All About the Moon Europa

Podcast Transcript

Located around the orbit of Jupiter is the moon Europa.

It is the smallest of the Gallelian moons and the second closest to Jupiter.

Despite being a moon, Europa might just be the most interesting body in the Solar System outside of Earth.

According to some, Europa might be the best place in our solar system outside of Earth to find life.

Learn more about Europa, what we know about it, and the future of its exploration on this episode of Everything Everywhere Daily.

Europa is one of the Galilean moons of Jupiter. It, along with the other three moons of Io, Ganymede, and Callisto, was discovered on January 8, 1610, by Galileo Galilei.

I provided an overview of Jupiter’s moons in a previous episode.

Galileo used one of the earliest astronomical telescopes and observed four points of light around the planet. These points of light moved over time, indicating that they orbited Jupiter, not the Sun or the Earth, which was a revelation at the time.

The points of light were given names based on their proximity to Jupiter, and for centuries, that is all we knew.

Europa is named after Europa, a figure from Greek mythology who was a Phoenician princess. In the myth, Europa was abducted by Zeus, who disguised himself as a white bull and carried her across the sea to Crete.

The name was chosen by Simon Marius in the early seventeenth century, shortly after Galileo discovered Jupiter’s large moons, following a convention of naming them after figures associated with Zeus, the Greek counterpart of the Roman god Jupiter.

This traditional name emphasized the close mythological relationship between the planet and its moons, with Europa fitting naturally as one of Zeus’s companions and reinforcing the classical naming scheme that is still used for Jupiter’s satellites today.

For several centuries, the moons’ names fell out of favor, and Europa was simply known as Jupiter II. The use of Europa came back into favor in the 20th century.

Our knowledge of Europa didn’t really change until the 20th century.

Meaningful exploration began in the space age, when NASA’s Pioneer missions in the early 1970s provided the first spacecraft data on the Jupiter system.

Flybys of Jupiter by Pioneer 10 in 1973 and Pioneer 11 in 1974 confirmed Europa’s icy nature through basic measurements of reflectivity and mass, setting the stage for future investigations.

The major leap forward came in 1979 with the flybys of Voyager 1 and Voyager 2. Their images revealed Europa’s striking surface, dominated by a bright ice shell crisscrossed with dark linear features and almost completely lacking large impact craters.

This unexpected youthfulness suggested that internal processes were renewing the surface, raising early speculation about a warm interior and possibly liquid water beneath the ice. Voyager data transformed Europa from an obscure icy moon into a geologically active world of high scientific interest.

The most important chapter in the exploration of Europa began with the arrival of the Galileo spacecraft in 1995. Over eight years, Galileo performed multiple close flybys of Europa, gathering high-resolution images, gravity data, and magnetic field measurements.

After Galileo’s mission ended in 2003, Europa exploration entered a quieter but still productive phase. Observations from Earth-based telescopes and the Hubble Space Telescope have been the primary means of collecting data on Europa for the last two decades.

Also, advances in computer modeling improved the understanding of tidal heating and the structure of Europa’s interior.

So, what have we learned over the last several decades of exploration and observation of Europa?

Our knowledge of Europa starts at the surface.

The surface of Europa is almost entirely water ice. It is effectually a giant snowball.

The white reflective surface makes the ice very evident. Unlike rocky bodies in the solar system, Europa has almost no craters on its surface, which, as I just mentioned, was one of the first things that researchers noticed.

All objects in the solar system get hit by meteors over time. If there is no activity on the surface, like on Earth’s moon, those meteor craters just accumulate over time.

On the Earth, craters don’t accumulate because of wind and water erosion and plate tectonics.

The lack of craters on Europa suggests that something on the surface is removing them. The nearby moons of Ganymede and Callisto are both postmarked with craters.

Europa has very few impact craters because its surface is continually renewed by internal processes driven by tidal heating. As Europa orbits Jupiter, gravitational flexing generates heat within the moon, keeping its ice shell warm and mobile.

This allows the icy crust to crack, shift, and slowly flow, erasing craters over time as fresh ice wells up from below or older surface ice is recycled and deformed. In addition, interactions between the ice shell and the subsurface ocean can break apart and refreeze the surface, further smoothing out impact scars.

The result is a surface that is constantly resurfaced on geological timescales, so most craters are effectively erased long before they can accumulate.

These cracks are clearly visible on the surface.

Many of these fractures are believed to penetrate deep into the ice shell, allowing warmer ice or liquid water from below to rise toward the surface. As this material reaches colder conditions, it refreezes, widening the crack and often forming raised, parallel ridges on either side.

Over time, repeated cycles of tidal flexing, cracking, and refreezing can extend these fractures for hundreds or even thousands of kilometers, creating the distinctive global network of dark lines that define Europa’s appearance.

We actually have direct evidence of liquid water coming up through the surface.

