September 19, 2017

Cassini is gone. For more than 13 years, the space probe revealed the secrets of Saturn. It ended its mission in a blaze of glory on Friday, September 15, crashing into the planet it had studied for so long.

Cassini photographed Saturn illuminated from behind by the sun. This panoramic view was created by combining 165 images taken by Cassini’s wide-angle camera over nearly three hours on Sept. 15, 2006. Credit: NASA/JPL/Space Science Institute

Cassini was a spacecraft sent to Saturn to study the planet and its rings and satellites. The United States National Aeronautics and Space Administration (NASA) launched Cassini on Oct. 15, 1997. The craft began orbiting Saturn on July 1, 2004. Engineers and scientists at NASA’s Jet Propulsion Laboratory built Cassini. The Italian Space Agency provided a large antenna and several other elements of the spacecraft. The craft was named for the Italian-born French astronomer Giovanni Domenico Cassini, who made major discoveries about Saturn in the late 1600’s.

Cassini regularly provided spectacular revelations about the Saturn system. It discovered lakes of liquid hydrocarbons on Saturn’s moon Titan, the first lakes observed outside of Earth. The probe also discovered vertical structures in Saturn’s rings, structures rising high above the equatorial plane.

This artist’s conception shows Cassini diving between Saturn and the planet’s innermost ring. Credit: NASA/JPL-Caltech

Perhaps its most important discovery, however, was the detection of favorable conditions for life within the icy moon Enceladus. Measurements from Cassini revealed that a global ocean of liquid water lies beneath Enceladus’s icy crust. Furthermore, the probe imaged—and even flew through—plumes of ice crystals created by geysers at the moon’s surface. Cassini determined that these geysers were powered by hydrothermal activity in the ocean floor and that the plume contained hydrocarbon compounds. With these discoveries, Enceladus leapt to the top of the list of other places in the solar system where life might have developed.

This photograph of Saturn’s northern hemisphere was among the last taken by NASA’s Cassini spacecraft on Sept. 13, 2017. Credit: NASA/JPL-Caltech/Space Science Institute

Nearing 20 years in space, Cassini was running low on fuel. Mission planners directed Cassini to hurtle into Saturn’s clouds. Scientists were concerned that if the probe ran out of propellant and orbited uncontrollably within the Saturn system, it could have crashed into Titan or Enceladus. If this had occurred, any microbes from Earth surviving on Cassini’s surface could have contaminated these moons. Cassini sent back data and images right up to its final plunge into Saturn’s atmosphere.

Saturn will be without a visiting spacecraft from Earth for some time. With NASA trying to develop both the James Webb Space Telescope and the Space Launch System within its tight budget, planetary exploration has fallen by the wayside. Such probes take at least 5 to 10 years to plan and build. Then, it would take another six years or so for a spacecraft to reach Saturn. Therefore, any follow-up probe to Saturn would be at least a dozen years in the future.

Cassini may be gone, but its impact on the study of Saturn will continue to be felt for many years. Over 3,000 scientific papers have already been published based on data gathered by Cassini, and more are on the way. Current and future scientists will be using data gathered by Cassini to make new discoveries about the ringed planet for decades to come.

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)

July 23, 2013

The people waving at Saturn knew they would be too far away to be seen, but that didn’t stop tens of thousands of Earthlings from posing for a rare portrait of our home planet and Saturn taken by the Cassini space probe. In the photograph–a mosaic of images released yesterday–Earth appears as a pale blue dot hanging in the darkness of space between several of Saturn’s outer rings. NASA planned the “Wave at Saturn” event to coincide with a rare opportunity for Cassini to photograph Saturn and its entire ring system in the same frame as it was backlit by the sun on July 19. Scientists hope the new photographs will reveal additional details about Saturn’s thinner rings and allow them to study how the rings have changed since 2006, the last time Cassini was able to capture all of the Saturn system at once. The new images, taken 898 million miles (1.44 billion kilometers) from Earth, also show the moon as a bright white dot.

Earth appears as a pale blue dot in an image of Saturn and its glorious rings taken by the Cassini space probe. (NASA/JPL-Caltech/Space Science Institute)

The 2006 images and the new Cassini photos mark only the second and third times, respectively, that Earth has been photographed from the outer solar system. The first image was taken by the Voyager 1 spacecraft in 1990, from about 3.7 billion miles (6 billion kilometers) away. NASA engineers ordered the spacecraft to take the photograph at the request of American astronomer Carl Sagan.

