Submarines hydrobots and cryobots

The presence of icy masses on certain bodies in the Solar System provides for an unusual class of spacecraft lander that has its operating location not at the surface of a body, but beneath it. While the engineering problems of having a whole spacecraft tunnel its way mechanically through rock and regolith are beyond current state of the art, it is possible to conceive of systems that can melt through an ice layer. The term 'cryobot' has been coined for such exploration craft.

Such thermal drilling is of particular interest in two locations - the Europan surface, and the Martian polar ice caps (e.g. Zimmerman et al., 2001). The latter may yield records of climate change, impact ejecta and volcanic activity.

While there is considerable heritage in the use of electrically powered melting probes for the exploration of terrestrial polar ice masses (e.g. Philberth, 1962; Aamot, 1967), there are two principal difficulties. First, especially on Europa, the absence of an atmosphere causes melt or even just warm ice to evaporate rapidly. Thus the drill must supply latent heat of evaporation as well as for melting, leading to a substantial increase in required power. Second, if an ice layer is dirty, the dirt can accumulate to form an insulating layer which reduces the penetration rate. Removing this material is challenging.

Note that the energy required for thermal drilling is typically much higher than for mechanical drilling. However, thermal drills are much easier to implement, and the heat need not even be generated from electricity, but could be provided directly from a sufficiently large radioisotope heat source.

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