Europa’s Oxygen and Aerobic Life

Few destinations in the Solar System have excited the imagination as much as Europa. Could a deep ocean beneath the ice support a biosphere utterly unlike our own? If so, we could be looking at a second emergence of life unrelated to anything on Earth, with implications for the likelihood of life throughout the cosmos. But so much depends on what happens as Europa’s surface and ocean interact. Alex Tolley, a fixture here on Centauri Dreams, today looks at new work suggesting the deeply problematic nature of Europa’s ocean from the standpoint of astrobiology. He also offers an entertaining glimpse at what Europa might become.

by Alex Tolley

Image: Plume on Europa’s Surface. Credit: NASA

With the abundance of newly discovered exoplanets, a fraction of them being both rocky and in their habitable zones (HZ), the excitement at finding life on such worlds is increasing. Given the ambiguous results of the attempt to detect life on Mars with the Viking experiments in 1976 and the subsequent NASA missions to look for proxies rather than direct detection, it is only to be expected that astrobiologists turned their attention to these exoplanets.

We tend to think of life primarily in terms of metazoa rather than unicellular organisms, so the search for life generally focuses on finding evidence of free oxygen (O 2 ), even though aerobic metazoa only appeared on Earth within the last billion years. But detecting proxies for life is not the same as studying it directly. Therefore a search for metazoa in our own solar system would offer the opportunity to sample extraterrestrial life well before we get such an opportunity from an exoplanet.

The discovery of ecosystems in the near lightless abyssal depths of Earth’s oceans around “hot smokers” has stimulated new hypotheses concerning abiogenesis and extended the known environments for extremophile life. Anaerobic bacteria feeding on the chemical brew from the vents become the primary food of aerobic metazoans living around these smokers. On Earth, all but a few metazoa are aerobic [5], as the higher energy from this mode of respiration allows faster growth and reproduction, as well as active behaviors. With the discovery that some icy moons around Jupiter and Saturn have subsurface oceans and active geologies, it seemed possible that these moons might harbor life too, and therefore offer a local, solar system destination to discover life and return samples. Because terrestrial metazoans are aerobes, the presence of oxygen in the icy moons would be a positive indication that there may be metazoan life forms as well as microbes.

Of the icy moons that might have oxidizing oceans Europa is the clear favorite.

Fictional Interlude 1

The Europa Oxygen and Life Surveyor (EOLiS) probe swung by Europa. Earthside Mission Control had done all it could to ensure the craft had successfully reached its target for orbital insertion. At 13:31:07 UTC the lander released itself from the orbiter, deployed its own magnetic radiation shield, and fired its braking rockets. Now the lander’s onboard AI became fully autonomous as it guided the craft towards the preselected surface destination, fusing its sensor data, radar and vision, to locate its surface landing point. The mother craft would remain in orbit and release 4 more communication relay satellites to maintain uninterrupted communications with Earth. The lander quickly reached the surface, mere meters from its preferred landing spot, and in the smoothest terrain within its target radius.

One of the mission objectives was to determine the depth profile of oxidants in the surface ice, a key variable for the oxygen levels in the subsurface ocean, and a factor for the evolution of metazoa. A nuclear fission-heated probe slowly melted its way down through the ice. Measurements of free O 2 , H 2 O 2 , and other oxidizing molecules were continually taken. Initial readouts indicated that the very high oxygen levels on the surface were not maintained below a few meters of the surface.

After 60 orbits around Jupiter, the lander had transmitted its findings back to mission control. Oxygen was mostly in the top meter of ice and snow, but in much lower concentrations down to at least the 3 kilometers its probe had penetrated. Despite this, the oxygen levels were still of the order of grams per cubic meter of ice. The planetologists inferred that the Europan ocean was likely still anoxic. The astrobiologists had to content themselves that maybe anaerobic microbes were still possible. The lander had performed well in its primary mission, so the extended mission included boring down to the base of the surface ice and into the ocean below.

Fact

Europa’s surface ice is subjected to about 0.125 W/m2 of ion radiation, radiolytically producing oxygen on the surface and a very thin atmosphere (the mechanism shown in figure 1).

