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and measurements that pertain to the possible presence of liquid water and its properties, would need to be made from a variety of locations on and beneath Europa's surface. As these locations cannot be determined a priori, but will require analysis of data from the surface, the ability to move from one place to another is required. Thus, in addition to acquiring global remote-sensing measurements from a low-altitude orbiting spacecraft, the following mobility modes are potentially required:
A surface rover capable of moving over long distances (perhaps the tens of kilometers necessary to cross the landing-error ellipse and reach areas of interest) to make pertinent geological, geochemical, and geophysical measurements, and to identify regions that might have a locally thin crust;
A multifunctional arm on a rover or lander to deploy and position instrument detectors or sampling devices;
Drilling and coring devices capable of penetrating to shallow depths (meters) beneath the surface. Mobility of this form would allow the deployment of instruments to measure subsurface temperature gradients, in situ composition measurement, and the collection of samples for analysis on board a lander or rover;
Devices for collecting coherent samples, and facilities to maintain them in a pristine thermal environment for eventual return to Earth;
A cryobot for melting into the ice shell of Europa, to depths on the order of kilometers, to deploy instruments either within the ice or within the underlying water, if it is present; and
A small submarine, deployed by the cryobot, to explore the subsurface water ocean, if it is present.
The range of mobility modes required suggests a progression of missions that each collect data relevant to the feasibility of later activities. Such a progression could begin with a relatively simple lander equipped with either an arm to collect samples or a similarly equipped rover. Additional surface missions, such as landers capable of performing more complex activities (e.g., drilling to relatively shallow depths), may be required before the deployment of cryobots capable of melting their way through a considerable thickness of ice and, possibly, penetrating the ice/water interface. Even the simplest of these activities is likely to present unique technological challenges due to Europa's extreme radiation environment and low surface temperatures.
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