Detecting salty ice and negatively charged water ions in the ice plume of Saturn’s
moon Enceladus, which primarily replenishes Saturn’s ring with material from
discharging jets, hints that it could harbor a reservoir of liquid water — perhaps
an ocean — beneath its surface. And, where’s there’s an ocean of liquid water, there’s
a higher probability of finding some form of life.
“The original picture of the plumes as violently erupting
Yellowstone-like geysers is changing,” said Frank Postberg, Cassini
scientist for the cosmic dust analyzer at the Max Planck Institute for
Nuclear Physics in Heidelberg, Germany. “They seem more like steady
jets of vapor and ice fed by a large water reservoir. However, we
cannot decide yet if the water is currently ‘trapped’ within huge
pockets in Enceladus’ thick ice crust or still connected to a large
ocean in contact with the rocky core.”
Scientists working on the Cassini space mission have found negatively charged water ions in the ice plume. Their findings, based on analysis from data taken in plume fly-throughs in 2008 provide evidence for the presence of liquid water. The Cassini plasma spectrometer, used to gather this data, also found other species of negatively charged ions including hydrocarbons.
"While it's no surprise that there is water there, these short-lived ions are extra evidence for sub-surface water and where there's water, carbon and energy, some of the major ingredients for life are present," said lead author Andrew Coates from University College London's Mullard Space Science Laboratory.
"The surprise for us was to look at the mass of these ions. There were several peaks in the spectrum, and when we analysed them we saw the effect of water molecules clustering together one after the other." The measurements were made as Cassini plunged through Enceladus' plume on March 12, 2008.
Enceladus thus joins Earth, Titan and comets where negatively charged ions are known to exist in the solar system. Negative oxygen ions were discovered in Earth's ionosphere at the dawn of the space age. At Earth's surface, negative water ions are present where liquid water is in motion, such as waterfalls or crashing ocean waves.
The Cassini plasma spectrometer, originally designed to take data in Saturn's magnetic environment, measures the density, flow velocity and temperature of ions and electrons that enter the instrument. But since the discovery of Enceladus' water ice plume, the instrument has also successfully captured and analysed samples of material in the jets.
Early in its mission, Cassini discovered the plume that fountains water vapour and ice particles above Enceladus. Since then, scientists have found that these water products dominate Saturn's magnetic environment and create Saturn's huge E-ring.
At Titan, the same instrument detected extremely large negative hydrocarbon ions with masses up to 13,800 times that of hydrogen. Titan, the largest hydrocarbon or nitrile ions are seen at the lowest altitudes of the atmosphere that Cassini flew (950 kilometers (590 miles). They suggest these large ions are the source of the smog-like haze that blocks most of Titan's surface from view and may be representative of the organic mix called "tholins" by Carl Sagan when he produced the reddish brew of prebiotic chemicals in the lab from gases that were known to be present in Titan's atmosphere. Tholins that may be produced in Titan's atmosphere could fall to the moon's surface and may even make up the sand grains of the dunes that dominate part of Titan's equatorial region.
Cassini discovered the water-ice jets in 2005 on Enceladus. These
jets expel tiny ice grains and vapor, some of which escape the moon’s
gravity and form Saturn’s outermost ring. Cassini’s cosmic dust
analyzer has examined the composition of those grains and found salt
within them. Cassini was launched from the Kennedy Space Center in 1997
and has been orbiting Saturn since July 2004.
“We believe that the salty minerals deep inside Enceladus washed out from rock at the bottom of a liquid layer,” said Postberg.
Scientists
on Cassini’s cosmic dust detector team conclude that liquid water must
be present because it is the only way to dissolve the significant
amounts of minerals that would account for the levels of salt detected.
The process of sublimation, the mechanism by which vapor is released
directly from solid ice in the crust, cannot account for the presence
of salt.
“Potential plume sources on Enceladus are an active area
of research with evidence continuing to converge on a possible salt
water ocean,” said Linda Spilker, Cassini deputy project scientist at
NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Our next
opportunity to gather data on Enceladus will come during two flybys in
November.”
The makeup of the outermost ring grains, determined when thousands of
high-speed particle hits were registered by Cassini, provides indirect
information about the composition of the plume material and what is
inside Enceladus. The outermost ring particles are almost pure water
ice, but nearly every time the dust analyzer has checked for the
composition, it has found at least some sodium within the particles.
“Our
measurements imply that besides table salt, the grains also contain
carbonates like soda. Both components are in concentrations that match
the predicted composition of an Enceladus ocean,” Postberg said. “The
carbonates also provide a slightly alkaline pH value. If the liquid
source is an ocean, it could provide a suitable environment on
Enceladus for the formation of life precursors when coupled with the
heat measured near the moon’s south pole and the organic compounds
found within the plumes.”
However, in another study published in
Nature, researchers doing ground-based observations did not see sodium,
an important salt component. That team notes that the amount of sodium
being expelled from Enceladus is actually less than observed around
many other planetary bodies. These scientists were looking for sodium
in the plume vapor and could not see it in the expelled ice grains.
