The viscous Fluid Mechanical Particle Barrier for the prevention of sample contamination on the Mars 2020 mission

The Mars 2020 mission will land a rover on the surface of Mars that will acquire, encapsulate, and cache scientifically selected samples of martian material for possible return to Earth by a future mission. The samples will be individually encapsulated and sealed in sample tubes. Each sample, and therefore each sample tube, must be kept clean of viable organisms with a terrestrial origin, which may adhere to the rover on their own and/or on other abiological particles. It is shown that contamination of the tubes by such terrestrial remnant particles as small as 0.15 mu m on the rover will be prevented using the Fluid Mechanical Particle Barrier (FMPB), a cylindrical enclosure within which each tube will be housed. The FMPB takes advantage of fluid viscosity to slow down the speed of the flow through a main thin annular orifice at the bottom of the device. An analytical solution of the fluid and particle dynamics in the FMPB has been developed and validated using 2-D and 3-D CFD simulations. Water tunnel tests have also been conducted that demonstrate the effectiveness of the FMPB to slow down the fluid through the orifice. It is found that for the flow speeds expected at the various phases of the mission, penetration of the smallest particles is not expected to exceed 10% of the orifice height. No penetration of particles >5 mu m is expected inside the orifice. Large margins on the already low contamination probability of the tubes are allowed by the presence of a large-volume cavity immediately downstream of the long annular orifice. The cavity further slows down the expanding flow and, in turn, minimizes particle penetration even at the most extreme conditions expected on Mars, For example at wind speeds of 75 m/s, characteristic of the largest and rarest dust devils that can form on Mars, 0.15-mu m particles are not expected to exceed a height larger than 3% of the cavity.