On Tuesday, March 2, 2010, NASA's Cassini spacecraft made its closest encounter yet with Saturn’s second largest moon. This is the mission’s second targeted flyby of the moon in the mission, so it is sometimes referred to as R-2.
Blazing through its closest pass of the Saturnian moon Mimas on Feb. 13, Cassini sent back prominent close-ups of the moon likened to the Death Star from "Star Wars" and the massive crater scarring its surface. The flyby also yielded solid data on the moon's thermal signature and surface composition.
Some of the raw, unprocessed pictures sent back from the flyby show the bright, steep slopes of the giant Herschel Crater, which measures about 140 kilometers (88 miles) wide. The icy slopes appear to be pitched around 24 degrees, which would possibly earn them a black- or double-black-diamond rating on Earth. Olympic downhill skiers could probably tear down these runs with ease, but it's clear Mimas is no place for bunny-slope beginners.
The images, which have the highest resolution so far, also explain jumbled terrain inside the crater and many craters within the crater. These features hint at a long history, which scientists will be working meticulously to analyze.
"This flyby has been like looking at a cell or an onion skin under the microscope for the first time," said Bonnie Buratti, one of the leads for the Satellite Orbiter Science Team. "We'd seen the large crater from afar because the early 1980s, but now its small bumps and blemishes are all obviously visible."
This encounter took the spacecraft as close as about 9,500 kilometers (5,900 miles) above Mimas. Cassini had to contrive through a dusty region to get in position, but survived the trip untouched, as expected.
The moon averages 396 kilometers (246 miles) in diameter. The walls of Herschel Crater are about 5 kilometers (3 miles) high, and parts of the floor are approximately 10 kilometers (6 miles) deep.
Unprocessed images of the flyby are available at http://saturn.jpl.nasa.gov/photos/raw/. More information about the Cassini mission is at http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.
NASA will extend the international Cassini-Huygens mission to explore Saturn and its moons to 2017. The agency's fiscal year 2011 budget provides a $60 million per year extension for continued study of the ringed planet.
"This is a mission that never stops providing us surprising scientific results and showing us eye popping new vistas," said Jim Green, director of NASA's planetary science division at NASA Headquarters in Washington. "The historic traveler's stunning discoveries and images have revolutionized our knowledge of Saturn and its moons."
Cassini launched in October 1997 with the European Space Agency's Huygens probe. The spacecraft arrived at Saturn in 2004. The probe was equipped with six instruments to study Titan, Saturn's largest moon. Cassini's 12 instruments have returned a daily stream of data from Saturn's system for nearly six years. The project was scheduled to end in 2008, but the mission received a 27-month extension to Sept. 2010.
"The extension presents a unique opportunity to follow seasonal changes of an outer planet system all the way from its winter to its summer," said Bob Pappalardo, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Some of Cassini's most exciting discoveries still lie ahead."
This second extension, called the Cassini Solstice Mission, enables scientists to study seasonal and other long-term weather changes on the planet and its moons. Cassini arrived just after Saturn's northern winter solstice, and this extension continues until a few months past northern summer solstice in May 2017. The northern summer solstice marks the beginning of summer in the northern hemisphere and winter in the southern hemisphere.
A complete seasonal period on Saturn has never been studied at this level of detail. The Solstice mission schedule calls for an additional 155 orbits around the planet, 54 flybys of Titan and 11 flybys of the icy moon Enceladus.
The mission extension also will allow scientists to continue observations of Saturn's rings and the magnetic bubble around the planet known as the magnetosphere. The spacecraft will make repeated dives between Saturn and its rings to obtain in depth knowledge of the gas giant. During these dives, the spacecraft will study the internal structure of Saturn, its magnetic fluctuations and ring mass.
The mission will be evaluated periodically to ensure the spacecraft has the ability to achieve new science objectives for the entire extension.
"The spacecraft is doing remarkably well, even as we endure the expected effects of age after logging 2.6 billion miles on its odometer," said Bob Mitchell, Cassini program manager at JPL. "This extension is important because there is so much still to be learned at Saturn. The planet is full of secrets, and it doesn't give them up easily."
