For geophysicist William B. Moore, the question of whether life
exists on Jupiter’s moon Europa boils down to whether the moon’s
center is chewy or crunchy. Many scientists doubt life can exist on
Europa’s surface because of extreme cold, lack of liquid water, the
tenuous atmosphere and intense bombardment from Jupiter’s radiation
belts. Moore believes distant Europa receives too little sunlight to
provide the energy needed for organisms to thrive on its apparently
icy surface. Others argue the chemical energy needed for life is
created when charged particles bombard Europa to produce oxidants.
Nevertheless, says Moore, Europa’s surface “would be a very difficult
place to make a living.” If Europan life exists at all it would most
likely be found within an ocean beneath the ice, where organisms
could get energy and mineral nutrients from eruptions of seafloor
volcanoes, says Moore, a postdoctoral researcher at the University of
California, Los Angeles, and member of the NASA Astrobiology
Institute.
If Europa has a hot, “chewy” center–that is, relatively low
viscosity–it would be similar to the soft, partly molten interior of
Jupiter’s moon Io, which is the most spectacularly volcanic body in
the solar system. So Moore says a “chewy” Europa likely would have
seafloor volcanoes producing conditions conducive to life–just like
the undersea volcanoes and hydrothermal vents along Earth’s mid-ocean
ridges.
If Europa has a cold, “crunchy” center–with high viscosity–it would
be rigid and volcanically dead like Earth’s moon. Undersea volcanoes
and life would be improbable, says Moore.
Moore argues Europa must either be chewy or crunchy–and nothing in
between–because of the way it orbits Jupiter and interacts with Io
and Ganymede, two of the three other major moons discovered
independently in 1610 by astronomers Galileo Galilei and Simon
Marius.
Radioactive decay is one potential source of internal heat for
planetary and lunar bodies. But, Moore says, it is inadequate to
cause volcanism in a body as small as Europa, which at 3,138
kilometers (1,950 miles) in diameter is a bit smaller than Earth’s
moon. Still, internal melting and volcanism could be triggered by
tidal forces, namely, the gravitational pull from Jupiter, Io and
Ganymede.
So Moore plans to conduct computer simulations of the rates at which
Europa and the other Galilean moons orbit Jupiter to find out if the
orbits are consistent with a chewy or crunchy center for Europa–and
thus with possible life or no life.
“I anticipate in the next three to six months we will have some
pretty solid results,” said Moore, who is working on the project with
UCLA postdoctoral researcher Ferenc Varadi and graduate student
Susanna Musotto.
If the simulation uses a crunchy Europa, and the result looks like
the existing orbits of Jupiter’s moons, that would tend to confirm
Europa indeed is crunchy or volcanically dead, Moore says. The same
would be true if a simulation with a chewy Europa resulted in orbits
radically different than seen today, he added.
If, however, a computer simulation with a chewy Europa results in
orbits that resemble reality, “then we just don’t know” what it
means, Moore says. So he says his experiment is a negative test–
able to identify a Europa that is crunchy and thus volcanically dead,
but not capable of proving it is chewy and volcanically active.
Indeed, if simulations using either a chewy or crunchy Europa both
resulted in orbits that looked like reality, it would raise questions
about the extent to which the orbits were influenced by Europa’s
internal viscosity.
“In principle, what he [Moore] is saying makes sense,” says Hal
Levison, staff scientist at the Southwest Research Institute in
Boulder, CO. Whether the viscosity of Europa’s center “has a big
enough effect to be measurable or not remains to be seen. But it’s a
great experiment to be doing.”
Moore doubts there is life on Europa, but “I don’t have good science
to back myself on that yet. That’s what I’m trying to do.”
Along with Mars and Saturn’s moon Titan, Europa long has been
considered one of the most likely places for life in our solar
system, largely because of the ocean believed to exist under its icy
outer shell. But Moore contends the presence of seafloor volcanism
as a source of energy and nutrients is far more important than water
in determining if Europa might harbor life.
