54 demanded small surface roughness (Ra = 0.1 / 0.2 μm) for all surfaces in contact with the sample to  improve cleaning efficiency and reduce sample contamination by contact transfer.  

The third group of design driver s are a result of  the environ mental conditions on Mars  and represents one  of the main drivers . The envir onmental conditions impose several restriction on the design, such as the  operative temperature range of -60°C to +40°C, and the dry low -pressure CO 2 atmosphere. Contrary to  the sterile vacuum in which most space mechanisms operate, the sample processing produces a very  dusty environment, imposing many challenges for the mechanism’s  tribological elements. The dry  atmosphere causes additional triboelectric charging of the particles , which can cause them to stick to all  surfaces they come into  contact with. The UCZ is thus exposed to an extremely dirty (but  uncontaminated) env ironment during sample handling [2] . 

Another group of design drivers is concerned with the  properties of the powdered sample material . For  instance, the cementation of the sample could lead to a blocking and therefore to a loss of a major  part of  the scientific  ExoMars  mission. The main reasons for cementation are long storage time and contact of  the sample with humidity . Sample cementation can lead to the creati on of large agglomerates  of sample,   which could cause a clogging of  the system.  

Design Description of the PSDDS  

The PSDDS has the following main functions:   • Collection and delivery of sample material to three different points along a circumference  • Delivery of sample into two different receptacle types or waste containers   • Delivery of the sample in defined amounts   • Provide the possibility to store one sample and process a second sample in parallel   • Active prevention of  sample cementation   • If the main delivery system has a defect or clogged , the sample delivery system must remain  operative albeit  possibly with degraded performance  • Provide a system that can remove an entire sample on a specifi ed surface on the next  mechanism   • Sealing of the Ultra Clean Zone    The mechanism (see Figure 2, left)  is actuated by a brushed DC motor that is modified and qualified by  the manufacturer Maxon to work in  the environmental conditions on Mars [3]. It is coupled to a gear -box  on whose output shaft a pre-torque device is mounted , which divides the torque to two spur gears that are  pre-torqued to minimize the mechanism backlash. The counterparts of each spur gear are located on the  input shafts of two identical  planetary gear -boxes in a parallel arrangement and also developed and  adapted by Maxon.  The output s of these two gear -boxes are connected via a gear mechanism to an  internal ring gear , which transfers the torque into carousel motion. To allow integration of the drive- train  after UCZ closure the entire drive- train a nd harness are integrated onto their own frame. This allows its  introduction as an entire unit into th e integrated hub. The motors, potentiometer and piezo,  which are  placed in the movable part of the mechanism , has to be connected to the harness of the PSDDS.  Therefore,  the frame also includes a flexible harness  (Printed Circuit Board).  The rotary degree of  freedom (DOF) of the hub is provided by a wire race bearing and incorporates the dynamic feed- through,  which guarantees the sealing and encapsulation of the UCZ with the entire drive -train remaining outside  of it [2].   The Dosing Station is mounted on the outside of the rotating hub of the PSDDS structure. The central  elements for each single D osing  Station  (see Figure 2, right)  are the dosing wheel and the inlet and outlet  funnels. The dosing wheel is part of the dosing shaft , providing two dosing chambers  shifted by 180°. The  inlet funnel collects the sampl e from the Crushing Station and guides it into  one of dosing chambers.  When the d osing wheel is turned the sample falls out of the dosing chamber  and the outlet funnel guides  the defined sample powder  doses  to the receptacles on the PSHS.  This dosing wheel is driven by a