Evidence for geysers on Europa comes from observations by the Hubble Space Telescope, which detected plumes of water vapor rising hundreds of kilometers above the moon’s surface near its south polar region.

These detections were supported by Galileo spacecraft data showing localized disturbances in the magnetic field and plasma environment consistent with jets of material venting into space.

All of these pieces of evidence point to what really makes Europa so interesting: liquid water beneath the surface.

Liquid water is what makes life possible on Earth. As far as we know, Europa is the only other place in the solar system, besides Earth, that has significant amounts of liquid water.

In fact, Europa might have more liquid water under its icy surface than in all the oceans on Earth.

Estimates of the ice shell thickness vary widely, ranging from a few kilometers to several tens of kilometers, depending on local heat flow and geological activity. Some models suggest that pockets of liquid water may exist much closer to the surface.

The potential habitability of Europa’s ocean has become one of the most exciting prospects in astrobiology. For life as we know it to exist, three essential ingredients are required: liquid water, energy sources, and the right chemical elements.

Europa appears to have all three. The subsurface ocean provides abundant liquid water in contact with a rocky seafloor, where hydrothermal vents similar to those on Earth might exist. Such vents on Earth support thriving ecosystems completely independent of sunlight, deriving energy from chemical reactions between seawater and hot rocks.

We are almost certain that liquid water exists, based on all the evidence we’ve gathered. Likewise, we are very certain about the heat produced via tidal heating, and we are pretty sure that there is a rocky core in the moon.

So, if there is life below the icy surface of Europa, how in the world can we find out?

Several missions are already planned or underway.

The most prominent is NASA’s Europa Clipper mission, which was launched in 2024 and is designed to perform dozens of close flybys of Europa while orbiting Jupiter.

Its instruments will map the ice shell in detail, probe the subsurface with radar, analyze the composition of the surface and tenuous atmosphere, and search for signs of active plumes. Europa Clipper is not intended to land, but it will identify the most promising locations for future surface or subsurface exploration.

It is expected to arrive at Europa in April 2030.

JUICE, short for Jupiter Icy Moons Explorer, is a European Space Agency mission that was launched in 2023 to study Jupiter and its large icy moons, with a particular focus on Ganymede, Callisto, and Europa.

JUICE will investigate the structure, composition, and potential habitability of these worlds by measuring their ice shells, subsurface oceans, magnetic environments, and geology, while also conducting detailed studies of Jupiter’s atmosphere and magnetosphere.

Although Europa is not its only target, JUICE will perform targeted flybys that complement NASA’s Europa Clipper mission and help place Europa within the broader context of the Jovian moon system.

Let’s assume that these missions, both of which are already underway, go well, and they confirm many of our suspicions about Europa.

We still won’t know what is beneath the ice.

To discover what lies beneath, we will have to send probes under the ice.

Several proposed missions aim to explore the water beneath Europa’s ice by directly penetrating its icy shell. This task represents one of the most formidable engineering challenges in planetary exploration.

One widely studied concept is the cryobot, a nuclear or electrically heated probe designed to slowly melt its way downward through the ice using thermal energy.

As it descends, the cryobot would trail a communications tether back to a surface lander, allowing data to be transmitted to an orbiter and then to Earth.

Once the cryobot reaches the subsurface ocean, it could release a smaller autonomous underwater vehicle, often called a hydrobot, capable of swimming through the ocean to measure temperature, salinity, chemistry, and potentially search for biological signatures.

Other mission concepts focus on accessing liquid water without drilling through the entire ice shell. Some models suggest that Europa’s ice may be thinner in certain regions or contain subsurface lakes only a few kilometers below the surface.

Proposed landers could target these areas and drill to shallower depths, dramatically reducing mission complexity. In addition, plume-sampling missions have been proposed that would fly spacecraft through water vapor jets thought to erupt from Europa’s surface, allowing direct chemical analysis of ocean material without landing or drilling at all.

Longer-term visions include hybrid systems combining surface stations, melt probes, and mobile ocean explorers capable of operating for months or years beneath the ice. These missions would focus not only on habitability but on detecting clear biosignatures such as organic molecules, isotopic ratios, or even cellular structures.

While none of these concepts has yet been approved for missions, ongoing technological development in nuclear power systems and autonomous navigation keeps the prospect of exploring Europa’s hidden ocean firmly within the realm of future exploration rather than science fiction.

Europa represents more than just another moon in our solar system. It exemplifies an entire class of ocean worlds that may be common throughout the universe.

If Europa harbors life, it would demonstrate that life can arise and thrive in environments vastly different from Earth’s surface, dramatically expanding the potential for life elsewhere in the cosmos.

Even if the ocean proves to be sterile, understanding why would provide crucial insights into the conditions necessary for life to emerge.

The study of Europa also contributes to our understanding of planetary formation, tidal heating, and the geological processes that shape icy worlds.

To learn the secrets of Europa, all we have to do is some extraterrestrial ice fishing.