Locked away beneath the surface of Mars are vast quantities of water ice. But the properties of that ice—how pure it is, how deep it goes, what shape it takes—remain a mystery to planetary geologists. Those things matter to mission planners, too: Future visitors to Mars, be they short-term sojourners or long-term settlers, will need to understand the planet's subsurface ice reserves if they want to mine it for drinking, growing crops, or converting into hydrogen for fuel.

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