THE NEWS FROM SATURN

On July 6, 2004, after a 7 ½-year journey of 3.5 billion kilometers (2.2 billion miles), the Cassini-Huygens space probe reached the planet Saturn, 9 ½ times farther than Earth from the sun.  Launched from Cape Kennedy, Florida, on October 15, 1957, the six-ton spacecraft passed twice by Venus, once by Earth, and once by Jupiter, the sun’s largest planet, on its way to Saturn.  Each close encounter provided a “gravity boost” whose cumulative effects allowed the spacecraft to reach Saturn, where it began a series of long, looping orbits around the planet that would carry it past Saturn’s larger moons.  This was the first spacecraft mission that did not simply fly past the Saturnian system, as did Pioneer 11 in 1973, Voyager 1 in 1980, and Voyager 2 in 1981.   Nearly twenty-five years after the last of these spacecraft visits, another interplanetary voyager has reached the sun’s second-largest planet, where it will spend several years investigating Saturn, its rings, its moons, and its magnificent magnetosphere.

            The Cassini portion of the probe immediately sent back stunning pictures of the rings around Saturn showing far more detail than any images obtained before.  The rings show complex patterns called density waves, induced by gravitational forces from relatively tiny satellites that orbit among the individual particles that comprise the rings.  Images of the planet itself revealed far more storm activity than the two Voyagers had seen, at least in part because of Cassini’s superior cameras.

            On June 11, 2004, Cassini passed within 21,000 kilometers (13,500 miles) of Saturn’s moon Phoebe, whose heavily cratered surface resembles that of our own moon.  Two differences are that Phoebe has a surface made of ice, not rock, and this moon is too small for its own gravity to deform it into a spherical shape.

Later that year, the spacecraft obtained photographs of two other moons, Tethys  and  Dione.  The most fascinating of Saturn’s lesser moons is Iapetus (pronounced YAH-peh-tuss), whose icy surface is half as dark as coal and half as bright as dirty ice.  The dark hemisphere is the one that leads as Iapetus orbits Saturn, which suggests that the satellite once encountered a cloud of dark, sticky material.  But when?  And what is that dark stuff?  Where did it come from?  No other moon in the solar system looks remotely like Iapetus.  Scientists hope that Cassini’s close-up views of this celestial domino will help to solve the riddle of its black-and-white dichotomy.  In addition, thin ridge runs halfway around the moon, the result of geological processes that are currently unknown.

            The most spectacular achievement by the Cassini-Huygens mission occurred when the Huygens portion of the space probe carried by the Cassini orbiter descended through the atmosphere of Titan, Saturn’s largest satellite by far, and landed on its surface.  Titan, which ties with Ganymede, Jupiter’s largest moon, for first place in size among solar-system satellites, is the only moon that possesses a significantly thick atmosphere.  Like the Earth’s, this atmosphere consists mainly of nitrogen molecules (N₂).  Because Titan’s atmosphere also contains smog-like particles, astronomers had never seen its surface until January 14, 2005, when the first images, traveling at the speed of light, arrived at Earth after an 80-minute journey through the solar system.  During its descent, Huygens secured images of a landscape with apparent fog and different types of terrain.  Once on the surface, where the temperature measures 94 Kelvin (-179 Celsius or -290 degrees Fahrenheit), Huygens was expected to communicate with Cassini for only three minutes, but succeeded in sending data for an hour.  In this time, Huygens revealed a surface strewn with boulders, which may consist of ice, made super-hard by the intense cold.

            The most amazing result from the Huygens probe resides in the image taken during its descent that apparently shows a system of river channels cut into the ice that constitutes “bedrock” on Titan . 

This strongly implies that liquid has flowed on Titan recently, and perhaps is flowing now.  This liquid cannot be water, which never liquefies on Titan, but may well be methane (CH₄), which we know as “natural gas.”  Methane is the second most abundant gas (after nitrogen molecules) in Titan’s atmosphere, and will condense as a liquid at the temperature on Titan’s surface.  Just after the probe landed, one of its instruments detected a sudden surge of methane gas that remained constant for the rest of the transmission.  Apparently the warm inlet on the probe vaporized liquid methane lying on or close to the surface. 

All in all, Titan resembles Earth to remarkable degree, though with ice instead of rock, liquid methane instead of water, and deposits of precipitated organic smog particles instead of dirt.  Future observations by the Cassini orbiter, which is equipped with a powerful radar system, will help to unravel the many mysteries that this moon continues to present.

Cassini will continue to orbit through the Saturnian system, making the following close encounters with Saturn’s moons during the remainder of 2005:

•     Titan -- March 31

•     Titan -- April 16

•     Enceladus -- July 14

•     Titan -- August 22

•     Titan -- September 7

•     Hyperion -- September 26

•     Dione -- October 11

•     Titan -- October 28

•     Rhea -- November 26

•     Titan -- December 26

Some Useful Websites:

Cassini-Huygens website at the European Space Agency

Cassini-Huygens website at NASA

Astronomy Picture of the Day: