Searching for the ingredients of life on Europa

When NASA’s Galileo spacecraft plunged into Jupiter’s swirling clouds of ammonia and water in 2003, the crash was no accident. Mission scientists had carefully calculated the craft’s trajectory into the planet to avoid it colliding with Europa. Why? The data Galileo sent home suggested the moon might support life, and the scientists wanted to keep the moon’s environment as pristine as possible.

Galileo, running on a radioisotope thermoelectric generator, flew past Europa 12 times during the 8 years it orbited Jupiter—thrice during the initial mission and nine more times in subsequent mission extensions. Each flyby revealed more about the mysterious moon. The images captured by Galileo’s optical camera confirmed the data sent home by Voyager 2 decades earlier: despite the moon being similarly sized to our own, its skin was not nearly as cratered by meteor strikes, suggesting the surface was consistently refreshing itself somehow.

Even more enticing was the magnetic field data. Scientific models predict that Europa is incapable of generating a magnetic field, but Galileo’s magnetometer clearly detected one encompassing the celestial body. The most likely explanation for the contradiction is the presence of a conductive layer of salt water under the moon’s icy surface—an all-encompassing ocean. As Europa orbits Jupiter, the planet’s powerful magnetic field interacts with this conductive layer to induce a magnetic field around the moon.

The possibility of a vast quantity of water beneath Europa’s surface combined with evidence of icy tectonics—or some other ongoing process of energy exchange—makes the moon an attractive candidate in the search for life beyond Earth. But the Galileo spacecraft was not created to investigate Europa or its potential habitability. Instead, the 11 instruments the spacecraft carried to the outer solar system in 1989 were designed primarily to probe Jupiter. So, as the spacecraft met its fiery end in the gas giant’s atmosphere, it left the scientists back home with a plethora of questions.

This year, NASA is going back for answers.

“Europa Clipper is the first spacecraft that will have dedicated instrumentation that’s designed with Europa specifically in mind,” says Cynthia Phillips, a Jet Propulsion Laboratory (JPL) project staff scientist and science communications lead on the Europa Clipper mission. Although this mission is decidedly not in search of life, the nine instruments aboard the solar-powered spacecraft will give scientists an up-close look at the moon, provide insights into the chemistry above and below the moon’s icy surface, and ultimately reveal if the moon does indeed have all the ingredients needed for life.

Seeing Europa

The craggy surface of Europa has intrigued scientists and inspired science fiction for decades. It may not come as a surprise, then, that a key part of Europa Clipper’s mission will be to capture better images of the moon.

“We’ve only seen about 10% to 15% of Europa at the moderate resolution we need to really understand the surface geology,” says Elizabeth Turtle at the Johns Hopkins University Applied Physics Laboratory, who is principal investigator of the Europa Imaging System, or EIS (pronounced “ice”). Most panoptic photos of Europa’s surface are mosaics of images captured by Voyager 2 and Galileo.

The images from Voyager 2—which flew past the moon in 1979—were just crisp enough for scientists to make out the spiderweb of large ridges across the moon’s surface but not high enough in resolution to look at the network of small cracks and crevices later revealed by Galileo. And although researchers were able to capture those more detailed images of Europa’s surface during Galileo’s extended mission, the spacecraft’s data transmission capabilities severely limited the scientists.

After Galileo’s launch, the ground team ran into a dire issue: the long, high-gain antenna was stuck and remained so despite multiple attempts to loosen it. The mission scientists scrambled to design a work-around and keep the mission on track. They ultimately designed new techniques to compress the data so Galileo’s functioning, but less-powerful, low-gain antennas could send them home. But Galileo’s ability to transfer data was severely limited.

This limitation directly affected how the science team decided to image the moon, Phillips recalls. She entered graduate school around the time Galileo entered Jupiter’s orbit, and she intentionally chose a school associated with the mission so that she could be involved. She spent her time analyzing the images coming from Galileo and comparing them with those captured by Voyager 2.

Galileo’s limited download capabilities made every image precious, Phillips says, so the team aimed to maximize the amount of area photographed even if that meant taking mostly low-resolution images. “There are 700-something pictures of Europa from Galileo,” Phillips says. “That’s it. That’s all we got.” She suspects that the number of photos captured by Europa Clipper’s EIS in just a few passes will surpass the entirety of those snapped by Galileo.

Two cameras will capture this abundance of images, Turtle says. One has a wide-angle field of view; the other captures a zoomed-in, narrow-angle view. The light sensors in both systems are the same as those in phone cameras. At just 8 megapixels (Mpx), the sensors may seem relatively low resolution compared with today’s top mobile phones, but the resolution is substantially larger than that of anything that has previously flown to the outer solar system, Turtle says.

As the wide-angle camera snaps images of large swaths of Europa’s surface, the narrow-angle camera will simultaneously capture high-resolution photos of smaller areas. This combination will allow researchers to use a single flyby to understand minute ground features in their overall context—something the lone Galileo camera required multiple flybys to achieve (Space Sci. Rev. 1992, DOI: 10.1007/BF00216864). By the end of the mission, the team will have mapped 90% of the moon’s surface in more detail than ever before. Europa Clipper will capture several dozen areas in superhigh resolution: when the spacecraft swoops down for its close flybys of the moon, EIS will snap photos with a resolution under 1 m/px.

Not only will the photos be detailed, but they will also allow researchers to map Europa’s true topography. Although current images of the moon may give viewers a sense of surface texture, researchers need multiple images of the same geological features from different angles to determine, for example, the true heights of mountains or depths of valleys.

To collect that information, the narrow- angle camera is mounted on a gimbal, allowing it to swivel and change the angle at which it takes an image. This kind of movement is not without risk. When designing spacecraft, engineers often hesitate to add more moving parts because they want to limit the number of things that could break. In this case, if all goes to plan, Turtle says, “we can have our cake and eat it too with a capable imager that has the ability to point.”

While EIS is busy snapping glamour shots of Europa, another instrument will be mapping the chemical composition of the moon’s surface. The Mapping Imaging Spectrometer for Europa, or MISE (pronounced “mize”) is a reflectance spectrometer that captures sunlight in the range of 800–5,000 nm as the light bounces off the surface of Europa, says the instrument’s principal investigator, Diana Blaney of JPL.

Light reflected from the surface will travel to a calcium fluoride lens, which will direct the light onto a curved diffraction grating in the heart of MISE. This grating will split the beam of infrared light into discrete wavelengths, like a prism splitting white light into a rainbow.

Those wavelengths will hit a detector and generate a line image of infrared data. Stacking these lines will generate whole images so that a full infrared spectrum is associated with each location. These spectra of the surface will reveal “salts, organics, radiologic products, and stuff that gets blown in from Io,” Jupiter’s volcanically active moon, Blaney says. “It’s just a really complicated and fun surface.”

This won’t be the first time scientists have tried to map the chemical composition of Europa’s surface (J. Geophys. Res.: Planets 1995, DOI: 10.1029/95JE01766). MISE is a descendant of the Near Infrared Mapping Spectrometer, or NIMS, that flew on Galileo, Blaney says. Although NIMS captured images of small regions of Europa’s surface, these were sparse and plagued by radiation noise. The longer wavelengths were particularly susceptible to noise from radiation, Blaney says, because the signal-to-noise ratio was low to begin with. Unfortunately, Europa’s radiation environment was not well understood at the time, and NIMS was not able to collect reliable data at wavelengths longer than 2,500 nm, she says.

Now researchers know that the intensity of the radiation environment close to Jupiter is the second most powerful in our solar system, superseded only by that of the sun. The planet’s mass generates a powerful magnetic field that catches and accelerates charged particles from surrounding space. Europa sits close enough to be constantly bombarded by these particles. Standing on the moon’s surface would be like standing inside a running nuclear reactor, one researcher told C&EN.

To survive this harsh environment, all the electronics controlling the instrumentation on Europa Clipper are stored in a tantalum-and-aluminum vault in the heart of the spacecraft. The centimeter-thick walls are lightweight enough to keep the spacecraft unencumbered while also providing enough protection to keep the electronics functioning. Europa Clipper’s flight path also provides some protection. Rather than orbiting Europa, it will fly in an ellipsoidal orbit around Jupiter, dipping into the higher-radiation environment during Europa flybys but leaving for safer space afterward.

With these radiation safety measures in place, MISE will be able to see the longer-wavelength regions that are most often associated with organics and that NIMS missed. “We’re bringing in a new wavelength region that is really going to be diagnostic for a lot of the lingering questions,” Blaney says. Combining improvements in instrument stability and calibration will enable MISE to provide a much more nuanced understanding of Europa’s surface composition, she says.

When overlapped with the infrared compositional maps, the optical images captured by EIS will help the MISE researchers determine the sources of any compounds they find on the surface. Molecules in and around craters caused by comet impacts can be assumed to come from space, Blaney says. There is also potential to find molecules originating from the ocean.

Of course, the most obvious place to find compounds from the oceans will be around saltwater plumes erupting from the surface, but there is no consistent evidence for such plumes. Instead, Blaney says, MISE will study other, more permanent geological features.

Photos from Galileo show places where the moon’s icy shell has pushed against itself or broken apart and moved around. These regions are relatively young. So when the MISE researchers find molecules on or around these tectonic features, Blaney says, they should be able to infer that those compounds come from the ocean.

Although Blaney is most excited about MISE’s ability to detect organics because that will provide new data, she also points out that organic molecules are very susceptible to degradation by radiation and will likely break down quickly on Europa’s brutal surface. Luckily, determining the habitability on Europa extends beyond searching for carbon-based molecules. “Habitability also requires chemical gradients,” she says, and understanding which ions are present in the subsurface ocean will be informative. These species might outlast any organics that accompany them as they travel up from the ocean through surface cracks.

Even though MISE has better sensitivity than its predecessor, the broad infrared peaks that many molecules share inherently limit MISE’s ability to differentiate specific molecular species. “Basically, I’m just looking for the primary stretches, like the C–H stretches at 3.4 [µm],” Blaney says. The two mass spectrometers flying with MISE, however, will be able to determine exactly what chemical species are present on Europa and in its sparse atmosphere. She says that “all the instruments work in synergy,” ultimately painting a complete picture of Europa’s chemistry.

Tasting Europa

Sampling the surface of a moon without actually landing on it may sound impossible, but that is exactly the task of Europa Clipper’s Surface Dust Analyzer, or SUDA (pronounced “soo-duh”). SUDA is a time-of-flight, reflectron-type impact mass spectrometer, says the instrument’s principal investigator, Sascha Kempf of the University of Colorado Boulder. The instrument was built at CU Boulder’s Laboratory for Atmospheric and Space Physics. It has a unique design that will allow SUDA to indirectly “taste” the surface of Europa as it whizzes through the moon’s thin atmosphere.

The gold-plated, bucket-like instrument will fly mouth open to collect the tiny particles of dust and ice that are ejected from the surface by a constant bombardment of micrometeorites. Aboard Europa Clipper, SUDA will be zipping along fast enough that these captured dust particles will ionize and form a plasma when they slam into the back of the instrument. The instrument’s electrical field then will steer the resulting ions into a detector in the center of the impact plate. As is the case with all time-of-flight mass spectrometers, the time between ionization and detection will reveal the masses and identities of the chemical species within the particle; heavier ions will lag behind lighter ones.

What makes SUDA different from an average time-of-flight mass spectrometer, according to Kempf, mostly comes down to controlling the injection and subsequent ionization of a sample. “For us, every single particle is different,” he says, “and we can’t predict when the particle will hit.”

To compensate for the randomness of each ionization, his team incorporated specialized electronics to immediately record when dust hits the back of the instrument. This sensitivity means that SUDA can accurately start the clock for the ions racing toward the final detector.

Testing the capabilities of the electronics and overall design of SUDA required specialized facilities capable of shooting dust at the same speed the instrument will be traveling, approximately 4–5 km/s. To conduct those simulations of flybys, the SUDA team used a dust accelerator operated by CU Boulder’s Institute for Modeling Plasma, Atmospheres, and Cosmic Dust (IMPACT).

IMPACT’s interdisciplinary facility houses two dust accelerators in a large, warehouse-like lab. The larger accelerator takes the main stage, with the long tube extending from its body stretching nearly the length of the room.

Instruments of all types can be placed inside a chamber at the end of the tube, says John Fontanese, an IMPACT researcher who operates the accelerator for visiting scientists. A 3 MV pulse of electricity sends randomly sized, metal-coated dust particles whizzing down the accelerator tube at various speeds—sometimes over 100 km/s—and these particles bombard instruments or materials placed in that chamber (Rev. Sci. Instrum. 2012, DOI: 10.1063/1.4732820).

Fontanese says making sure “we test like we fly” requires control over the speed of the impacting particles. So the researchers can filter the dust, two detectors register a particle’s velocity as it zips away from the accelerator and send a signal to open an electrostatic gate if the particle is traveling at the desired speed.

Testing at the IMPACT lab revealed that dust will not be the only material undergoing ionization from impact. Some dust particles may slam into the target plate with enough force to kick up metal ions from the surface, leaving microscopic craters.

To minimize the unwanted effects on the detector and the data, the team coated the titanium target with iridium. Iridium is hard, is stable, and generates heavy ions, Kempf says. So if a powerful impact does blow off metal ions into the detector, the resulting iridium lines in the mass spectrum will not overlap with any more-interesting ones, like those from hydrated salts or possibly amino acids.

“SUDA is specifically designed to detect amino acids,” Kempf says, and it is sensitive enough to detect the biomolecules at part-per-million concentrations within Europa’s ice. But the presence of individual amino acids is not a sure sign of habitability beneath Europa’s ice. What the team will really be searching for is the fingerprints of multiple amino acids in ratios that mirror what we see produced by life here on Earth.

Kempf and a team of collaborators observed these fingerprints when they ran an experiment simulating an exciting, and incredibly improbable, scenario: catching a speck of ice that encapsulates a bacterial cell (Sci. Adv. 2024, DOI: 10.1126/sciadv.adl0849). The results show that in cation mode, SUDA has the resolution required to see the pattern of multiple amino acids. When switched to anion mode, SUDA can detect fatty acids characteristic of bacterial cells. “If we happen to collect a particle that contains a frozen bacterium,” Kempf says, “we will know.”

