There’s an old saying that the clothes make the man. In planetary exploration, the instrument suite makes the mission. Fewer and simpler instruments can enable a lower cost mission but at the cost of restricting the richness of the scientific investigations.
Jupiter’s moon Europa has been a priority to explore because there’s good evidence that its vast ocean, hidden beneath an icy crust, may have the conditions needed to enable life. However, NASA’s managers have struggled to define a mission that is both compelling and affordable. Over the last several years, engineers at the Jet Propulsion Laboratory (JPL) and Applied Physics Laboratory (APL) have rethought the entire approach to exploring Europa. They started with a bare bones list of just three must have instruments (with a longer list of optional desired instruments). Their breakthrough was to plan a mission that would orbit Jupiter and make many brief swoops past Europa before swinging back out of the high radiation zone. NASA now has a concept that's affordable.
What would ultimately define the mission, though, would be the suite of instruments NASA’s managers choose. Designing instruments that can withstand the radiation has proven difficult. NASA’s managers could have decided on a minimal instrument suite to reduce mission costs and risks, in effect to fly an economy class mission to Europa.
As we learned last week (see here), however, they announced the selection of a rich instrument suite that will make this a first-class voyage. Not only is the list long—and includes everything on that original desired list—the instruments individually look to be highly capable. The resulting mission promises to be incredible.
NASA’s announcement was widely reported on and by now I expect that many of you have seen the instrument list. In this blog post, I’ll discuss how these instruments will work together to reveal Europa’s secrets. NASA did little more than announce the names of the instruments and said little about their capabilities. (This is standard; we usually learn the details about the instruments in the next year or two as their science team discuss them at scientific conferences.) Where possible, I’ve expanded upon the brief list of instruments with previously published information or from information published since NASA’s announcement. I’ve also provided comparisons with the instrument suite for the European Space Agency’s Jupiter Icy Moon Explorer (JUICE) mission, which will briefly study Europa but focus on the neighboring moon Ganymede and Jupiter itself.
Ice and Ocean
Europa is an ocean world that likely hosts twice as much water as Earth, capped by an icy crust a few kilometers to several tens of kilometers thick. Many of the instruments will focus on studying the ocean and the structure of the crust.
Europa is embedded within the powerful magnetosphere that surrounds Jupiter. A salty ocean would interact with the magnetic fields and reveal both its depth and salinity. NASA’s Galileo spacecraft all but proved the existence of Europa’s ocean by measuring the induced magnetic field from this interaction. Europa Clipper will refine Galileo’s measurements with its own magnetometer (Interior Characterization of Europa using Magnetometry (ICEMAG)—principal investigator Dr. Carol Raymond of JPL). The plasma fields carried within Jupiter’s magnetosphere locally modify the magnetic field, and the Europa Clipper will carry a basic plasma instrument to allow the modifications to be accounted for (Plasma Instrument for Magnetic Sounding (PIMS) – principal investigator Dr. Joseph Westlake of APL). NASA appears to have selected a core instrument set focused on the investigation of Europa’s ocean. ESA’s JUICE spacecraft will carry a richer set of investigations that will carry out broader investigations of Jupiter’s magnetosphere. However, if the Europa Clipper and JUICE operate at the same time, the Clipper’s instruments could enhance JUICE’s investigations by providing a basic measurement of the magnetic field at a second location. (JUICE will arrive at Jupiter in 2030; a date for the Clipper’s arrival has yet to be set.)
Galileo’s camera and spectrometers revealed that the icy crust is fractured and frequently covered with material that appears to have originated in the ocean below. The Clipper’s radar instrument (Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) – principal investigator Dr. Donald Blankenship of the University of Texas at Austin) will see below the surface to investigate the structure of the shell, potentially all the way to the interface with the ocean below.
Ground and ice penetrating radars face a fundamental trade off: Do they focus on the highest resolution to reveal fine structures (higher frequencies) beneath the surface at the cost of shallow penetration, or do they focus on maximizing the depth of penetration (lower frequencies)? Radar systems can focus on one measurement or the other based on the frequency at which they operate. The Clipper’s REASON instrument will use dual frequencies to enable both fine resolution in the upper layers of the icy shell and deep penetration. (JUICE’s radar system will use a single lower frequency.) The frequencies that allow deep measurements are subject to interference from radio waves emitted by Jupiter, and these measurements will be made only on the hemisphere of Europa that faces away from Jupiter where the bulk of the moon blocks Jupiter’s emissions. (At Mars, two spacecraft carry subsurface radar; NASA’s Shallow Radar (SHARAD) instrument uses higher frequencies while ESA’s Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument uses lower frequencies.)
