Europa Clipper Science Payload Selected
9 Instruments Will Evaluate Europa's Potential for Harboring Life
In a press conference on Tuesday, NASA announced the selection of scientific instruments for inclusion on the Europa Clipper, a spacecraft slated to investigate one of the solar system's most fascinating moons in the early 2020s.
Europa, slightly smaller than our own moon, is a bright, cracked ball of ice orbiting Jupiter. Its proximity to its massive host planet has a few major consequences. It orbits well inside the zone of energetic particles trapped by Jupiter's very powerful magnetic fields, meaning it is continuously bathed in enormous amounts of ionizing radiation, receiving an equivalent dosage of roughly 5.4 sieverts per day. For comparison, this chart compiled by Randall Munroe characterizes 4 sieverts as "Extremely severe radiation poisoning. Survival sometimes possible with prompt treatment."
A mosaic of Europa made from images taken by the Galileo probe in 1997. This "remastered" mosaic, released by NASA last November, approximates the colors a human eye would see. Credit: NASA
More importantly, Jupiter's gravitational clout wreaks havoc on Europa's innards. Since gravitational attraction between two objects decreases as the distance between them increases, the side of Europa that faces Jupiter experiences more attraction than the side facing away from it. This has the effect of pulling and stretching Europa, cracking its surface and generating enormous friction in its interior, in a process called "tidal flexing." The resulting energy from this process heats Europa's interior, keeping the moon geologically active and creating a massive subsurface ocean of liquid water from what would otherwise be solid ice. This salty ocean may be as deep as 100 kilometers, whereas Earth oceans only get as deep as about 11 km. Consequently, Europa's ocean could contain more than twice the volume of water present in Earth's ocean.
We now know that life can persist even where no sunlight is found, by tapping the energy provided by hydrothermal vents in Earth's seafloor, which carry up hot, mineral-rich liquids from below the crust. Since tidal flexing keeps Europa's ocean liquid and continually pumps thermal energy into the system, a similar mechanism could operate on its seafloor. The severe radiation at Europa's surface splits some of its frozen water into hydrogen and oxygen, and it's expected that some of the latter finds its way to the ocean below. A recent study also suggests that the dark brownish-red coloring of the moon's surface cracks could be caused by salt from the ocean, suggesting that there are conduits by which materials can be transported between the surface and the seafloor. All of this adds up to make Europa the most tantalizing target in the search for life in our solar system beyond Earth.
The goal of Europa Clipper, under development by NASA since the early 2000s, is to investigate the attributes that make it so potentially palatable to life, and, if at all possible, to detect any evidence of the presence of life. Yesterday's announcement told us what scientific instruments the Clipper will have at its disposal to accomplish these objectives. They are:
Europa Imaging System (EIS): A set of wide and narrow angle cameras that will image Europa's surface at 50 meters per pixel. Galileo imaged small patches of the surface at better resolutions than this in the 90s, but as a dedicated mission, the Clipper will have much greater coverage.
Europa Thermal Emission Imaging System (E-THEMIS): An infrared imager that will measure thermal variations at the surface, searching for warm spots that could indicate upwellings of warm ice or subsurface ocean material.
Interior Characterization of Europa Using Magnetometry (ICEMAG): A magnetometer that will investigate Europa's magnetic field. The presence and behavior of Europa's small magnetic field was a key piece of evidence in determining that a subsurface ocean existed.
Mass Spectrometer for Planetary Exploration/Europa (MASPEX): An instrument to measure the composition of the extremely thin Europan atmosphere, and analyze any potential surface or subsurface materials that have been flung into the space immediately surrounding Europa.
Mapping Imaging Spectrometer for Europa (MISE): An instrument to measure the spectra of various regions of Europa's surface, allowing mission scientists to create a detailed map of Europa's surface composition. This may also answer the question of what material causes the deep reddish coloring in the cracks and fissures.
Plasma Instrument for Magnetic Sounding (PIMS): Along with ICEMAG, PIMS can investigate currents of plasma surrounding Europa, which may allow a number of inferences about the interior structure of the moon. Since the ocean itself is thought to be the conducting material that causes Europa's magnetic field, more detailed knowledge of how the field behaves could determine the ocean depth and salinity.
Radar for Europa Assessment and Sounding: Ocean to Near-Surface (REASON): Radar will allow NASA to characterize the shape of the ice crust and identify structures embedded within it, such as the shallow subsurface lakes that are believed to exist. It may also be able to detect the topography of the seafloor.
