Europa’s Promise: Alien Oceans and Potential for Life
Searching for life in alien oceans is not science fiction; it’s science fact.
There are celestial bodies in our own solar system so alien that they feel as if they should belong in an entirely different galaxy. Europa — a moon of Jupiter — is one of these bodies: it is believed to have a global subsurface ocean containing more water than all of Earth’s oceans combined; this salty ocean can be up to 150 kilometers deep and is held beneath a shell of ice up to 25 kilometers thick.
For comparison, the famously deep Mariana Trench is only 11 km deep, and Antarctica’s ice is only 4.7 km at its thickest. The scale of Europa’s ocean is unlike anything on Earth and is remarkably impressive for a body smaller than our Moon. But what is even more remarkable is that the existence of Europa and its ocean is largely unknown by those who aren’t astronomers. Let’s change that.
The existence of liquid water anywhere in the universe is quite promising. But how can liquid — or even slushy — water exist on Europa? Jupiter is 5.2 AU from the Sun, meaning that it is over five times farther from the Sun than Earth. This means significantly less sunlight reaches Jupiter than Earth, so it is inherently too cold for liquid water.
Water generally needs to be above zero degrees Celsius to be found in liquid form, but it is less than -140 degrees Celsius on Europa’s surface. This explains the icy crust, but it means that Europa must be deriving its internal oceanic heat from other sources besides just the Sun. To understand this, we must first investigate Europa’s context and its origins.
Europa is one of the four Galilean moons of Jupiter — alongside Io, Ganymede, and Callisto. Europa and the other Galilean satellites likely formed around Jupiter in a very similar manner to how Earth and the other planets formed around the Sun. In this sense, Jupiter can often act like a scaled-down version of our solar system: Europa and its Galilean neighbors slowly condensed from the materials within Jupiter’s circumplanetary disc via collisions and pebble accretion — i.e., the gravitational accretion of dust grains to the proto-moon after having been disrupted by drag. This means that everything in this miniature system is of the same origin and approximately the same age.
This method of formation explains Europa’s water content as compared to the other Galilean moons. Like the solar system and the planets within it, we see the Galilean moons around Jupiter becoming less dense the larger their orbits are. Rock is denser than ice, so we see Io is composed largely of rock compared to Callisto, which is composed more substantially of ice. Again, like the rest of the solar system, this stratification based on water content is probably due to changes in temperature: ice could only condense in large amounts farther out where it is colder.
This is why we see Europa having an interesting mixed composition of rock and ice. The second Galilean moon from Jupiter, it was close enough for an iron core and rocky mantle to form but far enough away that water could still be present. Thus, we have a rocky body with a massive ocean and icy crust surrounding it. So, although the four Galilean moons are of the same origin, they have organized themselves into moons with different compositions and qualities.
The common origin of the moons helps us understand their relationship with each other as well as their host planet — and ultimately, how Europa can sustain liquid water. Io, Europa, and Ganymede are in a 4:2:1 resonance, meaning that every time Ganymede orbits Jupiter, Europa orbits exactly twice and Io orbits four times. This seems like magical harmony that could only be explained supernaturally, but it is actually a very common occurrence, well-understood by physicists. The Galilean resonance helps to perpetuate the eccentricity of Europa’s orbit, meaning that the orbit remains elliptical rather than circular in nature.
The more eccentric the orbit and the more massive the planet, the more dramatic the change in gravitational forces, and thus, more tidal heating. Tidal forces give us the tides of the seas on Earth, but when a moon is orbiting a planet as massive as Jupiter, the effects on that moon become more extreme. Jupiter is the most massive planet in the solar system, and this paired with Europa’s eccentric orbit promises substantial heating.
As Europa rapidly orbits its host, the magnitude of the gravitational force on the moon changes as the distance between the two bodies changes — resulting in a stretching pattern over Europa’s 3.5-day year. So, every few days Europa is being stretched and released by the gravity of its large planet, causing palpable internal friction. Friction entails heat, and thus, we believe that Europa’s interior is notably warmer than its exterior — warm enough for liquid water.
Europa’s neighbor Io is also subject to the massive tidal forces of Jupiter, but without very much water, all that force goes into heating its rocky interior, resulting in extreme volcanism. It is believed that a similar phenomenon may take place beneath Europa’s ocean, in the core and mantle. Europa’s internal heat is surely enough to keep at least some water liquid, but research is also being done on whether these tidal forces are substantial enough to produce volcanic activity in the mantle.
