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Ocean worlds Enceladus and Europa could be habitable for microbial life

Cassini flying through Enceladus' plumes

This artist’s rendering shows Cassini diving through the Enceladus plume in 2015. New ocean-world discoveries from Cassini and Hubble will help inform future exploration efforts as well as the broader search for life beyond Earth. Image Credit: NASA / JPL-Caltech

New data obtained by NASA’s Cassini mission and by the Hubble Space Telescope indicate Saturn’s moon Enceladus and Jupiter’s moon Europa, both referred to as ocean worlds because they harbor subsurface oceans, could be habitable for microbial life.

The developments were presented in an April 13, 2017, NASA press conference on oceans beyond Earth.

Enceladus hydrogen graphic

This graphic illustrates how Cassini scientists think water interacts with rock at the bottom of the ocean of Saturn’s icy moon Enceladus to produce hydrogen gas. (Click to enlarge) Credit: NASA / JPL-Caltech / Southwest Research Institute

Cassini’s Ion and Neutral Mass Spectrometer (INMS) detected hydrogen gas in icy plumes coming from Enceladus during its final and deepest fly into the plumes in October 2015. The finding confirms the plumes are composed of 98 percent water vapor, about one percent hydrogen gas, and the rest a mix of ammonia, methane, and carbon dioxide.

Discovery of hydrogen gas in the plumes is a major development because it is a potential source of energy required for the development of life.

Microbes have been found in hydrothermal vents on Earth’s ocean floors, in locations previously thought inhospitable to any life. They serve as the bottom of the food chain for ecosystems that include shrimp and other small marine life forms.

This finding raised the possibility of bacteria living in similar hydrothermal vents on other Solar System worlds that have subsurface oceans. If a potential source of energy is present, microbial life can survive even in the absence of sunlight.

Life as we know it requires three things in order to exist on a world: water, energy, and the appropriate chemistry, such as carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.

Finding hydrogen gas in Enceladus’ plumes, which were first discovered by Cassini in 2005, means the Saturnian moon is now known to have all three.

Production of hydrogen gas requires a geochemical source, which, in this case, is the interaction between water and rock beneath the ocean. Enceladus’ plumes are spewing from hot cracks visible near its south pole.

Through a chemical reaction process known as “methanogenesis”, potential microbes could obtain the energy they need by combining hydrogen with carbon dioxide dissolved in the water. Methane, a byproduct of this process, is of key importance to life on Earth.

Phosphorus and sulfur, two other major energy sources for microbes, have not been detected in the plumes emanating from Enceladus’ ocean, but scientists suspect they are present in the small moon’s rocky core, which has a composition similar to that of meteorites known to contain both elements.

In March 2015, Cassini mission scientists raised the possibility of hydrothermal activity in Enceladus’ ocean, produced by hot water interacting with rock below the ocean floor.

Cassini was not built to detect signs of life, and Enceladus’ plumes were not discovered until the spacecraft reached Saturn. INMS, which samples gases and determines their composition, was designed to fly through the atmosphere of Saturn’s largest moon, Titan.

“This is the closest we’ve come, so far, to identifying a place with some of the ingredients needed for a habitable environment,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington, D.C.

Hubble views Europa

These composite images show a suspected plume of material erupting two years apart from the same location on Jupiter’s icy moon Europa. Both plumes, photographed in UV light by Hubble, were seen in silhouette as the moon passed in front of Jupiter. Caption and Image Credit: NASA / ESA / STScI / USGS

“Confirmation that the chemical energy for life exists within the ocean of a small moon of Saturn is an important milestone in our search for habitable worlds beyond Earth,” said Cassini project scientist Linda Spilker of NASA’s Jet Propulsion Laboratory in California.

“Although we can’t detect life, we’ve found that there’s a food source for it,” said Hunter Waite, lead author of a study on the findings published in the journal Science.

Tiger Stripes on Saturn's Enceladus

This enhanced-color Cassini view of southern latitudes on Enceladus features the bluish “tiger stripe” fractures that rip across the south polar region. Image & Caption Credit: NASA / JPL-Caltech / Space Science Institute

Detecting life will require a probe that could land on Enceladus’ surface and drill through its icy shell into the ocean. An Enceladus Life Finder, which would fly through its plumes, has been proposed for NASA’s Discovery missions program.

The second discovery announced was confirmation of plumes being emitted on Jupiter’s moon Europa, which is known to have a global underground ocean.

A likely plume was first observed by the Hubble Space Telescope’s Imaging Spectrograph (STIS) in 2014, then not seen again until 2016.

This second sighting was found in the same location as the plume found two years earlier, a region where NASA’s Galileo spacecraft identified a hot spot during the late 1990s.

The plume discovered in 2014 extended approximately 62 miles (100 kilometers) above Europa’s surface while the one spotted two years later rose to a height of about 30 miles (50 kilometers).

As with Enceladus, the plumes appear to be emanating from cracks in the icy world’s crust located in an unusually warm region, suggesting water could be erupting from beneath the ocean and warming the surface nearby.

Alternatively, water emitted by the plume could be falling back onto the surface in mist form, keeping surface grains warm longer than other areas.

“The plumes on Enceladus are associated with hotter regions, so after Hubble imaged this new plume-like feature on Europa, we looked at that location on the Galileo thermal map,” said William Sparks of the Space Telescope Science Institute in Baltimore, who led both Hubble observations of the plumes. “We discovered that Europa’s plume candidate is sitting right on the thermal anomaly.”

STIS observed Europa’s plumes in ultraviolet light when the large moon passed in front of Jupiter. This configuration enabled Europa’s thin atmosphere to block Jupiter’s light, enabling the instrument to observe Europa in silhouette.

Findings of the Hubble study have been published in The Astrophysical Journal Letters.

NASA already has plans to explore Europa with the Europa Clipper mission, scheduled to launch around 2022. That mission will consist of a spacecraft with nine science instruments that will orbit Jupiter and make 45 close flybys of Europa.

Science goals for that mission include measuring the thickness of Europa’s ice shell, studying the depth and salinity of its ocean, and conducting an analysis of both the plumes and the moon’s thin atmosphere.

The probe will use an ultraviolet camera to analyze the same areas studied by Hubble but from a much closer vantage point.

Europa may be a stronger candidate for microbial life due to its age. Jupiter’s large moon is about four billion years old, meaning there was a lot of time for life to get started.

In contrast, Enceladus could be as young as 100 million years old, which may not be long enough for life to develop.

Plume on Enceladus

These composite images show a suspected plume on Europa around Jupiter. The images are from Hubble and Galileo, respectively. The image on the left shows an oval mark at the origin source for a plume NASA’s Hubble Space Telescope detected in 2014 and 2016; on the right is a heat map from NASA’s Galileo mission. Image Credit: NASA / ESA




Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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