The ultimate goal for Europa exploration will be to directly sample material from the ocean to determine whether it has the conditions likely to be habitable and whether complex organic molecules indicative of life are present. There are two ways to achieve this goal. The first will be to find a location where a future lander could sample material recently delivered from Europa’s ocean to the surface. The second would be to find plumes erupting from the surface that would be spewing the contents of Europa’s subsurface water into space.

Two instruments are dedicated to scouting out landing sites for future landers. The high resolution EIS camera will image sites at resolutions as fine as 1 meter to search for landing zones smooth enough for a safe landing. The Europa Thermal Emission Imaging System (E-THEMIS) (principal investigator Dr. Philip Christensen of Arizona State University) will map the surface in the thermal infrared to look for locations warmer than the surrounding ice that may indicate the presence of warmer water close to the surface. Locations with water near the surface may be sites where a future lander could drill beneath the to reach liquid water.

E-THEMIS appears to be based on the THEMIS instrument on the Mars Odyssey orbiter, which provides thermal imaging in multiple spectral to map the composition of the Red Planet. There’s no mention that I have found for E-THEMIS being used for composition mapping for Europa (the expected surface materials may not have characteristic spectra in these bands at the frigid temperatures of Europa’s surface).

Researchers using the Hubble telescope have observed a possible large plume erupting from Europa's surface. Repeat observations so far have failed to observe subsequent plumes, which may mean that these large plumes rarely erupt. However, plumes that would be too small to be seen by the Hubble’s telescope may be more common.

The Ultraviolet Spectrograph/Europa (UVS) (principal investigator Dr. Kurt Retherford of SwRI) will study the space above Europa’s surface to look for plumes and to more generally study the composition and structure of the rarefied atmosphere surrounding Europa. (That near-vacuum atmosphere will be the material that the MASPEX mass spectrometer will sample.) The location of any plumes also may be revealed by the E-THEMIS instrument by mapping very warm surface locations that could be the vent sources for plumes (as has been done for the plume sources for Saturn’s moon Enceladus).

If any plumes are discovered, the Clipper almost certainly would be retargeted to fly through them. The MASPEX and SUDA mass spectrometers would then be able to directly sample and analyze the composition of Europa’s subsurface water. (The source of any plumes could either be the ocean itself in the case of a deep fracture or a lake trapped in the ice below the surface but above the ocean.)

More to Come?

This instrument suite could be made even richer by future announcements. NASA has requested and received proposals for possible CubeSat spacecraft that the Europa Clipper spacecraft would carry to Europa and release. (See here.) These spacecraft, each likely the size of a loaf of bread, might enhance the mission by providing, for example, additional magnetometer measurements to study the ocean’s depth and salinity or by providing high resolution imaging as they fly into the surface.

NASA has also asked the European Space Agency if it would like to provide a daughter craft that might be a small lander or a probe that might fly through and analyze and plumes. (See here.) ESA’s managers have said that any contribution to NASA’s mission would have to come from a competition that would pit it against a number of excellent proposals for other science missions.

Also, one U.S. Congressman (who chairs the House subcommittee that funds NASA) has said that he thinks NASA’s mission should be enhanced with a capable lander. (See here.) While I appreciate his enthusiasm (which has led to hundreds of millions of dollars being added to NASA’s planning for its Europa mission), I’m skeptical that this will happen. Capable landers are expensive, and the interesting places on Europa’s surface look to be exceeding rugged. I doubt that a credible design could be put into place in time to launch with the Europa Clipper by the mid-2020’s even if the funding were provided.

NASA’s Europa spacecraft will also be joined by Europe’s JUICE spacecraft. The focus of the latter will be Europa’s neighboring icy-ocean world, Ganymede. Because Ganymede lies outside the intense Jovian radiation fields, the JUICE spacecraft will be able to orbit Ganymede for an extended period of close up studies. JUICE also will add to the Clipper’s studies of Europa by making two Europa flybys of its own.

In many ways, the instrument list for NASA’s Europa spacecraft and JUICE’s are similar. They both have cameras, imaging spectrometers, mass spectrometers, a magnetometer, ice penetrating radar, and a UV spectrometer. JUICE, however, will carry a laser altimeter to map the elevations of Ganymede’s surface that won’t be in NASA’s suite. The European mission has a goal to study Jupiter itself and therefore has a richer set of plasma instruments and a radio and plasma wave instrument to study Jupiter’s magnetosphere along with a submillimetre wave Instrument to study Jupiter’s atmosphere.

(It’s interesting to compare the estimated costs for NASA’s Clipper mission (~$2B) with those for JUICE, which is a similarly complex mission. NASA’s mission will be approximately twice as expensive, indicating how hard it is to design a spacecraft and instruments to survive the radiation fields at Europa.)

When to Fly?

The next key question for NASA’s Europa mission will be when it will launch. JPL’s design team are working towards a 2022 launch, provided the money can be found. The funding is the rub, though. While Congress has pushed for an early flight and backed that with generous funding, only this year has the President’s Office of Management and Budget, which sets the administration’s budget policy, agreed to make a trip to Europa an official NASA program and proposed a tiny down payment towards the mission’s cost. However, they and NASA’s managers, who ultimately work for the President and must publicly support the administration’s position, only speak vaguely of a launch in the mid-2020s or possibly later. (This reminds me of the father who tells his children that, yes, absolutely we will go to Disneyland someday (and means it), to get them to stop pestering him about the trip now.) The issue is that an earlier flight means either increasing NASA’s budget to pay for the mission or trading it for other work that is on NASA’s plate. (See these good background pieces by Casey Drier at the Planetary Society and Jeff Foust at the Space Review. I’ve also written about this.)

Over the last twenty years, I’ve watched NASA struggle to find a Europa mission that is both affordable and compelling. The Europa Clipper mission design achieves the affordable and the instrument suite NASA just announced provides the compelling. The instruments that NASA selected will enable a suite of complimentary studies that will allow us to understand Europa as an ocean world, judge whether it is likely to have conditions that would make it habitable, and scout for locations for the next logical mission, a lander. This is possible because NASA’s managers took the gutsy move and decided against an economy class mission that might have had just three or four instruments and selected the full set of instruments needed to do the job right.