## 2 Basic Structure of RECAS Sonde  
The RECAS sonde is a crucial component of the EFSOS system and serves as an actuator for drilling and sampling. It consists of four subsystems: a heating system, an inner winch system, a synthetic fiber-armored cable, and a drilling-parameter detection and control system. The heating system converts electrical energy into heat energy to melt ice during the drilling process, creating a safe pathway for the sonde. It consists of upper and lower melting tips and lateral heaters. The upper and lower melting tips were equipped with ten groups (20 pieces) of cartridge heaters that generated 6.5 kW heating power at a rated voltage of 650 V. By contrast, lateral heaters establish a uniform or gradient heating field on the sonde's radial outer surface, keeping the surrounding molten water in a liquid state. Direct contact between the thermal sonde surface and the borehole wall can present challenges. When the sonde is inclined, the hot surface of the upper part readily adheres  
Figure 2: Concept of EFSOS system. to the smooth ice borehole wall, leading to continuous deflection and a decline in drilling accuracy. To tackle this problem, the surface structure of the thermal sonde needs to be optimized. Organisms also encounter adhesion challenges at wet interfaces. For instance, earthworms have evolved a segmented body with a lubricating fluid on their surface to adapt to the soil environment [21]. The segmented, non-smooth grooved structure on the earthworm body surface decreases the contact angle, enhances hydrophobicity, reduces the contact area, and minimizes viscous friction [22; 23]. Inspired by this concept, a bionic surface with spiral grooves (Figure 3) was meticulously fabricated on a lateral heating tube. The grooves are designed with a height (h) of 2.5 mm and width (w) of 2 mm. This bionic surface structure effectively curtails the wet adhesion at the interface between the bionic surface, water, and ice. As a result, the likelihood of the lateral heating tube adhering to the borehole wall is substantially minimized. Moreover, a copper-nickel alloy wire was embedded in a double spiral winding within the grooves to prevent electromagnetic interference with the downhole sensors and control system during the activation of the lateral heaters. Based on lateral heater tests, when the ice temperature is \(-30\)\({}^{\circ}\)C, the power density needs to exceed 0.10 W/cm\({}^{2}\) to maintain the external water circumstance. The inner winch converts electrical energy into mechanical energy. It enables the sonde to descend or ascend steadily at a pre-determined speed by means of cable collection or release. To counterbalance the weight of the sonde, a multi-groove wheel-climbing mechanism was designed, which makes use of the frictional force between the driving wheel and cable. This mechanism effectively lessens the tension in the cable, thereby minimizing the structural strength and power requirements of the cable coil. The cable coil is driven by two separate motors, one motor is responsible for driving the drum, and the other drives the level winder, maintaining a fixed ratio. The controllable accuracy of the weight on bit (WOB) can be adjusted within the range of \(\pm 10\) kg, and because of this, the sonde penetration rate can be smoothly regulated between 0 and 10 m/h. The synthetic fiber armored cable serves a dual function: it supplies power and transmits signals to the sonde, and simultaneously provides the requisite strength to suspend the sonde. In the development process of the RECAS, the cable is one of the most critical components. Cable parameters such as diameter, minimum bending radius, working voltage, current, strength, and wear resistance, among other structural parameters, significantly influence the sonde's structural design.