Enceladus: A Lonely Snowball or a Haven of Life?
By Devin Tierney
Looking out into the night sky at the right time of the year, you might be lucky enough to
see a tiny speck of light known as Saturn. Around this little speck of light orbits a peculiar little
moon that has recently been stirring up quite a big story. This particular moon, Enceladus, has
been putting on a cosmic show that scientists are only now beginning to eagerly observe. Giant
plumes of icy particles, nearly 500 km tall, constantly erupt from the cold surface of the moon
stretching higher than one thousand Empire State Buildings stack on top of one another. While
this spectacular show is certainly entertaining, it leaves astronomers with a plethora of questions
to investigate. What is the source of energy driving these massive eruptions? Could a life form
inhabit Enceladus, harnessing this energy to survive? Since the onset of these lingering
questions, NASA has been searching for the answers.
Enceladus has been a mysterious body for most of modern history. First observed by
William Herschel in 1789, it was known to have an icy surface because of its high reflectivity.
However, that was the extent of knowledge about the moon for the next 200 years – until
NASA’s Voyager missions in the 1980s. The Voyager spacecraft flew through the area of the
solar system that Enceladus calls home and was able to capture low-resolution pictures of the
moon. Observations made by the Voyager missions allowed scientists to discover that
Enceladus’s orbit passes right through the outermost ring of Saturn, known as the E-ring. The
rings consist of fine particles locked in orbit around Saturn. Astronomers began to theorize that
Enceladus was somehow contributing material to the E-ring as the moon floated through it, but
they were not sure how.
The answer finally arrived in 2004 when the NASA Cassini-Huygens mission journeyed
to Enceladus. The Cassini spacecraft detected a thin atmosphere on the tiny moon and, most
intriguingly, huge towers of water and ice erupting from the southern pole - like the geysers of
Yellowstone, multiplied by ten thousand. These giant plumes of ice were spraying particles into
space that were then becoming a part of Saturn’s E ring. In addition, Cassini flew through one of
the monumental ice jets and detected the presence of water and carbon dioxide! While NASA
scientists were mapping the thermal features of Enceladus, a large heat signature was found
emanating from the same area as the ice geysers – meaning that Enceladus had some kind of
internal heat-producing mechanism, unlike most other objects in the solar system.
For life to exist on a celestial body it needs to be active; if the earth cooled and plate
tectonics stopped, many global processes vital to life would cease along with it. The discovery
of a moon that is somehow generating internal energy (right in our neighborhood!) started a
whole series of “what if” questions in the scientific community. As the impacts of these findings
still are not completely known, the question remains: what is driving the massive streams of ice
emerging from the moon and creating all the heat below the surface?
Richard Kerr, of the journal Science, outlines a theory to answer that question, proposing
that a large amount of liquid water is present below the icy crust of the moon. With a liquid
ocean beneath the surface, a process called tidal heating could potentially create enough energy
to drive the giant towers of ice spraying from the surface. Tidal heating is set in motion by the
pull of Saturn’s gravity on Enceladus creating tides in a subsurface ocean, in the same manner
that our moon creates tides in the oceans of Earth. The movement of the water in the ocean
creates heat, similar to the process of producing heat by bending a paperclip back and forth.
Alex Patthoff and Simon Kattenhorn of the University of Idaho detail how an ocean sloshing
about beneath the icy surface could cause the large fractures that are seen on the south pole of
Enceladus.
Additionally, high concentrations of salts have been detected in the material coming from
the geysers. Frank Postberg and his colleagues from Heidelberg University recently suggested in
Nature that such high concentrations of salt could only be explained by the presence of a liquid
ocean on the moon. The massive amount of heat coming from the planet cannot be entirely
explained by tidal heating. Dennis Matson from the Jet Propulsion Lab at the California Institute
of Technology claims that magma chambers remaining from the formation of the moon could
also be producing the heat seen on the moon through hydrothermal activity. If that is in fact true,
the possibility of life on Enceladus becomes a lot more feasible. The search for water and life
elsewhere has been the main focus of NASA planetary science. The hunt for life is like looking
for your keys - you know they are close but you struggle to find them.