Figure 1. A simplified sequence for radiolytic production and destruction of water, H 2 O 2 , and molecular oxygen. Some H 2 O 2 and O 2 become sequestered below the heavily processed surface. UV, ultraviolet. [Hand et al – [3]]

Hand estimated up to 7.6% oxidant contaminants in the surface ice, and up to 53% of the surface in the form of clathrate cages containing oxygen and other oxidants [3]. Most important for life is this trapping of oxygen that might find its way to the subsurface ocean. The relatively young surface of Europa is criss-crossed by ridges due to liquid or slush being pushed up from below, freezing and overlaying the surface. These clathrates would descend and oxygenate the subsurface oceans due to the resurfacing. The base of the ice crust melts into the ocean, a cycle that takes between 20 and 500 million years.

Hand’s estimates placed the rate of production of O 2 at 2E-7 to 8E-7 kg/m2/yr.

Greenberg was very much more positive about ocean oxygenation, suggesting that the oceans could reach Earth levels of O 2 saturation (around 10 mg/L) well within the rapid resurfacing rate times for the clathrates to reach the oceans, in about 10 million years [6].

In 2016, Vance and Hand continued to use Hand’s earlier O 2 production rates of 3E-7 to 3E-4 kg/ m2/ yr.

These estimates were based on assumptions about how the tenuous atmosphere was maintained. If it was primarily due to radiolysis, then subsurface radiolysis to produce O 2 would result in trapping of the O 2 for transport to the ocean. This would support the calculations by Vance of a rapidly oxidizing ocean that could support aerobic life, if not a rich as Earth’s, then at least within striking distance of abyssal life density.

These estimates may have been optimistic.

In recent papers [1,2], R E Johnson and A Oza call into question this model. They simulate the atmosphere and find that the best explanation for the atmosphere is thermal release of the O 2 from the surface ice by desorption. This implies that far less O 2 is trapped in the ice grains that can be subducted to the oceans.

Johnson:

This assumes that Europa’s ice regolith is permeated with trapped O 2 , which could also affect our understanding of the suggestion that the radiolytic products in Europa’s regolith might be a source of oxidants for its underground ocean.

While the O 2 is produced within the top meter of ice, gas diffusion prevents loss of O 2 , and regolith subduction and mixing draw down the O 2 into the lower depths. Gardening only allows mixing to about 10 meters, but resurfacing due to upwelling at the ridges results in the O 2 to be drawn down to the base of the ice sheet and enter the oceans below on timescales of tens to hundreds of billions of years.

Johnson et al:

Although direct diffusion to the depth of the ocean is likely problematic, geologic mixing and subduction of oxygen rich ice has been suggested as a possible source of oxidants for putative ocean biology.

Oza and Johnson’s previous paper [2] estimated production of O 2 on Europa was just 0.1-100 kg/s, or about 3E6 to 3E9 kg O 2 /yr (Earth year) or 1E-7 to 1E-4 kg/m2/yr. Their mechanism is explained in figure 3 below. They argue that thermally desorbed O 2 from the ice best explains the atmospheric dynamics over a Europan day, and therefore the O 2 at depth is less than previous estimates and models suggest.

Figure 2. Schematic diagram of O 2 trapping and thermal desorption: 1) Primary origin of O 2 (and H 2 ) is magnetospheric ion radiolysis. 2) Due to preferential loss of H 2 , the regolith becomes oxygen rich enhancing the production of O 2 . Formed and returning O 2 can become trapped at incomplete (dangling) H bonds (shown) as well as in voids (as shown and observed by Spencer & Calvin 2002). 3) The accumulated O 2 can then be thermally desorbed from the weak dangling bonds due to solar heating, maintaining a quasi vapor pressure equilibrium (Oza et al. 2018a), with a smaller gas-phase contribution from direct sputtering of O 2 . A fraction of the trapped O 2 . is likely subducted. [Johnson et al – [1]]

Fictional Interlude 2

The probe ran the samples from the bore hole through a battery of molecule and life detectors. While the usual mix of carbon compounds that could be found on any icy body, including comets and asteroids were present, none registered anything definitive for life. Asymmetry in organic molecules’ chirality was absent, as were odd lipid chain lengths. None of the growth experiments registered any change. Like the previous disappointment with Mars, the hopes of the early 21st century astrobiologists to find life in the icy moons were frustrated. Europa had so far proven sterile. Neither was there any unambiguous evidence of prebiotic chemistry.

The top kilometer of ocean below the ice crust proved still rather anoxic compared to the ice above it. The sheer volume of the ocean, plus the reducing nature of the vent emissions kept the oxygen levels well below that of the terrestrial oceans. Coupled with the absence of any signatures of microbial life, it was clear that there could not be multicellular life in that ocean.

