Science

Life beyond our own planet might exist under conditions that humans would struggle to imagine. We only know for certain the conditions that life needs here on Earth: a source of energy, appropriate chemical compounds, and water. Scientists suspect that Europa has all the necessary ingredients for life as we know it, including an enormous ocean under its thick, icy crust. Plus, Europa has time on its side: Scientists believe the moon has been harboring that subsurface ocean for 4 billion years — likely ample time for ingredients to “simmer” and for life to develop.

Here’s what we know about Europa’s potential ingredients for life: water, chemistry, and energy.

Scientists have known for a long time that Europa’s surface is made mostly of water ice, judging from ground-based telescopic observations that began in the 1960s. Then in 1979, NASA’s Voyager spacecraft captured images of the surface that showed bands and ridges crisscrossing each other, with dark gaps that looked like they had been filled with icy or watery material. These images also showed only a handful of large impact craters, which usually accumulate on the surface of planets and moons over time as they are battered by pieces of comets and asteroids. So scientists theorized that even though Europa is over 4 billion years old, its surface may be much younger. Some kind of geologic activity, which could include tectonics (fracturing and faulting) and cryovolcanism (flowing, slushy ice instead of molten lava) must have refreshed the surface.

Enter NASA’s Galileo mission, which began orbiting Jupiter in 1995. One of the most important measurements by the spacecraft showed that Jupiter’s magnetic field was altered in the space around Europa. This finding strongly implied that a special type of magnetic field had been created within Europa by a layer of fluid that conducts electricity. Based on the moon’s icy composition, scientists think the fluid most likely to create this magnetic signature is a global ocean of salty water. They estimate that the ocean holds about twice as much water as all of Earth’s oceans combined. Images from the Galileo spacecraft show how bizarre Europa’s surface is, with many different kinds of ridges, and with chaotic areas where the surface has moved and is jumbled.

All life on Earth is built from organic molecules, which are made from certain chemical elements, including carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Scientists know from observations from ground-based and space telescopes that there are non-ice materials on Europa’s surface, such as salts and carbon dioxide (dry ice). Organic molecules are expected, likely incorporated into Europa’s crust as the moon formed and, later, as pieces of asteroids and comets collided with the moon, possibly leaving more organic materials. However, scientists do not yet have direct observations of organic molecules at Europa or of all the chemical elements needed to build them.

Some essential chemical elements may be embedded in Europa’s icy shell. Others may originate from Europa’s core and the moon’s rocky interior. A process called tidal flexing, which happens as Europa orbits Jupiter and is intermittently squeezed by the massive planet’s gravity, heats Europa’s interior and may cycle water and nutrients between the moon’s rocky interior, ice shell, and ocean. This watery environment may have the right conditions for chemistry conducive to life.

NASA planetary science is guided by planetary science reports conducted every 10 years by the United States National Research Council. These decadal surveys, published in 2003, 2011, and 2022, recognized Europa as one of the best places in our solar system to look for signs of life. In 2015, NASA formally initiated the Europa Clipper project.

Europa Clipper is not designed to be a life-detection mission. Its primary science goal is to explore Europa to investigate its habitability. Europa Clipper will determine whether the moon is habitable — in other words, find out for certain if it has the ingredients for life and the potential to host it.

To evaluate Europa’s habitability, scientists need to better understand how the moon works. They will pursue three science objectives: assess the moon’s interior, composition, and geology.

Interior: Focusing on the ice shell and the ocean, scientists will be able to determine a range of measurements for how thick the shell is. Does water from the ocean rise through the icy shell to the surface? Does anything from the surface work its way down into the ocean? Does the icy shell itself contain pockets of water inside it? Scientists want to know how deep and salty the ocean is in order to understand its potential to host life.

Composition: Scientists will investigate what makes up Europa’s ocean, ice shell, surface, atmosphere, and space environment. They will learn about the chemistry of these materials and what their interaction says about the moon’s ability to sustain life.

Geology: Studying how surface features formed and if any of the surface was refreshed recently will tell scientists how active the moon is and what creates those features, including ridges, bands, and chaos. The mission will look for evidence of plumes that could be venting water into space, as well as signs of sliding crustal plates. In this way, investigation of the geology will lend insight into not just the exterior of the moon, but also how the surface interacts with any water inside or underneath the icy shell.

