Universe Today

As NASA’s Cassini spacecraft approached Saturn last July, it found evidence that lightning on Saturn is roughly one million times stronger than lightning on Earth.

That’s just one of several Cassini findings that University of Iowa Space Physicist Don Gurnett will present in a paper to be published Thursday, Dec. 16, in Science Express, an online version of the journal Science, and in a talk to be delivered Friday, Dec. 17, at a meeting of the American Geophysical Union in San Francisco.

Other findings include:
–Cassini impacted dust particles as it traversed Saturn’s rings.
–Saturn’s radio rotation rate varies.

The comparison between Saturn’s enormously strong lightning and Earth’s lightning began several years ago as the Cassini spacecraft prepared for its journey to Saturn by swinging past the Earth to receive a gravitational boost. At that time, Cassini started detecting radio signals from Earth’s lightning as far out as 89,200 kilometers from the Earth’s surface. In contrast, as Cassini approached Saturn, it started detecting radio signals from lightning about 161 million kilometers from the planet. “This means that radio signals from Saturn’s lightning are on the order of one million times stronger than Earth’s lightning. That’s just astonishing to me!” says Gurnett, who notes that some radio signals have been linked to storm systems observed by the Cassini imaging instrument.

Earth’s lightning is commonly detected on AM radios, a technique similar to that used by scientists monitoring signals from Cassini.

Regarding Saturn’s rings, Gurnett says that the Cassini Radio and Plasma Wave Science (RPWS) instrument detected large numbers of dust impacts on the spacecraft. Gurnett and his science team found that as Cassini approached the inbound ring plane crossing, the impact rate began to increase dramatically some two minutes before the ring plane crossing, then reached a peak of more than 1,000 per second at almost exactly the time of the ring plane crossing, and finally decreased to pre-existing levels about two minutes later. Gurnett notes that the particles are probably quite small, only a few microns in diameter, otherwise they would have damaged the spacecraft.

Finally, variations in Saturn’s radio rotation rate came as a surprise. Based upon more than one year of Cassini measurements, the rate is 10 hours 45 minutes and 45 seconds, plus or minus 36 seconds. That’s about six minutes longer than the value recorded by the Voyager 1 and 2 flybys of Saturn in 1980-81. Scientists use the rotation rate of radio emissions from the giant gas planets such as Saturn and Jupiter to determine the rotation rate of the planets themselves because the planets have no solid surfaces and are covered by clouds that make direct visual measurements impossible.

Gurnett suggests that the change in the radio rotation rate is difficult to explain. “Saturn is unique in that its magnetic axis is almost exactly aligned with its rotational axis. That means there is no rotationally induced wobble in the magnetic field, so there must be some secondary effect controlling the radio emission. We hope to nail that down during the next four to eight years of the Cassini mission.”

One possible scenario was suggested nearly 20 years ago. Writing in the May 1985 issue of “Geophysical Research Letters,” Alex J. Dessler, a senior research scientist at the Lunar and Planetary Laboratory, University of Arizona, argued that the magnetic fields of gaseous giant planets, such as Saturn and Jupiter, are more like that of the sun than of the Earth. The sun’s magnetic field does not rotate as a solid body. Instead, its rotation period varies with latitude. Commenting earlier this year on the work of Gurnett and his team, Dessler said, “This finding is very significant because it demonstrates that the idea of a rigidly rotating magnetic field is wrong. Saturn’s magnetic field has more in common with the sun than the Earth. The measurement can be interpreted as showing that the part of Saturn’s magnetic field that controls the radio emissions has moved to a higher latitude during the last two decades.”

The radio sounds of Saturn’s rotation — resembling a heartbeat — and other sounds of space can be heard by visiting Gurnett’s Web site at: http://www-pw.physics.uiowa.edu/space-audio

Cassini, carrying 12 scientific instruments, on June 30, 2004, became the first spacecraft to orbit Saturn and begin a four-year study of the planet, its rings and its 31 known moons. The $1.4 billion spacecraft is part of the $3.3 billion Cassini-Huygens Mission that includes the Huygens probe, a six-instrument European Space Agency probe, scheduled to land on Titan, Saturn’s largest moon, in January 2005.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology, Pasadena, Calif. manages the Cassini-Huygens mission for NASA’s Office of Space Science, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter. For the latest images and information about the Cassini-Huygens mission, visit: http://www.nasa.gov/cassini.

Original Source: UI News Release

Ice particles are key players in the ever-changing panorama at Saturn, according to a new study led by a University of Colorado at Boulder professor using an instrument on the Cassini-Huygens spacecraft now at the ringed planet.