One study found that melting of the seafloor has taken place “during most of Europa’s history due to the limited efficiency of internal cooling by thermal convection and the presence of radiogenic heating” (Behounkova et al.). Interactions between these melts and the ocean are bound to impact the ocean’s chemistry. This is crucial because the relationship between volcanic activity and the surrounding ocean could involve the release of volatiles into the water.
We believe that life on Earth may have started near hydrothermal vents on the seafloor, where ocean water can interact with minerals that are being released from the Earth’s crust. Such hydrothermal vents make “mineral-rich chimneys with alkaline and acidic fluids, providing a source of energy that facilitates chemical reactions between hydrogen and carbon dioxide to form increasingly complex organic compounds” (UCL). Complex organic compounds are the building blocks of life, and if life originated on Earth through the interaction of the ocean and seafloor volcanism, then the presence of both on Europa is incredibly promising for the existence of life in the moon’s ocean.
What else is needed for life, and do we find it on Europa? The ocean is the key to Europa’s promise. Where we find liquid water on Earth, we find life, so it follows that this would be the case in other parts of the solar system and the universe at large. This is why Europa has become one of the most scientifically promising locations in the solar system: what if there exists life beneath this alien moon’s icy crust?
If we find that the conditions in Europa’s ocean are like the conditions that we know are required for life, then it seems likely that life would have arisen there, just as it did billions of years ago on Earth. We believe that life’s fundamental ingredients are water, energy, and various chemical components such as carbon and hydrogen. There appears to be abundant water on Europa, so it is a matter of finding energy and the proper chemical ingredients to sustain life.
Solar energy is not a great candidate for life on Europa because Jupiter is rather far from the Sun; we will not find photosynthesizing plants beneath the icy crust, for there simply is not enough light penetrating to the ocean below. Chemosynthesis seems like a far more powerful candidate: radiation from Jupiter breaks the oxygen away from the water in Europa’s thin atmosphere, and if this oxygen were somehow able to penetrate the crust, then life in the ocean would have a chemical energy source.
Water and energy are not quite enough for life; it also requires the right chemical ingredients such as carbon, nitrogen, hydrogen, sulfur, phosphorus, etc. Some of these necessary elements are inherently a part of Europa’s composition, but other chemical ingredients — such as carbon, necessary for carbon-based life — could come from asteroid and comet impacts. More research is still being done on where other crucial elements could come from.
A study done by the Jet Propulsion Laboratory in 2015 determined that, based on Europa’s surface color, its ocean likely contains abundant sodium chloride — i.e., table salt. They looked at the geologically young cracks in the icy crust of Europa’s surface and found that they were consistently more of a yellow-brown color, corresponding to the presence of NaCl. These cracks bring up substances from the ocean below and are thus more representative of the ocean’s composition rather than that of the crust.
The presence of this salt suggests that “Europa’s ocean is interacting with a silicate seafloor, a critical consideration for assessing habitability” (Hand and Carlson). Thus, it appears that Europa should have all the ingredients necessary for life to exist within its liquid ocean; whether such life has come to be is still to be researched and uncovered.
These discoveries are all rather intriguing, but how will we truly find out if there is life on Europa? We can only find life if we go there, and this is where good space policy becomes crucial for our future discoveries. The Europa Clipper is a new orbiter that is intended to launch in 2024 and insert into Jupiter’s orbit in 2030. The goal of the project is to send a “radiation-tolerant spacecraft into a long, looping orbit around Jupiter to perform repeated close flybys of the icy moon” (NASA JPL, “Europa Clipper”).
The spacecraft is designed to perform up to 50 flybys approximately 25 km above the surface of Europa, which will help aid the discovery of three crucial unknowns: its composition, its geology, and the dynamics between Europa’s ocean and its crust. So, the goal of this mission is not to find life; it is to find the conditions necessary for life. Only when we have found the conditions necessary for life can we begin to worry about sending a lander to actually find it.
It is beautiful and exciting to see missions planned for Europa and its neighbors. There are many characteristics unique to Europa, but it is all these characteristics together that make the moon truly special. Its elliptical orbit around our star’s most massive planet gives it the ability to sustain liquid water, and water is the key to life. Life is in part why we have invested billions of dollars into studying Mars. Unlike Mars, however, Europa is very far from Earth, which is actually a rather important detail.
If we do end up finding life on Europa — or even just signs of life — it is more likely than not that this life arose independently from that which we see on Earth. It would be truly alien. And this would have remarkable implications, for it would imply that life is ubiquitous throughout the universe. If life arose independently on two bodies orbiting the same star, and there are billions of stars in our galaxy alone, then life must be commonplace everywhere.
Written by Curran Collier
2022–03–30
