MASPEX is under construction at the Southwest Research Institute in San Antonio. The principal investigator is Hunter Waite, who leads a team of deputy and co-investigators that includes Chris Glein (Glein is a pending co-I). Both Waite and Glein hail from SwRI, and have prior experience with Cassini's Ion Neutral Mass Spectrometer (Waite was the PI). The two were co-authors on a key paper announcing the discovery of molecular hydrogen in Enceladus' plumes, which bolstered the odds for the moon's habitability.

Plumes or no plumes?

The Hubble Space Telescope has looked for plumes around Europa several times, with mixed results. Twice, Hubble saw what looked like a plume coming from the same surface location, but during other observations it saw nothing. In the end, the results were slightly ambiguous, despite the fact that scientists pushed the telescope's capabilities to the limit.

When Cassini flew by Jupiter en route to Saturn in 2000, it saw what looked like a faint torus of water around Jupiter matching Europa's orbit. Something is definitely coming from the surface; the question is, what? Could the Europan definition of a plume be different from that of Enceladus? Perhaps the "plumes" could be weak outgassing from the moon's interior (Earth's Moon does something similar), or molecules getting knocked off the surface by Jupiter's intense radiation.

"It almost seems like it becomes a religious or philosophical point as to whether there are plumes or not," said Hunter Waite in a recent phone interview. "Whereas the answer is there are plumes—we just don't know how big they are."

Hubble saw about 1,000 kilograms of material per second coming from Europa. MASPEX on Clipper will be able to analyze a mere 1 kilogram of material per second—1,000 times less than Hubble. "We can look at a lot less dramatic features and obtain extremely interesting information," Waite said.

In the unlikely event MASPEX doesn't get to sniff anything plume-like, there's still probably ocean samples getting knocked off the moon's surface. Europa is littered with cracks and fissures where water and other compounds may ooze onto the surface. Jupiter's radiation splits that water apart, turning minerals and possibly organics dissolved in the water rust-colored, while also blasting some of them into space.

Because there are so many potential sources for what MASPEX sees, a fair amount of detective work, in tandem with other Clipper instruments, will be required to tell Europa's full story. Detecting hydrogen, for instance, could mean there are hydrothermal vents on the ocean floor (this was the conclusion at Enceladus, per the above-referenced paper). But hydrogen can also be created from the radioactive bombardment of surface materials, so the MASPEX team must be careful when pinpointing suspected sources of the compounds they detect. They can do this through a variety of methods, including correlating detections to surface geology, comparing the results to known abundances of compounds from other sources (like interstellar dust) and tracking the altitude, velocity and direction of the materials collected.