At first glance, it looks more like a freshly-fertilized chicken egg than Jupiter’s fourth largest moon. But Europa’s fractured brownish-colored surface also suggests activity beneath its surface. In fact, some astrobiologists think that under its icy shell, there may be an ocean of salty liquid water capable of spawning everything from microbial life to extraterrestrial squids.
NASA is even talking about sending a planetary orbiter there to learn more about whether it might currently harbor life.
About the size of our own moon, Europa is gravitationally-locked with Jupiter and orbits our solar system’s largest planet every 3.5 days. At the bottom of its 100 km-deep ocean, the moon is thought to have an iron core surrounded by a rocky mantle.
NASA says with abundant liquid water, and energy and chemistry provided by gravity-driven tidal heating, “Europa could be the best place in the solar system to look for present day life beyond earth.”
But as NASA JPL astrobiologist Kevin Hand points out, for life to evolve it needs at least three major quotients — liquid water; a handful of essential elements such as carbon, hydrogen, oxygen, nitrogen, phosphorous, sulfur; and energy that it can harness.
“Everything we’ve learned about life on Earth points toward Europa’s [ocean] being habitable and potentially inhabitated,” said Hand. “A null result at Europa would be almost equally or more profound than finding life there.”
Everyone loves a good yarn and while it’s tempting to run with this one, here are a few reasons why Europa may not harbor life.
“Because Europa’s not making use of sunlight, it’s very unlikely to have advanced octopuses” said Kevin Zahnle, a planetary scientist at NASA Ames Research Center.
Even though Zahnle says he’s not convinced that there is life on Europa, he says people who do presume that the rocky interior interacts with the ocean in such a way to create conditions under which organic compounds can form.
The idea is that such interactions, both at the top and bottom of its vast depths, could generate the building blocks for microbial life.
Europa’s orbital distance from Jupiter varies enough to create gravity-induced tidal flexing on its icy surface and perhaps even on its rocky subsurface. These gravitational tides could also create seafloor volcanic or hydrothermal activity that would in theory offer any burgeoning subsurface life a steady supply of nutrients. That is, if Europa’s tectonics are what some researchers hope.
“But there may be no energy source,” said Norm Sleep, a geophysicist at Stanford University. “The inner rock seafloor may be tectonically inactive and without tectonics and volcanism hydrothermal circulation would be weak. Tides may not be enough to maintain cracks in the rock.”
“You can imagine that you had fewer chances to roll the [astrobiological] die on Europa than you would have on early earth,” said Zahnle.
On Earth, Hand says the basis of our food chain is driven by photosynthesis. But Europa’s ice shell is going to make photosynthesis out of the question. So Hand says “chemosynthesis,” energy derived from the interaction of Europa’s rocky seafloor and its ocean may fill an entirely viable niche in an ocean beneath an ice shell.
Hand also says energetic particles that hit Europa’s would split surface water. He says these particles would likely create compounds like hydrogen peroxide and molecular oxygen, sulfate and carbon dioxide trapped in Europa’s surface ice.
“But too much oxygen and it could create an ocean that’s full of [oxygen] bleach in the form of hydrogen peroxide,” said Hand. “That could have been a problem for the origin of life on Europa.”
And as Ralph Lorenz, a planetary scientist at Johns Hopkins University Applied Physics Lab in Maryland, points out, there is more to life than water.
“It may be that Europa has too much water,” said Lorenz. “The depth of Europa’s internal ocean is 25 times earth’s average. That means that these essential elements are very dilute, which makes it hard to bring all the functions of life together.”
In contrast, life here on Earth may have started in coastal tidal pools where the ebb and flow of concentrations of vital organic compounds helped spark the evolution of our planet’s first microbial mats. As Lorenz points out, life’s metabolism, growth, information storage and replication requires molecules that act together. For this, they have to be physically in proximity.
Rather than being a “rich soup,” says Lorenz, Europa’s ocean may be just mineral water.
Even so, NASA is studying the idea of a $2 billion plus Jupiter-orbiting Europa Clipper mission that, if selected, could see launch by the end of the next decade; arriving at Europa by the 2030s.
“We’re hoping to get a lander on board but as designed the Clipper has no lander,” said Hand, who is on the Clipper’s science definition team. “It will fly by Europa more than 40 times and will [likely] have cameras, spectrometers, and ice penetrating radar.”
Such a mission would pass by Europa at altitudes ranging from 2700 to 25 km, in hopes of characterizing how Europa’s ocean and icy surface might interact.
However Sleep says the ice at Europa’s surface is likely to be tens of km thick. To sample Europa’s putative ocean, researchers would first have to find a place where the water has erupted through the ice.
“I’m skeptical that there’s even microbial life [on Europa],” said Sleep. “It’s like looking for platinum; a competent mining geologist may work for decades and not find a single economical ore deposit.”
Although Hand thinks Europa does harbor life, he points out that a negative result there would send shivers throughout the astrobiological community.
“If we were to go to Europa and not find life there, to me,” said Hand, “that would indicate the origin of life itself is very hard.”
Eight spacecraft have already flown by Europa over the past few decades, and before we head there again, it might behoove us to first put the final nails in Mars’ astrobiological coffin.
For his part, hand says “it’s quite likely” that Mars may have had life in the past and “may have life today.”
Thus, before spending billions in an attempt to further characterize Europa, it might make more sense to once and for all, determine whether Mars ever had life. After Mars has finally been checked off the list, then by all means proceed to Europa posthaste.