze of the moon's atmosphere.
The observations were made in December 2001 using the W. M. Keck II 10-meter and the Gemini North 8-meter telescopes atop Hawaii's Mauna Kea volcano. Both telescopes are outfitted with adaptive optics that provide unprecedented detail of features not seen even by the Voyager spacecraft during its flyby of Saturn and Titan.
Analyses of the observations are being published by the Caltech team in the Dec. 19 issue of Nature and by the UC Berkeley and NASA Ames team in the Dec. 20 issue of the Astrophysical Journal.
Titan is Saturn's largest moon, larger than the planet Mercury, and is the only moon in the solar system with a thick atmosphere. Like Earth's atmosphere, Titan's is mostly nitrogen. Unlike Earth, Titan is inhospitable to life because of a lack of atmospheric oxygen and because of its extremely cold surface temperature - minus 183 degrees Celsius (-297 degrees Fahrenheit). Along with nitrogen, Titan's atmosphere contains a significant amount of methane.
Earlier spectroscopic observations had hinted at the existence of clouds on Titan, but gave no clue as to their location. These early data were hotly debated, since Voyager spacecraft measurements of Titan appeared to show a calm and cloud-free atmosphere. Furthermore, previous images of Titan had failed to reveal clouds, finding only unchanging surface markings and very gradual seasonal changes in the haziness of the atmosphere.
Improvements in the resolution and sensitivity achievable with ground-based telescopes led to the present discovery. The observations used adaptive optics, in which a flexible mirror rapidly compensates for the distortions caused by turbulence in Earth's atmosphere. These distortions cause the well known twinkling of the stars. Using adaptive optics, details as small as 300 kilometers (186 miles) across can be distinguished at the enormous distance of Titan, 1.3 billion kilometers (820 million miles), equivalent to reading an automobile license plate from 100 kilometers (63 miles) away.
The images presented by the two teams clearly show bright clouds near Titan's south pole.
"We see the intensity of the clouds varying over as little as a few hours," said post-doctoral fellow Henry Roe, lead author for the UC Berkeley group. "The clouds are constantly changing, although some persist for as long as a few days."
Titan experiences seasons much like the Earth, though its year is 30 times longer due to Saturn's distant orbit from the sun. Titan is currently in the midst of southern summer, and its south pole has been in continuous sunlight for more than six Earth years. The researchers believe that this fact may explain the location of these large clouds.
"These clouds appear to be similar to summer thunderstorms on Earth, but formed of methane rather than water. This is the first time we have found such a close analogy to the Earth's atmospheric water cycle in the solar system," says Antonin Bouchez, one of the Caltech researchers.
In addition to the clouds above Titan's south pole, the Keck images, like previous data, reveal the bright continent-sized feature that may be a large icy highland on Titan's surface, surrounded by linked dark regions which are possibly ethane seas or tar-covered lowlands.
"These are the most spectacular images of Titan's surface which we've seen to date," says Michael Brown, associate professor of planetary astronomy and lead author of the Caltech paper. "They are so detailed that we can almost begin to speculate about Titan's geology, if only we knew for certain what the bright and dark regions represented."
In 2004, Titan will be visited by NASA's Cassini spacecraft, which will look for clouds on Titan during its multi-year mission around Saturn.
"Changes in the spatial distribution of these clouds over the next Titan season will help pin down their detailed formation process," says Imke de Pater, professor of astronomy at UC Berkeley. The Cassini mission includes a probe named Huygens that will descend by parachute into Titan's atmosphere and land on the surface near the edge of the bright continent.
The team conducting the Gemini observations consists of Roe and de Pater from UC Berkeley, Bruce A. Macintosh of Lawrence Livermore National Laboratory and Christopher P. McKay of the NASA Ames Research Center. The team reporting results from the Keck telescope consists of Brown and Bouchez of Caltech and Caitlin A. Griffith of the University of Arizona.
The Gemini Observatory is an international partnership managed by the Association of Universities for Research in Astronomy under a cooperative agreement with the National Science Foundation, represented in the U.S. by the National Optical Astronomy Observatory. The W.M. Keck Observatory is operated by the California Association for Research in Astronomy, a scientific partnership between the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. This research has been funded in part by grants from NSF and NASA.
We're nearly there.
LPL Press Release, November 1, 2002, from Carolyn Porco
Colleagues, Friends, and Fans of Cassini,
Cassini has sighted Saturn, and today the Imaging Team is releasing a color composite image of the planet and its moon, Titan, taken from a distance of 285 million kilometers....twice the distance from the Sun to the Earth.
Cassini Imaging Page
After an adventurous 7-year long tour among the planets, the Cassini-Huygens spacecraft will arrive at Saturn in July 2004. Once there, Cassini will parachute the Huygens probe to Saturn's biggest satellite, Titan. Titan is thought to have an atmosphere similar to the primitive Earth. However, both the probe and the Cassini-Huygens team are not in idle state until 2004. They have plenty of things to keep them busy.
PASADENA, Calif., Feb. 11 (AScribe Newswire) -- NASA's Cassini spacecraft continues to fly in good health with less than 29 months to go before it becomes the first Earth envoy to enter orbit around Saturn.
Last month, Cassini completed a 40-day period of data collection as part of a multi-year search for gravitational waves. The data comes from radio transmissions between Cassini and stations of NASA's Deep Space Network in California, Spain and Australia.