Larry Esposito of the Laboratory for Atmospheric and Space Physics said data from the Ultraviolet Imaging Spectrometer, or UVIS, indicates much of Saturn’s system is filled with ice, as well as atoms derived from water. Esposito is the principal investigator for the $12.5 million UVIS instrument riding on the craft.

Esposito said hydrogen and oxygen atoms are widely distributed in the planetary system, which extends millions of miles outward from Saturn. Cassini researchers are seeing large fluctuations in the amount of oxygen in the Saturn system, he said.

“A possible explanation for the fluctuation in oxygen is that small, unseen icy moons have been colliding with Saturn’s E ring,” said Esposito. “The collisions may have produced small grains of ice, which yielded oxygen atoms when struck by energetic, charged particles in Saturn’s magnetosphere. UVIS is able to identify these glowing atoms.”

A paper on the subject authored by Esposito and colleagues appears in the Dec. 16 issue of Science Express, the online version of Science magazine. Esposito also will give a presentation on the new results from the Cassini-Huygens mission at the Fall Meeting of the American Geophysical Union, being held this week through Friday in San Francisco.

Saturn’s ring particles may have formed originally from pure ice, Esposito said. But they have since been subjected to continual bombardment by meteorites, which has contaminated the ice and caused the rings to darken.

Over time, incessant meteorite bombardment has likely spread the dirty material resulting from the collisions widely among the ring particles, he said. But instead of uniformly dark rings, the UVIS instrument is recording “radial variations” that show brighter and darker bands in the individual rings.

“The evidence indicates that in the last 10 million to 100 million years, fresh material probably was added to the ring system,” he said. The research team proposed that such “renewal events” are from the fragmentation of small moons, each probably about 20 kilometers (12 miles) across.

“The interiors of the tiny moonlets, which have been shielded from contamination by the continual collisions with each other, are the source of purer water ice,” he said. “Both the oxygen fluctuation and the spectral variation in Saturn’s rings support a model of ring history in which small moons are continually destroyed to produce new rings.”

The ice grains released by the continual moonlet collisions are bathed by Saturn’s radiation belt, liberating the oxygen atoms that are seen by UVIS in the ultraviolet as they reflect sunlight in the immense cloud surrounding Saturn, said Esposito.

Other authors on the Science Express paper include LASP’s Joshua Colwell, Kristopher Larsen, William McClintock and Ian Stewart. Researchers from the University of Southern California, NASA’s Jet Propulsion Laboratory, Central Arizona University, the California Institute of Technology and the Max Planck Institute and Stuttgart University in Germany also co-authored the paper.

Launched in 1997, the Cassini-Huygens spacecraft achieved Saturn orbit June 30. During the spacecraft’s four-year tour of the Saturn system, the UVIS team will continue to track the dynamic interactions of the planet’s rings, moons and radiation belts, Esposito said.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, Calif., manages the Cassini-Huygens mission for NASA’s Science Mission Directorate in Washington, D.C.

Original Source: UCB News Release

Cassini’s second close flyby of Titan completes a ‘before’ and ‘after’ look at the fuzzy moon and provides the first direct evidence of changing weather patterns in the skies over Titan.

In images obtained less than two months ago, the Titan skies were cloud free, except for a patch of clouds observed over the moon’s south pole. In images taken Monday, Dec. 13, during Cassini’s second close flyby of Titan, several extensive patches of clouds have formed.

“We see for the first time discrete cloud features at mid-latitudes, which means we see direct evidence of weather, and we can get wind speeds and atmospheric circulation over a region we hadn?t been able to measure before,” said Dr. Kevin Baines, Cassini science-team member with the visual and infrared mapping spectrometer, from NASA’s Jet Propulsion Laboratory, Pasadena, Calif.

The latest data and other results from Cassini’s close observations of Saturn’s moons Titan and Dione were presented today at a news conference during the American Geophysical Union fall meeting in San Francisco.

Cassini swept within 1,200 kilometers (750 miles) of Titan’s surface on Monday, and took a close look at the icy moon Dione just one day later. During the flyby, Cassini captured a stunning view of Titan’s night side with the atmosphere shimmering in its own glow. This allows scientists to study the detached haze layers, which extend some 400 kilometers (249 miles) above Titan.

Images from Cassini’s cameras show regions on Titan that had not been seen clearly before, as well as fine details in Titan?s intermittent clouds. The surface features may be impact related, but without information on their height, it is too soon to know for sure. No definitive craters have been seen in these images, though several bright rings or circular features are seen in dark terrain.