The bulk of Europa is a rocky world containing an ocean that may be around 100 kilometers deep covered by an icy shell that likely is kilometers thick. Measuring variations in Europa’s gravity field can reveal this moon’s internal structure to its core. Differences in the gravity field would arise from variations in the thickness of the icy shell, variations in the topography of the ocean floor, or deep variations in the structure of the rocky body. The Clipper engineering team has put considerable effort into enabling sensitive measurements of the gravity field from multiple flybys by measuring slight differences in the Doppler shifts of the radio. NASA did not announce a science team for gravity measurements when it announced the instrument suite. However, according to NASA’s program manager for Europa instruments, Curt Niebur, “We have spent considerable effort accommodating gravity science into the mission design. At this point NASA is considering options as to how best to inject the necessary expertise to the science team. But gravity science remains a key investigation of the Europa mission.”
While Europa’s potentially life-bearing ocean lies hidden beneath the icy shell, the surface of that shell is streaked and spotted with darker stains. Scientists believe that the staining materials are likely materials such as salts or potentially organic molecules brought to the surface when the ice shell fractures and ocean material erupts to the surface. Three of the instruments will examine the composition of these materials.
The composition of the surface will be mapped across the globe by the Mapping Imaging Spectrometer for Europa (MISE) (principal investigator Dr. Diana Blaney of JPL). Cameras are typically optimized to provide sharp images but record only a selected few colors (or specific spectral bands). Mapping spectrometers trade imaging resolution for the fidelity of their spectral measurements across many spectral bands. MISE will use the spectra of light reflected from Europa to map the distribution of “organics, salts, acid hydrates, water ice phases, and other materials” across this moon’s surface.
Various processes such as sublimation and micrometeoroid impacts expel material from the surface to form a tenuous cloud around Europa. The Europa mission’s two mass spectrometers will directly sample this material and determine its composition by “weighing” the atoms and molecules they encounter. By doing so, they will be able to directly taste the ices, salts, and any organic molecules present on the surface and will provide more sensitive measurements of surface composition than MISE (but without MISE’s ability to map composition across the surface).
The MAss SPectrometer for Planetary EXploration/Europa (MASPEX) (principal investigator Dr. Jack (Hunter) Waite, of the Southwest Research Institute (SwRI)) will measure the composition of gases, ices, and organic molecules. (MASPEX is the current state of the art and will be more sensitive than the equivalent instruments that have been flown on the Cassini Saturn and Rosetta comet missions. I’ve seen this instrument included in a number of current proposals for new missions.)
The SUrface Dust Mass Analyzer (SUDA) (principal investigator Dr. Sascha Kempf of the University of Colorado Boulder) will measure dust and salt particles ejected from the surface.
Previous generation equivalents to these two instruments were included in the Cassini mission and in combination have provided the data from measuring the composition of the plumes of Enceladus to show that its internal ocean may provide a habitable environment.
The surface of Europa’s icy shell records a history of fracturing and interaction with the ocean below. Globally mapping the surface will provide scientists with clues to what processes shaped the surface and how material is exchanged between the ocean and surface.
The wide angle camera in the Europa Imaging System (EIS) (principal investigator Dr. Elizabeth Turtle of APL) will map the surface of Europa at 50 meter resolution in color to document the surface structure. While 50 meter resolution may seem to be coarse compared to the high resolution images we have come to expect from Mars and the Moon, for a first dedicated mission to a world this will be detailed global coverage. The best global maps of Venus from the Magellan mission are approximately 300 m resolution. (Mars has better coverage, but it has been the focus of many missions. At Mars, over 90% of the surface has been mapped at 6 meter resolution (as of November 2014) in black and white by the Context Camera (CTX) on NASA's Mars Reconnaissance Orbiter while 61% has been mapped in color at 20 meters by the High Resolution Stereo Camera (HRSC) camera on ESA's Mars Express orbiter (as of February 2013).)
During each of the planned 45 flybys, the spacecraft will travel close to the surface of Europa. At each encounter, the wide and narrow angle EIS cameras will record the surface geology in high resolution recording details as small as one meter. The MISE spectrometer likely will provide high resolution composition maps across the narrow strips of terrain that the spacecraft will traverse during its close encounters.
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.