Europa Could be Pulling Oxygen Down Below the Ice to Feed Life - Beyond The World

Europa Could be Pulling Oxygen Down Below the Ice to Feed Life

Europa, Jupiter’s moon, is a prime candidate in the search for life. There is a subterranean ocean beneath the frozen moon, and evidence suggests it is warm, salty, and rich in life-enabling chemical.

According to new research, the moon is pulling oxygen down below its frozen cover, where it may be supporting primitive life.

The question of whether Europa’s subsurface ocean can support life is highly disputed, and the argument will remain in uncertainty until NASA ships the Europa Clipper there. The journey to Europa must be precisely planned, and NASA focuses part of that planning on the exact questions scientists want the Clipper to answer. We can’t launch a spaceship to Europa and instruct it to look for life.

NASA missions are designed to answer huge questions, but they can only answer smaller, more specialized questions. As a result, scientists are investigating various characteristics of Europa and running simulations to fine-tune the questions that the expedition will need to ask.

One of those questions revolves around oxygen. It could be the deciding factor in determining Europa’s habitability.

Europa has, or we believe it has, the majority of what life requires to survive. Water is the main element, and it has plenty of it in its underground ocean. Europa contains more water than the oceans of Earth. It also contains the necessary chemical nutrients. Life requires energy, and Europa’s energy supply is Jupiter’s tidal flexing, which heats its interior and keeps the water from freezing solid. These are pretty well-established facts to most scientists.

Another fascinating indicator of habitability is the presence of oxygen on the icy moon’s surface. When sunlight and charged particles from Jupiter strike the moon’s surface, oxygen is produced. But there’s a problem: Europa’s vast ice cover acts as a barrier between the ocean and oxygen. Because Europa’s surface is frozen solid, any life must exist in its huge ocean.

How does oxygen go from the surface to the ocean?

When charged particles strike Europa's surface, they split water molecules apart. The lighter hydrogen floats away into space, but the oxygen stays behind. If the oxygen somehow makes its way to the ocean, it could possibly provide chemical energy for microbial life. Image Credit: NASA

When charged particles collide with Europa’s surface, they separate water molecules. The lighter hydrogen floats out into space, but the heavier oxygen remains. If the oxygen makes its way into the water, it may supply chemical energy to microbial life. NASA provided the image.

Pools of seawater in Europa’s frozen shell may be delivering oxygen from the surface to the ocean, according to a new study letter. The study was published in the journal Geophysical Research Letters as “Downward Oxidant Transport Through Europa’s Ice Shell via Density-Driven Brine Percolation.” Marc Hesse, a professor in the Department of Geological Sciences at the University of Tennessee at Jackson, is the study’s lead author.

These briny pools exist in places in the shell where some ice melts due to convection currents in the ocean. Europa’s famous and photogenic chaos terrain forms above these pools.

Chaos terrain makes up nearly a quarter of Europa’s frozen surface. Chaos terrain is made up of jumbled ridges, cracks, faults, and plains. The specific causes of chaos terrain are unknown, however they are most likely related to uneven subsurface heating and melting. This curiously lovely feature is highlighted in some of Europa’s most memorable photos.

Image of Europa’s ice shell, taken by the Galileo spacecraft, of fractured “chaos terrain.” A tunnelling robot would likely be sent to this type of surface area. Image Credit: NASA/JPL-Caltech

The Galileo spacecraft captured this image of Europa’s ice shell with cracked “chaos terrain.” Saltwater pools beneath chaotic terrain could be carrying oxygen to the moon’s ocean. NASA/JPL-Caltech contributed to this image.

Scientists estimate that Europa’s ice sheet is 15 to 25 km (10 to 15 miles) thick. According to a 2011 study, chaos terrain on Europa might be found over large lakes of liquid water as little as 3 km (1.9 miles) below the ice. These lakes do not have a direct connection to the underlying ocean, although they can drain into it. According to this new study, briny lakes can mix with surface oxygen and transport enormous amounts of oxygen to the deeper subsurface ocean over time.

This figure from the study shows how oxidants are generated and distributed in Europa's surface ice. Radiolysis sputters H2O into H2 and O, with O recombining into O2. Some of the O2 is released into the moon's atmosphere, but most of it returns to the icy regolith and is trapped in bubbles. The bubbles are the dominant near-surface reservoir for oxidants. Over thousands of years, the bubbles can make their way down to the ocean. Image Credit: Hesse et al. 2022.

This study’s figure depicts how oxidants are created and spread over Europa’s surface ice. H2O is sputtered into H2 and O by radiolysis, with O recombining to form O2. Some of the O2 is released into the moon’s atmosphere, but the majority of it is trapped in bubbles in the frozen regolith. The major near-surface reservoir for oxidants is bubbles. The bubbles can make their way down to the ocean over thousands of years. Image courtesy of Hesse et al., 2022.

“Our research puts this process into the realm of the possible,” said Hesse. “It provides a solution to what is considered one of the outstanding problems of the habitability of the Europa subsurface ocean.”