Cassini imaging scientists are intrigued by the complex braided structure of surface fractures on Dione. To the surprise of scientists, the wispy terrain features do not consist of thick ice deposits, but bright ice cliffs created by tectonic features. ?This is one of the most surprising results so far. It just wasn?t what we expected,? said Dr. Carolyn Porco, Cassini imaging team leader, Space Science Institute, Boulder, Colo.

Other Cassini results presented at the meeting included observations made by the ultraviolet imaging spectrograph instrument, which indicates that the nearby environment of the rings and moons in the Saturn system is filled with ice, and atoms derived from water. Cassini researchers are seeing large changes in the amount of oxygen atoms in the Saturn system. A possible explanation for the fluctuation in oxygen is that small, unseen icy moons have been colliding with Saturn’s E ring,” said Dr. Larry Esposito, principal investigator of the imaging spectrograph instrument, University of Colorado, Boulder, Colo. “These collisions may have produced small grains of ice, which yielded oxygen atoms.” Esposito presented these findings at the meeting, and a paper on the subject appears in the online version of the journal Science.

According to Esposito, Saturn’s ring particles may have formed originally from pure ice. But they have since been subjected to continual bombardment by meteorites, which has contaminated the ice and caused the rings to darken. Over time, continuous meteorite bombardment has likely spread the dirty material resulting from the collisions over a wide area in the rings. “The evidence indicates that in the last 10 to 100 million years, fresh material probably was added to the ring system,” said Esposito. These renewal events are from fragments of small moons, each probably about 20 kilometers (12 miles) across.

Images and more information about the Cassini mission are available at http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini .

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the Cassini mission for NASA’s Science Mission Directorate, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter. The European Space Agency built and managed the development of the Huygens probe and is in charge of the probe operations. The Italian Space Agency provided the high-gain antenna, much of the radio system and elements of several of Cassini’s science instruments.

Original Source: NASA/JPL News Release

Cassini captured Dione against the globe of Saturn as it approached the icy moon for its close rendezvous on Dec. 14, 2004. This natural color view shows the moon has strong variations in brightness across its surface, but a remarkable lack of color, compared to the warm hues of Saturn’s atmosphere. Several oval-shaped storms are present in the planet’s atmosphere, along with ripples and waves in the cloud bands.

The images used to create this view were obtained with the Cassini spacecraft wide-angle camera at a distance of approximately 603,000 kilometers (375,000 miles) from Dione through a filter sensitive to wavelengths of ultraviolet light centered at 338 nanometers. The Sun-Dione-spacecraft, or phase, angle is 34 degrees. The image scale is about 32 kilometers (20 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, 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 imaging team is based at the Space Science Institute, Boulder, Colo.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org.

Original Source: NASA/JPL/SSI News Release

Using adaptive optics on the Gemini North and Keck 2 telescopes on Mauna Kea, Hawai’i, a U.S. team has discovered a new phenomenon in the atmosphere of Saturn?s largest moon Titan.

Unlike previous observations showing storms at the south pole, these new images reveal atmospheric disturbances at Titan?s temperate mid latitudes?about halfway between the equator and the poles. Explaining the unexpected activity has proven difficult, and the team speculates that the storms could be driven by anything from short-term surface events to shifts in global wind patterns.

?We were fortunate to catch these new mid-latitude clouds when they first appeared in early 2004,” said team leader Henry Roe (California Institute of Technology). “We are not yet certain how their formation is triggered. Continued observations over the next few years will show us whether these clouds are the result of a seasonal change in weather patterns or a surface-related phenomenon.”

The causes of these storms might include activities that disturb the atmosphere from the surface. It?s possible that geysers of methane ?slush? are brewing from below, or a warm spot on Titan?s surface is heating the atmosphere. Cryovolcanism?volcanic activity that spews an icy mix of chemicals?has also been suggested as one mechanism that would cause disturbances. It?s also possible that the storms are driven by seasonal shifts in the global winds that circulate in the upper atmosphere. Hints about what is happening on this frigid world could be obtained as the Huygens probe from the Cassini mission drops through Titan?s atmosphere in mid-January, 2005.

The Gemini-Keck II observations were the result of good timing and telescope availability. According to Gemini scientist Chad Trujillo, Titan?s weather patterns can be stable for many months, with only occasional bursts of unusual activity like these recently discovered atmospheric features. The chances of catching such occurrences depend largely on the availability of flexible scheduling like that used at Gemini. “This flexible scheduling is absolutely critical to Titan meteorology studies,? he said. ?Imagine how hard it would be to understand the Earth’s diverse meteorological phenomena if you only saw a weather report a few nights every year.”

