The planets are beautiful. There is no denying that. But as we explored the solar system, we were struck by the uniqueness the smaller worlds had to offer. Moons around the largest planets have surfaces early astronomers couldn’t even fathom. NASA’s New Horizons team shows us even Pluto can blow us away.
Jupiter’s moon Europa sits near the top of the list for worlds I want to see explored. An icy shell may hide one of the most important components for life – water.
How can a moon so far from the sun have water? Jupiter’s strong gravitational grip on Europa creates an effect called tidal dissipation. Because the moon’s orbit is elliptical around Jupiter, the moon icy surface flexes back and forth. This constant flexing creates more heat than the sun can offer the distant moon.
Christine McCarthy, who led the research when she was a graduate student at Brown, compares the effect to you repeatedly bending a metal coat hanger.
“If you bend it back and forth, you can feel it making heat at the junction,” said McCarthy. “The way it does that is that internal defects within that metal are rubbing past each other, and it’s a similar process to how energy would be dissipated in ice.”
Scientists have known about tidal dissipation for a while, but they don’t know the details of the process all that well. Previous research “weren’t getting the kind of heat fluxes that would create these tectonics,” said McCarthy. “So we ran some experiments to try to understand this process better.”
With the help of Reid Cooper, a professor of Earth, environmental and planetary sciences at Brown, McCarthy placed ice samples into a compression apparatus.
This piece of equipment put the samples under cyclical loads similar to how Europa’s surface flexes as its orbit gets closer to Jupiter. As it flexed back and forth, the ice deformed and then rebounded a certain amount. The researchers measured the time between when the load was applied and the deformation of the ice. With these measurements, McCarthy could get an idea of how much heat was created.
What she found could shake up the understanding of what’s causing the heat generation. It’s been assumed that the heat builds up from friction at the boundaries between ice grains. Meaning, the size of the ice grains dictate the amount of heat generated. But McCarthy found the results didn’t see big changes even when she changed the grain size in her samples by a large amount.
Instead of grain size, the experiments suggest heat is generated from the defects in the ice’s structure that occur as a result of the flexing. These defects create much more heat than the grain boundaries.
Cooper explains why understanding Europa’s icy shell is imperative to unlocking its grander secrets.
“The beauty of this is that once we get the physics right, it becomes wonderfully extrapolative,” Cooper said. “Those physics are first order in understanding the thickness of Europa’s shell. In turn, the thickness of the shell relative to the bulk chemistry of the moon is important in understanding the chemistry of that ocean. And if you’re looking for life, then the chemistry of the ocean is a big deal.”
We still don’t have all the answers, but the research offers scientists a new tool to figure out what could be going on beneath the moon’s icy surface.
NASA’s mission to Europa
We’ll probably know what’s going on beneath Europa’s icy surface one way or another in the next decade or so. Last June, NASA announced its mission concept to the moon completed its first major review and is entering the development phase.
“Today we’re taking an exciting step from concept to mission, in our quest to find signs of life beyond Earth,” said NASA’s John Grunsfeld at the time. “Observations of Europa have provided us with tantalizing clues over the last two decades, and the time has come to seek answers to one of humanity’s most profound questions.”
The mission would begin its journey towards Jupiter in the 2020s. 45 close flybys will give scientists the best look at Europa’s icy surface. Here’s a lecture from February that dives into what the Europa mission team will be looking for during their time around the moon.