Water from the subsurface ocean of Saturn’s moon Enceladus sprays from huge fissures out into space. NASA’s Cassini spacecraft, which captured this image in 2010, sampled icy particles and scientists are continuing to make new discoveries from the data. NASA/JPL-Caltech/Space Science Institute
A study zooms in on data that NASA’s Cassini gathered at Saturn’s icy moon
and finds evidence of a key ingredient for life and a supercharged source of
energy to fuel it.
Scientists have known that the
giant plume of ice grains and water vapor spewing from Saturn’s moon Enceladus
is rich with organic compounds, some of which are important for life as we know
it. Now, scientists analyzing data from NASA’s Cassini mission are taking the evidence for habitability
a step further: They’ve found strong confirmation of hydrogen cyanide, a
molecule that is key to the origin of life.
The researchers also uncovered
evidence that the ocean, which is hiding below the moon’s icy outer shell and
supplies the plume, holds a powerful source of chemical energy. Unidentified
until now, the energy source is in the form of several organic compounds, some
of which, on Earth, serve as fuel for organisms.
The findings, published Thursday, Dec. 14, in Nature Astronomy, indicate there may be
much more chemical energy inside this tiny moon than previously thought. The
more energy available, the more likely that life might proliferate and be
sustained.
“Our work provides further evidence that Enceladus is host to some of the most important molecules for both creating the building blocks of life and for sustaining that life through metabolic reactions,” said lead author Jonah Peter, a doctoral student at Harvard University who performed much of the research while working at NASA’s Jet Propulsion Laboratory in Southern California. “Not only does Enceladus seem to meet the basic requirements for habitability, we now have an idea about how complex biomolecules could form there, and what sort of chemical pathways might be involved.”
NASA’s Cassini spacecraft captured this image of reflective Enceladus, seen at center, as it orbits Saturn. Also in the 2007 image are two other moons: Pandora, a bright speck hovering near the rings, and Mimas, at lower right. NASA/JPL/Space Science Institute
Versatile and Energetic
“The discovery of hydrogen cyanide
was particularly exciting, because it’s the starting point for most theories on
the origin of life,” Peter said. Life as we know it requires building blocks,
such as amino acids, and hydrogen cyanide is one of the most important and
versatile molecules needed to form amino acids. Because its molecules can be
stacked together in many different ways, the study authors refer to hydrogen
cyanide as the Swiss army knife of amino acid precursors.
“The more we tried to poke holes in
our results by testing alternative models,” Peter added, “the stronger the
evidence became. Eventually, it became clear that there is no way to match the
plume composition without including hydrogen cyanide.”
In 2017, scientists found evidence
at Enceladus of chemistry that could help sustain life, if present, in its
ocean. The combination of carbon dioxide, methane, and hydrogen in the plume
was suggestive of methanogenesis, a metabolic process that produces methane.
Methanogenesis is widespread on Earth, and may have been critical to the origin
of life on our planet.
The new work uncovers evidence for additional energy chemical sources far
more powerful and diverse than the making of methane: The authors found an
array of organic compounds that were oxidized, indicating to scientists that
there are many chemical pathways to potentially sustain life in Enceladus’
subsurface ocean. That’s because oxidation helps drive the release of chemical
energy.
“If methanogenesis is like a small
watch battery, in terms of energy, then our results suggest the ocean of
Enceladus might offer something more akin to a car battery, capable of
providing a large amount of energy to any life that might be present,” said
JPL’s Kevin Hand, co-author of the study and principal investigator of the
effort that led to the new results.
Math Is the
Way
Unlike earlier research that used
lab experiments and geochemical modeling to replicate the conditions Cassini
found at Enceladus, the authors of the new work relied on detailed statistical
analyses. They examined data collected by Cassini’s ion and neutral mass spectrometer, which studied the gas, ions, and ice grains around
Saturn.
By quantifying the amount of
information contained in the data, the authors were able to tease out subtle
differences in how well different chemical compounds explain the Cassini
signal.
“There are many potential puzzle
pieces that can be fit together when trying to match the observed data,” Peter
said. “We used math and statistical modeling to figure out which combination of
puzzle pieces best matches the plume composition and makes the most of the
data, without overinterpreting the limited dataset.”
Scientists are still a long way
from answering whether life could originate on Enceladus. But as Peter noted,
the new work lays out chemical pathways for life that could be tested in the
lab.
Meanwhile, Cassini is the mission
that keeps giving – long after it revealed that Enceladus is an active moon. In
2017, the mission ended by deliberately plunging the spacecraft into Saturn’s
atmosphere. “Our study demonstrates that while Cassini’s mission has ended, its
observations continue to provide us with new insights about Saturn and its
moons – including the enigmatic Enceladus,” said Tom Nordheim, a JPL planetary
scientist who’s a co-author of the study and was a member of the Cassini team.
More About the
Mission
The Cassini-Huygens mission was a
cooperative project of NASA, ESA (European Space Agency), and the Italian Space
Agency. JPL, a division of Caltech in Pasadena, California, managed the mission
for NASA’s Space Mission Directorate in Washington. JPL designed, developed,
and assembled the Cassini orbiter.
For more information about Cassini, visit: http://nasa.gov/cassini
Source: NASA Study Finds Life-Sparking Energy Source and Molecule at Enceladus - NASA
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