Aperture Valve for the Mars Organic Molecule Analyzer (MOMA) Charles Engler*and John Canham** Abstract NASA's participation in the multi-nation ExoMars 2018 Rover mission includes a critical astrobiology  Mass Spectrometer Instrument on the Rover called the Mars Organic Molecule Analyzer (MOMA). The  Aperture Valve is a critical electromechanical valve used by the Mass Spectrometer to facilitate the  transfer of ions from Martian soil to the Mass Spectrometer for analysis. The MOMA Aperture Valvedevelopment program will be discussed in terms of the initial valve design and subsequent improvements that resulted from prototype testing. The initial Aperture Valve concept seemed promising, based on calculations and perceived merits. However, performance results of this design were disappointing, due  to delamination of TiN and DLC coatings applied to the titanium base metals, causing debris from the coatings to seize the valve. While peer reviews and design trade studies are important forums to vet a concept design, results from testing should not be underestimated. Despite the lack of development progress to meet requirements, valuable information from weakness discovered in the initial Valve design was used to develop a second, more robust Aperture Valve. Based on a check-ball design, the ETU / flight valve design resulted in significantly less surface area to create  the seal. Moreover, PVD coatings were eliminated in favor of hardened, non-magnetic corrosion resistant alloys. Test results were impressive, with the valve achieving five orders of magnitude better sealing leak rate over end of life requirements. Cycle life was equally impressive, achieving 280,000 cycles without failure. Introduction By studying organic molecules (the chemical building blocks of life as we know it), MOMA is designed to  help answer questions about whether life existed on Mars, as well as its potential origin, evolution, and distribution on the Red Planet. The MOMA Instrument is principally a mass spectrometer used to detect  the masses and relative concentrations of atoms and molecules in a substance. MOMA is designed to analyze the types and amounts of chemicals that make up organic and inorganic compounds found in  rock and soil samples on Mars. The instrument will address a top science goal seeking the signs of past  or present life on Mars. Since the surface of Mars is bathed in ultraviolet and cosmic radiation, complex organics in the uppermost surface layers could be degraded. The 2018 Rover will be equipped with a drill to obtain samples from as deep as 2 meters in the hope that organics at that depth would still be viable. Core samples of Martian soil will be ablated by a UV laser, producing ions which pass thru the Aperture  Valve and are subsequently trapped and analyzed by the Mass Spectrometer to determine the molecular  composition of the ion, which then determines the molecular composition of the sample. The MOMA Mass Spectrometer illustrated in Figure 1. Principle components are the Mass Spectrometer  Housing, 266-nm UV Laser Head, and Aperture Valve. The Martian sample is moved to the ultra-clean zone directly below the Aperture Valve. A laser mounted onto the Mass Spectrometer Housing creates a focused beam of energy which oblates the soil and ionizes soil particles. The valve opens, allowing ions to flow into the Mass Spectrometer, then quickly closes. Ions are trapped by a Linear Ion Trap, scanned for molecular composition, then ejected.  *NASA Goddard Space Flight Center, Greenbelt, MD **ATK Space Systems, Beltsville, MD 211