Story highlights
- Cassini detected the presence of complex organic molecules in Enceladus' ocean
- This is a potential indicator of habitability on Saturn's icy moon
(CNN)Last year, astronomers announced that ocean worlds like Europa and Enceladus may be the best chance for finding life outside of Earth in our solar system. Now, the discovery of complex organic molecules in plumes that rise from Enceladus' subsurface ocean further suggests that the moon could support life as we know it.
Before NASA's Cassini mission studied Saturn and its moons for 13 years, beginning in 2004, Enceladus held many secrets. Cassini revealed that there was a global ocean between the moon's icy crust and its rocky core.
Although the Cassini mission ended in a blaze of glory in September when it disintegrated in Saturn's atmosphere, surprises from the data it collected will be released for years to come. A study detailing the latest discovery from Cassini data was published Wednesday in the journal Nature.
"We are, yet again, blown away by Enceladus," study co-author Christopher Glein, a Southwest Research Institute space scientist specializing in extraterrestrial chemical oceanography, said in a statement. "Previously we'd only identified the simplest organic molecules containing a few carbon atoms, but even that was very intriguing. Now we've found organic molecules with masses above 200 atomic mass units. That's over ten times heavier than methane. With complex organic molecules emanating from its liquid water ocean, this moon is the only body besides Earth known to simultaneously satisfy all of the basic requirements for life as we know it."
An alien ocean
Cassini's Cosmic Dust Analyzer and the Ion and Neutral Mass Spectrometer sampled and measured the ice grains and vapor being ejected by cryo-volcanic plumes from warm cracks in the ice crust. They also sampled Saturn's E ring, which was formed by ice grains that escaped Enceladus' gravity.
In 2015, when Cassini flew the closest it ever came to Enceladus, molecular hydrogen was detected in the plume. Scientists believe the molecular hydrogen forms as a result of the interaction between water and rocks when it's in a hydrothermal environment.
"Hydrogen provides a source of chemical energy supporting microbes that live in the Earth's oceans near hydrothermal vents," study coauthor Hunter Waite, program director at the Southwest Research Institute and the Ion and Neutral Mass Spectrometer principal investigator, said in a statement. "Once you have identified a potential food source for microbes, the next question to ask is 'what is the nature of the complex organics in the ocean?' This paper represents the first step in that understanding -- complexity in the organic chemistry beyond our expectations!"
When looking at the potential habitability of an environment, NASA's checklist includes water, organics and a food or energy source toward the top, Waite said in an email.
Measurements of methane, molecular hydrogen and carbon dioxide in the global ocean show that the ocean has the chemical energy necessary for microbes to produce methane -- if there are microbes, Waite said. Recent studies show that there are microbes like this in our own ocean.
The new study adds the detection of large, carbon-rich organic molecules to the previous findings, surprising the researchers. They believe that the molecules are caused by chemical reactions between Enceladus' rocky core and the ocean's warm water. Based on their data, these molecules could be sitting in a thin film on top of the ocean, where bubbles can rise and burst.
"They may represent life's chemical building blocks or even the remains of past microbial populations or there may be simple abiotic ways to explain the origin of the materials," Waite said. "In any event, specific identification of these organic compounds is the next step in our search for life in Enceladus' ocean. The complexity of the organic compounds identified was beyond our wildest expectations -- non-soluble complex organics floating as a film on an alien ocean. Wrap your head around that."
Photos: NASA says farewell to Cassini
On September 15, 2017, the 20-year Cassini mission ended in a "death dive" into Saturn's upper atmosphere, collecting data until the spacecraft broke apart and became part of the planet it set out to explore.
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Photos: NASA says farewell to Cassini
This is the last image taken by NASA's Cassini spacecraft before it broke apart in Saturn's atmosphere on September 15, 2017. RIP, Cassini!
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Photos: NASA says farewell to Cassini
This is Cassini's final resting place. NASA says the montage of images was created using data from Cassini's visual and infrared mapping spectrometer. What you see in the photo are clouds in the atmosphere, silhouetted against that inner glow, according to NASA.
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Photos: NASA says farewell to Cassini
Cassini captured this image of Saturn's northern hemisphere on September 13, 2017. It is among the last images Cassini sent back to Earth.
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Photos: NASA says farewell to Cassini
Cassini took this final image of Saturn's rings on September 13, 2017 while the spacecraft was 684,000 miles (1.1 million kilometers) away from the planet.
