Sniffing the Enceladus Plume: The High Ice Flux Instrument (HIFI) Compositional Analyzer

The Cosmic Dust Analyzer (CDA) on the Cassini spacecraft has convincingly demonstrated the scientific value of mass spectra of ice particles ejected by the plume on Saturn's ice moon Enceladus. Trace amounts of organic and inorganic molecules embedded in ice particles revealed invaluable insight into the chemical composition of the ocean beneath the moon's icy crust. However, it became quickly obvious that to address open questions about the astrobiological nature of the ocean requires impact ionisation mass spectrometers with a considerably higher mass resolution than that of the CDA instrument of m/Δm ~ 50. Other CDA shortcomings include target cleanliness issues and the low detection cadence of 1 impact per second.

 

The High Ice Flux Instrument (HIFI) is a reflectron-type impact mass spectrometer specifically designed for such applications. It has a mass resolution of 1000 to 2000 and has been optimized for using the electronics of the Surface Dust Analyser instrument on Europa Clipper for recording the spectra. To ensure a high mass resolution HIFI has a long drift region and uses a set of electrostatic Einzel lenses to prevent the ion beam from diverging before entering the single stage reflectron region. The reflectron optics is composed of 23 precision machined electrostatic electrodes to guarantee a smooth reflecting field. In contrast to previous reflectron impact mass spectrometers such as CIDA enter the impacting particles the spectrometer through the reflectron to strike the target at a right angle. The target itself is a highly polished Titanium carrier coated with 250 nm of high purity Iridium (nm surface roughness). The high atomic mass of Iridium ensures that no target lines as well as target cluster lines appear in the mass range ≤ 200 u relevant for the compositional analysis of mineral and ice particles.

The instrument performance has been verified through the impact of metal particles at high velocities. Additionally, experiments were conducted with ice particles to illustrate the capacity of HIFI to discern minute quantities of salts and organics in the spectra of water ice.