But one of Jupiter’s icy moons is considered the most likely place in our solar neighborhood where extraterrestrial life might exist.

Why Europa?

“It has a global ocean underneath an icy shell,” says NASA astronomer Trina Ray on X. Scientists estimate Europa has about twice as much water as all of Earth's oceans combined. “And that ocean has been there for 4 billion years. It's full of chemistry, its full of energy, and maybe the ingredients for life.”

That’s why one of the most anticipated missions scheduled for launch this year is the Europa Clipper, a spacecraft filled with a suite of sensitive instruments that will help unlock the secrets of this enticing, distant moon. Currently scheduled to launch in October 2024, Europa Clipper should arrive in the Jupiter system no earlier than 2030. During its nominal four year mission, it will perform nearly 50 close flybys of Europa.  

Europa is 1,940 miles (3,100 kilometers) across, just slightly smaller than Earth’s Moon. This moon is considered an ocean world, as decades of analysis strongly suggest an ocean of liquid water lies beneath a surface of ice. Images from previous mission show an ice-covered surface full of cracks, ridges and faults, but hardly any craters. While the surfaces of most moons are covered in craters, a crater-free surface indicates there's an active geologic process taking place, erasing any evidence of impact. Scientists say the cracks are an external signature of a vast internal salty ocean, with ice plates that have broken apart, shifted position, and been refrozen. All these characteristics would be extremely difficult to explain if the moon does not contain a global subsurface ocean.

Europa Clipper will be able to investigate Europa’s ice shell and subsurface water, as well as study the chemical elements and energy sources that might exist there. All nine of the science instruments for the mission have now been installed on the spacecraft, which is being assembled at NASA’s Jet Propulsion Laboratory. 

“The mission will confirm — or conceivably, refute — the existence of Europa’s subsurface ocean, and it will further characterize the moon through multiple investigations in order to constrain its habitability,” the mission science team wrote in a recent paper. “The goal of the Europa Clipper mission is not to detect life itself, but to assess Europa’s ability to support life as we know it by harboring essential chemical compounds and sources of energy in addition to liquid water.”

Much of the evidence is likely to be buried deep beneath the moon’s thick icy crust. While Europa Clipper is an orbiter and will not be landing or sending a probe to the surface, many instruments will allow for studying the moon ‘from the inside out.’ Additionally, plumes of water vapor have been detected erupting from below the icy surface of Europa, bringing the ocean to the surface and above. Several instruments should be able to analyze these plumes, allowing for the study of the moon’s interior ocean.

Unlike other missions where the various instruments are often studying different targets, NASA says the hallmark of Europa Clipper’s science investigation is how all the instruments will work in sync while collecting data. During each flyby, all the instruments will gather measurements and images that will be combined to paint the full picture of Europa.

“The instruments work together hand in hand to answer our most pressing questions about Europa,” said JPL’s Robert Pappalardo, the mission’s project scientist. “We will learn what makes Europa tick, from its core and rocky interior to its ocean and ice shell to its very thin atmosphere and the surrounding space environment.”

Pappalardo added, “The science is better if we obtain the observations at the same time. What we’re striving for is integration, so that at any point we are using all the instruments to study Europa at once and there is no need to have to trade off among them.”

Let's take a look at the various instruments onboard Clipper, and what they should be able to reveal about Europa:

Even before the spacecraft reaches the Jupiter system, the science cameras will begin their work to send back high-resolution color and stereoscopic images of Europa. The Europa Imaging System (EIS) includes a wide-angle camera and a narrow-angle camera, each with an eight-megapixel sensor. These images will allow for the study of geologic activity — and any changes in the moon’s surface during the mission — as well as measuring surface elevations, and providing context for other instruments. The visible-light cameras will map Europa at far better resolution than previous missions. This will provide a better understanding of Europa’s surface. 

The spacecraft’s two infrared cameras, the Europa Thermal Emission Imaging System (E-THEMIS) uses infrared light to find warmer regions on Europa where liquid water may be near the surface or might have erupted onto the surface. It will also measure surface texture to understand the small-scale properties of the surface, mapping the moon’s surface composition, temperature, and roughness. 