While disappointing to the biologists, this finding indicated that there would be no violation of a putative “Prime Directive” should colonization be attempted.

Fact

Whatever the amount of O 2 trapped in the ice, it is the production rate of O 2 that determines the steady state in a biosphere, even if accumulation can create highly oxic conditions in the oceans suitable for aerobic life to exist.

Therefore the key question is just how much O 2 is produced by radiolysis? Let me put that in perspective, given the earlier conclusions, especially the optimistic ones of Greenberg.

On Earth, photolytic O 2 production is insignificant compared to that from photosynthesis. Earth’s environment was largely anoxic for billions of years, with aerobic, multicellular life only appearing in the fossil record less than a billion years ago and flowering in the Cambrian as O 2 levels increased. This was a result of the evolution of photosynthesis, which is the dominant source of Earth’s O 2 .

On Earth, net primary production (carbon fixation by photosynthesis minus plant respiration) creates about 3E14 kg O 2 /yr, or about 0.65 kg /M2/yr averaged over the total Earth’s surface. It is about a tenth as much if all respiration from heterotrophs and saprophytes is included.[7].

Therefore the rate of O 2 production on Europa is 3-6 orders of magnitude lower than Earth’s net primary production of released O 2 . The difference between Earth’s and Europa’s O 2 production is somewhat larger than Vance’s suggestion that Europa’s O 2 production is about 1% of Earth’s. Therefore, even without any other sinks, Europa’s O 2 production is 1/1000th that of Earth, at best, on an area based comparison, and possibly just a millionth at worst. Johnson’s analysis of likely lower O 2 concentrations in the surface would further reduce the subduction rate of O 2 .

The implication for life in Europa is that the production of oxygen via radiolysis is clearly insufficient to replace photosynthetic organisms that produce the oxygen in quantities to support aerobic life on Earth, even those most adapted to low concentrations, such as sessile invertebrates.

While Greenberg has suggested that photosynthetic life might reach just below the surface to add primary production to the oceanic organisms below the surface, it is more likely that if life exists at all, it is going to be anaerobic bacteria, like those of the Archaean in Earth’s history. If that is correct, any ocean vents may have bacteria, but aerobic metazoa will not be present around them as they are on Earth now.

How does this impact the search for life in Europa? If life is either absent or anaerobic, the fanciful suggestion by Freeman Dyson that we might look for fish remains ejected from the ocean is likely futile. As all but a few terrestrial metazoa are aerobic, the lack of significant O 2 production seems to diminish any likelihood that Europa hosts large animals as suggested by Clarke [9]:

Suddenly, a vast bulk broke through the surface of the ocean and arched into the sky. For a moment, the whole monstrous shape was suspended between air and water. The familiar can be as shocking as the strange – when it is in the wrong place. Both captain and doctor exclaimed simultaneously: ‘It’s a shark!’

Image: Europa deep ocean vent visited by a robotic submersible. Credit: NASA/JPL

However, there is the possibility that without a sink via consumption, free O 2 could just accumulate over the eons and possibly jumpstart the greening of Europa’s oceans.

The O 2 level in Earth’s oceans is saturated around 10 mg/L at 0 degrees C and declines with rising temperatures. Active vertebrates like fish need around 4 mg/L, much more than the 1% saturation required by sessile invertebrates like sponges.

Using Oza and Johnson’s estimated range of 0.1 – 100 kg/s O 2 production on Europa by radiolysis, and ignoring issues of reduced surface concentration levels, and other sinks for O 2 , Europa’s ocean would reach saturation at 10 mg/L in 3 million to 3 billion years. The time required is due to the immense volume of the estimated 100 km deep ocean, 2-3x as great as Earth’s oceans.

However, the rich O 2 levels in the ice might range from 0.01 to 10 kg/m3. Melted, this ice would provide for a more than adequate level of oxygen saturation for terrestrial fish. Adding terrestrial life to such lakes would quickly deplete the O 2 levels. New oxygen would have to be added by either maintaining a rate of ice melting or adding photosynthetic organisms.

If this analysis is correct, while it seems to rule out a rich aerobic ecology today, it does not preclude one tomorrow, if the production rates of O 2 could be enhanced.

