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In October, NASA’s Europa Clipper embarked on a long journey in search of alien life in the cosmos. The spacecraft isn’t due to arrive at Europa until 2030, and even then it will only sail around the moon at close range, without alighting on its surface. It will take many more years for scientists to determine if the dark subsurface ocean there could be capable of sustaining extraterrestrial life. But if life is there, scientists will have to drill down through kilometers of dense icy exterior to find it.
Kris Zacny is one of the leading contenders for the job. A roboticist at Honeybee Robotics, Zacny has been developing a drill called the SLUSH probe specifically for Europa for about 10 years. The instrument would combine a drill to break through the ice with a heat probe that would melt the ice chips, forming slush as it descends and preventing the drill from freezing in place.
“We’re kind of drilling and melting as we’re going down,” says Zacny, who has been working on drilling in mines and on oil rigs for more than 20 years and has authored several books about drilling in space and other extreme environments. The SLUSH project is partially funded by NASA, though the agency is also considering other proposals for drilling on Europa, including a cryobot developed at the Jet Propulsion Laboratory.
SLUSH stands for Search for Life Using Submersible Head, and ultimately, the probe would drop a submersible into the ocean to explore its depths, sending data up to the surface via conductive microfilaments and optical fiber cable. (If the cable were to break, the microfilaments could be used as antennae to transmit data to a lander.) The descent through the ice alone could take two years, says Zacny, as the probe can only drill about 6.5 feet an hour at top speed, and it will have to descend to a distance of between 6 and 20 miles.
The SLUSH probe looks like a light saber, or a bit for a handheld drill fit for giants.
While many moons, including Saturn’s moon Enceladus, show evidence of oceans beneath their icy crusts, astrobiologists believe that Europa may be the best place to find extraterrestrial life in our solar system. Other subsurface oceans may exist only as pockets of water within layers of ice. But scientists think Europa’s ocean lies above a rocky core that could infuse it with essential chemicals. It is also possible the core of Europa contains hydrothermal vents that release heat and nutrients to the dark ocean. Europa has another advantage in that it has a much stronger force of gravity than Enceladus—Europa is almost as big as Earth’s own moon, whereas Enceladus is small enough to fit within the length of the United Kingdom. Any drill would need gravity to draw it down.
The SLUSH probe looks like a light saber, or a bit for a handheld drill fit for giants: a narrow cylinder a little over 16 feet in length, with four spiraling flutes at one end that form a rotating auger to bite down into the ice. The probe’s cylindrical body contains a nuclear power source, motors, gears, batteries, and a “science bay” for analyzing samples of water. Above that lie spools for a thin fiber-optic cable that would allow communication with a lander at the surface. The cable would be unwound as the probe heats and drills its way down.
Zacny reasons that the heat generated by the probe in the near-vacuum of Europa’s atmosphere would be both an advantage and a drawback. On one hand, this heat could be funneled into the ice surrounding the probe to encourage melting and facilitate drilling. On the other hand, the probe would need precise thermal control to prevent the instrument from overheating and even melting, he says. So far, Honeybee has successfully tested prototypes for the SLUSH probe in an ice tower erected in a walk-in freezer and on Devon Island in the Arctic.
Drilling on Europa will be very difficult, says Ralph Lorenz, a planetary scientist at Johns Hopkins Applied Physics Laboratory who is not involved in the SLUSH project. “It is theoretically possible, but I think is not likely to happen for 30 years or more,” he says. “It would be a hugely demanding undertaking technologically, even if money were no object.” He notes evidence that the outer crust on Europa is more than 6 miles thick—an impractical depth for regular drills.
Instead, “the only method that seems even remotely feasible is thermal drilling,” powered by a nuclear battery, he says. As well, the probe will need protection from the strong radiation that floods Europa; and studies suggest Europa’s surface terrain is rough, which could create problems during landing and drilling. Even in the best case, “it would be a massive mission, and massively expensive,” Lorenz says.
Studies suggest the crust on Europa is mainly salty ice. “There will be some subterranean lakes, and there may be some sediments,” Zacny says. But “I don’t think we are going to see any big boulders,” which could interfere with drilling. If all goes well, the probe would reach the subsurface ocean and then use its onboard instruments—probably including a mass spectrometer, to determine the types of chemicals in its water, says Zacny.
The information from the analysis would be relayed to the lander at Europa’s surface, and then transmitted to scientists on Earth. But the probe would also conduct scientific measurements during its descent through Europa’s ice, just in case its trailing fiber-optic cable turns out to be too short, for example, or if the probe fails for some other reason.
“You may get stuck forever—you may never reach the ocean,” says Zacny. For now, finding life on Europa remains a distant dream. 
Lead image: muratart / Shutterstock
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