Subsurface access is an essential feature of planetary exploration for both scientific understanding and in situ resource utilization. Water, in particular, is extremely valuable as both a science target and resource.
It is generally accepted that the geologic history of a region can be discerned from careful study of an appropriate selection of unweathered rock samples. Similarly, the study of ice layers can provide important information on the climate history of the planet. Finally, on a number of planetary bodies, penetration of both rock and ice is needed to reach areas deep below the surface where liquid water is thought to exist and possibly harbor life.
For in situ resource utilization, the first step is similarly the assay of appropriate samples. Reaching and securing such samples will likely require subsurface access.
Near-term lunar and planetary missions are often envisioned to secure samples from depths of a few centimeters to a meter or so, but many of the key scientific questions can be answered only by drilling to depths comparable to those attainable on Earth (e.g., kilometers). For example, Mars and several of the moons of Jupiter are thought to have liquid-water aquifers at depths of a few to many kilometers. Such aquifers probably represent the only plausible abode of extant extraterrestrial life in the solar system.
JPL has worked on a variety of subsurface-access concepts for different applications. For shallow access in soft soils, an augering system is shown in Figure 1. In this system, the tail assembly reacts to the torque of the screw and has instrument heads looking out through sapphire windows in the tail vanes. Sapphire prevents the windows from becoming scratched by quartz in the regolith. (Technically, "soil" includes embedded organics and so should be called "regolith" on other planets that lack such organics.) Screw-type subsurface explorers are limited in their maximum depth to a few meters because the overburden pressure causes the screw-vane friction to exceed the strength of available materials.
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| Fig. 2: Testing of small, commercial, rotary-percussive drill motor to evaluate required axial force, torque, and vibration. |
Fig. 3: 'Self-contained Pile Driver' able to penetrate to 8-m depth in sand in a few hours. |
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For shallow access into rock with drilling or coring bits, JPL has demonstrated new techniques for low-force penetration. In testing of small, commercial, rotary-percussive drill motors as shown in Figure 2, drilling and coring have been accomplished with only 30 Newtons of axial force. This small force allows relatively lightweight robot arms to hold small sampling devices at arm's length and yet still be able to sample rocks to a depth of 10 cm or more. Shock isolation has been shown to mitigate vibration induced back into the arm and to other instruments.
To go deeper, JPL has developed a "self-contained pile driver," shown in Figure 3. It has an internal hammer that is propelled forward so that it strikes the interior of the nose. The resulting impact causes the entire vehicle to advance by a few millimeters. This vehicle has successfully penetrated to the bottom of an 8-m vertical test facility containing sand within about 3 hours. This prototype was powered by compressed air, although an electrically-powered unit has also been designed.
To go even deeper, it is necessary to actively destroy any solid rocks that are encountered, reducing them to small rubble. This process of reducing rock to fine particles is called "comminution." JPL has experimented with long-life drill bits based on a matrix of diamond and tungsten particles embedded in a copper-silver alloy, as shown in Figure 4. These bits have been shown to wear at a rate such that they will drill through thousands of meters.
Finally, to penetrate water ice or other frozen volatiles, Figure 5 shows the JPL Cryobot, which melts its way through the ice. A key problem for such systems is to prevent dust or other refractory materials embedded in the ice from becoming concentrated in the meltwater ahead of the vehicle. This can form a refractory "plug" that can eventually surround the hot nose of the cryobot and prevent further melting and descent. JPL has successfully demonstrated a system of hot-water jets that can stir this sediment so that it passes to the rear of the vehicle instead of building up at the nose.
