Coring basalt under Mars low pressure conditions

Abstract

Background: Future exploratory drilling on Mars will ideally employ drill bits that are capable of penetrating any terrain from hard rocks such as basalt to ice or ice-bound soils. Candidate drill bits include diamond impregnated or surface-set types, and bits with large individual cutters, typically made of polycrystalline diamond or tungsten carbide composites. Each has disadvantages, but bits with large individual cutters seem more likely to be capable of improvement. Drilling in hard rocks poses difficulties for these bits, so we have investigated their performance in basalt. Basalt rock was chosen as it is the most prevalent type of rock found on the surface of Mars, and any future drilling mission to Mars will probably encounter basalt while drilling a hole (even if drilling basalt was not the mission objective). It is also one of the hardest types of rock, which makes the design of a Mars drilling system very challenging, especially because a lander or a rover will be constrained to low mass and limited power. Method: Custom designed core bits with discrete cutters (Polycrystalline Diamond or PDC elements) were used to drill high strength basalt rock under conditions of low temperature and pressure appropriate to Mars. The experimental arrangement allowed for remote control of the weight-on-bit and the rotational speed of the drill string. Acquired data included weight-on-bit, rotational speed of the drill, reaction torque, drilling power, penetration rate, bit temperature and the temperature of the drilled formation. Conclusion: Penetration in the basalt was initially rapid, but increasing wear required progressive increases in the applied weight-on-bit to maintain the rate of penetration. Eventually, the wear was so great that penetration could not be maintained with 500 N, the maximum weight that can reasonably be expected on a small exploration vehicle (~150-200 kg). In the present case, for a core bit with an outside diameter of 38 mm and an inside diameter of 25 mm, the wear limit was reached after a depth of five centimeters had been penetrated. Additional penetration to a depth of over 15 cm was possible only after increasing the Weight on Bit to 950 N. The drilling power was in the range of 100 W. Improvements in drilling performance could potentially be achieved by reducing the bit diameter, changing the bit design and replacing a PDC cutter with a more wear resistant material. The bit performance has also been modeled, and a consistent explanation for the bit behavior was found.