Cassini's travel scrapbook includes more than 210,000 images; information gathered during more than 125 revolutions around Saturn; 67 flybys of Titan and eight close flybys of Enceladus. Cassini has revealed unexpected details in the planet's signature rings, and observations of Titan have given scientists a glimpse of what Earth might have been like before life evolved.
Scientists hope to learn answers to many questions that have developed during the course of the mission, including why Saturn seems to have an inconsistent rotation rate and how a probable subsurface ocean feeds the Enceladus' jets.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the project for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL.
NASA'S Cassini spacecraft will return to Titan's southern hemisphere on a flyby tomorrow, Jan. 12, dipping to within about 1,050 kilometers (about 670 miles) of the hazy moon's surface. During this pass, the onboard radar gadget will scan Ontario Lacus, the largest lake in the southern hemisphere, in a quest to learn more about the liquid methane and ethane in the lake and obtain more comprehensive topographical information about the shoreline. Titan is the only other body in the solar system in addition to Earth that is known to have stable liquid on its surface.
This will also be the most southern pass in the mission for the ion and neutral mass spectrometer device, which will probe the composition and density of the atmosphere near Titan's south pole. The atmospheric data collected on this pass will be paired with a similar sampling mission near Titan's north pole during the most recent flyby, 16 days earlier.
Cassini last flew by Titan on Dec. 27, 2009 California time, or Dec. 28 Universal Time. Although this newest flyby is dubbed "T65," planning changes early in the orbital tour have made this the 66th targeted flyby of Titan. This flyby also comes two days before the fifth anniversary of the landing of the Huygens probe on the surface of Titan.
The Cassini-Huygens mission is a supportive 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 Huygens probe, built and managed by the European Space Agency, was bolted to Cassini and rode along during its nearly seven-year journey to Saturn, before being released for its descent through Titan's atmosphere.
Blobs of warm ice that occasionally rise to the surface and churn the icy crust on Saturn's moon Enceladus explain the unusual heat behavior and intriguing surface of the moon's south polar region, according to a new paper using data from NASA's Cassini spacecraft.
"Cassini appears to have caught Enceladus in the center of a burp," said Francis Nimmo, a planetary scientist at the University of California Santa Cruz and a co-author of the new paper in Nature Geoscience. "These confused periods are rare and Cassini happens to have been examine the moon during one of these special epochs."
The south polar region captivates scientists because it hosts the fissures known as "tiger stripes" that spray water vapor and other particles out from the moon. While the newest paper, released on Jan. 10, doesn't link the churning and resurfacing directly to the formation of fissures and jets, it does fill in some of the blanks in the region's history.
"This episodic model helps to solve one of the most puzzling mysteries of Enceladus," said Bob Pappalardo, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., of the research done by his colleagues. "Why is the south polar surface so young? How could this quantity of heat be pumped out at the moon's south pole? This idea assembles the pieces of the mystery."
About four years ago, Cassini's amalgamated infrared spectrometer instrument detected a heat flow in the south polar region of at least 6 gigawatts, the corresponding of at least a dozen electric power plants. This is at least three times as much heat as an standard region of Earth of similar area would produce, despite Enceladus' small size. The region was also later found by Cassini's ion and unbiased mass spectrometer instrument to be swiftly expelling argon, which comes from rocks decaying radioactively and has a well-known rate of decay.
Calculations told scientists it would be not possible for Enceladus to have continually produced heat and gas at this rate. Tidal movement – the pull and push from Saturn as Enceladus moves around the planet – cannot explain the release of so much energy.
The surface ages of diverse regions of Enceladus also show great diversity. Heavily cratered plains in the northern part of the moon appear to be as old as 4.2 billion years, while a region near the equator known as Sarandib Planitia is between 170 million and 3.7 billion years old. The south polar area, however, appears to be less than 100 million years old, perhaps as young as 500,000 years.