Europa, Io and Ganymede are influenced by tidal forces because their
orbits around Jupiter are slightly oval-shaped or “eccentric” instead
of perfectly circular. Those moons orbit Jupiter in what is called a
Laplace resonance, which means “every time Ganymede goes around once,
Europa goes around twice and Io goes around four times,” Moore says.
“This means they keep meeting up at the same place over and over
again.”
Tidal forces from that resonance tend to “pump” the orbits of
Ganymede, Europa and Io so they become more oval-shaped. It is “just
like if you push a kid on swing at the high point of his swing,”
Moore says. “He keeps going higher and higher because you are
pushing at the time you can speed him up.”
The moons tend to return to more circular orbits by wobbling to
dissipate tidal energy internally, which produces heat. Tides occur
not only in oceans, but also in solid rock–even on Earth.
Moore says other researchers have estimated that Europa gets only
seven percent of the tidal “squishing and squeezing” that Io receives
because Europa is farther from Jupiter. How well that tidal energy
heats up Europa’s interior depends on the viscosity of material
within Europa.
“The chewier [less viscous] something is, the more efficiently this
squishing and squeezing turn into heat,” Moore says.
In contrast, Earth’s moon “is getting squished and squeezed by tides
due to the Earth, but it is not volcanically active,” he adds. “It
is not dissipating tidal energy [as heat] because it’s a cold,
crunchy [viscous] object.”
Although tidal forces on the moon are smaller than tidal forces on
Europa, Moore says the amount of force exerted on each moon is not
the critical factor. Rather, a chewy object like Io will warm up due
to tidal “squishing and squeezing” while a crunchy object like the
moon will not, he says.
Previous pencil-and-paper mathematical calculations of the orbits of
Jupiter’s moons assumed a solid or crunchy interior for Europa–with
little heating due to tidal forces. Moore says his computer
simulations will try to determine if those assumptions are valid.
“Hopefully, the results can either say definitively that Europa is
crunchy, or they say nothing,” he says.
Moore says his computer simulation will be more complex, with fewer
assumptions, than previous mathematical calculations. For example,
earlier calculations assumed lunar orbits were perfectly elliptical;
his computer simulations will include minor “bulges” in those
ellipses induced by the fact gravity from more than two objects is
involved.
But “the major difference will be that I will try out this
alternative case–a chewy Europa–that simply was skipped before,” he
adds.
A crunchy Europa should produce orbits that resemble reality and
earlier calculations; a chewy Europa should be closer to circular to
stay in resonance with Io and Ganymede, and thus would not resemble
reality.
Moore’s effort to use computer modeling to determine whether Europa
is crunchy and dead or chewy and perhaps volcanically active “is
subject to a lot of uncertainty” and unlikely to give a definitive
answer, says planetary scientist Paul Geissler of the University of
Arizona’s Lunar and Planetary Laboratory.
One uncertainty is how any silicate rock beneath Europa’s presumed
ocean behaves when squeezed by tides. Another is the possibility
that there are changes over time in how Io, Europa and Ganymede
“push” on each other–changes that could make Europa’s rocky interior
either hot or cool, Geissler adds.
“Computer modeling is the best we can do right now,” Geissler says.
“But in the long run, the answer is going to come from further
observation.”
What’s next?
NASA hopes to launch a Europa Orbiter mission in 2008, with the
primary goal of determining if there indeed is a global, subsurface
ocean. But as far as whether Europa is chewy, volcanic and conducive
to life, “it won’t be able to determine much,” Moore says. “The
Europa Orbiter is stuck on the outside looking in. The ice blocks
observation of the rocky interior, and the ocean prevents you from
sensing much about the interior” because it allows the rocky interior
and icy shell to move independently under tidal forces.
Geissler, however, says the Europa Orbiter’s radar sounding device
should be able to detect a reflected radar echo from an ice-ocean
boundary if the ice is only a few miles thick. If Europa is
volcanically dead, the ice should be tens of miles thick. But
Geissler believes a thin shell of ice would be evidence of heat
rising from a volcanically active seafloor, preventing formation of
thicker ice.