They argue that if the plume vapor does come from ocean water, the
evaporation must happen slowly deep underground, rather than as a
violent geyser erupting into space.
“Finding salt in the plume
gives evidence for liquid water below the surface,” said Sascha Kempf,
also a Cassini scientist for the cosmic dust analyzer from the Max
Planck Institute for Nuclear Physics. “The lack of detection of sodium
vapor in the plume gives hints about what the water reservoir might
look like.”
Determining the nature and origin of the plume
material is a top priority for Cassini during its extended tour, called
the Cassini Equinox Mission.
Scientists at Jet Propulsion Lab in
California, the University of Colorado and the University of Central
Florida in Orlando teamed up to analyze the plumes of water vapor and
ice particles spewing from Saturn’s Moon, Enceladus. They used data
collected by the Cassini spacecraft’s Ultraviolet Imaging Spectrograph
(UVIS).
The team, including, found that the source of plumes
may be vents on the moon that channel water vapor from a warm, probably
liquid source to the surface at supersonic speeds.
“There are
only three places in the solar system we know or suspect to have liquid
water near the surface,” said UCF Assistant Professor Joshua Colwell.
“Earth, Jupiter’s moon Europa and now Saturn’s Enceladus. Water is a
basic ingredient for life, and there are certainly implications there.
If we find that the tidal heating that we believe causes these geysers
is a common planetary systems phenomenon, then it gets really
interesting.”
The team’s findings support a theory that the
plumes observed are caused by a water source deep inside Enceladus.
This is not a foreign concept. On earth, liquid water exists beneath
the 15-million year-old ice at Lake Vostok, in Antarctica.
Scientists
suggest that in Enceladus's case, the ice grains would condense from
the vapor escaping from the water source and stream through the cracks
in the ice crust before heading into space. That's likely what
Cassini's instruments detected in 2005 and 2007, the basis for the
team's investigation.
The team’s work also suggests that another
hypothesis is unlikely. That theory predicts that the plumes of gas and
dust observed are caused by evaporation of volatile ice freshly exposed
to space when Saturn's tidal forces open vents in the south pole. But
the team found more water vapor coming from the vents in 2007 at a time
when the theory predicted there should have been less.
“Our
observations do not agree with the predicted timing of the faults
opening and closing due to tidal tension and compression,” said Candice
Hansen, the lead author on the project. “We don't rule it out entirely
. . . but we also definitely do not substantiate this hypothesis.”
Instead,
their results suggest that the behavior of the geysers supports a
mathematical model that treats the vents as nozzles that channel water
vapor from a liquid reservoir to the surface of the moon. By observing
the flickering light of a star as the geysers blocked it out, the team
found that the water vapor forms narrow jets. The authors theorize that
only high temperatures close to the melting point of water ice could
account for the high speed of the water vapor jets.
Although
there is no solid conclusion yet, there may be one soon. Enceladus is a
prime target of Cassini during its extended Equinox Mission, underway
now through September 2010.
“We still have a lot to discover and
learn about how this all works on Enceladus,” Colwell said. “But this
is a good step in figuring it all out.”
Until the two Voyager
spacecraft passed near Enceladus, the sixth-largest moon of Saturn, in
the early 1980s, very little was known about this small moon except for
the identification of water ice on its surface. The Voyager missions
showed that Enceladus is only 500 km in diameter and reflects almost
100% of the sunlight that strikes it. Voyager 1 found that Enceladus
orbited in the densest part of Saturn’s diffuse E ring, indicating a
possible link between the two, while Voyager 2 revealed that despite
the moon’s small size, it had a wide range of terrains ranging from
ancient, heavily cratered surfaces to young, tectonically deformed
terrain, with some regions with surface ages as young as 100 million
years old.
The Cassini spacecraft performed several close flybys
of Enceladus in 2005, revealing the moon’s surface and environment in
greater detail. In particular, the probe discovered a water-rich plume
venting from the moon’s south polar region. This discovery, along with
the presence of escaping internal heat and very few (if any) impact
craters in the south polar region, shows that Enceladus is geologically
active today.
Given the level of tectonic resurfacing found on
Enceladus, a critical factor in the evolution of life on Earth, has
been an important driver of geology on this small moon. Enceladus the
fourth body in the solar system to have confirmed volcanic activity,
along with Earth, Neptune’s Triton, and Jupiter’s Io.
There are
three ecosystems discovered on Earth that could mirror possible
lifeforms on Enceladus. Two are based on methanogens, which belong to
an ancient group related to bacteria, called the archaea — the hardy
survivalists of bacteria that thrive in harsh environments without
oxygen. Deep volcanic rocks along the Columbia River and in Idaho Falls
host two of these ecosystems, which pull their energy from the chemical
interaction of different rocks. The third ecosystem is powered by the
energy produced in the radioactive decay in rocks, and was found deep
below the surface in a mine in South Africa.
