Quadrupole Ion Trap Mass Spectrometer for Ice Giant Atmospheres Exploration

To date, a variety of dierent types of mass spectrometers have been utilized on missionsto study the composition of atmospheres of solar system bodies, including Venus, Mars,Jupiter, Titan, the moon, and several comets. With the increasing interest in future smallprobe missions, mass spectrometers need to become even more versatile, lightweight, compact,and sensitive.For in situ exploration of ice giant atmospheres, the highest priority composition measurementsare helium and the other noble gases, noble gas isotopes, including 3He/4He, andother key isotopes like D/H. Other important but lower priority composition measurementsinclude abundances of volatiles C, N, S, and P; isotopes 13C/12C, 15N/14N, 18O/17O/16O;and disequilibrium species PH3, CO, AsH3, GeH4, and SiH4. Required measurement accuraciesare largely dened by the accuracies achieved by the Galileo (Jupiter) probe NeutralMass Spectrometer and Helium Abundance Detectors, and current measurement accuraciesof solar abundances [1].An inherent challenge of planetary entry probe mass spectrometers is the introduction ofmaterial to be sampled (gas, solid, or liquid) into the instrument interior, which operates ata vacuum level. Atmospheric entry probe mass spectrometers typically require a speciallydesigned sample inlet system, which ideally provides highly choked, nearly constant mass-ow intake over a large range of ambient pressures. An ice giant descent probe would haveto operate for 1-2 hours over a range of atmospheric pressures, possibly covering 2 or moreorders of magnitude, from the tropopause near 100 mbar to at least 10 bars, in an atmosphericlayer of depth beneath the tropopause of about 120 km at Neptune and about 150 km atUranus.The Jet Propulsion Laboratory's Quadrupole Ion Trap Mass Spectrometer (QITMS) [2] isbeing developed to achieve all of these requirements. A compact, wireless instrument with amass of only 7.5 kg, and a volume of 7 liters (7U), the JPL QITMS is currently the smallestight mass spectrometer available for possible use on planetary descent probes as well assmall bodies, including comet landers and surface sample return missions. The QITMS iscapable of making measurements of all required constituents in the mass range of 1 - 600atomic mass units (u) at a typical speed of 50 mass spectra per second, with a sensitivityof up to 1013 counts/mbar/sec and mass resolution of m=m = 18000 at m/q=401. The QITMS features a novel MEMS-based inlet system driven by a piezoelectric actuator thatcontinuously regulates gas ow at inlet pressures of up to 100 bar.In this paper, we present an overview of the QITMS capabilities, including instrument designand characteristics of the inlet system, as well as the most recent results from laboratorymeasurements in dierent modes of operation, especially suitable for ice giant atmospheresexploration.