## 4 ARCHES LIBS Module as LRU2 Payload  
We integrated the LIBS module as a modular payload of the lightweight rover unit (LRU) for increased flexibility of the measuring possibilities. The manipulator of the rover can dock to the payload and position the instrument on samples in the reachable workspace: on rock-bed, walls and even on the ceilings of caves. For laser-safety reasons, however, in the ARCHES mission scenario, the LIBS module was only positioned in a way that the laser was targeting downwards. The entire process of instrument deployment is fully autonomous [40]. The LRU is equipped with two stereo systems that capture depth information around the system. The images are segmented into individual objects using a neural network [41] and transmitted to mission control for further analysis. Mission control selects a rock in the image, and the algorithms on the rover calculate various instrument positions on the sample with two constraints: (1) the contacted rock surface is flat, and (2) it can support the instrument during the measuring process. The manipulator docks to the LIBS instrument on the rover's carrier, and the planning algorithms compute and execute a collision-free motion to the target position. A Cartesian impedance controller monitors the final contact motion with the sample to ensure adequate forces during deployment. Once in position, the rover queries the control software of the instrument to trigger the measurement as described in Section 2. For immediate feedback on the measurement quality, the instrument software computes a signal-to-noise ratio of the measurement. After the measurement, the manipulator retracts the instrument and stores it in the rover payload carrier. The recorded data are downloaded from the instrument to the rover and uploaded to mission control. Due to the supported autonomy, the system is capable of recording LIBS data for approximately five to seven different samples per hour. The execution of the LIBS measurement task followed the modular approach presented in [30]. With several hardware and software modular components integrated into both the LRU and the payload module, this strategy provided versatility and robustness to the ARCHES scientific mission. The use of the LIBS module represents a strong application of the guiding paradigm of the LRUs, i.e., a universal, lightweight rover specialized in navigation, exploration, and manipulation of payloads and tools. This paradigm contradicts today's approach to rover-based planetary exploration, which involves large and heavy mobile systems equipped with permanently installed sub-components to cover their complete mission extent. The increasing number of components, however, escalates the system's complexity, launch cost, energy consumption, and the risk of failures. A key element that enables the LRU to fulfill a multi-role capability during its missions by handling payloads and tools is the so-called ENVICON docking interface, initially developed during the ROBEX project [42]. The docking interface is used together with a Kinova Jaco Manipulator Arm capable of manipulating 2.5 kg. Its diameter is 102 mm, and it is designed around the manipulator arm's tool center point (TCP) in such a way that the distance between the payload's center of gravity and the front face of the TCP is kept as short as possible. As depicted in Figure 7, the docking interface makes use of a set of flexible sheet metal springs that perform a transition from a more position-tolerant pre-docking regime to a precise and stable fixation. Therefore, the springs open up in front of the barrel-shaped docking interface facing towards their coupling partner. At the beginning of the docking cycle, they form a funnel-like structure that promotes a safe coupling approach (see Figure 7). Within the axial center of both coupling partners, a coaxial connector (male and female counterpart) is situated, enabling the host system to provide power and communication to the payload.