Ion-Optical Design of a Mass Spectrometer for Analyzing Complex Molecules during Fast Flybys

Future deep space missions visiting the moons Io or Enceladus plan to analyze these objects' tenuous upper atmospheres to detect complex (bio-) molecules. Furthermore, the sampling of eruption plumes could reveal detailed information on subsurface chemical and biological processes. However, many planned or proposed mission designs foresee high flyby velocities on these objects, typically exceeding 5 km/s and amounting up to 20 km/s, referred to as hypervelocity. These high relative encounter velocities with respect to the atmospheric gas complicate the detection and unambiguous identification of complex molecules using state-of-the-art mass spectrometers due to molecular fragmentation caused by hypervelocity impact induced bond-dissociation. Additional ambiguity is caused by the constant presence of gas outgassing from the spacecraft, as this introduces an undesired background to the measurements that challenges the assignment of each compound to its origin, i.e., the spacecraft or the exosphere. Here, we present the ion-optical design of a time-of-flight mass spectrometer, referred to as OpenTOF, using a novel gas inlet system where species enter the mass analyzer on a direct trajectory, without any surface contact, at velocities up to 20 km/s. This prevents hypervelocity impact induced fragmentation. Furthermore, the novel gas inlet system causes both the mass resolution of the instrument and the flight times of ions to depend on the velocity with which the species enter the ion-optical system. As a consequence, OpenTOF provides the unique capability to separate species originating from spacecraft outgassing from the gas originating from the object's exosphere. The optimized ion-optical system design covers a mass range of m/z 1 to 800 and provides a mass resolution up to m/Δm = 1,000 (full width at half maximum). OpenTOF is comparable in size (250 mm characteristic length), weight (3.2 kg + 3 kg shielding), and performance (6 decades dynamic range in 10 s, about 12 W) to the Neutral and Ion Mass spectrometer (NIM) launched on-board ESA's Jupiter Icy Moons Explorer (JUICE/ESA), thanks to heritage of its electronics. Thanks to its two novel features, a contactless ion inlet at hypervelocity, and the capability to separate spacecraft background, OpenTOF will be able to perform highly reliable and unambiguous composition measurements of tenuous upper atmospheres and plumes, providing key data to improve our understanding of the status, origin, and evolution of the Solar System.