Samantha Trumbo (PhD ’21) couldn’t take her eyes off of the launchpad. There, balanced on top of a SpaceX Falcon Heavy rocket, was the project she had been working on since her time at Caltech: NASA’s Europa Clipper. If everything went according to plan, the spacecraft would soon be on its way to the Jupiter system. “It was both very exciting and very nerve-racking,” Trumbo says of seeing the October 2024 launch from the viewing stands at the Kennedy Space Center with other members of the mission’s science team. She remembers the roar of liftoff and watching as the Clipper grew smaller and smaller before disappearing into the outer reaches of the atmosphere. Then the waiting began.
In the spring of 2031, the spacecraft will finally reach its destination: Europa, an icy moon orbiting the largest planet in the solar system. But when it does, Trumbo, now an assistant professor of astronomy and astrophysics at the University of California, San Diego, and her colleagues on the NASA science team will have an unprecedented view of the planetary body first discovered by Galileo Galilei more than 400 years ago. “We’ve known about Europa’s existence for a long time, but with spacecraft exploration, we’ve come to understand it as a really fascinating place,” Trumbo says. “Galileo saw this little point of light, and now we see this whole world.”
Describe Europa for us.
Samantha Trumbo: It’s an amazing place. Europa has very few craters, so we know that the icy surface that we see must be geologically recent; it hasn’t had enough time to accumulate a lot of impact scars. Between 40 and 90 million years is the approximate age estimate. And there are places on the surface called “chaos terrain,” which is where the ice has broken apart into different plates that have moved around and gotten jumbled up and there’s been apparent upwelling of material from below. And what’s really fascinating about Europa is that we think it has abundant liquid water under the surface. There’s evidence from NASA’s Galileo mission, which went to the Jupiter system in the 1990s, that beneath that pretty thick crust of ice, Europa probably has a global subsurface ocean—a truly alien ocean. It’s about the size of Earth’s moon, but there’s probably more water there than in all of Earth’s oceans combined.
There’s evidence from NASA’s Galileo mission, which went to the Jupiter system in the 1990s, that beneath that pretty thick crust of ice, Europa probably has a global subsurface ocean—a truly alien ocean.
How do you study a moon that’s 500 million miles from Earth and an ocean that’s under its surface?
ST: In my work, I mostly use “earth-based” observations—which means either ground-based telescopes or space-based telescopes like the Hubble Space Telescope or the James Webb Space Telescope. I am looking at the sunlight reflected off of Europa’s surface to understand its composition. Light is a spectrum and essentially what I’m looking for is absorption—missing light that tells me that there was a compound on the surface that absorbed the light at that specific wavelength. If we can understand what these compounds are and where they are located on the surface in relation to geologic features or processes, we can make reasonable interpretations about the internal composition of Europa, too. For instance, we can see absorptions due to things like salts, which we interpret as having been sourced from Europa’s ocean originally.
You’ve also seen signs of carbon dioxide on Europa. Why is that important?
ST: We’ve known since the Galileo mission that Europa and some of Jupiter’s other icy satellites have carbon dioxide on their surfaces, but it was a mystery where it was coming from. Was it native to Europa or was it delivered externally? Or was it produced by particle bombardment? Those are important questions because carbon dioxide is the only known carbon-bearing component at Europa, and carbon is an essential element for life. The James Webb Space Telescope, which launched in 2021, was our first opportunity since the Galileo mission to answer this question; we can’t see carbon dioxide on Europa from any ground-based telescope because the carbon dioxide in our own atmosphere interferes with measurements. We hypothesized that the carbon dioxide on Europa’s surface was coming from the interior, from the ocean, and would be associated with the same geological terrain where we see salt. And that’s what we saw.
What could we learn when the Europa Clipper reaches Europa in 2031?
ST: The goal of the mission is to understand Europa’s potential habitability. I’ll be doing the same type of spectra analysis I’ve done with other datasets, but the Europa Clipper will achieve much higher spatial resolution than the Webb telescope, so we’ll be able to see things we could never see from Earth. And I’ll have an opportunity to link that data with other types of measurements. There are instruments on the Clipper that can measure the gases in the atmosphere and possibly collect salt-bearing dust particles. There’s also an ice-penetrating radar that can look into the ice shell for things like pockets of liquid water. We’ll be able to paint a complete picture.
Then the big question is: What if Europa is habitable?
ST: The question of whether there’s life beyond Earth is probably one of humanity’s biggest. Determining if a place is habitable is not answering the question of whether a place has or does not have life. It’s answering the question “could it support life if life happened to be there?” I think if the answer turns out to be yes, that would be transformative. We would know that there is a place in our own backyard, within reach, that, as far as we understand it, could support life. I think that would motivate a lot of continued exploration.
Because it’s not just Europa: We’re starting to understand that our solar system is littered with places that might have liquid water. There’s Ganymede and Callisto, two other satellites of Jupiter, which show evidence of subsurface oceans of liquid water. There’s Enceladus, which is a little tiny moon of Saturn that’s erupting water vapor and salty particles into space. There’s reason to think that Titan, another moon of Saturn, probably has an interior ocean, and there are other candidate ocean worlds further out in the solar system. We’re going to Europa, and we’re going to build a really complete understanding of a fascinating, potentially habitable world, but it’s one of several that are also worthy of consideration.