Craig O'Neill of Macquarie University in Sydney, Australia, and Nimmo, who was partly funded by the NASA Outer Planets Research program, adapted a model that O'Neill had developed for the convection of Earth's crust. For Enceladus, which has a surface totally covered in cold ice that is fractured by the tug of Saturn's gravitational pull, the scientists stiffened up the crust. They picked a strength somewhere between that of the malleable tectonic plates on Earth and the rigid plates of Venus, which are so strong, it appears they never get sucked down into the interior.
Their model showed that heat building up from the interior of Enceladus could be released in episodic bubbles of warm, light ice rising to the surface, akin to the growing blobs of heated wax in a lava lamp. The rise of the warm bubbles would send cold, heavier ice down into the interior. (Warm is, of course, relative. Nimmo said the bubbles are possibly just below freezing, which is 273 degrees Kelvin or 32 degrees Farenheit, whereas the surface is a frigid 80 degrees Kelvin or -316 degrees Farenheit.)
The model fits the activity on Enceladus when the churning and resurfacing periods are unspecified to last about 10 million years, and the quiet periods, when the surface ice is undisturbed, last about 100 million to two billion years. Their model suggests the active periods have occurred only 1 to 10 percent of the time that Enceladus has existed and have recycled 10 to 40 percent of the surface. The active area around Enceladus's south pole is about 10 percent of its surface.
The Cassini-Huygens mission is a supportive 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 mission for NASA's Science Mission Directorate in Washington. The Cassini orbiter was intended, developed and assembled at JPL.
NASA's Cassini Spacecraft has captured the first flash of sunlight reflected off a lake on Saturn's moon Titan, confirming the existence of liquid on the part of the moon dotted with many large, lake-shaped basins.
Cassini scientists had been looking for the sparkle, also known as a specular reflection, since the spacecraft began orbiting Saturn in 2004. But Titan's northern hemisphere, which has more lakes than the southern hemisphere, has been obscure in winter darkness. The sun only began to in a straight line illuminate the northern lakes recently as it approached the equinox of August 2009, the start of spring in the northern hemisphere. Titan's misty atmosphere also blocked out reflections of sunlight in the majority wavelengths. This unexpected image was captured on July 8, 2009, using Cassini's visual and infrared mapping spectrometer.
In 2008, Cassini scientists using infrared data established the presence of liquid in Ontario Lacus, the largest lake in Titan's southern hemisphere. But they were still looking for the smoking gun to confirm liquid in the northern hemisphere, where lakes are also larger. Katrin Stephan, of the German Aerospace Center (DLR) in Berlin, an associate member of the Cassini visual and infrared mapping spectrometer team, was processing the initial image and was the first to see the glint on July 10th.
"I was instantly excited because the glint reminded me of an image of our own planet taken from orbit around Earth, showing a reflection of sunlight on an ocean," Stephan said. "But we also had to do more work to make sure the glint we were seeing wasn't lightning or an erupting volcano."Team members at the University of Arizona, Tucson, processed the image further, and scientists were able to compare the new image to radar and near-infrared-light images acquired from 2006 to 2008.
They were able to compare the reflection to the southern shoreline of a lake called Kraken Mare. The extensive Kraken Mare covers about 400,000 square kilometers (150,000 square miles), an area larger than the Caspian Sea, the largest lake on Earth. It is located around 71 degrees north latitude and 337 degrees west latitude.
The finding shows that the shoreline of Kraken Mare has been steady over the last three years and that Titan has an ongoing hydrological cycle that brings liquids to the surface, said Ralf Jaumann, a visual and infrared mapping spectrometer team member who leads the scientists at the DLR who work on Cassini. Of course, in this case, the liquid in the hydrological cycle is methane rather than water, as it is on Earth."These results remind us how exceptional Titan is in the solar system," Jaumann said. "But they also show us that liquid has a universal control to shape geological surfaces in the same way, no matter what the liquid is."
The Cassini-Huygens assignment is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was planned, urbanized and assembled at JPL. The visual and infrared mapping spectrometer team is based at the University of Arizona, Tucson.
