The planet Saturn in astronomy

The 1998 Hubble photo above shows Saturn in reflected infrared sunlight. The different colors indicate varying heights and compositions of cloud layers believed to consist of ammonia ice crystals. The bright stripe barely discernable within the left ring shadow is infrared sunlight streaming through a large gap in the rings. Two of Saturn’s 62 known moons appear as tiny dots of light: Tethys on the upper right edge of Saturn’s disk, and Dione in the lower left. All of the gas giant planets (Jupiter, Saturn and Neptune) have rings; Saturn's are just by far the most pronounced.

Saturn is the last planet that can be see with the naked eye, although in extremely clear seeing conditions Uranus can be seen with good eyesight. It is also the least dense of all the planets, and would float in water. Even though Saturn has 9.5 times the Earth's diameter—and therefore about 760 times the Earth's volume—its low density gives it a surface gravity less than that on Earth: on the surface of Saturn we would weigh less than we do here. The only problem is that there is nothing there to stand on. Saturn doesn't really have a surface; it's composed of 99% hydrogen and helium with a little methane, ammonia, and water vapor thrown in. Saturn's low density and its 11-hour day—extraordinarily fast rotation for a planet 75,000 miles in diameter—also make it the least spherical of all the planets. It's over 7300 miles thicker at the equator than it is at the poles.

Eclipsing the Sun, the night side of Saturn is partly lit by light reflected from its own rings. Visible above in spectacular detail is Saturn’s E ring: the diffuse, outermost ring created by the newly discovered ice-fountains of the moon Enceladus. The Earth itself is a barely visible pale blue dot at about 10:00, outside the bright, wide inner rings and just inside the first single ring (that is itself within the outermost E ring). Taken by the robotic Cassini spacecraft from the opposite side of Saturn, APOD January 11, 2009.

At 888 million miles from the Sun, it takes sunlight 1 hour and 20 minutes to reach the ringed planet. That's also how long it took our control signals to reach the Cassini-Huygens mission when we were sending it instructions to separate and parachute to the surface of Titan. Because of its great distance, the average cloud top temperature on Saturn is about 300 degrees below zero Fahrenheit! It takes Saturn 29.5 years to circle the Sun.

These ultraviolet aurorae tower more than 1000 miles over Saturn’s cloud tops at both poles. Launched Oct. 15, 1997, the Cassini mission which took this picture took 7 years to reach Saturn (with gravitational assists from Venus, Earth and Jupiter on the way), arriving July 1 2004. The Huygens probe then separated from the Cassini main body, parachuting to Titan (picture below) on Jan. 14, 2005.

Second only to Jupiter’s Ganymede in size, Titan is the only satellite with an atmosphere: thick, smoggy, and mostly Nitrogen like Earth. Methane rain, evaporating lakes, flowing rivers, and water ice-volcanoes all likely exist there, at about -300° Fahrenheit. The dark lakebed across the bottom, thought to be dry at the time of the photo, recently contained a flowing liquid, possibly methane. APOD Jan. 24, 2005.

Fresh tiger stripes on Saturn’s moon Enceladus might be spewing fresh ice into space from its icy interior, creating a cloud of fine particles over the moon’s S. pole and Saturn’s mysterious E-ring. APOD September 6, 2005.

This Planet-Saturn-in-astronomy page and much of this 600-page website are excerpted from the personalized Fine Art Book You and the Universe.