Hydrogen Gas Can Seep Through Rock Providing Food to Bacteria. Another Place to Look for Life On Other Worlds.

Spin Google Earth around until you’re looking down at the nation of Oman. Ancient rock in that country is the backdrop for a new study with consequences for our search for life. Water reacts with this rock to produce hydrogen, which could be an energy source for bacteria. Could this happen on other worlds?

Ocean moons like Europa might be our best bet for finding life elsewhere. That’s why NASA is developing the Europa Clipper mission, and the ESA is developing the Jupiter Icy Moons Explorer mission. But detecting life in Europa’s ocean—or the conditions for life—requires specific methods and instruments because the ocean is buried under kilometres of ice. Studies like this one can help inform instrument and mission design and can provide a valuable context in interpreting results from missions to Europa and elsewhere.

The study is “Energetically Informed Niche Models of Hydrogenotrophs Detected in Sediments of Serpentinized Fluids of the Samail Ophiolite of Oman,” and it’s published in JGR Biogeosciences. The lead author is Alta Howells, now at NASA’s Ames Research Center.

An ophiolite is a section of the Earth’s oceanic crust and a piece of the underlying upper mantle. It’s an ophiolite when it’s lifted above sea level and exposed. Ophiolites often become embedded in the continental crust.

The Samail Ophiolite is a well-known geological feature in the Arabian Peninsula. It’s the largest and best-preserved ophiolite in the world, and it formed during Earth’s Late Cretaceous Period, which spanned from about 65 mya to 100 mya. It’s commercially valuable because of its copper ore, but it’s also scientifically valuable because of what it tells scientists about Earth’s geological history.

This image shows the location of the Semail Ophiolite on the eastern corner of the Arabian Peninsula. Image Credit: By Sadeghm2010 at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=54524205

The Semail Ophiolite is important in this study because of the portion of the upper mantle rock it contains. Rock from the upper mantle is reduced, which means there’s a lack of oxygen in the rock. There are more ionic sites in the rock that could react with oxygen if it were available. For example, there’s more ferrous iron than ferric iron because there’s not enough oxygen to bond with all of the iron. This results in a mass of rock basically “waiting” for oxygen to fill all of its ionic sites.

This is what’s happening with the Semail Ophiolite. It reacts with water in a process called serpentinization. The serpentinization produces H2, which is molecular hydrogen. The H2 is food for microbes called hydrogenotrophs, which can metabolize it using oxygen and use it as an energy source.

This diagram from the study shows the lithology of the Oman Ophiolite. Study sites are marked with SJA-03, WDA-17, etc. Image Credit: Howells et al 2022.

There are different types of hydrogenotrophs, and they form hydrogenotroph communities. In this study, the researchers wanted to “… determine geochemical and biological factors that may influence hydrogenotroph community composition …” To do that, they examined the relationship between the “… distribution of hydrogenotrophs among pools of serpentinized fluid in Oman and the chemical energy supplies used by their metabolisms.”

The researchers found that the chemical surroundings limited the distribution of different hydrogenotrophs. They focused on methanogens, which are hydrogenotrophs that produce methane. Methanogens have a toxic reaction to O2 that limits their spread to relatively oxygen-poor areas. The team characterized the “… environmental thresholds for energy and substrates…” that may restrict methanogen and hydrogenotroph survival.

These are pictures of some of the sample sites in the Oman Ophiolite. (A) shows surface water running through serpentinized fluid. (B) shows the same thing where the surface water is brown. The white is serpentinized fluid that’s white because of carbonate precipitation. (C) shows surface water titrating into the serpentinized fluid. (D) shows only serpentinized fluid. Its pH is over 11.5 and it’s an example of an oxygen-rich, hyperalkaline fluid. Image Credit: Howells et al. 2022.

What does this mean for ocean moons like Europa and our search for life?

“It is believed that processes like serpentinization may exist throughout the universe, and evidence has been found that it may occur on Jupiter’s moon Europa and Saturn’s moon Enceladus,” lead author Howells said in a press release.

In this study, the researchers found that not all serpentinization-hosted ecosystems may support methanogens. Where there are no methanogens, sulphate-reducing organisms may instead be prevalent. This can be important in mission design because the sulphate-reducers don’t produce methane, which is often cited as a potential key biosignature.

“Because sulphate-reducers don’t produce methane, this can have a big influence on the instrumentation we develop and deploy on missions to detect life on other planets,” Howells said.

This diagram from the study illustrates the serpentinization process. Serpentinized fluid, surrounding surface water, and storm runoff can mix at the surface, resulting in geochemical gradients. The inset picture shows one of the sites in this study where surrounding surface water gently mixes into serpentinized fluid. Image by Howells et al. 2022

The results also touch on the energy needed for life and how that plays into our efforts on Europa. At the Semail Ophiolite, the methanogens living in the serpentinized fluids need more energy than methanogens found in fresh water and marine sediments. The researchers aren’t sure why this is, but it may be due to the high pH of serpentinized fluids, or it may be due to low amounts of carbon dioxide, which is their primary electron acceptor.

The methanogens’ appetite for energy when living in serpentinized fluid can also help inform mission design.

“A requirement for energy is fundamental to all life on Earth,” Howells said. “If we can develop simple models with energy supply as a parameter to predict the occurrence and activity of life on Earth, we can deploy these models in the study of other ocean worlds.”

Scientists have been thinking about serpentinization on other worlds like Mars and Europa for a long time. Since serpentinization produces an energy source for life here, could it do the same on Europa? That’s a tough but compelling question.

In a 2020 study on serpentinization on other worlds, the authors wrote, “In Europa, volcanism or serpentinization may provide hydrogen as a redox couple to oxygen generated at the moon’s surface.” We don’t yet know if serpentinization occurs on Europa, but it’s possible.

Europa has a rocky core surrounded by an ocean, capped off with ice up to 30 km (19 miles) thick. Scientists know that there’s oxygen on Europa’s surface, though not much of it. It comes from charged particles in Jupiter’s atmosphere that irradiate the icy moon’s surface. If oxygen makes its way through the ice somehow and into the ocean, it could contact the rocky core. But the rock has to be reduced to react with the oxygen to produce hydrogen as an energy source for life.

This graphic shows what the interior of Europa might look like. Credit: NASA/JPL-Caltech/Michael Carroll

But as the Semail Ophiolite shows us, only rock low in oxygen, or reduced, can serpentinize to produce hydrogen. On Europa, radiogenic decay of the rock might expose reduced rock to oxygen. If the rock expands thermally, then microfractures might provide an opportunity for the oxygen in the water to contact the reduced rock. “In this bulk geophysical model,” the 2020 paper states, “planetary cooling from radiogenic decay implies the infiltration of water to greater depths through time, continuing to the present. Comparing the computed hydrogen and surface-generated oxygen delivered to Europa’s ocean reveals redox fluxes similar to Earth’s.”

So there’s at least some evidence that serpentinization could produce an energy source for microbes in Europa’s ocean. When can we test these models and deepen our understanding of serpentinization, particularly on Europa?

NASA’s Europa Clipper is scheduled to launch in October 2024. But it’ll take until April 2030 to arrive at Jupiter and enter into an orbit. A separate lander mission might launch in 2025 if approved, so by around 3031, we may be gathering data from the icy ocean moon’s surface.

ESA’s Jupiter Icy Moons Explorer (JUICE) is scheduled for launch in 2023 and should arrive at Jupiter in 2031. JUICE will study three of Jupiter’s moons: Ganymede, Callisto, and Europa. It’ll gather data on Europa, but its main target is Ganymede, the Solar System’s largest moon. Ganymede also has an underground saltwater ocean.