The quest to discover life beyond Earth is one of humanity’s most compelling endeavors. Among the celestial bodies in our solar system, Jupiter’s moon Europa stands out as a particularly promising location to potentially find present-day environments conducive to life. Astrobiology, the scientific field dedicated to studying the origin, evolution, distribution, and future of life in the universe, guides this search. Scientists have identified three essential ingredients for life as we know it: liquid water, carbon-based molecules, and a source of energy, such as sunlight. Europa appears to possess all of these elements. However, to truly grasp why Europa, and by extension, similar celestial bodies are such captivating targets in the search for life, it’s insightful to consider other planets and moons within our solar system, particularly when compared to Jupiter and Saturn. While gas giants like Jupiter and Saturn are colossal and fascinating, it’s planets like Venus and Mars that, alongside icy moons, offer more compelling avenues in our search for life beyond Earth, possessing characteristics that, in some ways, have greater implications for astrobiological studies. The story, in many ways, begins with water.
Probing Planetary Pasts: Venus and Mars, Once Watery Worlds
Water is fundamental to life as we understand it. On Earth, wherever we find water, we often find life. Therefore, the presence, or former presence, of water becomes a primary indicator when assessing a world’s potential habitability. Intriguingly, our planetary neighbors, Venus and Mars, may have had significantly more water in their pasts than they do today.
Venus, for instance, might have been far different billions of years ago. For up to two billion years, liquid water could have graced much of its surface. However, present-day Venus is anything but hospitable. It’s a scorching, high-pressure environment where atmospheric pressure would be lethal to humans, and surface temperatures soar to a staggering 864 degrees Fahrenheit (462 degrees Celsius). Even if evidence of past or present life were readily available on the surface, the extreme conditions would pose immense challenges for robotic probes, let alone human exploration. The planet’s climate is as hostile to robotic missions as it would be to humans.
Europa, one of Jupiter’s moons, is positioned as a key target in the search for life beyond Earth due to its potential for harboring liquid water.
Mars, the Red Planet, presents a somewhat more forgiving picture. Landing on Mars remains a complex endeavor, yet, once there, the environment is more tolerable for robotic exploration than that of Venus. Decades of missions involving rovers and landers have unveiled a wealth of information about Mars. Billions of years ago, an ocean may have encompassed as much as one-fifth of the Martian surface. Scientists now believe Mars was once a habitable world. Over vast stretches of time, however, Mars underwent a dramatic transformation, losing much of its surface water. The solar wind and radiation gradually stripped away its atmosphere. A planet that could have supported life is now a cold, arid desert with a very thin atmosphere.
However, the solar system holds more exotic possibilities further from the Sun, worlds where vast oceans exist even in the present day. Europa is one such world.
It Matters Where We Find Life: Ocean Worlds Beyond the Goldilocks Zone
Around each star, there exists a region known as the habitable zone, or “Goldilocks zone,” where conditions are theoretically ideal for a planet to sustain liquid water on its surface. However, our understanding has expanded significantly. Scientists now know that liquid oceans can exist on worlds situated well outside this traditional habitable zone, including several moons in our outer solar system.
Certain moons orbiting Saturn and Jupiter, including Europa, are now recognized as ocean worlds. These bodies possess oceans hidden beneath their icy surfaces. In fact, the discovery of life in one of these subsurface oceans could be even more profound than finding evidence of life on Mars. It could fundamentally alter our understanding of the prerequisites for life in the cosmos and suggest that life may be far more common than previously imagined.
This graphic illustrates the estimated depth of Europa’s subsurface ocean in comparison to Earth’s ocean, highlighting the significant volume of water potentially present beneath Europa’s icy shell.
Theoretically, even basic life forms could potentially travel between planets. A meteorite impact on a planet could eject surface material containing microbes into space. These fragments could then journey through space and, in some instances, collide with another planet, potentially seeding life on a new world.
If life were to be discovered on both Earth and Mars, it wouldn’t automatically imply independent origins. Life could have originated on one planet and then been transported to the other. We know that Mars and Earth exchange rocks; Martian rocks have been found on Earth.
However, the outer solar system presents a different scenario. The vast distances between planets make the transfer of rocks far less likely. For example, the orbits of Mars and Earth are separated by approximately 34 million miles (54.6 million kilometers). But the orbit of Jupiter, the next planet outward, is about three times further away.
Therefore, if life exists in Europa’s ocean, it is highly probable that it evolved entirely independently from life on Earth. Discovering life that arose separately on two different worlds orbiting the same star would have profound implications. It would strongly suggest that life emerges readily and frequently when conditions are favorable. This, in turn, would lead us to infer that life might be widespread throughout the universe.
Icy Moons as Ocean Worlds: Europa’s Promising Position
The outer solar system is home to several ocean worlds, including some of Europa’s neighbors among Jupiter’s moons.