Additional World Book articles:

• Close Encounters with Saturn (a special report)
• Exploring the Suburban Solar System (a special report)

May 1, 2013

Scientists have discovered a hurricane on Saturn that is 20 times as large as the average hurricane on Earth. NASA’s Cassini spacecraft has provided scientists with the first close-up views of the enormous storm swirling around Saturn’s north pole. The hurricane swirls inside a mysterious six-sided feature first photographed in 2006. The hexagon is unlike anything seen on any other planet. The Cassini probe was able to photograph the storm in sunlight only after the planet’s north pole emerged from the darkness of its polar winter. Cassini’s orbit was also shifted so the probe would pass directly over the north pole.

The eye of the hurricane is about 1,250 miles (2,000 kilometers) wide. The clouds circling around the outer edge of the storm are moving at 330 miles  (530 meters) per hour. By contrast, the strongest hurricanes on Earth have winds of up to 200 miles (320 kilometers) per hour. Also, unlike hurricanes on Earth, which generally move, the Saturnian hurricane is locked onto the planet’s north pole. Scientists believe it has been churning for years.

A hurricane swirls around the north pole of Saturn, in a false-color infrared image taken by the Cassini space probe at a distance of about 261,000 miles (419,000 kilometers). Clouds shown in red are closer to the surface than those shown in green. (NASA/JPL-Caltech/SSI)

“We did a double take when we saw this vortex because it looks so much like a hurricane on Earth,” noted Cassini imaging team member Andrew Ingersoll. “But there it is at Saturn, on a much larger scale, and it is somehow getting by on the small amounts of water vapor in Saturn’s hydrogen atmosphere.”

Hurricanes on Earth feed off warm ocean water. But there is no body of water close to these clouds high in Saturn’s atmosphere. Scientists suggest learning how these Saturnian storms use water vapor could provide greater understanding of how hurricanes on Earth are generated and sustained.

Additional World Book articles

• Jet Propulsion Laboratory
• Close encounters with Saturn (a special report)
• Probing the Planets (a special report)
• Space exploration 1997 (a Back in Time article)
• Space exploration 2004 (a Back in Time article)
• Space exploration 2008 (a Back in Time article)

December 13, 2012

Evidence of a river in the north polar region of Saturn’s moon Titan has been reported by scientists with NASA’s Cassini mission. The river, which has several smaller tributaries, looks like a smaller version of the Nile River on Earth. As revealed in radar images taken on Sept. 26, the “mini Nile” winds through a river valley more than 200 miles (400 kilometers) from its source to a large sea called Ligeia Mare. The relative straightness of the river indicates that it follows at least one fault (crack) in Titan’s crust (outer layer), similar to the path followed by other rivers running into Ligeia Mare. The Cassini scientists concluded that the river is filled with liquid due to the dark coloration of the surface in the images.

The dark, branching channels in an image of Titan's surface were almost certainly carved by rivers of liquid methane. The image was taken by the probe Huygens as it parachuted to Titan's surface in January 2005. The channels appear to cross a bright plateau (top of image), then descend into a darker area (bottom) which may be a lake bed. (European Space Agency/NASA/JPL/University of Arizona)

Titan is the only other body in the solar system known to have stable liquid on its surface. On Earth, the most abundant surface liquid is water. But because Titan’s atmosphere is extremely cold, no liquid water would be found there. Instead, the liquids on Titan are such hydrocarbons as ethane and methane. These liquids have a very low freezing point. In the past several years, Cassini scientists have found evidence of rainfall, large lakes, and other rivers on Titan. Such evidence supports the theory that Titan has a cycle of hydrocarbons similar to the water cycle on Earth.

Scientists have speculated that the hydrocarbons could possibly form an environment in which life could develop. However, the thick and hazy atmosphere shrouds the moon from most forms of imaging. Detecting life or the evidence of life is not likely to be confirmed or refuted anytime in the near future.

Additional World Book articles:

• Porco, Carolyn
• Satellite
• Close encounters with Saturn (a special report)
• Astronomy 1944 (Back in Time article)
• Astronomy 1988 (Back in Time article)
• Astronomy 2004 (Back in Time article)
  
• Astronomy 2005 (Back in Time article)
• Astronomy 2006 (Back in Time article)