Fictional Interlude 3

“Sub One operating nominally,” intoned a somewhat bored Thomas Roberts. He was lead eco-engineer at Nagata base on Callisto, well clear of the intense, deadly radiation from Jupiter that was the key to the greening of Europa. Roberts team was monitoring the newly created subsurface lake christened Dodon Lake, known more colloquially as “dee-el” by his co-workers. It was situated in the Conamara Chaos and radar imaging had indicated it was now about 1 kilometer long and half a kilometer wide, with a maximum depth of 10 meters, laying just 20 meters below Europa’s surface near what appeared to be an old plume vent. The shallow depth of the lake beneath Europa’s surface ensured both sufficient radiation shielding as well as relatively easy access via the fractured ice in the vent.

The lake had started out as a natural fracture below the surface. Nuclear generators had melted the ice at the base of the fracture, creating a freshwater lake that was saturated with oxygen, and with more than enough extra oxygen to fill the void above it with breathable air. Preliminary tests indicated the water column was now mostly freshwater, not unlike that of L. Vostok in Antarctica, although with more dissolved CO 2 and SO 4 . The dissolved O 2 was at saturation. All that was missing was enough nitrogen and phosphorus, as well as trace minerals, to make this a living lake like those in Northern Canada, albeit without the summer mosquitoes. It was as dark as any subterranean cave on Earth, although that would soon change. 10 submersibles with high intensity LED lamps had been lowered down from the surface and had swarmed out across the surface of the lake, guided by the swarm intelligence of their onboard AI. The juice needed to power the motors and lights came from small nuclear reactors which fed waste heat to the lake bottom to increase the O 2 release.

When the first sub powered up its lights, for the first time since its formation, the lake became a wonderland, illuminated with purple light, whose red and blue wavelengths were suited to maximize the photosynthesis that was to come. A cocktail of single cell algae originally sourced from subsurface Antarctic and Greenland lakes and cryogenically stored during transit, was released from the subs and soon began to photosynthesize and reproduce near the lights.

After a week, the crystal clear water started to become faintly cloudy as the density of algae increased to become the needed food for the large variety of invertebrates that followed. After a month, Thomas was certain that there would be no need to tweak the nutrients that had been added to the lake. The relatively simple starter ecosystem was on the predicted growth path that would reach its stable state cycle in 2 years. Within a decade, it was expected that a stable ecology would be established with sufficient oxygen production to maintain the first seeding of fish. But that was a job for the next crew of engineers to baby. The first steps to the greening of Europa had begun.

References

1. Johnson, R.E. et al (2019) “The Origin and Fate of O2 in Europa’s Ice: An Atmospheric Perspective,” Space Sci Rev (2019) 215:20 DOI 10.1007/s11214-019-0582-1

2. Oza A P et al (2019) “Dusk Over Dawn O2 Asymmetry in Europa’s Near-Surface Atmosphere,” Planetary and Space Science 167 23-32

3. Hand, K. P., Chyba, C. F., Carlson, R. W., & Cooper, J. F. (2006). “Clathrate Hydrates of Oxidants in the Ice Shell of Europa,” Astrobiology, 6(3), 463–482. doi:10.1089/ast.2006.6.463; Davis, J C, (1975) “Minimal Dissolved Oxygen Requirements of Aquatic Life with Emphasis on Canadian Species: a Review,” J. Fish Res. Bd. Can. Vol. 32(12)

4. Danovaro, R., Dell’Anno, A., Pusceddu, A., Gambi, C., Heiner, I., & Kristensen, R. M. (2010). “The first metazoa living in permanently anoxic conditions,” BMC Biology, 8(1), 30. doi:10.1186/1741-7007-8-30

5. Greenberg, R., (2010) “Transport rates of radiolytic substances into Europa’s ocean – Implications for the potential origin and maintenance of life,” Astrogiology Vol. 10, Number 3, 2010. DOI: 10.1089/ast.2009.0386

6. Huang J, et all (2018) “The global oxygen budget and its future projection,” Science Bull.. v63:18 pp1180-1186 https://doi.org/10.1016/j.scib.2018.07.023

7. Vance, S. D., K. P. Hand, and R. T. Pappalardo (2016), “Geophysical controls of chemical disequilibria in Europa,” Geophys. Res. Lett., 43, 4871–4879, doi:10.1002/2016GL068547.

8. Clarke, A C. 2061: Odyssey 3. Ballantine Books, 1987.