Scientists believe that Jupiter’s moons Ganymede and Callisto also possess subsurface oceans sandwiched between layers of ice. High-energy particles around Jupiter generate chemical compounds on the surfaces of these moons, which could potentially be utilized by living organisms. However, the outer ice shells of both Ganymede and Callisto are estimated to be around 100 miles (150 kilometers) thick. Furthermore, neither moon shows significant signs of current geologic activity that could transport these surface-generated compounds down to their oceans. It’s also hypothesized that Ganymede and Callisto may have a layer of ice at the seafloor, potentially hindering the transfer of chemical nutrients from the rocky mantle below into the oceans.
In contrast, Europa’s ocean is thought to be relatively close to its surface. Its icy shell is estimated to be, on average, only 10 to 15 miles (15 to 25 kilometers) thick. Europa also exhibits evidence suggesting present-day geologic activity, which could facilitate the transport of surface compounds into its ocean.
An illustration depicting Europa’s interior, showing its icy crust above a global ocean, and a rocky mantle underneath, highlighting the potential for water-rock interactions.
Unlike Ganymede and Callisto, Europa’s ocean is also likely in direct contact with warm rock at the seafloor. This interaction could provide a source of hydrogen and other chemicals to the ocean environment. While life on Earth primarily derives its energy from the Sun, Europa’s energy input might originate from surface chemistry and water-rock interactions occurring at its ocean floor.
Ocean worlds are found even further out in the solar system than Jupiter. Enceladus, a small moon of Saturn, nearly twice as far from the Sun as Jupiter, boasts a global saltwater ocean that vents into space as plumes of icy particles. Titan, Saturn’s largest moon, may also harbor a subsurface water ocean. NASA has approved the Dragonfly mission to Titan, which will deploy a rotorcraft to explore Titan’s atmosphere and surface in search of prebiotic chemical processes. Scientists are also developing concepts for a mission to Enceladus.
However, Europa has already been the subject of study by six spacecraft, beginning about five decades ago. Pioneer 10 and 11, Voyager 1 and 2, and the Galileo spacecraft all visited the Jupiter system. The Hubble Space Telescope has also conducted observations of Europa from Earth orbit.
Furthermore, the Juno spacecraft, currently orbiting Jupiter since 2016, is also contributing to our understanding of Europa. NASA extended the Juno mission beyond its initial phase (which concluded in July 2021) to continue operations through September 2025, or until the spacecraft’s end of life. This extended mission allows Juno to explore beyond its primary target, Jupiter, and investigate the planet’s rings and moons. Juno now has planned flybys of Europa, as well as Ganymede and Io.
The European Space Agency is also targeting Europa with its upcoming JUpiter ICy moons Explorer (JUICE) mission.
Europa’s Thick Dossier: Decades of Investigation
The exploration of Europa began with Pioneer 10’s flyby of Jupiter in 1973, which provided the first survey of the planet and its moons. In 1979, Voyager 2 captured the first close-up images of Europa, revealing a surface crisscrossed with ridges and cracks. These images also hinted at the possibility that Europa might be geologically active in the present day.
An image showcasing the intricate network of linear features, ridges and cracks, that characterize Europa’s icy surface, suggesting underlying geological processes.
From 1995 to 2003, the Galileo spacecraft orbited Jupiter, conducting extensive studies of the Jovian system. Galileo performed 12 close flybys of Europa, providing more detailed views of the icy moon. Crucially, it gathered the strongest evidence to date for the existence of a Europan ocean: an “induced magnetic field” that strongly indicated the presence of a salty water layer beneath the ice.
In 2012, the Hubble Space Telescope observed tantalizing hints of water vapor plumes potentially erupting from Europa’s surface. In 2018, scientists re-analyzed data from the Galileo mission and found further supporting evidence for these plumes.
The wealth of information gathered about Europa has enabled scientists to formulate specific, focused questions.
Does Europa truly possess a liquid water ocean? What are the ocean’s depth, temperature, salinity, acidity, and overall chemical composition? Are compounds essential for microbial life being transported from the moon’s surface and rocky mantle into the ocean? If Europa’s ocean is indeed venting into space as plumes, what can analyzing these plumes reveal about the ocean’s composition?
Answering these questions could pave the way to addressing the ultimate question – the BIG question.
The Big Question: Does Europa Harbor Life?
Does Europa harbor life?
Currently, we lack definitive proof of life on Europa, but…
Between 50 and 80 percent of all life on Earth resides in the oceans. This includes not only familiar organisms like plants, coral, fish, and mammals, but also extremophiles – organisms capable of thriving in extreme environments. These extremophiles inhabit frigid regions beneath arctic sea ice and endure the extreme temperatures and pressures around deep-sea hydrothermal vents.
Europa also appears to possess another vital ingredient for life: time.
The oldest known fossils on Earth are approximately 3.5 billion years old. This suggests that life emerged on our planet relatively soon after conditions became suitable for its survival. Europa’s ocean may have existed for billions of years.
Considering our understanding of the essential ingredients for life, the apparent rapidity with which life arose on Earth once conditions allowed, and the remarkable ability of life to adapt and survive in extreme environments, Europa stands as the most compelling and premiere location in our solar system to search for life beyond Earth. While Jupiter and Saturn are magnificent planets, it’s worlds like Venus and Mars, and especially icy moons like Europa, that have greater potential to revolutionize our understanding of life in the cosmos.
