Based
on new data from the W. M. Keck Observatory about Jupiter's moon
Europa, astronomers hypothesize that chloride salts bubble up from the
icy moon's global liquid ocean and reach the frozen surface where they
are bombarded with sulfur from volcanoes on Jupiter's largest moon, Io.
This illustration of Europa (foreground), Jupiter (right) and Io
(middle) is an artist's concept. Credit: NASA/JPL-Caltech. Hi-Res Image
KAMUELA, Hawaii—With data collected from the mighty W. M. Keck
Observatory, California Institute of Technology (Caltech) astronomer
Mike Brown — known as the Pluto killer for discovering a Kuiper-belt
object that led to the demotion of Pluto from planetary status — and
Kevin Hand from the Jet Propulsion Laboratory (JPL) have found the
strongest evidence yet that salty water from the vast liquid ocean
beneath Europa’s frozen exterior actually makes its way to the surface.
The data suggests there is a chemical exchange between the ocean and
surface, making the ocean a richer chemical environment, and implies
that learning more about the ocean could be as simple as analyzing the
moon’s surface. The work is described in a paper that has been accepted
for publication in the Astronomical Journal.
The findings were derived from spectroscopy delivered from the Keck
Observatory, which operates the largest and most scientifically
productive telescopes on Earth.
“We now have the best spectrum of this thing in the world,” Brown says. “Nobody knew there was this little dip in the spectrum because no one had the resolution to zoom in on it before.”
“We now have the best spectrum of this thing in the world,” Brown says. “Nobody knew there was this little dip in the spectrum because no one had the resolution to zoom in on it before.”
Ten-meter Keck II, fitted with Adaptive Optics (AO) to adjust for the
blurring effect of Earth’s atmosphere, and its OH-Suppressing Infrared
Integral Field Spectrograph (OSIRIS) produced details not capable of
collection when NASA’s Galileo mission (1989–2003) was sent to study
Jupiter and its moons.
“We now have evidence that Europa’s ocean is not isolated—that the
ocean and the surface talk to each other and exchange chemicals,” says
Brown, the Richard and Barbara Rosenberg Professor and professor of
planetary astronomy at Caltech. “That means that energy might be going
into the ocean, which is important in terms of the possibilities for
life there. It also means that if you’d like to know what’s in the
ocean, you can just go to the surface and scrape some off.”
“The surface ice is providing us a window into that potentially
habitable ocean below,” says Hand, deputy chief scientist for solar
system exploration at JPL.
Since the days of the Galileo mission, when the spacecraft showed
that Europa was covered with an icy shell, scientists have debated the
composition of Europa’s surface. The infrared spectrometer aboard
Galileo was not capable of providing the detail needed to definitively
identify some of the materials present on the surface. Now, using
current technology on ground-based telescopes, Brown and Hand have
definitively identified a spectroscopic feature on Europa’s surface that
indicates the presence of a magnesium sulfate salt, a mineral called
epsomite, that could only originate from the ocean below.
“Magnesium should not be on the surface of Europa unless it’s coming
from the ocean,” Brown says. “So that means ocean water gets onto the
surface, and stuff on the surface presumably gets into the ocean water.”
Europa’s ocean is thought to be 100 kilometers deep and covers the
entire globe. The moon remains locked in relation to Jupiter, with the
same hemisphere always leading and the other trailing in its orbit. The
leading hemisphere has a yellowish appearance, while the trailing
hemisphere seems to be splattered and streaked with a red material.
The spectroscopic data from that red side has been a cause of
scientific debate for 15 years. It is thought that one of Jupiter’s
largest moons, Io, spews volcanic sulfur from its atmosphere, and
Jupiter’s strong magnetic field sends some of that sulfur hurtling
toward the trailing hemisphere of Europa, where it sticks. It was also
clear from Galileo’s data that there is something other than pure water
ice on the trailing hemisphere’s surface. The debate has focused on what
that other something is—i.e., what has caused the spectroscopic data to
deviate from the signature of pure water ice.