Europa Clipper’s visible-light cameras (extending slightly into near-infrared and ultraviolet wavelengths) will capture color and stereoscopic images of Europa’s ridges, grooves, bands, and other surface features in unprecedented detail. EIS, which includes a narrow- and a wide-angle camera, will map about 90% of Europa at better than 330 feet (100 meters) per pixel. The imaging system will look for evidence of recent surface changes, which will tell scientists about geologic activity and how it relates to the materials on the surface.

The thermal imager uses infrared light to distinguish warmer regions on the moon, where water may be near the surface or might have erupted onto the surface. Together with EIS, E-THEMIS will reveal much about Europa’s geologic activity.

Atoms and molecules emit, absorb, and reflect light in telltale ways. Scientists use spectrometry to dissect wavelengths of light and learn about the composition of surfaces and particles in space. By collecting ultraviolet light with a telescope and creating images, Europa-UVS will help determine the composition of Europa’s atmospheric gases and surface materials. It also will monitor Europa’s thin atmosphere and search for signs of plumes being emitted from the moon’s surface.

The infrared spectrometer will reveal details about the chemistry of the top layer of Europa’s ice shell. MISE will map the distribution of ices, salts, and organic compounds, and those maps will help scientists better understand the moon’s geologic and compositional processes and history.

The spacecraft’s magnetometer will be key to characterizing the ocean under Europa’s icy shell. Jupiter’s magnetic field is the largest of the solar system’s planets and movement relative to Europa induces a magnetic field at Europa, most likely via electric currents flowing in the ocean. With a sensor deployed on a boom 25 feet (8.5 meters) long, the ECM will tell scientists more about the ocean, including how conductive and how deep it is — as well as how thick the ice shell is.

The magnetometer will work hand-in-hand with PIMS to help scientists be sure they’re studying clean, accurate measurements of Europa’s induced magnetic field. They need to know more about the plasma — charged particles in ionized gas — that is trapped around Jupiter to understand how it affects Europa’s magnetic field.

Europa’s radar instrument will be able to “see” below the moon’s surface into the icy shell to search for any pockets of water inside it — and to determine how the ice and water within interacts with the ocean below. REASON will also study the moon’s surface elevations.

To identify the chemical makeup of the gases in Europa’s extremely thin atmosphere, MASPEX will collect gases around the moon, including those in any potential plumes of water vapor. The instrument will then analyze their chemical makeup. Understanding the gases near Europa tells scientists more about the moon’s ocean, how it interacts with the surface, and how radiation alters materials on the surface.

Scientists expect Europa Clipper to encounter not just gases but also dust particles that form in a thin cloud around the moon. Some of these particles will fly into the SUDA instrument and be vaporized, permitting the instrument to analyze their chemical makeup. SUDA data will tell scientists whether this material came from Europa or from elsewhere in the Jupiter system — possibly detecting organic compounds and helping scientists gauge the ocean’s salinity.

Gravity and Radio Science

While not a science instrument, the spacecraft’s telecommunication system will help scientists investigate the interior of Europa. By analyzing the radio signals sent back and forth through space via the spacecraft’s low-gain antennas, scientists can study how the frequency of these signals changes. Understanding that fluctuation will offer details about the motion of the spacecraft, which is affected by Europa’s gravity field. Analyzing the gravity field, in turn, will reveal more details about the internal structure of the moon, including confirming the presence of the ocean that scientists suspect is there.

Europa Clipper isn’t the only spacecraft set to study Europa in the 2030s. In April 2023, ESA launched its Jupiter Icy Moons Explorer (Juice), which will arrive in Jupiter orbit in 2031. Juice’s main focus is Ganymede, which it will orbit for at least a year, conducting two prior flybys of Europa and 21 flybys of Callisto as well. Scientists for each mission are collaborating to design investigations that complement one another and can take advantage of the unprecedented opportunity of having two spacecraft in the Jupiter system at the same time. For example, collecting and comparing data simultaneously in locations near Europa could lend insight that neither mission team would gain on its own.