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Even in the most inhospitable regions on Earth—where light fails to penetrate, food is scarce, and the temperature hovers just above freezing—life finds a way. Nightmarish creatures such as kraken-esque colossal squid, chitinous giant isopods, and jellyfish a meter in diameter have flourished under these harsh conditions. One trick that has helped them thrive: an adaptation known as gigantism, common in the deep seas and frigid waters of the poles.
On Earth, disparate species evolve similar tools to tackle similar problems—a phenomenon called convergent evolution. So if size is a boon in the cold and spooky parts of Earth’s oceans, perhaps it could have convergently evolved among the life forms dwelling in the cold and spooky parts of other worlds?
Millions of kilometers away, on Jupiter’s cold, dark, and icy moon Europa, life may exist. And if it does, it could resemble the deep-sea monsters of Earth’s abyssal regions, says Tim Shank, a biologist with the Woods Hole Oceanographic Institution (WHOI) in Massachusetts.
Jupiter is the largest planet in the solar system. The push and pull of the gas giant on the small moon could be injecting enough energy to cause water to melt deep beneath Europa’s frozen surface, according to NASA. Indeed, scientists at the organization have observed Europa spewing liquid into space, supporting the hypothesis that water lies below. If Europa does harbor a hidden ocean beneath its icy exterior, NASA expects it to be extremely deep—extending as far as 150 kilometers below the frozen surface, which itself is 25 kilometers thick. The deepest point in Earth’s ocean—the Mariana Trench—is a mere 11 kilometers down.
On Earth, life has taken some truly bizarre forms, Shank says. From dinosaurs with long necks or clubbed tails, to the giant (and very extinct) shark megalodon, marine creatures have come up with all manner of strange adaptations. Much more recently, scientists have discovered single-celled organisms called xenophyophores that grow to the size of softballs in the deep, Shank says. Such bizarre adaptations might be found in Europan life forms, he adds.
According to Amy Moran, a marine ecologist at the University of Hawaiʻi at Mānoa, there are several hypotheses to explain deep-sea and polar gigantism. One suggests being big makes a creature a more effective predator, and less likely to be preyed upon—an important adaptation in environments where food is so scarce. Another suggests the cold slows animals’ metabolisms, causing them to steadily grow larger.
Moran says polar and deep-sea species likely have more than one reason for growing large. “I think the simple answer is that there’s a lot going on and we don’t know all of it by any means,” she says.
These same forces would likely be at play on Jupiter’s frozen moon. Would Europa’s marine dwellers also be giants as a result?
“You would have to come up with a rationale why it couldn’t happen, and I can’t do that,” Shank says.
The hunt for extraterrestrial life is well underway. Later this decade, NASA plans to send the Europa Clipper probe to collect a sample from the jets of water Europa occasionally sends into space to test its suitability for life. NASA also has early plans for a possible Europa Lander, which would look for signs of life from the surface of the icy moon.
Scientists, says Shank, should also look to Earth’s oceans to inform the search for life on other worlds. WHOI is making steps in this direction with its plan to explore Earth’s hadal zone—located six to 11 kilometers below the ocean’s surface—using a lightweight class of autonomous underwater vehicle known as Orpheus, which was co-designed by NASA. Shank hopes this effort, called the HADEX program, will get underway this year.
“These Orpheus vehicles we built are going to be the great, great, great, great grandmother of whatever goes into the Europan ocean,” Shank says.