Smelling Europa

A second time-of-flight mass spectrometer will neighbor SUDA on board Europa Clipper. Rather than tasting solids kicked up from Europa’s surface, the Mass Spectrometer for Planetary Exploration/Europa, or MASPEX (pronounced “mass-pecks”) will “smell” the neutral gases in Europa’s thin atmosphere (Space Sci. Rev. 2024, DOI: 10.1007/s11214-024-01061-6).

The instrument’s original design was made nearly 20 years ago for a Mars mission, says Greg Miller of the Southwest Research Institute, who is the leader of MASPEX’s mass spectrometer design team. But NASA chose to launch a smaller quadrupole mass spectrometer instead, leaving the time-of-flight device in an earthbound lab. Now the instrument’s time has come.

For the Europa Clipper mission, a time-of-flight mass spectrometer is a clear choice over a quadrupole, Miller says. Quadrupoles inherently require more time to separate and detect ions because they must scan through a range of electric voltages to generate a full mass spectrum. This process is slow, and time is of the essence for Europa Clipper scientists. By the time a quadrupole reaches the correct voltage for separating interesting chemical species, the spacecraft may have already left those molecules behind. A time-of-flight mass spectrometer can measure the masses of all ions simultaneously—no scanning is required—making it ideal for zipping through Europa’s potentially nonuniform atmosphere.

MASPEX’s electron ionization source is designed to fragment neutral gases into ions and direct them to a flight tube bookended by two reflectrons. These specialized components act like mirrors for charged species, causing them to bounce back and forth and increasing the distance ions will fly without physically adding length to the flight tube. How long the scientists let the ions ping-pong between the reflectrons depends entirely on how much separation they want to achieve.

“We can bounce between the two reflectrons for tens of microseconds to tens of milliseconds,” Miller says. “That’s equivalent in some cases to storing ions for almost a kilometer of length.” Ultimately, he says, this will allow researchers to easily separate ions as close in mass as carbon monoxide and dinitrogen, molecules with masses that differ by just thousandths of a mass unit.

The MASPEX design team added a couple of extra components to increase the instrument’s capabilities. One device is a cryocooler, a type of minifridge that traps and stores a breath of Europa’s atmosphere at under 100 K. The sample can be sent to the mass spectrometer when Europa Clipper is at the farthest point in its orbit around Jupiter, Miller says. At that position, the spacecraft and its instruments will experience lower levels of radiation, and MASPEX may be able to detect low-concentration chemical species that would otherwise be drowned out in radiation noise near Europa, Miller says.

The researchers also fitted MASPEX with an onboard calibration system to help quantify the data. Specifically, the team integrated a hermetically sealed titanium canister of the commonly used calibration compound perfluorotributylamine. When the time comes, the scientists will apply a current to crack open the seal, release controlled amounts of the perfluorotributylamine, and recalibrate or tune the instrument.

“[MASPEX] has a lot of capabilities, probably more than anybody would have ever thought of putting on the instrument,” Miller says. “We say it has everything and the kitchen sink.”

Europa Clipper sets off

In total, nine scientific instruments will share the space aboard Europa Clipper. A radar, magnetometer, and plasma instrument will each send home detailed information about Europa’s internal structure. A thermal imager and an ultraviolet spectrograph will fill holes in the data captured by EIS and MISE. Together, these instruments will answer the lingering questions from past missions.

All nine of these instruments have already been designed, built, and calibrated in labs across the US. At the time of writing, all have undergone rigorous environmental testing and been individually attached to Europa Clipper. Now the completed spacecraft is being prepared for the last leg of its earthbound journey: traveling from the clean room at the JPL in California to the Kennedy Space Center in Florida.

Florida, of course, means humidity, so the MISE team has made sure that any water that hitches a ride from the launch at Cape Canaveral will evaporate before MISE begins imaging and thus won’t interfere with the moon’s own icy IR signature. “When we launch, we turn on heaters in the instrument to keep us at a temperature above where water will condense on our optics,” MISE team lead Blaney says. After a few years, all the water should be lost to space.

The insulation around MISE has plenty of time to dry out—although it is launching later this year, Europa Clipper is not scheduled to do its first flyby of Europa until 2031. Many of the instrument scientists will be there to send it off when it starts that journey, but there will be one more chance to bid the spacecraft adieu.

Two years after launch, Europa Clipper will fly past Earth using the planet’s mass for one final gravity assist. “It’ll look like a bright speck in the sky,” project staff scientist Phillips says. “It’s not going to be huge, but yes, it will be observable by telescopes or maybe the naked eye.” Anyone watching is encouraged to wave goodbye.