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Module 1 - The Solar System

Finally... Spokes!

After much anticipation, Cassini has finally spotted the elusive spokes in Saturn's rings. Spokes are the ghostly radial markings discovered in the rings by NASA's Voyager spacecraft 25 years ago. Since that time, spokes had been seen in images taken by NASA's Hubble Space Telescope but had not, until now, been seen by Cassini. These three images, taken over a span of 27 minutes, show a few faint, narrow spokes in the outer B ring. The spokes are about 3,500 kilometers (2,200 miles) long and about 100 kilometers-wide (60 miles). The motion of the spokes here is from left to right. They are seen just prior to disappearing into the planet's shadow on the rings. At the bottom left corner of the left and center images, the bright inner edge of the A ring is visible. Continuing radially inward (or toward Saturn) are several bands that lie within the Cassini Division, bounded by the bright outer edge of the B ring. The rounded shadow of Saturn cuts across the rings in the image at right. Cassini's first sighting of spokes occurs on the unilluminated side of the rings, in the same region in which they were seen during the Voyager flybys. Although the most familiar Voyager images of spokes showed them on the sunlit side of the rings, spokes also were seen on the unilluminated side. In Voyager images, when spokes were seen at low phase angles, they appeared dark; when seen at high phase angles, they appeared bright. The spokes seen here are viewed by Cassini at a very high phase angle, which is about 145 degrees at the center of each image. Imaging team members will be studying the new spoke images and will maintain their vigil for additional spoke sightings. These images were taken using the clear filters on Cassini's wide-angle camera on Sept. 5, 2005, at a mean distance of 318,000 kilometers (198,000 miles) from Saturn. The radial scale on the rings (the image scale at the center of each image) is about 17 kilometers (11 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 [ http://saturn.jpl.nasa.gov ]. For additional images visit the Cassini imaging team homepage http://ciclops.org [ http://ciclops.org ]. Image Credit: NASA/JPL/Space Science Institute

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Saturn
Saturn

Saturn was the most distant of the five planets known to the ancients. In 1610, Italian astronomer Galileo Galilei was the first to gaze at Saturn through a telescope. To his surprise, he saw a pair of objects on either side of the planet. He sketched them as separate spheres, thinking that Saturn was triple-bodied. Continuing his observations over the next few years, Galileo drew the lateral bodies as arms or handles attached to Saturn. In 1659, Dutch astronomer Christiaan Huygens, using a more powerful telescope than Galileo's, proposed that Saturn was surrounded by a thin, flat ring. In 1675, Italian-born astronomer Jean-Dominique Cassini discovered a "division" between what are now called the A and B rings. It is now known that the gravitational influence of Saturn's moon Mimas is responsible for the Cassini Division, which is 4,800 kilometers (3,000 miles) wide.
This is Mimas (397 kilometers across) as viewed by Cassini, the spacecraft, on March 21, 2006.
Like Jupiter, Saturn is made mostly of hydrogen and helium. Its volume is 755 times greater than that of Earth. Winds in the upper atmosphere reach 500 meters (1,640 feet) per second in the equatorial region. In contrast, the strongest hurricane-force winds on Earth top out at about 110 meters (360 feet) per second. These super-fast winds, combined with heat rising from within the planet's interior, cause the yellow and gold bands visible in the atmosphere.
In the early 1980s, NASA's two Voyager spacecraft revealed that Saturn's rings are made mostly of water ice, and they imaged "braided" rings, ringlets, and "spokes" - dark features in the rings that form and initially circle the planet at different rates from that of the surrounding ring material. Saturn's ring system extends hundreds of thousands of kilometers from the planet, yet the vertical height is typically about 10 meters (30 feet) in the main rings. During Saturn's equinox in autumn 2009, when sunlight illuminated the rings edge-on, Cassini spacecraft images showed vertical formations in some of the rings; the particles seem to pile up in bumps or ridges more than 3 kilometers (2 miles) tall.
At Saturn's center is a dense core of rock, ice, water, and other compounds made solid by the intense pressure and heat. It is enveloped by liquid metallic hydrogen, inside a layer of liquid hydrogen - similar to Jupiter but considerably smaller. Saturn's magnetic field is smaller than Jupiter's but still 578 times as powerful as Earth's. Saturn, the rings, and many of the satellites lie totally within Saturn's enormous magnetosphere, the region of space in which the behavior of electrically charged particles is influenced more by Saturn's magnetic field than by the solar wind. Aurorae occur when charged particles spiral into a planet's atmosphere along magnetic field lines. On Earth, these charged particles come from the solar wind. Cassini showed that at least some of Saturn's aurorae are like Jupiter's and are largely unaffected by the solar wind.
As we will see, Saturn's moons Enceladus and Titan will also be discussed in detail due to its importance in later modules.
Saturn's largest satellite, Titan, is a bit bigger than the planet Mercury. (Titan is the second-largest moon in the solar system; only Jupiter's moon Ganymede is bigger.) Titan is shrouded in a thick, nitrogen-rich atmosphere that might be similar to what Earth's was like long ago. Further study of this moon promises to reveal much about planetary formation and, perhaps, about the early days of Earth. Saturn also has many smaller "icy" satellites. From Enceladus, which shows evidence of recent (and ongoing) surface changes, to Iapetus, with one hemisphere darker than asphalt and the other as bright as snow, each of Saturn's satellites is unique.
The next chapter in our knowledge of Saturn is being written right now by the Cassini-Huygens mission. The Huygens probe descended through Titan's atmosphere in January 2005, collecting data on the atmosphere and surface. The Cassini spacecraft, orbiting Saturn since 2004, continues to explore the planet and its moons, rings, and magnetosphere. The Cassini Equinox Mission studied the rings during Saturn's autumnal equinox, when the Sun was shining directly on the equator, through 2010. Now the spacecraft is seeking to make exciting new discoveries in a second extended mission called the Cassini Solstice Mission, which continues until September 2017.
Update June 2019: Cassini was retired in 2017 and deliberately disposed of via a controlled fall into Saturn's atmosphere, marking the end of a spectacularly successful mission.