Like Earth, Titan is surrounded by a thick atmosphere of mostly nitrogen. Conditions on Earth allow water to exist in liquid, solid, or vapor states, depending on localized temperatures and pressures. The phase changes of water between these states are an important factor in the formation of weather in our atmosphere. Titan’s atmosphere is so cold that any water is frozen solid, but conditions are such that methane can move between liquid, solid, and gaseous states. This leads to a methane meteorological cycle on Titan in analogy to the water-based weather cycle on Earth.

As it does on Earth, seasonal solar heating can drive atmospheric activity on Titan, and this could be the mechanism behind the previously observed south polar clouds. However, the new temperate-latitude cloud formations cannot be explained by the same solar heating process If a seasonal circulation shift is causing the newly discovered features, the team theorizes that they will drift northward over the next few years as Titan?s year progresses through the southern summer and into autumn. If it is being caused by geological changes, such as methane geysers or a geologic ?warm? spot on the surface, the feature should stay at the observed 40-degree latitude as the surface activity spurs changes in atmospheric convection and methane cloud formation. Continued storm formations will be easily distinguishable in future ground-based observations using Gemini, Keck and other adaptive-optics enabled telescopes.

?Using adaptive optics from the Earth allows us to see things that just a few years ago would have been invisible,? said Keck Scientist Antonin Bouchez. ?These observations show that ground-based telescopes are a perfect complement to space missions like Cassini.?

This research is scheduled for publication in the January 1, 2005 issue of the Astrophysical Journal.

Original Source: Gemini News Release

Dione is a small heavily cratered moon 560 km (350 miles) in diameter orbiting Saturn once every 2.73 days, which is actually the same period as its axial rotation. The moon lies at a distance of 377,400 kilometres (234,555 miles) from its parent planet. Dione is also known to share its orbit around Saturn with a small asteroid named Helene, that occupies a stable point ahead of it in orbit. The surface temperature of Dione is very similar to that of Titan at -186 degrees centigrade, and cryogenic activity is known to have helped shape the moon?s surface, although, unlike Titan, Dione has no known atmosphere.

The Cassini flyby this week has helped to confirm that some time in the moon?s past there were two episodes of cryo-volcanic flooding widely spaced in time that affected different regions. It is believed these episodes may be the result of tidal heating caused by the orbital interaction with another of Saturn?s moons named Enceladus.

As the Cassini Orbiter passed by Dione it took detailed images of an uncharted surface region of the area known as the ?Trailing Hemisphere,? centered on Latitude 0?, and Longitude 270? that is dominated by three craters, one large named Amata, with two smaller craters nearby named Catillus and Coras. Geologically speaking, this is the most interesting area of the moon for planetary astronomers, because this region of Dione is marked by two distinct white ray systems.

The largest, Palatine Linea, streaks down towards the moon?s south polar region that ends with a large unnamed crater. The new Cassini images show this area to be composed of long linear groves, and rills, with intermittent small craters at varying distances.

The second white linear feature, named Padua Linea, is about half the size Palatine Linea, and is also crossed with linear rills that stretch from Dione?s equator at Longitude 240 ? down to the southeast, ending at Latitude -20?. The largest crater dominating the area named Cassandra is shown prominently in the Cassini photographs.

Cassini also imaged the ?Leading Hemisphere? for the first time between Longitude 180 ? 145 degrees, at Latitude of around 40 degrees either side of the moon?s equator, and again it is shown to be covered in small impact craters.

All of the eight images returned by Cassini today show that much of Dione is heavily potholed with small craters with intermittent large impact craters at wide intervals, all of which are uncharted and unnamed. Therefore, Cassini has confirmed what planetary scientists had believed all along; that the resurfacing events on Dione must have taken place long before the resurfacing of Enceladus, because Dione?s least cratered areas have far more craters than those on Enceladus itself.

While the images of Dione returned today are the best ever close range images taken of this moon, the Cassini space craft is due to fly by Dione even closer next October, when it will pass just 500 Km (311 miles) above the moon?s surface.

The moon Dione has a visual magnitude of +10.4 so that it’s visible in medium sized telescopes, and amateur astronomers can view the moon over the next few months for themselves. Saturn is at opposition on 13 January, and lies in the zodiac constellation of Gemini (The Twins).

By Science Correspondent Richard Pearson