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Frozen Enceladus, one of the moons of Saturn, has what could could almost pass for the off-Earth version of the fountain show at the Bellagio in Las Vegas. Spectacular plumes of water reach hundreds of miles into space. These were easily detected by Cassini (RIP) and proven to contain actual H2O. That could mean the presence of life (at least as we know it). Europa is also suspected to spew plumes like this, but none have actually been observed yet.
Launching in 2024, the Europa Clipper mission is going to investigate whether there are plumes on Europa, though that is not the only evidence it will need to gauge the Jovian moon’s habitability. Just don’t expect a clone of Enceladus. NASA planetary JPL scientist Erin Leonard is currently leading the development of a new global geological map of Europa and working closely with the Europa Clipper mission.
"If plumes are present on Europa, the composition of the plume could give us hints at the composition and current state of the subsurface ocean," Leonard told SYFY WIRE. "If Europa Clipper were to fly through a hypothetical plume, our first observations of the plume composition might come from the ultraviolet spectrograph remote sensing instrument, Europa-UVS."
The Europa Clipper spacecraft is a lab in itself. If a plume is detected and UVS shines a beam right at it, the light emitted from and shining right through the plume could possibly give away the composition of that plume. Its REASON radar instrument will be able to find out whether there is liquid water under all that ice, while its SUDA dust analyzer and the MASPEX mass spectrometer could find out what is in subsurface material right there, without scientists on Earth waiting on a sample return for years. SUDA would measure particles in Europa's space dust while MASPEX would measure gases.
So what might all that reveal about the unknown expanse of ocean believed to lie beneath the surface?
Europa and Enceladus are not exactly mirror images of each other. They both are thought to have a subsurface ocean beneath miles of icy crust, but that is pretty much where the similarities end. Europa is much larger than Enceladus. Something a little smaller than our Moon (Europa) beats an object only a seventh of the Moon’s diameter (Enceladus). Europa’s ocean may be deeper — meaning, possibly twice as much water as all the oceans on Earth.
It is possible that hydrothermal vents at the bottom of Europa’s ocean provide enough heat for life to thrive as it does on Earth. The gravitational forces Jupiter and its other moons keep exerting on Europa may keep it heated deep inside. UVS will be able to make out chemicals on the surface, which may give a better idea of this moon’s habitability in the strange waters below all that ice. Hydrogen gas in a plume could mean there really are hydrothermal vents in the deep, because it indicates chemical reactions between salty water and rocks at the bottom of the ocean.
"The presence of large organic molecules would be particularly interesting and could (depending on the organics) hint at potential habitability of the subsurface ocean and icy shell," Leonard said. "However, it is important to note that the composition of a potential plume may not directly reflect the composition of a subsurface ocean. Plumes could originate from a shallow liquid reservoir that is disconnected from the ocean."
Whether any beasts, or microscopic beasties, actually live there or at least could be living there is what scientists are anxious to find out. It isn’t going to be easy. Plumes may not necessarily reveal what is going on beneath the surface, but if there are any on Europa, they would look more like a backyard fountain compared to a spectacle in Vegas. Hypothetical Europan plumes may be nearly impossible to observe. They may not go off nearly as often as those from Enceladus. Because Europa is a larger moon with more gravity, they may also be much closer to the surface, harder to see.
Europa Clipper will squint all it can to find something. Its EIS camera suite will use visible light to beam back hi-res color and stereo maps back to Earth. It will capture images of plumes and possible deposits left behind, also keeping an eye out for silhouettes of plumes on the edge of Europa when it passes in front of Jupiter. From behind, the planet’s light may reveal things that would otherwise disappear in the darkness. The Europa Thermal Emission Imaging System, aka E-THEMIS, will seek out surface hotspots that may indicate plumes were or are actively erupting from the area.
"We want to ensure that if and when the instruments measure something interesting, we will be confident in the result," said Leonard. "This is particularly important for habitability investigations. We want to avoid a situation where we get data back that indicates a potentially habitable environment, but then not be able to trust it due to some flaw with the instrumentation, or to find out that a scientific inference is flawed because we were asking the wrong questions."
Then there are the possibilities of what a plume (or lack thereof) can actually mean. Any water that makes it to the glacial surface of Europa won't stay liquid for long because it will soon turn to ice. Plumes could also be caused by intense pressure exerted on a bubble of liquid water within the icy crust. Even that liquid water would eventually freeze depending how close to the surface it is. If Europa's core is heated, anything that far away would be frozen solid in no time, which leads to another way a plume could get through. If liquid water inside the crust freezes, it would cause expansion and therefore fracturing of the ice, which would potentially allow liquid water to get through.
Even if all this next-gen tech does not glimpse actual plumes, that doesn’t mean Europa is lifeless. There may be life as we know it. There may be life as we never imagined it. Europa may even have plate tectonics like Earth, and if it does, that could mean that plates sliding over and under each other keep pushing around nutrients needed for life. Plumes may amaze us, but that doesn’t necessarily mean aliens — or no aliens at all. Leonard looks forward to finding out the habitability potential of Europa, whether or not any strange microbes swim to the surface.
"The real power of Europa Clipper will come from the combination of several datasets and several lines of investigation in order to constrain Europa’s habitability," she said. "Even the spacecraft’s telecommunications system and the radiation dosage monitor will contribute to our understanding of Europa as a potentially habitable world."