Interpreting Impact Ionization Spectra From Icy Worlds

Impact ionization time-of-flight (TOF) mass spectrometry has been shown to be a powerful in situ technique for analyzing the composition of icy bodies in the outer Solar System, providing information inaccessible to remote sensing alone. Instruments such as the SUrface Dust Analyzer (SUDA) aboard Europa Clipper will sample ice grains in Europa’s exosphere, enabling compositional mapping of Europa's surface. Interpretation of these measurements is complicated by the physics of hypervelocity impacts, as fragmentation pathways, ion clustering, and relative ion yields depend strongly on both impact velocity and target composition. Reliable interpretation of spacecraft data, including measurements from Cassini’s Cosmic Dust Analyzer, therefore requires high-fidelity laboratory analogue experiments.

Here we present laboratory TOF mass spectra generated from hypervelocity dust impacts into cryogenic ice targets using the dust accelerator facility at the University of Colorado Boulder. These experiments produce reference spectra from known ice compositions that serve as analogue datasets for spaceborne measurements. The materials investigated include ammonia ices, carbon dioxide ices, and salt-rich water ices, all of which have been proposed as relevant to Europa’s icy shell.

A central advancement of this work is the use of isotopically labeled ice compositions to reduce ambiguity in spectral interpretation. By generating spectra from ¹⁵NH₃ and ¹³CO₂ ices, predictable mass shifts are observed in diagnostic ion and cluster peaks originating from the impacted material. These shifts provide direct confirmation of molecular contributions within complex spectra and significantly improve confidence in peak assignments.

The experiments were conducted by accelerating micron-scale dust particles to velocities of 1-50 km s^-1 and impacting them into thin ice films grown under ultra-high vacuum on cryogenically cooled substrates. Volatile ices were deposited from high-purity gases. And salt-rich water ices were produced from aerosolized liquid particles using novel operating conditions. Impact-generated plasmas were analyzed using TOF mass spectrometry. Isotopically labeled datasets were used to confirm molecular assignments via predictable mass shifts.

These results demonstrate that laboratory analogue measurements can reproducibly capture composition-dependent features in impact ionization spectra across a wide range of relevant velocities. Ammonia-bearing ices provide a useful test case for isotopic validation. Our early results indicate that this approach is broadly applicable to diverse icy-world compositions. Ongoing and future work will expand this reference database to additional salts and isotopically marked water ice, chemical compositions relevant to Europa and other ocean worlds.