Together, the cameras and other instruments will reveal much about Europa’s chemistry and geologic activity. 

Three instruments will help investigate the atmosphere and surface of Europa using spectrometry, which analyzes the chemical spectrum of materials. While this moon’s atmosphere is faint, with only 100 billionth the pressure of Earth’s atmosphere, scientists expect that it holds clues about the moon. 

A mass spectrometer called MAss Spectrometer for Planetary EXploration/Europa (MASPEX) will analyze gases in Europa’s faint atmosphere and in the possible plumes. It will study the chemistry of the moon’s suspected subsurface ocean, and how the ocean and surface might exchange material. 

“Dogs love to go for a ride and the best part is to stick their heads out the window and take a big sniff as the world goes by,” said science team member Dr. Kelly Miller, at a talk she gave at the Southwest Research Institute in 2023. “They can learn a lot about their environment doing that and that's essentially what we're going to do with MASPEX. We're going to sniff Europa as we fly by and learn as much as we can.”

The Mapping Imaging Spectrometer for Europa (MISE) is an infrared spectrometer that will map the distribution of ices, salts, organics, and find the warmest hotspots on Europa. The maps will help scientists understand the moon’s geologic history and determine if Europa’s suspected ocean is suitable for life.

Europa-UVS, an ultraviolet spectrograph, will collect ultraviolet light to search for plumes and identify how the properties of the dynamic atmosphere change over time. If any plumes are erupting, they could be observable by this instrument.  

Jupiter’s magnetic field, the largest in the solar system, fills the entire Jupiter system and beyond. The Galileo mission to the Jovian system in the 1990s made a big discovery: the magnetic field of Jupiter is disturbed by Europa. Scientists determined this is likely caused by the movements of Europa’s salty subsurface ocean, which changes the surrounding magnetic field. A magnetometer called Plasma Instrument for Magnetic Sounding (PIMS) will study this by analyzing Europa’s ionosphere. 

The Europa Clipper Magnetometer (ECM) should be able to confirm that Europa’s ocean exists. It will measure its depth and salinity, and measure the moon’s ice shell thickness. It will also study Europa’s ionized atmosphere and how it interacts with Jupiter’s ionized atmosphere.

Working in tandem, the two instruments will analyze the plasma (charged particles) around Europa, which will provide clues to the structure of the moon’s interior. 

A big part of the mission is gaining a better understanding of Europa’s ice shell. Estimated to be about 10 to 15 miles (15 to 25 kilometers) thick, this outer casing may be geologically active, which would explain the fracture patterns that are visible at the surface. The Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) will use ice-penetrating radar to probe Europa’s icy shell for the moon’s suspected ocean and study the ice’s structure and thickness. It will also study the moon’s surface elevations, composition, and roughness, and search the moon’s atmosphere for plumes. 

Europa and its gravity field can affect radio signals. The Gravity/Radio Science instrument will use the spacecraft’s telecommunications equipment and analyze frequency shifts in the spacecraft’s signals to Earth. This will help study Europa’s internal structure. 

While MASPEX will study the chemistry of the moon’s suspected subsurface ocean, scientists also believe that tiny meteorites eject bits of Europa’s surface into space.

The SUrface Dust Analyzer (SUDA) will identify that material’s chemistry and area of origin, which should offer clues to Europa’s ocean salinity.

The Europa Clipper mission has been under development since 2015, and scientists are ready for their spacecraft to set sail. 

“Planetary scientists are so excited about ocean worlds like Europa,” said Miller, “because they're a really tantalizing opportunity to answer that millennia-old question, are we alone?"

"This question dates back at least to the ancient Greeks and it has profound implications for the origins of life on Earth, and our understanding of our place in the universe. Either answer — yes or no — is going to have a huge impact not only on science but also on the culture and philosophy of our world.” 

The Europa Clipper mission will launch on a SpaceX Falcon Heavy from from Kennedy Space Center in Florida.