Quadrupole Ion Trap Mass Spectrometer for ice giant atmospheres exploration

To date a variety of different types of Mass Spectrometers has been utilized on missions to study the composition of atmospheres of many solar system bodies including Venus, Mars, Jupiter, Titan, the moon and several comets. For in-situ exploration of ice giant atmospheres, the highest priority composition measurements are helium and the other noble gases, noble gas isotopes, and other key isotopes including ³He/⁴He and D/H. Other important but lower priority composition measurements include 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 accuracies are largely defined by the accuracies achieved by the Galileo (Jupiter) probe Neutral Mass Spectrometer and Helium Abundance Detectors, and current measurement accuracies of solar abundances^[1].

The Jet Propulsion Laboratory’s Quadrupole Ion Trap Mass Spectrometer (QITMS)^[2] is a compact, wireless instrument with a mass of only 7.5 kg, designed to meet these requirements and challenges specific to the planetary probe missions. It is currently the smallest flight MS available, capable of making measurements of all required constituents in the mass range 1-600Da, with a sensitivity of up to 10¹³ counts/mbar/sec and resolution of m/∆m=12000 at 40Da.

During a fly-by or a descent mission, the time available to perform an in-situ measurement is usually short. This makes it challenging to measure the abundances of minor constituents for which long integration times are needed. Mass spectrometers largely employ a non-discriminatory electron impact ionization of sampled gas mixtures for creating ions, which means the probability to create and trap ion fragments of trace species is very low and further destabilized by space charge effects due to an excessive number of ions from dominant species. A selective resonant ejection technique was employed to lower the amount of major constituent species, while keeping the minor constituents intact, which resulted in higher accuracy measurements of minor species.

Another inherent challenge of planetary entry probe mass spectrometers is the introduction of material to be sampled into the instrument interior, which operates at vacuum. Atmospheric entry probe mass spectrometers typically require a specially designed sample inlet system, which ideally provides highly choked, nearly constant mass-flow intake over a large range of ambient pressures. An ice giant descent probe would have to operate over a range of atmospheric pressures covering 2 or more orders of magnitude, 100 mb to 10+ bars, in an atmospheric layer of ~120 km at Neptune to ~150 km at Uranus. The QITMS features a novel MEMS based inlet system driven by a piezo-electric actuator that continuously regulates gas flow at inlet pressures of up to 100 bar.

In this paper, we present an overview of the QITMS capabilities including instrument design and characteristics of the inlet system, as well as the most recent results from laboratory measurements in different modes of operation.

[1] Mousis, O., et al., Pl. Sp. Sci., 155 12–40, 2018.

[2] Madzunkov, S.M., Nikolic, D., J. Am. Soc. Mass Spectrom. 25(11), 2014.