Searching for Life’s Simple Necessities Across the Asteroid Belt

So far, scientists have one place to study the life forms in the universe: Earth. However, across our planet, life is anything but homogenous. From the watery, dark depths of the ocean to frigid glaciers at the planet’s poles, organisms survive and thrive. This suggests that life can take hold in a variety of environments with some common denominators. In all of Earth’s examples of life, organisms require three things: water, a source of energy, and organic molecules. It’s possible that other places even within our solar system provide these as well.

Currently, some of the best contenders for life in Earth’s backyard are celestial bodies that have subsurface water oceans, which planetary scientists call ocean worlds. One of these candidates is Europa, an ice-covered moon orbiting Jupiter. Although researchers have some information about this moon from satellites and ground-based instruments, they still lack detailed knowledge about its environment. Soon, they may have answers. In October, the National Aeronautics and Space Administration (NASA) will send Europa Clipper, a satellite orbiter with nine analytical instruments, to Europa to study its environment and assess its habitability. 

What we see on Earth is that anytime water and hot rock come into contact with one another, it’s capable of supporting life without sunlight and without oxygen.

 —James Holden, University of Massachusetts at Amherst

When Clipper arrives at Jupiter and begins surveying the icy moon, it will not focus on discovering life. Instead, Clipper’s mission is to characterize many of Europa’s features above and below the ice. “By looking at Europa as a whole, by studying all of these species, that gives us a much better picture of what real chemistry could be going on,” said Morgan Cable, a chemist at NASA’s Jet Propulsion Laboratory working on the Europa Clipper mission. 

An Ocean World

“There are a few different reasons why Europa is exciting,” said Kathleen Craft, a planetary scientist at Johns Hopkins University Applied Physics Laboratory working on the Europa Clipper mission. One of these is the likely existence of a salty global ocean indicated by a previous satellite mission, Galileo.^1,2 Unlike Earth, Europa does not produce its own magnetic field, however, data from Galileo showed that the moon’s orbit through Jupiter’s magnetic field induces one on the moon. That suggested that there is a conductor beneath Europa’s ice, most likely a salty ocean “Even though it’s about the size of our moon, there’s a thick enough water layer that it has more water than all the surface water of the Earth,” Craft said. 

One of Clipper’s objectives is to confirm the existence of this subsurface ocean, and importantly, determine its depth, salinity, and composition. Radar and gravity measuring instruments will help determine how deep the water body is under the ice. Meanwhile, Clipper’s magnetometer will help scientists figure out the salinity of Europa’s ocean and determine whether or not the moon is a possible place to find life; it may also point to the type of life that could be found. The average salinity of an ocean on Earth is three and a half percent, and a plethora of species call the salty seas home. However, in colder regions of the planet, for water to remain liquid, the salt content is higher, in some cases, up to 24 percent. Against expectations, researchers found microbes, cryophiles, living in these conditions.^3,4

“If you are a cryophile, it means that you also have to be a salt loving organism,” explained Lyle Whyte, a microbial ecologist at McGill University who is not affiliated with the Europa Clipper mission. He studies microbes that live in regions of the High Arctic and in Antarctica. “It’s not like a lush, green Amazon jungle of microbial life. It’s more like a desert of microbial life, but there’s life there,” he said. 

Scientists classify organisms like cryophiles as extremophiles, but considering a universal perspective, this may be a misnomer. “We’re kind of the extremophiles using oxygen,” said Robert Pappalardo, a planetary scientist on the Europa Clipper mission at NASA’s Jet Propulsion Laboratory. “Out there in the universe, it’s probably a lot more common that you don’t have an oxygen rich atmosphere.” 

Additionally, life on Europa would likely be capable of surviving in the dark. “We’re not counting on sunlight as the energy source,” Pappalardo said. “Instead of photosynthesis, it’s chemosynthesis that we’re interested in.” 

Unlike much of life on terrestrial Earth, cryophiles do not use sunlight or sugars to obtain energy. Instead, these organisms are lithoautotrophs that produce energy from methane or fix carbon dioxide with the help of sulfur species.^5,6  This may make these organisms excellent models for the possible types of life on a world like Europa. “If you’re looking for life on the icy moons or Mars, what would Mars or an icy moon microbe look like?” Whyte asked. “It’d have to be a cryophile and highly salt tolerant and anaerobic.”

Chemicals Between Us

Although sunlight may be dispensable for cryophiles, any habitable world still needs to provide some form of energy to sustain life, such as heat to maintain a liquid ocean or organic molecules. Scientists suspect that the moon’s metallic core and rocky mantle generates energy as a result of its orbit around Jupiter.⁷ “It’s whipped around Jupiter, and Jupiter pushes and pulls and flexes Europa at different times at different points in its orbit,” explained Lynnae Quick, a planetary scientist at NASA’s Goddard Space Flight Center who studies icy moons and a scientist working on the Europa Clipper mission. “That pushing and pulling and flexing creates friction because you have Europa’s innards that are sliding past each other.” 