In their simulation, the researchers demonstrated how oxygen is delivered through the ice. A porosity wave transports the oxygen-rich brine to the subsurface ocean. A porosity wave distributes the brine through the ice by briefly enlarging the pores before immediately closing them again. These porosity waves carry the oxygen-rich brine to the ocean over thousands of years.

The researchers’ physics-based model depicts a porosity wave (spherical form) moving brine and oxygen from Europa’s surface through the moon’s ice cover to the liquid water ocean below. The graph depicts the passage of time (in thousands of years) as well as the depth of the ice shell (in kilometres). The color red indicates increased oxygen levels. Blue indicates lower oxygen levels. Hesse and colleagues, 2022

The connection between chaos terrain and oxygen transport is not entirely established. However, scientists believe that convective upwellings induced by tidal warmth partially melt the ice, resulting in the surface’s tangled chaos terrain. For the oxygen-rich brine to drain into the ocean, the ice beneath the brine must be molten or partially molten. “For these brines to drain, the underlying ice must be permeable and thus partially molten. Previous studies show that tidal heating increases the temperature of upwellings in the convecting portion of Europa’s ice shell to the melting point of pure ice,”  the authors write.

“Given that chaotic terrains likely form over diapiric upwellings, it is plausible that the underlying ice is partially molten,” the letter concludes. The presence of NaCl in the connecting ice most likely contributes to the melting.

The surface of Europa is very cold, but not so freezing cold that oxygen cannot be carried through brines. The temperature at the moon’s poles never rises above minus 220 ° C (370 F.) However, the model’s results “… show that refreezing at the surface is too slow to stop brine outflow and prevent oxidant delivery to the internal ocean.” Though the ice on Europa’s surface is solid, the ice beneath it is convective, delaying freezing. Furthermore, some study indicates that the seafloor may be volcanic.

This illustration shows how volcanism in Europa's interior might work to maintain a liquid ocean. Credit:  NASA/JPL-Caltech/Michael Carroll

This illustration depicts how volcanism in Europa’s interior might act to keep the water liquid. Photographer: NASA/JPL-Caltech/Michael Carroll

According to the findings, around 86 percent of the oxygen taken up at Europa’s surface makes its way to the ocean. That percentage could have altered dramatically over the moon’s history. However, the researchers’ model produces an oxygen-rich ocean that is quite similar to Earth’s. Is there something living beneath the ice?

Artist's impression of a hypothetical ocean cryobot (a robot capable of penetrating water ice) in Europa. Credit: NASA

Artist’s impression of a hypothetical ocean cryobot (a robot capable of penetrating water ice) in Europa. Credit: NASA

“It’s fascinating to imagine some kind of aerobic life living just beneath the ice,” said co-author Steven Vance, a research scientist at NASA’s Jet Propulsion Laboratory (JPL) and the supervisor of its Planetary Interiors and Geophysics Group.

Kevin Hand is one of many scientists who are excited about Europa, its potential for life, and the planned Europa Clipper expedition. Hand is a NASA/JPL scientist who studies Europa. He believes Hesse and his colleagues have solved the problem of oxygen in the frozen moon’s oceans.

“We know that Europa has useful compounds like oxygen on its surface, but do those make it down into the ocean below, where life can use them?” he asked. “In the work by Hesse and his collaborators, the answer seems to be yes.”

What questions may the Europa Clipper be able to ask to corroborate these findings?

The Clipper mission is the first dedicated to Europa. We believe we know a lot about Europa but haven’t been able to confirm it. The Clipper is intended to achieve three major goals:

  • Examine the ocean’s composition to see if it contains the elements required to support life.
  • Investigate the moon’s geology to learn how the surface, particularly the chaos landscape, developed.
  • Determine the thickness of the ice shell and whether or not there is liquid water within and beneath it. They will also influence how the ocean interacts with the surface: Is there anything in the ocean that rises to the surface through the shell? Is there any material from the surface that makes its way into the ocean?

That last statement refers to the possibility of oxygen transmission from the surface to the water. The Europa Clipper will transport eleven instruments that will collaborate to answer these issues.

When it comes to oxygen transport on Europa, the MAss SPectrometer for Planetary EXploration/Europa (MASPEX) is especially interesting.

“MASPEX will gain crucial answers from gases near Europa, such as the chemistry of Europa’s surface, atmosphere, and suspected ocean,” the instrument’s web page explains. “MASPEX will study how Jupiter’s radiation alters Europa’s surface compounds and how the surface and ocean exchange material.”

MASPEX and the other Europa Clipper instruments may confirm oxygen flow from the surface to the ocean, where life, if it exists, may use it. But we’ll have to wait a little longer. Europa Clipper is set to launch in October 2024 and will not reach the Jupiter system for another 5.5 years. Its science phase is scheduled to span four years once there. So it’s possible that we won’t obtain all of the data until 2034.

Meanwhile, studies like this one will whet our appetites.

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