Showing papers by "J. Hunter Waite published in 2018"

Macromolecular organic compounds from the depths of Enceladus

Abstract: Saturn’s moon Enceladus harbours a global water ocean1, which lies under an ice crust and above a rocky core2. Through warm cracks in the crust3 a cryo-volcanic plume ejects ice grains and vapour into space4–7 that contain materials originating from the ocean8,9. Hydrothermal activity is suspected to occur deep inside the porous core10–12, powered by tidal dissipation13. So far, only simple organic compounds with molecular masses mostly below 50 atomic mass units have been observed in plume material6,14,15. Here we report observations of emitted ice grains containing concentrated and complex macromolecular organic material with molecular masses above 200 atomic mass units. The data constrain the macromolecular structure of organics detected in the ice grains and suggest the presence of a thin organic-rich film on top of the oceanic water table, where organic nucleation cores generated by the bursting of bubbles allow the probing of Enceladus’ organic inventory in enhanced concentrations.

Microchannel Plate Detection Efficiency to Monoenergetic Photons Between 0.66 and 20 MeV

Abstract: Southwest Research Institute investigated the response of a microchannel plate (MCP) detector to isotropic radioactive source emissions at photon energies of 0.662 and approximately 1.25 million electronvolts (MeV) and to a beam of monoenergetic photons (gamma rays) at 2.5, 5, 7, 10, 13, and 20 MeV in the Free Electron Laser Laboratory at Duke University. These measurements were performed to quantify anticipated noise levels of a mass spectrometer instrument for space exploration in a harsh radiation environment and included various incident angles of radiation on the MCP. Measured photon detection efficiencies at 0° incident angle to 0.662 and 1.25 MeV were approximately 0.3%–0.4% and are bracketed by previously published data. In the 2.5- to 20-MeV energy range for which comparable published data are not available, measured detection efficiencies were on the order of 0.02%–0.2%. Radiation transport simulations were compared to the experimental results and showed decent agreement. The measured detection efficiency increased as the incident photon angle was changed from being normal to the MCP surface (0°) to being aligned with the MCP edge (90°). At greater off-axis angles, photons were incident on the side of the detector cartridge and generated secondary radiation from photon interactions in the cartridge materials that subsequently registered MCP counts.

Limits on the Contribution of Endogenic Radiolysis to the Presence of Molecular Oxygen in Comet 67P/Churyumov-Gerasimenko

Abstract: Radiolytic production has been proposed as a potential source for the molecular oxygen observed in comet 67P/Churyumov-Gerasimenko. Radiolysis can be exogenic or endogenic, the latter due to radionuclides present in the dust constitutive of the comet nucleus. We investigated the possibility of forming a significant amount of molecular oxygen through endogenic radiolysis. We applied a model of radiolytic production, developed for an Earth rock-water mixture, and improved it to account for the effect of the size of a radionuclide-bearing grain on the net radiation deposited in its ice mantle. We calculated the possible production of molecular oxygen considering the available experimental values of radiolytic yields. We found that endogenic radiolysis cannot account for the totality of the 3.8% (relative to water) O2 abundance derived from the ROSINA observations, with an end member case of our model producing at most a 1% abundance. By contrast, we predict H2O2 production leads to an abundance up to two orders of magnitude above observed values.