The spacecraft will measure the tidal forces the moon experiences at different positions around Jupiter using gravity science.⁸ This will allow scientists to model the moon’s internal composition, such as the depth of its ocean and the tidally-induced heating.⁹ 

The heat produced from this flexing could dissipate into the mantle, which may melt ice or create hydrothermal vents.^10,11 On Earth, seafloor volcanoes generate reduced metals and reductants like hydrogen or hydrogen sulfide and organic molecules such as methane and other hydrocarbons when cooler saltwater accesses the heated rock through cracks and undergoes chemical reactions.^12-16 These could provide an ocean with molecules for organisms to use in metabolism. 

     

Hydrothermal vents support the synthesis of amino acids and other organic compounds, leading some scientists to surmise that they are where life began on Earth.^17-19 “What we see on Earth is that anytime water and hot rock come into contact with one another, it’s capable of supporting life without sunlight and without oxygen,” said James Holden, a microbiologist at the University of Massachusetts at Amherst who studies microbes living at hydrothermal vents. Holden is not affiliated with the Europa Clipper mission.

Unlike most terrestrial organisms, much of the microbial life at hydrothermal vents use the reduced compounds produced by these structures as energy sources in place of sunlight. Depending on the vent, oxygen may or may not be present. Where it is absent, anaerobic organisms rely on other electron acceptors such as carbon dioxide or compounds rich in sulfur or iron. While distinct from life above water, these ocean hot springs are full of organisms.^20-22 “You go to these hydrothermal vents, and it’s this amazing rich oasis, really high biomass,” Holden said. “That’s because you’ve got this really great electron source mixing with this great electron acceptor source.”

However, living things rely on exchanging materials, namely electrons. Hydrothermal vents may provide a source of reductants, but without electron acceptors, chemical reactions to sustain life won’t be possible. One source of oxidants could be Europa’s surface.

Jupiter’s magnetic field irradiates the icy shell. While this radiation is bad for life, it can create oxidizing agents, either reacting with the ice itself to release molecular oxygen or with sulfur from another moon, Io, that dusts Europa’s surface.^23,24 The ability for water to access the surface would be important for bringing chemical products produced on the ice into the ocean.²⁵ Scientists see good evidence that this occurs.

Long cracks and fissures scar Europa’s surface, but it bears few larger craters. The most likely explanation for these features is geological activity from an ocean that shifts the ice from below and possibly even accesses the surface.²⁶ Domed structures and pits on the moon’s surface point to the ability for water to invade the ice to some extent.²⁷ Recent studies from Galileo data and images from the Hubble Space Telescope even suggest that there could be plumes of water vapor erupting on Europa.^28,29 

I’m most excited about the things that we can’t even anticipate that we will learn.

 —Morgan Cable, NASA Jet Propulsion Laboratory

“Anytime, as planetary scientists, we can find a world that is currently geologically active, where we may have plumes, or we may have dome like structures forming on the surface, that really excites us, especially if it’s made of slushy ice stuff,” said Quick. “In order to have plumes, or in order to form geological structures on the surface, the slushy ice stuff is still moving around, and in some way, it’s moving from the ocean to the surface.” 

Previous studies also detected salt and minerals on the surface of Europa but could not determine where these originated.^30,31 Clipper’s range of instruments will help to answer what compounds are on the ice, and if there is water movement that could carry them to the ocean below. 

One instrument that will help determine this movement is a thermal imager, which will identify warmer regions of the ice that indicate where water is closer to the surface or has even leaked through cracks and plumes. Cameras will take images in visible, ultraviolet (UV), and infrared wavelengths to study the moon’s features in detail. 

Clipper also carries an ultraviolet spectrograph and infrared spectrometer. “Reflectance spectroscopy is really incredibly powerful in that it helps to identify different compounds,” Holden explained. “Maybe it’ll identify salts. Maybe it’ll identify even organic compounds. And so having the ability to take a closer look at what’s actually trapped on the surface of Europa would be incredibly valuable.” Meanwhile, two mass spectrometers will identify the molecular species from gases and dust particulate near the icy world.

“One of the powers of Europa Clipper is that it’s bringing a whole suite of instruments that will be able to make measurements at the same time in the same place,” Cable said. “That’s important for being able to distinguish what we call exogenous versus endogenous compounds.” Scientists will use these simultaneous assays to determine if a sample came from a non-Europan source, like Io’s sulfur, or if it came from within the moon. 

For example, if Clipper identifies a plume with its UV and radar instruments, it could study the water composition on the moon. Plumes may come from shallow water bodies or melt lenses within the crust created from the shifting ice and passing water.³² “That could also be a habitable environment completely distinct from the ocean itself,” said Cable. “If it’s somewhere shallow enough that potentially sunlight could access it, you could even consider photosynthetic organisms.”

