Blood Falls, Antarctica, where microorganisms live under the ice. Iron and other elements in the subglacial bedrock are oxidized as they interact with air, resulting in a rusty red color. **jill mikucki
The search for life beyond Earth has fascinated many and inspired some big questions: Are we really alone in the universe? Is our planet unique? Is it possible that life outside of Earth could actually be far away from the little green aliens and closer to the microbial life with which we share Earth?
Single-celled organisms were the first life forms to evolve on Earth billions of years ago and have been around much longer than humans and other multicellular organisms. They also have a diverse metabolism and can thrive in environments that we humans consider extreme, such as in the hot hydrothermal vents of the ocean floor, in extremely salty lakes, and even in rocks.
The first place to find life outside of Earth is within our solar system, and the distance between us and the potentially habitable world is still manageable for spacecraft flybys and even sampling missions. Astrobiologists are interested in Venus, Mars, and the many moons of Jupiter and Saturn, although Europa, one of Jupiter's 95 moons, is a particularly promising candidate for the world. Europa is an icy marine world with streams of water gushing out of the ocean under a thick crust of ice.
Although Europa's surface temperature is forever latent below -220°F, many astrobiologists are excited about Europa as a possible site for life in the solar system due to its subglacial ocean. As we know, water is important for the habitability of the planet; Polar solvents like water are essential to drive the biochemical reactions of all life on Earth and can also provide a thermally stable habitat for the survival and evolution of living organisms.
As we know, carbon is another important component of life. All the macromolecules necessary for life are carbon-based – sugars, proteins, DNA, and lipids are all made up of carbon atoms arranged in various shapes, including rings, sheets, and chains.
In September 2023, two independent teams of scientists discovered that solid carbon dioxide (CO2) most likely originated in its subglacial ocean, as its location on the surface coincided with geological features, suggesting that the material was transported from under the ice.
One team also hypothesized that the oceans are oxidized, a chemical condition that supports Earth's current biosphere and therefore favors the habitability of life as we know it. Although scientists have not been able to determine that the **2 of carbon monoxide is on Europa, the confirmation of the presence of carbon on Europa has ignited the enthusiasm of astrobiologists who believe that it may host microbial life.
Signs of life such as organic carbon and water are widely known as biological signatures and are chemical or physical markers that specifically require biological origin. While no single biosignature is sufficient to claim life in distant worlds, the discovery of many complementary biosignatures on celestial bodies like Europa could reinforce the thesis that life may exist beyond Earth in some form.
Illustration of NASA's Europa Clipper, scheduled to launch in October 2024. **nasa/wikimedia commons
From Europa to Antarctica – studying subglacial microbes.
As a microbial fieldwork site, Europa is almost out of reach – it's more than 3900 million miles, and it's unfathomably cold. So how can we be sure that life can survive in European conditions? One idea is to study Earth-based simulation sites - extreme environments on Earth, the conditions of which mimic those of distant worlds.
By describing microbial life in these ecosystems, we can gain insight into how life persists in places where most other life forms are completely inhospitable. Studying the mimetic sites can also give us clues as to what types of biofeatures may be important in different contexts and help understand what researchers are looking for in the data of future Europa missions.
Dr. Jill Mikucki, an associate professor at the University of Tennessee, Knoxville, studied a simulated site: Blood Falls, a coloring feature of the terminus of Taylor Glacier in Antarctica's McMurdo Dry Valleys. There, a saline subglacial groundwater ecosystem leaks iron-bearing brine to the surface. Iron oxidizes when it comes into contact with air, staining the brine that flows out of it a rusty red and giving the Blood Falls an eerie appearance and name to match.
Working and camping in a dry valley feels otherworldly," Mikucki said. "It could be very quiet. So thorough. But if the wind rises, it roars.
Part of Blood Falls' appeal as a simulation game comes from its unique geography and hydrological features. "I think Blood Falls is a good analogy for the study of the marine world because it is one of the few places where liquids cross from under the ice to the surface," Mikucki explained. "Plus, it's salty, so it's like a mini ocean world with the occasional spill of an aliquot of the liquid under the ice – and its microbial content.
These features are reminiscent of Europa plumes erupting from under the ice. "At Blood Falls, we can study what life is like under the ice, what transport to the surface is involved, and what survival at the surface looks like," Mikucki said.
In 2009, Mikucki and colleagues published a paper detailing how microbes under Taylor Glacier circulate sulfur and use iron as a terminal electron acceptor, which is the role of oxygen on many organisms on the Earth's surface.
This metabolism occurs under anaerobic conditions (when oxygen is limited), which can occur in some environments of photosynthetic organisms that produce O2 in absence. This ecosystem is buried deep under the ice and may have been cut off from the outside world for more than 1 million years.
Mikucki has been studying the subglacial environment for more than two decades, but is still shocked by some of the discoveries she and her team have made. For example, microbial cells grow very slowly under ice and can take a year or more**.
Everything was unbelievable to me," she says with a laugh. "I want to know how long this salt water has been trapped under Taylor Glacier and how, under what circumstances, it was produced. How do these microbial communities persist throughout this physical and chemical journey? Can life persist on Europa in a similar way? The jury has not yet been determined, but efforts to collect more data are ongoing.
Over the next few decades, we will get to know Europa better through two missions: the European Space Agency's JUICE (Jupiter's Ice Moon Probe) and NASA's Europa Clipper. The JUICE mission, launched in April 2023, is designed to characterize Europa and two other Jupiter moons, while NASA's Clipper mission (scheduled to launch in October 2024) will focus on Europa.
The goal of the Clipper is to measure the thickness of the ice crust and the exchange between the surface and the ocean, as well as to study Europa's composition and geology. The two spacecraft should reach their goal in the 2030s, after which they can begin to collect and send back data.
The possibility of life beyond Earth – and it may well be very different from life here – is both exciting and humbling. If we had never found life outside of Earth, it would mean that what happened here was very special. If we do, it may upend what we think we know about life and show us that we are not alone in the vastness of the universe.