Among Jupiter’s 79 moons, Europa jumps out at planetary scientists. Covered in an ice crust, it has the smoothest surface of any object in the solar system. Spacecraft have detected water vapor plumes wafting from the Jovian moon. Scientists estimate an ice crust 62 miles thick covers a subsurface ocean.
Down there, at the bottom of that ocean, may be the first place we find life off Earth.
If we do, Everett Shock will recognize it right away.
The Arizona State University environmental biogeochemist spent the past summer peering down into the depths of the Pacific Ocean and Yellowstone National Park hot springs. In both places he studied chemical energy sources and the microorganisms that live off them.
NASA has planned a mission to Europa to fly sometime in the next decade. Plenty of planetary scientists are more excited about Europa than Mars. Shock is involved with the mission.
“I don't expect to find the same organisms,” he said. “I expect to find the same kind of process … whatever's going on that produces these kinds of energy supplies. And then, if something's living there — whatever living there means on another planet — what an organism might be like.”
Most of Earth’s volcanoes are underwater, in the oceans. Put a volcano in an ocean and you’re going to get some hot water. It’s an energy supply being delivered by a normal geologic process.
“It's like, yeah, this is normal,” Shock said. “That's how our planet works.”
We only have one data point of a planet with life. (You’re on it right now.) We know organisms used chemical energy sources before they figured out how to use sunlight.
“So, based on our one data point, you're placing a bet,” Shock said. “How does it work on another planet? OK, maybe it also starts with chemical energy. … So that's a really exciting mission.”
Shock is a professor in the School of Life Sciences with joint appointments in the School of Molecular Sciences and the School of Earth and Space Exploration. He has been conducting research at the Yellowstone hot springs for 20 years now. He travels up with 10 students. They camp in the tourist campground, because it’s cheap and convenient and fun, but they follow a carefully planned research program.
They don’t think that Yellowstone is the early Earth or that it’s Europa. They go there to understand how these chemical energy sources become available.
On the Gorda Ridge expedition almost two miles below the Pacific, Shock studied sea vents which feed tiny organisms. (He was in a control room, studying instruments' readouts and live video images.)
Hot water escaped the vents and mixed with chilly ocean water. Elements mix, providing energy sources that microorganisms tap into. And it all happens in the dark.
There's an aspect of the Yellowstone hot springs that is quite similar. There is a temperature limit on photosynthesis. It depends on the pH of the water, but there are high temperature parts of many of the hot springs systems where there's no photosynthesis.
It's quite similar to the Pacific seabed, but not because it's dark. The hot spring is sitting out in the sunlight, but photosynthetic organisms are not there. So once again, you can study a system that has chemical energy sources supporting it. There's life there: microorganisms, bacteria, archaea, those sorts of things. They’re eating sulfur.
At the big picture level, many of the same chemical energy sources exist. If you go down into the details, the life at the bottom of the ocean and the hot springs are really different. But, in both cases the planet is supplying chemical energy and microorganisms are taking advantage.
Research permit in hand, Shock and his group head to Yellowstone, where they take samples and make some measurements which have to be done in the field, like temperature. They sample sediments, because that’s usually where the microorganisms are. Their water sample kit has eight different little bottles in it, each one destined for a different type of analysis. The biological sample has at least that many smaller vials.
It's a coordinated effort. There's one person who's in charge of making the basic measurements of pH and temperature and so forth and keeping records of all those as somebody else is making the chemical measurements that need to be done in the field. Another person is just trying to get that dissolved gas sample. Another person's doing the water sampling and the fifth person in this group is doing the biological sampling.
They do things in a certain order to minimize pain and suffering. (You don’t want to take water sample after the sediment guy has stirred up a mess, for instance.) Every day there is a whole plan of where they’re going and which hot springs they’ll be sampling and why that is. That's a couple of weeks of field work.
“Yellowstone is always dealing me a new hand,” Shock said. “I’m definitely never bored. … Even the places I've been before, they change a little or they change a lot and I can now stand in an area where I've been there many times and remember what it was like and how it is now and then, ‘Oh wow, this part has really changed.’ So there's an advantage to having been to the same places many times.”
Top photo: Sapphire Pool, Biscuit Basin, Yellowstone National Park. Photo by Jim Peaco. Courtesy of the National Park Service.