The experiment used frequencies both in the X-band, which is the band commonly used by interplanetary spacecraft, and in the higher-frequency Ka-band, a new band for the Deep Space Network. Data was successfully collected for 90 percent of the possible transmission time in the Ka-band, a promising beginning for future uses of that band by Cassini and other spacecraft. In the traditional X-band, data was received for 98 percent of the possible time over the 40-day
Gravitational waves are ripples in the fabric of space and time that are set off by acceleration of massive bodies, such as black holes or supernovas. Their existence has been confirmed indirectly, but never detected experimentally. This search assesses the Doppler effect on radio waves traveling between Cassini and Earth. The Doppler effect is how the frequency of a transmission is affected by the relative speed between the sender and receiver, such as the raised pitch of an approaching train's whistle.
Scientists are looking for barely perceptible fluctuations that would be caused in Cassini's speed relative to Earth if gravitational waves of certain wavelengths were traveling through the solar system. They expect analysis of the data to take months. Cassini will be used for two more periods of gravitational wave investigation before it reaches Saturn.
Engineers are making progress at correcting a problem of haze on the spacecraft's narrow-angle camera. Warming the camera for a week to a temperature just above freezing has significantly reduced the problem, so that treatment will be repeated for a longer period beginning March 5.
"We're fully confident it is going to get better," said Robert Mitchell, Cassini-Huygens program manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif.
The usual operating temperature for the camera is minus 90 Celsius (minus 130 Fahrenheit). Haze on its optics appeared when it was cooled to that temperature after a routine-maintenance heating of the instrument to 30 C (86 F). That occurred following flawless imaging of Jupiter for several months of 2000 and 2001. Heating the camera again, but to only 4 C (39 F), is removing the haze. Test images taken of a star in late January showed the improvement.
Cassini will reach Saturn on July 1, 2004, and release its piggybacked Huygens probe about six months later for descent through the thick atmosphere of the moon Titan on Jan. 14, 2005. Cassini-Huygens is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Office of Space Science, Washington, D.C. Additional information about Cassini-Huygens is available online at http://saturn.jpl.nasa.gov .
University of Arizona Press Release
Astronomers have discovered 12 more moons around Saturn. And they have evidence that these once were just 3 or 4 moons, minding their business, orbiting the planet like all regular saturnian moons do today.
The 12 new-found moons are in irregular orbits that suggest they are the collisional remnants of larger parent moons, once securely captured in, but later blasted from, their saturnian orbits.
Using several medium-to-large sized telescopes, large-format CCD arrays that photograph big areas of sky, and computers that process multiple gigabytes of data each night, teams of astronomers collaborated last fall in a search for so-called "irregular" moons around the gas giant.
Saturn was known to have six relatively large moons and 12 minor moons. All except one minor moon, Phoebe, discovered in 1898, are classified as regular satellites because they move along nearly circular orbits in the planet's orbital plane, revolving in the same direction as the planet spins.
The 12 new-found satellites are irregular - meaning they orbit outside the plane of Saturn's equator - and it appears that their orbits cluster in three, possibly four, distinct groups, said Carl W. Hergenrother of the UA Lunar and Planetary Laboratory (LPL).
"We think we're seeing orbits cluster, that is, orbits of several moons fall in the same general plane, just as asteroids cluster," Hergenrother said. "And with asteroids that cluster, the belief is they are pieces of what once was a big asteroid that got hit by something. It's possible that we're seeing the same thing with the satellites."
Brett Gladman of the Observatoire de la Cote d'Azur in France, J.J. Kavelaars of McMaster University in Canada, and Matthew Holman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., discovered the irregular saturnian moons in August, September and November, 2000, using the 2.2-meter (87-inch) European Southern Observatory in Chile, the 3.6-meter (142-inch) Canada-France-Hawaii Telescope in Hawaii, and the 1.2-meter (48-inch) Mount Hopkins telescope in Arizona.
Hergenrother, Stephen M. Larson and Rob Whiteley - all of the LPL - and Dennis Means of the UA Steward Observatory used the Steward Observatory's 1.5-meter telescope (61-inch) in the Santa Catalina Mountains north of Tucson and the 2.3-meter (90-inch) Bok Telescope on Kitt Peak to observe the moons for more precise information on their orbits.
Others doing this "recovery" work to help define the satellite orbits used the 4-meter Kitt Peak telescope, the 5-meter Palomar telescope and 2-to-3-meter class European telescopes.
The research is reported in the article, "Discovery of 12 satellites of Saturn exhibiting orbital clustering," in the July 12 Nature.
Astronomers in 1997 and 1999 discovered five irregular satellites around Uranus, and in 1999 - 2000 discovered another 12 irregular satellites around Jupiter, previously known to have eight. The UA Spacewatch on Kitt Peak discovered one of the new-found jovian moons.
Almost all of the irregulars discovered since 1997 cluster in easily discernible groupings, the astronomers note in their article.
"The difficult question is whether the disruptions occurred during the capture process itself when the planets formed long ago, or whether intact moons were captured at that time into orbits near the present grouping and these single moons were subsequently shattered and scattered by intruding comets or asteroids during the subsequent (more than 4-billion-year solar system history)," they wrote.
The most probable theory is that each cluster is the remains of a once-intact moon smashed by a collision sometime after the planets were formed, according to their analysis.
Saturn must have captured the original parent moons during planetary formation, as the objects passed through Saturn's surrounding proto-planetary gas cloud, Hergenrother said.
An alternative theory is that the moons were captured when Saturn suddenly increased in mass - in which case the moons would all be prograde, moving around the planet in the same direction as the planet moves around the sun.
"But we are seeing just as many retrograde as prograde irregular moons at Saturn," Hergenrother said. Objects captured as moons would move in either prograde or retograde orbits depending on their direction as they passed through and were slowed by proto-Saturn's gas cloud.
Satellites in orbital clusters could range in size from one to 100 kilometers in diameter, he added.
"Right now, we see irregular satellites as small as 3 kilometers around Saturn, but there may be many smaller than that. These may go on a continuum in size all the way down to the size of dust. "