For life as we currently know it to exist, a few main ingredients are needed: water, an
energy source, and organic molecules. Enceladus could be the winning lottery ticket judging by
what is known of the moon. Water definitely exists on the moon of Saturn - it is just a question
of what form it is in. The surface in much too cold to have liquid water, but, insulated under tens
of kilometers of ice and kept warm through tides, an immense ocean may be hidden beneath the
surface. There is some sort of massive energy source on the moon creating enough force to
make the magnificent explosions of icy particles on the southern pole. The energy released as
the core of the moon cools might be playing a role, and life could harness that energy.
Organisms exist at hydrothermal vents here on Earth, capturing the energy of our planet itself, as
detailed by John Breier of the Woods Hole Oceanographic Institution. Maybe microorganisms
similar to the ones found in Earth’s oceans are alive in the ocean of Enceladus as well.
When most people think of life on our planet they picture fish, birds, or flowers. Deep
down at the bottom of the ocean, where not even a small sliver of sunlight makes it, a different
kind of life thrives. Entire ecosystems have found a way to survive at the bottom of the sea, their
survival depending on hydrothermal vents rather than energy from the sun. Water interacting
with the ocean crust near the hydrothermal vents becomes enriched with many different minerals
and organic molecules. Brier et al. (2010) describe how chemosynthetic bacteria clustered
around these vents produce organic molecules, which they can metabolize, from dissolved
inorganic minerals in the water. Dissolved organics, such as methane and carbon dioxide, occur
at high levels near hydrothermal vents as a result of the water being superheated.
Chemosynthetic organisms use the energy released from the breakdown of these organic
molecules for sustenance. These bacteria get a free meal on the Earth’s ticket, but their joyride
does not last forever.
Chemosynthetic bacteria make up the base of the food chain at hydrothermal vent
communities and are the staple of the diet for many other life forms living where the sun doesn’t
shine. In this way, energy from the cooling of the earth is turned into food for a variety of
organisms.
Methane and carbon dioxide are the fundamental organic molecules used by bacteria at
deep sea hydrothermal vents on Earth. Both of these molecules have been detected from the
plumes on Enceladus, so perhaps organisms similar to the ones found at hydrothermal vents on
Earth have found a way to survive there. From observations of Enceladus, it seems that all the
basic ingredients necessary for life are present. Whether or not those ingredients have been
cooked up into living organisms will remain a mystery until NASA is able to further uncover the
secrets of the little moon Enceladus during future projects. Enceladus is putting on a dazzling
show, beckoning us to look more intently. Maybe those keys that NASA has been searching for
have been right in their pocket the whole time.
References
Breier, J. A., White, S. N., & German, C. R. (2010). Mineral-microbe interactions in deep-sea
hydrothermal systems: A challenge for raman spectroscopy. Philosophical Transactions of the Royal
Society A-Mathematical Physical and Engineering Sciences, 368(1922), 3067-3086.
doi:10.1098/rsta.2010.0024
House, C. Geosc 484. Pennsylvania State University. 2011.
Jones, W., Stugard, C., & Jannasch, H. (1989). Comparison of thermophilic methanogens from
submarine hydrothermal vents. Archives of Microbiology, 151(4), 314-318.
doi:10.1007/BF00406557
Kerr, R. A. (2011). Enceladus now looks wet, so it may be ALIVE! Science, 332(6035), 1259-1259.
Matson, D.L., et al. (2006). Enceladus’s Interior and Geysers- Possibility for Hydrothermal Geometry and
N2 Production, 37th Annual Lunar and Planetary Science Conference.
NASA.GOV (2011) Enceladus: Overview. Solar System Exploration. NASA. August 31, 2011.
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Enceladus
Patthoff, D. A., & Kattenhorn, S. A. (2011). A fracture history on enceladus provides evidence for a global
ocean. Geophysical Research Letters, 38, L18201. doi:10.1029/2011GL048387
Postberg, F., Schmidt, J., Hillier, J., Kempf, S., & Srama, R. (2011). A salt-water reservoir as the source of
a compositionally stratified plume on enceladus. Nature, 474(7353), 620-622.
doi:10.1038/nature10175
Postberg, F. (2008). “Organic molecules in saturnian E-ring particles. Probing subsurface oceans of
Enceladus?” Proceedings of the International Astronomical Union (1743-9213), pp 317.