Europa, beneath its frozen exterior, is believed to harbor a salty ocean in contact with a molten interior, potentially creating a habitable environment. "We think, based on our own planet, that Europa may have conditions that can support life," says Holden. NASA's Europa Clipper mission, now underway, is designed to evaluate Europa's potential habitability.
Holden, who has studied deep-sea volcanoes since 1988, uses manned and robotic submersibles to collect microbial samples from depths a mile below the ocean surface. His lab simulates these extreme environments, replicating conditions without light or oxygen. The microbes he studies thrive by extracting energy from gases and minerals released from hydrothermal vents.
"Because Europa's conditions might be similar to the conditions these microbes come from," Holden explains, "we think that Europan life, if it exists, should look something like our own hydrothermal microbes."
While Earth's microbes provide a model, Holden emphasizes that Europa's life, if it exists, may differ significantly. "So, we need to figure out the different chemical processes that Europan microbial life might be using in order to create energy," he says.
On Earth, vent microbes use enzymes called hydrogenases to break down hydrogen and extract energy. However, different hydrogenases perform varied roles in different cell types, and the ways in which iron, sulfur, and carbon interact with hydrogen to produce energy remain only partially understood.
"Our research will be to determine how the different chemical process contribute to an organism's physiology," says Holden, as he investigates how diverse biochemistries could support alien microbial life in environments beyond Earth.
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