Surface Dust Mass Analyzer (SUDA): This instrument will analyze the size and composition of dust particles kicked up by small impactors that collide with Europa's surface.
Ultraviolet Spectrograph/Europa (UVS): Two years ago the Hubble space telescope detected evidence that Europa may emit geysers or plumes of water into space, as has been observed on Saturn's moon Enceladus. UVS will allow the Clipper to search for and analyze these plumes of water, which could tell it more about the composition of the subsurface ocean, if that's where the plumes originate.
These instruments will combine to give us a more complete scientific picture of Europa than we've ever had before. Lovely though it is in mosaics like the above, having the best knowledge we can collect about its inner workings will tell us whether it could possibly support some form of life, and may even, after extensive study of the data returned, allow us to identify indirect measurements of the presence of life itself on Europa. The grimmest possibility for humanity has always been that life has only arisen once in the entire universe -- us. It's the only case for which we have observational evidence. But if we were to detect the presence of life on Europa, we would know not only that life has arisen twice in the history of the universe, but that it has arisen twice in this single star system alone. That is, undeniably, a paradigm shift, one that allows us to speculate with much more certainty about the existence of life elsewhere in this galaxy and in others.
At the very least, the Clipper will tell us whether Europa merits a follow-up mission for further investigation. And with detailed surface and subsurface maps from the Clipper, we will have a leg up on identifying safe and scientifically valuable sites for a possible lander.
Political machinations may be conspiring to accelerate Europa Clipper's timetable, as well. Members of the House of Representatives Committee on Science, Space, and Technology have been advocating for a launch date of 2022, bumped up substantially from the previously expected 2025 date, though the Clipper team still considers it achievable. And the recent budget passed by the House allotted a bump in funding to the Clipper mission but mandated that it be launched on the Space Launch System (SLS), NASA's in-development super heavy lifter. Though SLS's primary purpose is to launch the crewed Orion spacecraft to destinations beyond Earth orbit, such as the Moon and Mars, using its massive capacity on a much lighter probe like the Clipper will allow injection into a direct-to-Jupiter trajectory. This would eliminate the need for Venus and Earth gravity assists to get to Jupiter, which require long, looping trajectories around the solar system and would add as many as 5 years to the transit time.
Boeing concept art showing the Europa Clipper mounted on the SLS rocket for launch. Note the massive size of the second stage (large fuel tank and engine in the zoomed-in view) compared to the probe itself. The increased available propellant will allow injection into a direct-to-Jupiter trajectory instead of relying on gravity assists. Credit: Boeing
Upon arrival at Jupiter, the Clipper will not enter orbit around Europa. The aforementioned extreme radiation environment is extremely hazardous to electronics, even when shielded to the extent that they are on our spacecraft. High energy particles can damage instruments and sensors with potentially disastrous consequences for a mission. The Pioneer, Voyager, and Galileo probes that passed through Jupiter's intense radiation belts all sustained damage to their electronics and saw a substantial uptick in errors in the operation of their software. The Clipper would not last long orbiting Europa, constantly awash in the ionizing radiation.
Instead, Europa will stay in a very wide orbit of Jupiter, most of which is well clear of the radiation. It will continually tweak this orbit so that its perijove (point in the orbit closest to Jupiter) will intersect Europa's orbit. In this way, it can dip in to the radiation belt, take scientific measurements while flying by Europa, and then continue on its orbit back out of the radiation, where it can dedicate time to transmitting the collected data back to Earth. By continually tweaking this orbit, the Europa Clipper can perform many fly-bys of Europa (45 is currently the planned number) at varying altitudes while minimizing the time spent in hazardous radiation conditions and maximizing the amount of time it can dedicate to transmitting science back to Earth.
Conceptual illustration of Europa fly-bys by the Europa clipper. Each line segment is one fly-by, and represents a small section of the Clipper's orbit of Jupiter that brings it close to the Europan surface. Credit: NASA
Construction of the instruments selected will begin very soon, and assembly will proceed with the goal of meeting the 2022 launch date. If SLS development proceeds as planned, we may be getting data back from Europa Clipper by the mid-2020s, representing our best chance yet at evaluating the potential for life beyond Earth. It will be well worth the wait.
Cover image credit: NASA.