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 Analyzer 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 (5 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. 

Impact ionization mass spectrometers characterize the composition of microscopic particles hitting the instrument's metal target by recording Time-Of-Flight (TOF) mass spectra of the plasma created upon impact. The resulting TOF spectra show a strong dependence on the impact velocity. At very low speeds around 1 km/s, the spectra are dominated by lines of elements with very low ionization energy, mainly sodium and potassium. At velocities of a few km/s or more, the spectra are dominated by the mass lines associated with the impactor material and the target material.

Because dust impacts in interplanetary space are rare events, previous impact ionization mass spectrometers, such as the Surface Dust Analyzer (SUDA) on the payload of NASA's next flagship mission to the Jovian moon Europa, the Interstellar Dust Experiment (IDEX) on the payload of NASA's IMAP mission, or the Dust Detection System (DDS) on the payload of JAXA's Destiny+ mission to the asteroid Phaethon, have been optimized for a large sensitive area. However, there are dust environments where the dust number density is so high that the TOF mass spectra of successive impacts recorded by large area dust detectors begin to overlap. The Cosmic Dust Detector (CDA) onboard the Cassini spacecraft  recorded impact rates exceeding 1000 s^-1 in the ice particle plume of the Saturnian moon Enceladus (sensitive area ~36 cm²). Therefore, large area dust instruments are not capable of recording high quality mass spectra in dust-rich environments, such as cryovolcanoes on the Saturnian moon Enceladus  or comet trails. To prevent mass spectra from overlapping, instruments optimized for high impact rates must have a very small sensitive area.

The ideal ion optics for such an instrument is a classical reflectron optimized for impact plasma analysis. Each TOF spectrometer consists of an accelerator region, which provides all ions with the energy U_acc, and an ion detector at a distance L from the accelerator region - the drift length - to measure the flight times of the ions t = t₀ + a m^1/2, where t₀ is the ion launch time and a is the stretch parameter of the instrument. The initial thermal energy of an ion species of mass m leads to a spread Δt in their flight times, which decreases the resulting mass resolution m/Δm = t/2Δt of the instrument. A reflectron uses a repulsive electric field to reduce Δt. Ions launched with excess energy ΔU > 0 penetrate deeper into the repulsive field region and thus have a longer flight path than ions launched with zero initial energy, which partially compensates for the nonzero excess energy.

Cosmic dust particle impacts are a stochastic phenomenon, which means that there is no prior knowledge of either the impact or the ion launch time t₀. Therefore, techniques such as the extraction pulse method  for reducing the initial energy spread cannot be used in impact ionization mass spectroscopy.

Current state-of-the-art impact mass spectrometers have a mass resolution of 100 to 300, which is not sufficient to identify astrobiologically relevant materials such as amino acids embedded in ice grains. For example, in an impact mass spectrum of a shock-frozen ice grain from a 0.1 M NaCl solution containing 100 ppm argenine, which is a realistic assumption for the composition of the Enceladus ocean in the presence of life, argenine appears as the sodiated ion [Arg_Na]⁺ (m ~ 219.08 u), which is close to the salt cluster line [(NaCl)₂(NaOH)₂Na]⁺ (m ~ 218.89 u). The separation of the two lines requires a mass resolution of m/Δm \sim 219 u / 0.19 u = 1,153.

Here we report on the High Ice Flux Instrument (HIFI), which combines a mass resolution sufficient for the unambiguous identification of astrobiologically relevant traces of organic matter in ice particles with a small sensitive area. The instrument is a Time-Of-Flight (TOF) impact mass spectrometer with a single-stage reflectron ion optics, optimized for the acquisition of high-resolution mass spectra of ice particles during close spacecraft flybys through dust-rich environments.