Alternatively, if that plume connects to the ocean, scientists could even evaluate if the ocean supports the presence of hydrothermal vents, as such analyses did when exploring Saturn’s icy moon, Enceladus.^33,34 Detecting the compounds present might also indicate a cozy home for a cryophile hiding under the icy crust.³⁵ “When I looked at the list [from the Enceladus plume], I said, ‘Geez, you could put that into a beaker and add some bugs from the Canadian High Arctic, and something would grow on that stuff,’” Whyte said.

Exploring Habitability on Europa Scientists suspect that the ice-encrusted Jupiter moon, Europa, is potentially inhabitable. Current data indicate that it likely has liquid water present under the ice, suggesting a heat source within the moon, and chemicals generated above and below the ice could provide the foundation for life. To investigate this moon’s environment, a team at NASA planned a mission to evaluate if this world has the necessary materials to sustain living organisms. With an array of scientific instruments, Europa Clipper will study the water, energy, and chemistry present on the moon.       1) Europa Clipper Europa Clipper will use cameras that detect visible, infrared, and ultraviolet light to study the moon’s surface. It will also study the moon using radar and magnetometry. Lastly, Clipper has two mass spectrometry instruments to survey the moon’s chemistry. All of these instruments work together to give a full picture of Europa’s environment. 2) Plumes If plumes are identified, scientists could study the molecular composition of the water by using mass spectrometry. This can narrow down the types of chemistry available for life on Europa. 3) Subsurface Lakes Water trapped in the ice could form lakes. These may present sites for life to take hold, and if it is near enough for sunlight to reach, organisms could have evolved photosynthesis. 4) Europa’s Surface Jupiter’s magnetic field irradiates Europa’s surface. This could create oxidants important to living organisms by breaking apart water to generate oxygen or reacting with other molecules, such as sulfur deposited from Jupiter’s nearby moon, Io. 5) Mineral-rich Cracks Cracks along Europa’s surface indicate that there is geological activity on the moon that pulls sheets of ice apart. Some of these cracks are discolored, and scientists suspect that this results from debris from a water source below reacting with the harsh surface conditions. Researchers hope that Clipper will help answer what forms these features and what is in them. 6) Hydrothermal Vents Heat and friction from the moon’s inner core maintain its liquid ocean and may create hydrothermal vents. The reaction between heated rock and cold ocean water produces many reduced metals and other reducing compounds that microorganisms on Earth use for chemosynthesis.

Exploring Possibilities

One of the outstanding questions in biology is how life began. On Earth, scientists suspect that life began in the oceans, so by exploring those depths, researchers gather clues about how prebiotic chemistry evolved into biochemistry and biology. Using Clipper, scientists will make an unbiased assessment of the compounds present on Europa. “We can get an idea of what is most common, and what may stand out as being uniquely enriched or depleted in a way that could tell us [what] prebiotic chemistry or biochemistry might be going on,” Cable explained. 

As Clipper zeroes in on Europa, scientists can compare that to Earth-based examples and prepare to hunt for its parallel on an icy world across the asteroid belt. “It would just spark a whole new line of investigation with the certain conditions that now we know better about what they are,” Craft said.

“It’s going to take the community here on Earth—the science community that does modeling, that does experimental work, that does field work—to be able to help us put that into a perspective where we really can trace the composition of things that we see on the surface and back down into what might be in the ocean,” Cable said. 

Scientists expect that Clipper will confirm and define many of Europa’s features, such as its ocean composition and what compounds are present on and around the moon. However, with so much data to collect, they expect the mission to yield plenty of surprises. “I’m most excited about the things that we can’t even anticipate that we will learn,” Cable said. 

