Europa: Chewy or Crunchy?

Cutout of Europa
If Europa has a "chewy" (low viscocity) core, then it likely would have seafloor volcanoes producing conditions conducive to life – just like the undersea volcanoes and hydrothermal vents along Earth’s mid-ocean ridges.
Credit: William B. Moore

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.

Heat Flux vs. Temperature
Equilibrium is achieved when heat production balances heat transfer. The dependence of tidal dissipation and convective heat transfer on viscocity (and therefore temperature) results in multiple equilibria as shown above. When the convective heat flux exceeds the heat production, the system cools, while the the temperature increases when the heat production exceeds the convective heat flux. Thus, the central equilibrium is unstable, and two very different states exist: hot or cold, tidally or radiogenically dominated, Io-like or Moon-like, chewy or crunchy.
Credit: William B. Moore

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, Colo. 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.

Eccentric Orbits
The orbits of Io, Europa and Ganymede are locked in a 1:2:4 mean-motion resonance known as the Laplace resonance. It is this resonant configuration that makes Europa’s and Io’s orbits eccentric and drives dissipation.
Credit: NASA

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.

Europa Orbiter
NASA hopes to launch a Europa Orbiter mission in 2008, with the primary goal of determining if there indeed is a global, subsurface ocean.
Credit: NASA

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.


Related Web Pages

Focus on Europa (NAI)
Through Thick or Thin: Exploring Europa’s outer layer of ice. (NAI)
Jovian Moons (NAI)
Jupiter and satellites (NASA)
Planetary Science Research Discoveries (University of Hawaii, Manoa)
Calvin J. Hamilton’s Views of the Solar System (Solarviews)
The Partially Watery World of Europa (ASU)
Europa Orbiter site (NASA)