May 17, 2017

Enceladus, an icy moon orbiting Saturn, is quickly becoming one of the hottest spots in the search for life beyond Earth. A group of scientists led by J. Hunter Waite of the Southwestern Research Institute in San Antonio, Texas, has determined that hydrothermal vents (flows of heated water) likely exist in a global ocean beneath Enceladus’s icy crust. These vents could possibly be home to life forms. Waite and his team published their findings last month in the journal Science.

Enceladus, a moon of Saturn, contains geysers at its south polar region that spray water vapor and grains of water ice into space. Credit: NASA/JPL/Space Science Institute

Enceladus is the sixth largest moon of Saturn. Its surface, made of bright ice, makes it the most reflective object in the solar system. Like only a few known satellites, Enceladus shows signs of ongoing geological activity. Scientists have observed a plume of particles erupting from Enceladus’s south polar region. The plume is fed by several individual jets on the surface of the moon that release mostly water vapor and grains of water ice. These jets are in turn fed by a global ocean of liquid water that exists deep beneath the moon’s surface. The jets spray material onto Enceladus’s surface and into space.

The icy surface of Enceladus, seen here in a Cassini probe image, is continually smoothed by active geysers and shows few craters. Credit: NASA/JPL/Space Science Institute

Much of what is known about the moon comes from data gathered by the Cassini spacecraft launched by the United States National Aeronautics and Space Administration (NASA) in 1997. Cassini first visited Enceladus in 2005. The ship sampled Enceladus’s geysers in 2008 and again in 2015. It detected Enceladus’s global subsurface ocean in 2015.

This artist’s conception shows Cassini nearing the rings of Saturn. Credit: NASA/JPL

Waite and his coauthors analyzed the data obtained by Cassini and found molecular hydrogen (H₂) in Enceladus’s plumes. Such hydrogen is thought to be rare in planetary bodies as small as Enceladus, which has a circumference of roughly 1,000 miles (1,600 kilometers), making it less than 1/6th the size of Earth’s moon. Waite and his team determined that the hydrogen most likely came from hydrothermal vents on a sea floor within Enceladus. On Earth, hydrothermal vents are places where heated water flows from the ocean floor. Some deep-sea vents support diverse and exotic communities of marine life. Certain microbes can use the heat of the water to produce energy. These microbes are in turn consumed or harbored by larger organisms. Scientists suspect that life could flourish around such vents on other planets or moons.

Cassini has revealed tantalizing details about Enceladus, but it will not be able to determine if life exists beneath the moon’s icy surface. Cassini’s mission is coming to an end because the craft is running out of fuel. Cassini is now making a series of orbits between Saturn and its nearest ring. In September, when Cassini completes these orbits, it will crash into Saturn. Even if Cassini could continue its mission, it would be unable to gather more detailed information on the moon’s water jets. When the probe was launched 20 years ago, astronomers had not yet observed Enceladus’s polar plumes, so they could not have included instruments to study it specifically. Energized by these new findings, engineers are now working to design a probe that can sample Enceladus’s jets in greater detail. Such a probe would carry instruments designed to look for and study organic (carbon-containing) molecules in the plumes to determine if the molecules came from living things.

July 30, 2014

Geysers in the solar polar region of Saturn’s moon Enceladus are actually fountains erupting from an underground ocean of liquid water, according to a new study by scientists working with the Cassini space probe. The discovery solves a 10-year puzzle about the source of the geysers, which shoot water vapor and tiny particles of ice hundreds of kilometers into space. Discovered by Cassini in 2005, the geysers erupt from four “tiger-stripe” fractures, each of which is some 80 miles (130 kilometers) long and 1 1/2 miles (2.4 kilometers) wide. Cassini scientists confirmed the existence of the ocean in 2013.

Plumes of ice particles and water vapor erupt from Enceladus in a false-color image taken by Cassini. Some of the particles become part of one of Saturn's rings. (NASA/JPL/Space Science Institute)

Heat-detection equipment on Cassini had found that the fractures were warmer than the surrounding terrain. Some scientists had theorized that heat created as the sides of the fractures rubbed against one another under Saturn’s gravitational pull was producing the geysers. The new study has revealed that the geysers are actually producing the surface heat by transporting water vapor and water droplets from deep inside the moon. As the water vapor condenses near the surface, it also releases heat. The excess heat causes the water to erupt in geysers. Buried beneath about 20 miles (32 kilometers) of surface ice, the sea is centered on the moon’s south pole, though it may extend much farther around the moon.

The "tiger-stripe" fractures from which the geysers erupt score a basin surrounding Enceladus's south pole. (NASA/JPL-Caltech/SSI)

Geysers appear as colored bars in a three-dimensional model of Enceladus's south polar region. (ASA/JPL-Caltech/Space Science Institute)

Additional World Book articles:

• Cassini, Giovanni Domenico
• Satellite
• Space exploration
• Close Encounters with Saturn (a Special Report)
• Probing the Planets (a Special Report)

April 4, 2014

The discovery of a plume of water vapor shooting from the surface of Enceladus in 2005 provided the first clue that this tiny moon of Saturn might hold liquid water. Now, radar surveys of Enceladus have revealed strong evidence of a subterranean sea with at least as much water as Lake Superior, the largest body of fresh water on Earth. The sea, which is buried beneath about 20 miles (32 kilometers) of surface ice, is centered on the moon’s south pole. But it may actually extend much farther, perhaps across the entire moon. Lined by a rocky floor, the sea may be as much as 5 miles (8 kilometers) deep.

The radar surveys that detected the sea were conducted by scientists working with the Cassini space probe, which has been in orbit around Saturn since 2004. Cassini has flown close to Enceladus several times. Each pass has allowed scientists to use the gravitational pull of the moon on the probe to help map the moon’s internal structure.

Geysers of water vapor and ice erupt from Saurn's moon Enceladus. (Cassini Imaging Team, SSI, JPL, ESA, NASA)

The discovery of the sea puts Enceladus in a select group of extraterrestrial bodies in the solar system that likely hold liquid water. These include Mars and Jupiter’s moons Callisto, Europa, and Ganymede. Since 2005, Cassini scientists have confirmed that the water vapor from Enceladus’s plumes contains carbon and nitrogen, two essential chemical elements for life as we know it. Could there be life there? Planetary scientist Jonathan Lunine, a member of the study team, said the presence of the sea makes Enceladus “a very attractive potential place to look for life.” In recent years, Enceladus and Europa had become space scientists’ favorite destinations for space missions seeking signs of extraterrestrial life in the solar system. However, due to funding limitations at NASA, only one mission has been approved–to Europa. This new evidence may help get a mission to Enceladus back on the drawing board.

Additional World Book articles:

• Cassini, Giovanni Domenico
• Satellite
• Space exploration
• Close Encounters with Saturn (a Special Report)
• Probing the Planets (a Special Report)