References

1. Vance SD, et al. Magnetic induction responses of Jupiter’s ocean moons including effects from adiabatic convection. J Geophys Res. 2021;126(2):e2020JE006418
2. Kivelson MG, et al. Galileo magnetometer measurements: A stronger case for a subsurface ocean at Europa. Science. 2000;289(5483):1340-1343
3. Niederberger TD, et al. Microbial characterization of a subzero, hypersaline methane seep in the Canadian High Arctic. ISME J.2010; 4(10):1326-1339
4. Mykytczuk NCS, et al. Bacterial growth at −15 °C; molecular insights from the permafrost bacterium Planococcus halocryophilus Or1. ISME J. 2013;7(6):1211-1226
5. Magnuson E, et al. Active lithoautotrophic and methane-oxidizing microbial community in an anoxic, sub-zero, and hypersaline High Arctic spring. ISME J. 2022;16(7):1798-1808
6. Murray AE, et al. Microbial life at −13 °C in the brine of an ice-sealed Antarctic lake. Proc Natl Acad Sci. 2012;109(50):20626-20631
7. Anderson JD, et al. Europa’s differentiated internal structure: Inferences from four Galileo encounters. Science. 1998;281(5385):2019-2022
8. Mazarico E, et al. The Europa Clipper gravity and radio science investigation. Space Sci Rev. 2023;219(4):30
9. Cascioli G, et al. Leveraging the gravity field spectrum for icy satellite interior structure determination: The case of Europa with the Europa Clipper mission. Planet Sci. 2024;5(2):45
10. Pappalardo RT, et al. Geological evidence for solid-state convection in Europa’s ice shell. Nature. 1998;391:365-368
11. Squyres SW, et al. Liquid water and active resurfacing on Europa. Nature. 1983;301:225-226
12. Jannasch HW, Mottl MJ. Geomicrobiology of deep-sea hydrothermal vents. 1985;229(4715):717-725
13. Jones WJ, et al. Methanococcus jannaschii sp. nov., an extremely thermophilic methanogen from a submarine hydrothermal vent. Arch Microbiol. 1983;136:254-261
14. Ding K, et al. In situ measurement of dissolved H2 and H2S in high-temperature hydrothermal vent fluids at the Main Endeavour Field, Juan de Fuca Ridge. Earth Planet Sci Lett. 2001;186(3-4):417-425
15. Kelley DS, Früh-Green GL. Abiogenic methane in deep-seated mid-ocean ridge environments: Insights from stable isotope analyses. J Geophys Res. 1999;104(B5):10439-10460
16. Hold NG, Charlou JL. Initial indications of abiotic formation of hydrocarbons in the Rainbow ultramafic hydrothermal system, Mid-Atlantic Ridge. Earth Planet Sci Lett. 2001;191(1-2):1-8
17. Amend JP, Shock EL. Energetics of amino acid synthesis in hydrothermal ecosystems. Science. 1998;281(5283):1659-1662
18. Shock EL. Geochemical constraints on the origin of organic compounds in hydrothermal systems. Orig Life Evol Biosph. 1990;20:331-367
19. Baross JA, Hoffman SE. Submarine hydrothermal vents and associated gradient environments as sites for the origin and evolution of life. Orig Life Evol Biosph. 1985;15:327-345
20. Stewart LC, et al. M. ethanocaldococcus bathoardescens sp. nov., a hyperthermophilic methanogen isolated from a volcanically active deep-sea hydrothermal vent. Int J Syst Evol Microbiol. 2015;65(4):1280-1283
21. Jannasch HW, et al. Chemoautotrophic sulfur-oxidizing bacteria from the Black Sea. Deep-Sea Res. 1991;38(2):S1105-S1120
22. Lin TJ, et al. Pyrodictium delaneyi sp. nov., a hyperthermophilic autotrophic archaeon that reduces Fe(III) oxide and nitrate. 2016;66(9):3372-3376
23. Johnson RE, et al. The production of oxidants in Europa’s surface. Astrobiology. 2004;3(4):823-850
24. Alvarellos JL, et al. Transfer of mass from Io to Europa and beyond due to cometary impacts. Icarus. 2008;194(2):636-646
25. Hesse MA, et al. Downward oxidant transportation through Europa’s ice shell by density-driven brine percolation. Geophys Res Letters. 2022;49(5):e2021GL095416
26. Leonard EJ, et al. Analysis of very-high-resolution Galileo images and implications for resurfacing mechanisms on Europa. Icarus. 2018;312(15):100-120
27. Manga M, Michaut C. Formation of lenticulae on Europa by saucer-shaped sills. Icarus. 2017;286:261-269
28. Jia X, et al. Evidence of a plume on Europa from Galileo magnetic and plasma wave signatures. Nat Astron. 2018;2:459-464
29. Roth L, et al. Transient water vapor at Europa’s south pole. Science. 2014;343(6167): 171-174
30. Trumbo SK, et al. A new UV spectral feature on Europa: Confirmation of NaCl in leading-hemisphere chaos terrain. Planet Sci. 2022;3(2):27
31. McCord TB, et al. Salts on Europa’s surface detected by Galileo’s near infrared mapping spectrometer. Science. 1998;280(5367):1242-1245
32. Chivers CJ, et al. Thermal and chemical evolution of small, shallow water bodies in Europa’s ice shell. J Geophys Res. 2021;126(5):e2020JE06692
33. Wait JH, et al. Cassini ion and neutral mass spectrometer: Enceladus plume composition and structure. Science. 2006;311(5766)1419-1422
34. Brown RH, et al. Composition and physical properties of Enceladus’ surface. Science. 2006;311(5766):1425-1428
35. Taubner R-S, et al. Biological methane production under putative Enceladus-like conditions. Nat Commun. 2018;9:748