This illustration shows Cassini diving through the Enceladus plume in 2015. New ocean world discoveries from Cassini and Hubble will help inform future exploration and the broader search for life beyond Earth.
Credits: NASA/JPL-Caltech
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Two veteran NASA
missions are providing new details about icy, ocean-bearing moons of
Jupiter and Saturn, further heightening the scientific interest of these
and other "ocean worlds" in our solar system and beyond. The findings
are presented in papers published
Thursday by researchers with NASA’s Cassini mission to Saturn and Hubble Space Telescope.
In the papers, Cassini scientists announce that a form of chemical
energy that life can feed on appears to exist on Saturn's moon
Enceladus, and Hubble researchers report additional evidence of plumes
erupting from Jupiter's moon Europa.
“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 at Headquarters in Washington. ”These results demonstrate
the interconnected nature of NASA's science missions that are getting us
closer to answering whether we are indeed alone or not.”
The paper from researchers with the Cassini mission,
published in the journal Science, indicates hydrogen gas, which could
potentially provide a chemical energy source for life, is pouring into
the subsurface ocean of Enceladus from hydrothermal activity on the
seafloor.
The presence of ample hydrogen in the moon's ocean means that
microbes – if any exist there – could use it to obtain energy by
combining the hydrogen with carbon dioxide dissolved in the water. This
chemical reaction, known as "methanogenesis" because it produces methane
as a byproduct, is at the root of the tree of life on Earth, and could
even have been critical to the origin of life on our planet.
Life as we know it requires three primary ingredients: liquid water; a
source of energy for metabolism; and the right chemical ingredients,
primarily carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.
With this finding, Cassini has shown that Enceladus – a small, icy moon a
billion miles farther from the sun than Earth – has nearly all of these
ingredients for habitability. Cassini has not yet shown phosphorus and
sulfur are present in the ocean, but scientists suspect them to be,
since the rocky core of Enceladus is thought to be chemically similar to
meteorites that contain the two elements.
"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 Linda Spilker, Cassini project
scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena,
California.
The Cassini spacecraft detected the hydrogen in the plume of gas and
icy material spraying from Enceladus during its last, and deepest, dive
through the plume on Oct. 28, 2015. Cassini also sampled the plume's
composition during flybys earlier in the mission. From these
observations scientists have determined that nearly 98 percent of the
gas in the plume is water, about 1 percent is hydrogen and the rest is a
mixture of other molecules including carbon dioxide, methane and
ammonia.
The measurement was made using Cassini's Ion and Neutral Mass
Spectrometer (INMS) instrument, which sniffs gases to determine their
composition. INMS was designed to sample the upper atmosphere of
Saturn's moon Titan. After Cassini's surprising discovery of a towering
plume of icy spray in 2005, emanating from hot cracks near the south
pole, scientists turned its detectors toward the small moon.
Cassini wasn't designed to detect signs of life in the Enceladus
plume – indeed, scientists didn't know the plume existed until after the
spacecraft arrived at Saturn.
"Although we can't detect life, we've found that there's a food
source there for it. It would be like a candy store for microbes," said
Hunter Waite, lead author of the Cassini study.
The new findings are an independent line of evidence that
hydrothermal activity is taking place in the Enceladus ocean. Previous
results, published in March 2015, suggested hot water is interacting
with rock beneath the sea; the new findings support that conclusion and
add that the rock appears to be reacting chemically to produce the
hydrogen.
The paper detailing new Hubble Space Telescope findings,
published in The Astrophysical Journal Letters, reports on observations
of Europa from 2016 in which a probable plume of material was seen
erupting from the moon’s surface at the same location where Hubble saw
evidence of a plume in 2014. These images bolster evidence that the
Europa plumes could be a real phenomenon, flaring up intermittently in
the same region on the moon's surface.
The newly imaged plume rises about 62 miles (100 kilometers) above
Europa’s surface, while the one observed in 2014 was estimated to be
about 30 miles (50 kilometers) high. Both correspond to the location of
an unusually warm region that contains features that appear to be cracks
in the moon’s icy crust, seen in the late 1990s by NASA's Galileo
spacecraft. Researchers speculate that, like Enceladus, this could be
evidence of water erupting from the moon’s interior.
“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. We discovered that Europa’s plume
candidate is sitting right on the thermal anomaly," said William Sparks
of the Space Telescope Science Institute in Baltimore, Maryland. Sparks
led the Hubble plume studies in both 2014 and 2016.
The researchers say if the plumes and the warm spot are linked, it
could mean water being vented from beneath the moon's icy crust is
warming the surrounding surface. Another idea is that water ejected by
the plume falls onto the surface as a fine mist, changing the structure
of the surface grains and allowing them to retain heat longer than the
surrounding landscape.
For both the 2014 and 2016 observations, the team used Hubble's Space
Telescope Imaging Spectrograph (STIS) to spot the plumes in ultraviolet
light. As Europa passes in front of Jupiter, any atmospheric features
around the edge of the moon block some of Jupiter’s light, allowing STIS
to see the features in silhouette. Sparks and his team are continuing
to use Hubble to monitor Europa for additional examples of plume
candidates and hope to determine the frequency with which they appear.
NASA's future exploration of ocean worlds is enabled by Hubble's
monitoring of Europa's putative plume activity and Cassini's long-term
investigation of the Enceladus plume. In particular, both investigations
are laying the groundwork for NASA's Europa Clipper mission, which is
planned for launch in the 2020s.
“If there are plumes on Europa, as we now strongly suspect, with the
Europa Clipper we will be ready for them,” said Jim Green, Director of
Planetary Science, at NASA Headquarters.
Hubble's identification of a site which appears to have persistent,
intermittent plume activity provides a tempting target for the Europa
mission to investigate with its powerful suite of science instruments.
In addition, some of Sparks' co-authors on the Hubble Europa studies are
preparing a powerful ultraviolet camera to fly on Europa Clipper that
will make similar measurements to Hubble's, but from thousands of times
closer. And several members of the Cassini INMS team are developing an
exquisitely sensitive, next-generation version of their instrument for
flight on Europa Clipper.
For more information on ocean worlds in our solar system and beyond, visit:
https://www.nasa.gov/specials/
ocean-worlds