Close-up image from NASA's Galileo spacecraft of a portion of Europa's icy surface, cracked in patterns suggesting floating sea ice. (NASA/JPL-Caltech/SETI Institute)
NASA is setting its sights on getting a much closer, deeper look at Jupiter's tantalizing moon, Europa, and the mysterious ocean hidden beneath its icy crust. With two orbital missions already in the works, by NASA and the European Space Agency, NASA is looking further into the future toward a possible mission to put a robot on the surface.
Europa's ocean, which may lie under only a few miles of ice—perhaps only a few hundred feet in some places—may be as deep as 30 miles, and contains more water than in all of Earth's oceans. With the possibility of some form of hydrothermal vents supplying heat and life-supporting chemicals on the ocean's floor, like those on Earth, the tiny moon has become one of the hottest subjects in the search for extraterrestrial life in the solar system.
In advance of issuing a call for formal proposals, NASA is priming the scientific community to begin thinking about what set of scientific instruments should be included on a lander. Ultimately, ten proposals will be selected to proceed to a competitive concept study, which carries funding of $1.5 million for each selected group, who will have a year to submit their technical proposals.
Most recently, the Hubble Space Telescope detected plumes of water vapor erupting from Europa's surface, further wetting scientists' appetites to explore the liquid realm beneath.
Europa was discovered by Galileo in January 1610, along with three other large moons, the "Galileans." Through a small telescope like Galileo's, they appear as mere star-like dots, and for over 300 years that's about all that humans could see of them. Only when the first robotic probe, Pioneer 10, passed through the Jupiter system did we get a closer picture.
Envisioning the Lander
How would you design a robot to probe Europa's deep dark waters from a landing site on the icy surface above? That's the primary framing question for any competitive design concept. NASA will only select proposals that address specific scientific goals—namely, to look for evidence of life and a habitable environment in Europa's ocean.
What instruments would you include? Ground-penetrating radar to survey structures in the moon's crust? Chemical detectors to sniff for water vapor and other volatiles that may seep up from the ocean? Motion sensors to measure the movements of floating ice? Sensitive microphones to listen for any sounds penetrating the ice from below? NASA has issued just the right challenge to get imaginations churning….
It's not technically feasible at present to deliver a probe directly to the waters under Europa's ice crust—let alone to the ocean floor tens of miles below that—so a lander will need to do its job without direct visual inspection. Pictures of Europan jellyfish swimming around would be nice—but we can't get a camera down there yet. It's difficult enough exploring the depths of Earth's oceans….
What Else Does the Lander Need?
The final design of the lander ultimately will comprise more than the scientific instruments it carries. There will be plenty of engineering challenges and considerations further along in the design process.
Simply landing the probe will require some serious thought. Unlike landings on Venus, Mars, and Saturn's moon Titan, parachutes are not an option since Europa has no atmosphere to help slow a lander down. Some form of rocket-powered descent and soft touchdown might work.
Deciding where to land requires planning as well. A landing site where the ice is thin and the waters relatively near the surface could make it easier for the lander to measure properties of the ocean, but we know very little about the nature of Europa's surface terrain. Other than a few pictures taken by passing spacecraft, Europa is mostly unexplored. Planners should get some guidance from surveys by upcoming missions--NASA's Europa Clipper and ESA's "JUICE"—but right now the icy moon is terra incognita.
Another design consideration is a lander's end-of-mission disposition. The Galileo spacecraft was deliberately incinerated in Jupiter's atmosphere at the close of its mission, to prevent it from crashing into a moon like Europa and contaminating a potentially life-bearing environment with any biological material that may have hitched a ride with the spacecraft. Even if the lander were completely sterilized, an outer space version of environmental "green" responsibility seems like a good idea.
Scientists preparing concept proposals don't need to concern themselves with the robot's landing or safe-self-destruct modes right now--just its scientific capabilities. Now is the time for dreaming up how the lander might tell us what, if anything, is swimming in the waters of Europa.
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