How to build an ice-div­ing probe that won’t freeze up

Rather than bor­ing through rock-hard ice, fu­ture ro­botic ex­plor­ers to ice worlds will melt their way in

Sky at Night Magazine - - BULLETIN - LEWIS DARTNELL is an as­tro­bi­ol­ogy re­searcher at the Univer­sity of West­min­ster and the au­thor of The Knowl­edge: How to Re­build our World from Scratch (www.the-knowl­ LEWIS DARTNELL was read­ing… Melt­ing probe tech­nol­ogy for sub­sur­face ex­plo­ration o

Some of the most in­trigu­ing ob­jects in the So­lar Sys­tem are ice worlds. The moons Europa and Ence­ladus are hid­ing large bod­ies of liq­uid wa­ter deep un­der their sur­faces, sealed be­neath a shell of hard-frozen ice. Liq­uid wa­ter is thought to be one of the fun­da­men­tal re­quire­ments for sup­port­ing life, and so many plan­e­tary sci­en­tists are very keen to explore th­ese alien oceans.

The chal­lenge, though, will be pen­e­trat­ing down through a sub­stan­tial thick­ness of rock-hard ice to ac­cess th­ese ex­cit­ing en­vi­ron­ments. On Earth, this sort of oper­a­tion (like drilling down into Lake Vos­tok buried deep be­neath the Antarc­tic ice) is ac­com­plished with huge, in­dus­trial ma­chin­ery and a team of hu­man en­gi­neers to main­tain it: a mis­sion us­ing a setup of this type is sim­ply not fea­si­ble in the near-term of space ex­plo­ration.

So how will we be able to reach the in­te­rior of th­ese icy worlds with cur­rent ro­botic tech­nol­ogy? The most promis­ing so­lu­tion is a melter probe. A roughly cylin­dri­cal can­is­ter holds a suite of sci­en­tific in­stru­ments, and sports a heated plate at the front to slowly melt its way down.

But there re­main a num­ber of en­gi­neer­ing prob­lems that need to be solved, say Kai Schüller and Ju­lia Kowal­ski from RWTH Aachen Univer­sity in Ger­many. For one, the probe needs to melt the ice im­me­di­ately be­neath it, whilst en­sur­ing that it doesn’t re­freeze too quickly in the chan­nel that’s been cre­ated and trap the probe. While de­signs can be tested on Earth, the much colder ice and lower grav­ity of the icy moons would slow the down­wards pro­gres­sion of the probe and so in­crease the risk of such a lock-in.

Schüller and Kowal­ski have been study­ing how to make the most ef­fi­cient use of the heat­ing power a small probe could de­liver and en­sure it doesn’t be­come stuck. They’ve mod­elled a sim­ple de­sign to see how pa­ram­e­ters like the length of the probe, the power of its heated head and the phys­i­cal prop­er­ties of the icy world af­fect its abil­ity to bur­row down. “To pre­vent it­self getting stuck an ex­trater­res­trial melter probe will al­most cer­tainly need heaters along its sides as well”

They’ve found that the grav­i­ta­tional pull of a world, and hence the down­wards force ex­erted by the probe, does in­deed have a sig­nif­i­cant im­pact. To pre­vent getting it­self stuck, an ex­trater­res­trial melter probe about a me­tre long and de­liv­er­ing a rea­son­able power to its front heater will al­most cer­tainly also need heaters along its sides as well. And for some­where like Ence­ladus, with its low grav­ity, an ad­di­tional mech­a­nism to keep push­ing the heat­ing head for­ward into close con­tact with the ice needs to be con­sid­ered.

Re­cent dis­cov­er­ies have made just this kind of melter-probe tech­nol­ogy even more rel­e­vant. In July of this year, sci­en­tists an­nounced the dis­cov­ery of a large lake of wa­ter be­neath the ice cap on the south pole of Mars. This too could be a po­ten­tial habi­tat for life, and so is an­other ex­tremely invit­ing tar­get for ex­trater­res­trial sub­glacial ex­plo­ration. Just this sort of melter probe de­sign, orig­i­nally con­ceived for Europa, would there­fore make an ideal mis­sion for the Mar­tian pole.

An artist’s im­pres­sion of a probe melt­ing itsway into Europa

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