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Photos: NASA says farewell to Cassini
Two days before its death plunge into Saturn, Cassini took this image of Saturn's A ring. The ring features what scientists call a lone "propeller" -- a feature created by small moonlets in the rings.
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Photos: NASA says farewell to Cassini
This image of Saturn's moon, Titan, was among the last obtained by Cassini's narrow-angle camera on September 13, 2017. The images were taken two days before Cassini plunged into Saturn's atmosphere.
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Photos: NASA says farewell to Cassini
Cassini took this image of Saturn's moon Titan in 2012. NASA scientists say they have detected acrylonitrile in Titan's atmosphere. The chemical could possibly form cell membranes. That means, according to scientists, Titan could have the right conditions for life to develop.
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Photos: NASA says farewell to Cassini
On July 19, 2013, Cassini snapped a very special vista of our home world. The spacecraft slipped into Saturn's shadow and, with the sun blocked, it was able to image not only Saturn, but seven of its moons, its inner rings -- and, in the background, Earth and our moon.
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Photos: NASA says farewell to Cassini
Springtime on Saturn: Cassini's wide angle camera shot 75 images showing Saturn, its rings, and some of its moons just after the Spring equinox. An equinox occurs when the sun's disk is exactly over a planet's equator. It takes 30 years for Saturn to orbit the sun, so an equinox occurs every 15 Earth years, NASA says. These images were taken on August 12, 2009, a little more than a day after the exact equinox.
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Photos: NASA says farewell to Cassini
Saturn appears to sit in a nest of rings in this composite of 45 images Cassini took on May 9, 2007. The spacecraft was about 700,000 miles (1.1 million kilometers) from Saturn when the images were taken.
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Photos: NASA says farewell to Cassini
Saturn's moon, Enceladus, is a small, icy world. For scientists, it's also one of the most interesting places in our solar system. Cassini discovered Enceladus is an active moon with a global ocean of liquid salty water beneath its crust. Planetary scientists now think Enceladus may possibly be hospitable to life. "Enceladus discoveries have changed the direction of planetary science," said Cassini project scientist Linda Spilker. This mosaic was created from 21 false-color images taken during Cassini's close approaches to Enceladus on March 9 and July 14, 2005.
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Photos: NASA says farewell to Cassini
This mosaic of nine images shows Saturn's moon Titan during Cassini's first very close flyby on October 26, 2004. The spacecraft was at distances ranging from about 200,000 miles (320,000 kilometers) to 400,000 miles (640,000 kilometers) from Titan when the images were taken.
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Photos: NASA says farewell to Cassini
Saturn's pale colors and its rings come into view as Cassini approaches on May 7, 2004. This composite was made from images taken when Cassini was about 18 million miles (29 million kilometers) from Saturn. It also shows some of Saturn's moons.
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Photos: NASA says farewell to Cassini
The Cassini spacecraft was launched from Cape Canaveral, Florida, on a Titan IVB/Centaur rocket on October 15, 1997. It began orbiting Saturn in 2004.
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Europa or Enceladus?
Proposals for missions to further study Enceladus were submitted to NASA last year, and they're being evaluated. Waite suggests that the next step would be a new mission to fly through the E ring and the plumes of Enceladus to provide data.
"A future spacecraft could fly through the plume of Enceladus, and analyze those complex organic molecules using a high-resolution mass spectrometer to help us determine how they were made," Glein said. "We must be cautious, but it is exciting to ponder that this finding indicates that the biological synthesis of organic molecules on Enceladus is possible."
Meanwhile, NASA plans to further explore ocean worlds in our solar system through the Europa Clipper mission, the first to explore an alien ocean. The Europa Clipper, named for the innovative streamlined ships of the 1800s, will launch in the 2020s and arrive at Jupiter's moon Europa after a few years. Europa is also an ice-covered moon with a subsurface salty liquid ocean that shoots plumes above the surface.
Europa Clipper will be able to gather material from plumes if it can fly through them. This would allow scientists a first look at the material inside Europa's ocean that's spewing through the icy crust to determine whether it's habitable and supports life.
Europa Clipper's instruments will be capable of "sniffing" the atmosphere of Europa, with more than 40 planned flybys. The flybys will be less than 228 miles above the surface, in the observed range of the plumes, which can reach 124 to 228 miles above the surface.
So which is the best candidate for supporting life: Europa or Enceladus?
"Verification of plumes on Europa suggest that with Europa Clipper's improved instrumentation, Europa could leap frog Enceladus in the search for alien life," Waite said. "The Clipper is tuned to look for signs of life in Europa's plume, something we never expected with Cassini."
