Detection of the Amino Acid Histidine and Fragmentation Products in Hypervelocity Impact Ice Spectra: Implications for Flyby Mission Velocities

Impact ionization time of flight mass spectrometer (TOF-MS) instruments (such as the SUDA instrument on the Europa Clipper) study molecular composition of space-borne dust grains by impacting them at several km/s to ionize molecules for TOF-MS analysis. However, there have been questions about whether flyby impacts can shatter organics, if there is some critical velocity below which fragmentation is minimized, and if water ice reduces fragmentation. In particular, it would be beneficial to know if amino acids or other organics can be studied with this type of instrument during flybys of Enceladus or Europa. More generally, the ability of these instruments to assess the inventory and distribution of organics is important for understanding habitability across the solar system and the origins of terrestrial life.

Here we describe experiments using a novel airbrushing technique for ice creation at the Colorado Dust Accelerator. Experiments were performed with a pure surface of histidine mono-hydrochloride and with an identical pure surface with 60 nm of water ice deposited on top. These surfaces were kept at ~80 Kelvin and impacted by <2 um iron dust particles at velocities >3km/s. The resulting impact plumes were studied using TOF-MS.

A plot of the ion yields of breakup products relative to the parent molecule ions as a function of velocity is shown in Figure 1. Purple triangles denote bare histidine data, while blue circles denote histidine under 60 nm of water ice. The dotted lines represent linear fits to the first three data points, below 6 km/s. The solid lines represent linear fits to the high velocity data. Fragmentation is minimal below 6 km/s, with little variation between datasets. With bare histidine, the fragmentation rises significantly beyond 6.1 km/s, while ice-covered histidine shows significant increases beyond 8.5 km/s. These results imply that there is a critical flyby velocity between 6 and 8.5 km/s and that water ice layers reduce fragmentation rates, both in absolute terms and as a function of velocity. We further observe characteristic fragmentation products at 81-83 and 110 AMU, the dominant breakup products associated with histidine in existing literature. This would indicate that fragmentation products may be used to enhance detection of organics when they are observed alongside the parent molecule.