“From Galileo’s spectra, people knew something was there besides
water. They argued for years over what it might be—sodium sulfate,
hydrogen sulfate, sodium hydrogen carbonate, all these things that look
more or less similar in this range of the spectrum,” says Brown. “But
the really difficult thing was that the spectrometer on the Galileo
spacecraft was just too coarse.”
Brown and Hand decided that the latest spectrometers on ground-based
telescopes could improve the data pertaining to Europa, even from a
distance of about 400 million miles. Using the Keck II telescope on
Mauna Kea, they first mapped the distribution of pure water ice versus
anything else on the moon. The spectra showed that even Europa’s leading
hemisphere contains significant amounts of nonwater ice. Then, at low
latitudes on the trailing hemisphere—the area with the greatest
concentration of the nonwater ice material—they found a tiny dip in the
spectrum that had never been detected before.
The two researchers racked their brains to come up with materials
that might explain the new spectroscopic feature, and then tested
everything from sodium chloride to Drano in Hand’s lab at JPL, where he
tries to simulate the environments found on various icy worlds. “We
tried to think outside the box to consider all sorts of other
possibilities, but at the end of the day, the magnesium sulfate
persisted,” Hand says.
Some scientists had long suspected that magnesium sulfate was on the
surface of Europa. But, Brown says, “the interesting twist is that it
doesn’t look like the magnesium sulfate is coming from the ocean.” Since
the mineral he and Hand found is only on the trailing side, where the
moon is being bombarded with sulfur from Io’s, they believe that there
is a magnesium-bearing mineral everywhere on Europa that produces
magnesium sulfate in combination with sulfur. The pervasive
magnesium-bearing mineral might also be what makes up the nonwater ice
detected on the leading hemisphere’s surface.
Brown and Hand believe that this mystery magnesium-bearing mineral is
magnesium chloride. But magnesium is not the only unexpected element on
the surface of Europa. Fifteen years ago, Brown showed that Europa is
surrounded by an atmosphere of atomic sodium and potassium, presumably
originating from the surface. The researchers reason that the sodium and
potassium chlorides are actually the dominant salts on the surface of
Europa, but that they are not detectable because they have no clear
spectral features.
The scientists combined this information with the fact that Europa’s
ocean can only be one of two types—either sulfate-rich or chlorine-rich.
Having ruled out the sulfate-rich version since magnesium sulfate was
found only on the trailing side, Brown and Hand hypothesize that the
ocean is chlorine-rich and that the sodium and potassium must be present
as chlorides.
Therefore, Brown says, they believe the composition of Europa’s sea
closely resembles the salty ocean of Earth. “If you could go swim down
in the ocean of Europa and taste it, it would just taste like normal old
salt,” he says.
Hand emphasizes that, from an astrobiology standpoint, Europa is
considered a premier target in the search for life beyond Earth; a
NASA-funded study team led by JPL and the Johns Hopkins University
Applied Physics Laboratory have been working with the scientific
community to identify options to explore Europa further. “If we’ve
learned anything about life on Earth, it’s that where there’s liquid
water, there’s generally life,” Hand says. “And of course our ocean is a
nice salty ocean. Perhaps Europa’s salty ocean is also a wonderful
place for life.”
The Astronomical Journal paper is titled “Salts and radiation
products on the surface of Europa.” The work was supported by the
Richard and Barbara Rosenberg Professorship at Caltech and by the NASA
Astrobiology Institute through the Astrobiology of Icy Worlds node at
JPL.
The W. M. Keck Observatory operates the two biggest and most
scientifically productive telescopes on Earth. The twin 10-meter
optical/infrared telescopes located on the summit of Mauna Kea on the
Island of Hawaii, feature a world-leading suite of advanced instruments
including imagers, multi-object spectrographs, high-resolution
spectrographs, integral-field spectroscopy and a laser guide star
adaptive optics system. The Observatory is a private 501(c) 3 non-profit
organization and a scientific partnership of the California Institute
of Technology, the University of California and NASA.
Science Contact:
Contact:
Deborah Williams-Hedges
California Institute of Technology
(626) 395-3227
debwms@caltech.edu
Observatory Contact:
W. M. Keck Observatory
(808)881-2837
sjefferson@keck.hawaii.edu
Steve Jefferson