Like Earth and other planets, Saturn's orbit is not completely circular, but is instead somewhat elliptical and oblique. Because of this, during its southern summer, Titan is about 12 percent nearer to the sun than during the northern summer. As a result, northern summers are extensive and subdued; southern summers are short and extreme.
"Like Earth, Titan has tens-of-thousands-of-year variations in climate driven by orbital motions," Aharonson said. On Earth, these variations, well-known as Milankovitch cycles, are associated to changes in solar radiation, which concern global redistribution of water in the form of glaciers, and are believed to be responsible for ice-age cycles. "On Titan, there are long-term climate cycles in the worldwide movement of methane that make lakes and carve lake basins. In both cases we discover a record of the process embedded in the geology," he added.
"We may have established an example of long-term climate change, similar to Milankovitch climate cycles on Earth, on another object in the solar system," he said.
In the auroras above the northern latitudes of Saturn, Cassini has spotted the tallest known "northern lights" in the solar system, flickering in shape and brightness high above the ringed planet.
The new video reveals changes in Saturn's aurora every little minute, in high resolution, with three dimensions. The images show a formerly unseen vertical profile to the auroras, which ripple in the video like tall curtains. These curtains reach more than 1,200 kilometers (750 miles) over the edge of the planet's northern hemisphere.
An Aurora
"The auroras have put on a amazing show, shape-shifting rapidly and exposing curtains that were suspected to be there, but hadn't seen on Saturn before," said Andrew Ingersoll of the California Institute of Technology in Pasadena, who is a member of the Cassini imaging team that processed the latest video. "Seeing these things on a new planet helps us understand them a little better when we see them on Earth."
Auroras appear generally in the high latitudes near a planet's magnetic poles. When charged particles from the magnetosphere the magnetic bubble surrounding a planet plunge into the planet's higher atmosphere, they cause the atmosphere to glow. The curtain shapes prove the paths that these charged particles take as they flow along the lines of the magnetic field between the magnetosphere and the topmost part of the atmosphere.
An aurora above the northern part of Saturn
The height of the curtains on Saturn exposes a key distinction between Saturn's atmosphere and our own, Ingersoll said. While Earth's atmosphere has a lot of oxygen and nitrogen, Saturn's atmosphere is composed chiefly of hydrogen. Because hydrogen is very light, the atmosphere and auroras reach extreme out from Saturn. Earth's auroras tend to flare only about 100 to 500 kilometers over the surface.
The speed of the auroral changes in the video is similar to some of those on Earth, but scientists are still working to understand the processes that produce these quick changes. The height will also help them discover how much energy is necessary to light up auroras.
Ultraviolet and infrared instruments on Cassini have captured images of and data from Saturn's auroras before, but in these newest images, Cassini's narrow-angle camera was able to confine the northern lights in the noticeable part of the light spectrum, in higher resolution.
The images were initially obtained in black and white, and the imaging team highlighted the auroras in false-color orange. The oxygen and nitrogen in Earth's upper atmosphere add to the colorful flashes of green, red and even purple in auroras. But scientists are still operational to determine the true color of the auroras at Saturn, whose atmosphere lacks those chemicals.
It shows the moon's south polar region, where jets of water vapor and other particles spew from fissures on the surface.
"These first raw images are stunning, and paint an even more attractive picture of Enceladus," said Bob Pappalardo, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The Cassini teams will be delving into the data to better appreciate the workings of this bizarre, active moon."
Scientists are chiefly interested in the tiger stripes, which are fissures in the South Polar Region, because they discharge jets of water vapor and other particles hundreds of kilometers, or miles, from the surface. This flyby was scientists' last peek at the tiger stripes before the South Pole fades into the dusk of winter for numerous years. The thermal imaging work focused on the tiger stripe known as Baghdad Sulcus.
It shows the ridges and fractures on the surface of the icy moon.
The Nov. 21 encounter, which is occasionally called "E8" since it is the eighth targeted flyby of Enceladus, brought Cassini to within about 1,600 kilometers of the moon's surface, at around 82 degrees south latitude. Cassini is now cruising toward Rhea, another one of Saturn's moons, for additional imaging and mapping work.