NASA’s Cassini
spacecraft discovered a surprising organic brew erupting in geyser-like
fashion from Saturn’s moon Enceladus during a close flyby on March 12,
2008. Scientists were stunned that this tiny moon is so active, “hot”
and teeming with water vapor and organic chemicals.
“Enceladus
has got warmth, water and organic chemicals, some of the essential
building blocks needed for life,” said Dennis Matson, Cassini project
scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “We
have quite a recipe for life on our hands, but we have yet to find the
final ingredient, liquid water, but Enceladus is only whetting our
appetites for more.”
“A completely unexpected surprise is that
the chemistry of Enceladus, what’s coming out from inside, resembles
that of a comet,” said Hunter Waite, principal investigator at the
Southwest Research Institute in San Antonio. “To have primordial
material coming out from inside a Saturn moon raises many questions on
the formation of the Saturn system.”
“Enceladus is by no means a
comet. Comets have tails and orbit the sun, and Enceladus’ activity is
powered by internal heat while comet activity is powered by sunlight.
Enceladus’ brew is like carbonated water with an essence of natural
gas,” said Waite.
The Casssini Ion and Neutral Mass Spectrometer
saw a much higher density of volatile gases, water vapor, carbon
dioxide and carbon monoxide, as well as organic materials, some 20
times denser than expected. This dramatic increase in density was
evident as the spacecraft flew over the area of the plumes.
New
high-resolution heat maps of the south pole by Cassini’s Composite
Infrared Spectrometer show that the so-called tiger stripes, giant
fissures that are the source of the geysers, are warm along almost
their entire lengths, and reveal other warm fissures nearby. The
warmest regions along the tiger stripes correspond to two of the jet
locations seen in Cassini images.
“These spectacular new data
will really help us understand what powers the geysers. The
surprisingly high temperatures make it more likely that there’s liquid
water not far below the surface,” said John Spencer, Cassini scientist
on the Composite Infrared Spectrometer team at the Southwest Research
Institute in Boulder, Colo.
Previous ultraviolet observations
showed four jet sources, matching the locations of the plumes seen in
previous images. This indicates that gas in the plume blasts off the
surface into space, blending to form the larger plume.
At closest
approach, Cassini was only 30 miles from Enceladus. When it flew
through the plumes it was 120 miles from the moon’s surface. Cassini’s
next flyby of Enceladus is in August.
The first step toward
answering the question of whether life exists inside the subsurface
aquifer of Enceladus is to analyze the organic compounds in the plume.
Cassini’s March 12 passage through the plume provided some measurements
that help us move toward an answer, and preliminary plans call for
Cassini to fly through the plume again for more measurements in the
future. Ultimately, another mission in the future could conceivably
land near the plume or even return plume material to Earth for
laboratory analysis.
Organic chemicals were part of the raw
material from which Enceladus and Saturn’s other moons formed. The
origin of Enceladus’ heat is less clear, but there are several
possibilities that could have given Enceladus a layer of liquid water
that persists today. Early on, it could have been heated by decay of
short-lived radioactivity in rocks, with the heating prolonged by tidal
influences.
Or perhaps an earlier oblong orbit could have brought
more tidal heating than exists there today. A past tidal relationship
with another moon could have caused the heat. Another theory says the
heat could have been produced from a process called serpentization,
where chemical binding of water and silicate rock could occur at the
upper layer of the moon’s core. This increases the volume of the rock
and creates energy in the form of heat.
Any of these heating
mechanisms might have created a liquid subsurface aquifer solution rich
in organics, allowing Enceladus to serve up a suitable prebiotic soup.
The
deep sea vent theory for the origin of life on Earth might apply to
Enceladus as well. In this scenario, life on Earth began at the
interface where chemically rich fluids, heated by tidal or other
mechanisms, emerge from below the sea floor. Chemical energy is derived
from the reduced gases, such as hydrogen-sulfide and hydrogen coming
out from the vent in contact with a suitable oxidant, such as carbon
dioxide. Hot spots on an Enceladus sea floor could be locales for this
type of process.
We don’t know how long it takes for life to
start when the ingredients are there and the environment is suitable,
but it appears to have happened quickly on Earth. So maybe it was
possible that on Enceladus, life started in a “warm little pond” below
the icy surface occurring over the last few tens of millions of years.
Edited and posted by Casey Kazan from materials provided by NASA.
Related Galaxy posts:
Links:
For images and more information, visit http://www.nasa.gov/cassini or http://saturn.jpl.nasa.gov/ .
http://www.sciencedaily.com/releases/2008/04/080420122601.htm
Source: http://news.ucf.edu/UCFnews/index?page=article&id=00240041e030fdf011daaca44450078d1.
http://www.nasa.gov/mission_pages/cassini/media/cassini-20090624.html
More information: http://saturn.jpl.nasa.gov/index.cfm
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