Enceladus

Enceladus, large moon of the planet Saturn. Enceladus orbits Saturn at a distance of about 238,000 km (about 147,900 mi), completing an orbit about once every 32.9 hours. Enceladus’s orbit parallels Saturn’s equator and is only slightly elliptical.

The Cassini space probe to Saturn discovered geysers of water vapor and ice erupting in plumes from Enceladus’s south pole, along with evidence of organic (carbon-based) molecules such as methane. These findings make Enceladus of special interest to astrobiologists as a prime candidate for investigating whether a moon could support life.

Enceladus is not quite spherical, with an average radius measuring about 252 km (157 mi), or about one-sixth the radius of Earth’s moon. Measurements of the density of Enceladus combined with its bright surface suggest to planetary scientists that the moon is made almost entirely of water ice, with a small rocky core. Observations by the Cassini probe indicate that the ice contains chemicals similar to those found in comets, such as carbon dioxide, carbon monoxide, and organic compounds.

Enceladus’s surface is extremely smooth, with few craters. This means that Enceladus’s surface is young compared to the surfaces of most of the other moons of Saturn—in fact, planetary scientists estimate that Enceladus’s surface is younger than 100 million years. The surface is very reflective, like newly fallen snow. The moon reflects more of the light that falls on it than any other body of significant size in the solar system. Cracks in the surface associated with the geyser-like plumes of water are also very young. Dubbed “tiger stripes,” the cracks may only be from 10 to 1,000 years old and have deposits of fresh crystalline ice from the plumes. Measurements from Cassini indicate that the stripes are much warmer than the rest of Enceladus—180 K compared to 60 K in inactive regions of the moon.

The gravitational forces created by the nearby moons Tethys and Dione, combined with Saturn’s gravitational pull, may squeeze and stretch Enceladus enough to keep it warm inside. Rocks inside Enceladus rub together, producing heat, which melts ice to produce dramatic geologic features on the moon. Some researchers also think that heat from the decay of short-lived radioactive elements may also have warmed the core of the moon shortly after it formed around 4.6 billion years ago. Other radioactive elements could have kept the interior warm for billions of years. Scientists believe that the interacting gravitational forces, possibly in combination with radioactive heat, may account for the geysers on Enceladus, which were first observed by the Cassini spacecraft in January 2005. Cassini’s cameras captured towering plumes jetting out from the south polar region and reaching about 418 km (about 260 mi) into space. Operated by the National Aeronautics and Space Administration (NASA), Cassini’s instruments also detected oxygen atoms, along with methane and carbon dioxide.

NASA scientists concluded that the best explanation for the geysers was the existence of liquid water just below the moon’s icy surface. When cracks open up in the surface, the water is ejected out as vapor and ice, somewhat like the Old Faithful geyser at Yellowstone National Park, but icy, not boiling. The water molecules then break down into oxygen and hydrogen atoms. Enceladus joins a small number of bodies in the solar system that show evidence of some kind of active volcanism, including Earth and Jupiter’s moon Io. The water-ice spewing from Enceladus also appears to supply particles to Saturn’s outermost E ring.

The Cassini probe also discovered that gas particles from the geysers on Enceladus apparently affect the rotation rate of Saturn’s magnetic field. The particles ejected into space become electrically charged and interact with the planet’s magnetic field, forming a disk of hot, ionized gas or plasma around Saturn’s equator that may slow the rotation of the magnetic field. Attempts to determine Saturn’s rotation rate using radio signals from the planet’s magnetic field gave inconsistent results, possibly in part because of effects from the gas particles emitted by Enceladus.

Organic molecules had previously been detected on the surface of Enceladus. The existence of liquid water, heat, and organic molecules are the three ingredients essential to life as it is known on Earth. Although astronomers have detected evidence for possible liquid water oceans on other moons in the solar system, some scientists noted that what makes Enceladus unique is that liquid water exists in pockets close to the surface.

Elsewhere on the moon portions of Enceladus’s crust are covered by alternating ridges and valleys similar to those observed on Jupiter’s moon Ganymede. Planetary scientists believe these ridges and valleys formed when sections of the crust broke into plates of sheet ice and bumped into each other. Some of Enceladus’s few craters are flooded by water that washed in from outside the crater and froze.

Enceladus is theoretically too small to hold onto an atmosphere, but the Cassini spacecraft detected a tenuous atmosphere composed of water molecules. Scientists speculated then that some source on the surface must continually renew this atmosphere. The finding that Enceladus has water-spewing geysers appeared to confirm that speculation.

Enceladus was discovered in 1789 by German-born English astronomer Sir William Herschel, who named the moon for a Titan defeated in battle by the goddess Athena. Individual features on Enceladus are named for characters and places from English writer Sir Richard Burton’s translation of the collection of stories Arabian Nights.