Sampling Accelerated Micron Scale Ice Particles with a Quadrupole Ion Trap Mass Spectrometer

Abstract

The Enceladus plume is a target of astrobiological interest in planetary science since it may carry signs of extraterrestrial life entrapped in ice grains formed from the subsurface ocean of this moon of Saturn. Fly-by mission concepts have been proposed to perform close investigations of the plume, including detailed in situ measurements of chemical composition with a new generation of mass spectrometer instrumentation. Such a scenario involves high-velocity collisions (typically around 5 km/s or higher) of the instrument with the encountered ice grains. Postimpact processes may include molecular fragmentation, impact ionization, and various subsequent chemical reactions that could alter the original material prior to analysis. In order to simulate Enceladus plume fly through conditions, we are developing an ice grain accelerator and have coupled it to the quadrupole ion trap mass spectrometer (QITMS) developed for flight applications. Our experimental setup enables the creation and acceleration of ice particles with well-defined size, charge, and velocity, which are subsequently directed into the QITMS, where they impact the surface of the mass analyzer and the analysis of postimpact, volatilized molecules takes place. In this work, we performed mass spectral analysis of ice grains of ca. 1.3 μm in diameter, accelerated and impacted at velocities up to 1000 m/s, with an upgrade of the accelerator in progress that will enable velocities up to 5000 m/s. We report the first observations of ice grain impacts measured by the QITMS, which were recorded as brief increases in the abundance of water molecules detected within the instrument.

Production and Impact Characterization of Enceladus Ice Grain Analogues

“…The AIS was previously used to generate ice particles for initial investigation of their spectral signatures using a quadrupole ion trap mass spectrometer developed at the Jet Propulsion Laboratory. 17 This capability to create water-ices will be reviewed here in brief. Highly charged water droplets were produced using ESI of deionized water before being injected into vacuum through an aerodynamic lens (ADL).…”

“…Pulses from each image charge detector trigger a set of high-voltage switches for each stage that are preprogrammed to accelerate particles based on precalculated pulse sequences using spectrometric information collected in the NET. 17 With the 41 element LINAC and 400 kV total acceleration potential, a particle diameter of ∼560 nm is needed to reach 5 km/s. Continued efforts to decrease the particle size are being performed to achieve the 5 km/s velocities desired for ice particle impact measurements.…”

“…The AIS was previously used to generate ice particles for initial investigation of their spectral signatures using a quadrupole ion trap mass spectrometer developed at the Jet Propulsion Laboratory . This capability to create water-ices will be reviewed here in brief.…”

“…Accelerated grains can then be injected into vacuum-compatible instruments for further studies and analysis. 17 Future missions have the potential to achieve a wide range of interaction velocities with plume-based particles. The orbital velocity of the spacecraft is usually significantly greater than the initial velocity achieved by the particles exiting the moon; 18 therefore, the interaction velocity of any sensor with the ejecta will be almost entirely dictated by the orbital trajectories used to sample the plume.…”

“…The particles produced with the ESI source are charged and can be accelerated to high velocities in a controlled manner using a linear accelerator. Accelerated grains can then be injected into vacuum-compatible instruments for further studies and analysis . Future missions have the potential to achieve a wide range of interaction velocities with plume-based particles.…”