Murthy S. Gudipati

New experimental capability to investigate the hypervelocity micrometeoroid bombardment of cryogenic surfaces

Ice is prevalent throughout the solar system and beyond. Though the evolution of many of these icy surfaces is highly dependent on associated micrometeoroid impact phenomena, experimental investigation of these impacts has been extremely limited, especially at the impactor speeds encountered in space. The dust accelerator facility at the Institute for Modeling Plasmas, Atmospheres, and Cosmic Dust (IMPACT) of NASAs Solar System Exploration Research Virtual Institute has developed a novel cryogenic system that will facilitate future study of hypervelocity impacts into ice and icy regolith. The target consists of a copper block, cooled by liquid nitrogen, upon which layers of vapor-deposited ice, pre-frozen ice, or icy regolith can be built in a controlled and quantifiable environment. This ice can be grown from a variety of materials, including H2O, CH3OH, NH3, and slurries containing nanophase iron. Ice temperatures can be varied between 96 K and 150 K and ice thickness greater than 150 nm can be accurately measured. Importantly, the composition of ion plumes created during micrometeoroid impacts onto these icy layers can be measured even in trace amounts by in situ time-of-flight mass spectroscopy. In this paper, we present the fundamental design components of the cryogenic target chamber at IMPACT and proof-of-concept results from target development and from first impacts into thick layers of water ice.

Simulation of Titan’s atmospheric photochemistry - Formation of non-volatile residue from polar nitrile ices

We studied the photochemistry of frozen ice of a polar Titan’s atmospheric molecule cyanodiacetylene (HC5N) to determine the possible contribution of this compound to the lower altitude photochemistry of haze layers found on Titan. We used infrared analysis to examine the residue produced by irradiation of solid HC5N at > 300 nm. The resulting polymer is orange-brown in color. Based on theoretical analysis and the general tendency of HC5N and C4N2 to undergo similar ice photochemistry at longer wavelengths accessible in Titan’s lower atmosphere, we conclude that Titan’s lower atmosphere is photochemically active in the regions of cloud, ice, and aerosol formation. C4N2 is a symmetric molecule with no net dipole moment whereas, HC5N has a large dipole moment of 4 D. Consequently, though both these molecules have very similar molecular weight and size, their sublimation temperatures are di erent, HC5N subliming around 170 K compared to 160 K for C4N2. Based on our studies we conclude that in Titan’s atmosphere the cyanoacetylene class of molecules (HCN, HC3N, HC5N, etc.) would condense first followed by the dicyanoacetylenes (C2N2, C4N2, C6N2, etc.), leading to fractionation of di erent class of molecules. From the fluxes used in the laboratory and depletion of the original HC5N signals, we estimate Titan’s haze ice photochemistry involving polar nitriles to be significant and very similar to their non-polar counterparts.

Macromolecular organic compounds from the depths of Enceladus

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.The detection of complex organic molecules with masses higher than 200 atomic mass units in ice grains emitted from Enceladus indicates the presence of a thin organic-rich layer on top of the moon’s subsurface ocean.

Observing outer planet satellites (except Titan) with JWST: Science justification and observational requirements

The James Webb Space Telescope (JWST) will allow observations with a unique combination of spectral, spatial, and temporal resolution for the study of outer planet satellites within our Solar System. We highlight the infrared spectroscopy of icy moons and temporal changes on geologically active satellites as two particularly valuable avenues of scientific inquiry. While some care must be taken to avoid saturation issues, JWST has observation modes that should provide excellent infrared data for such studies.

The evolution of Titan’s high-altitude aerosols under ultraviolet irradiation

The Cassini–Huygens space mission revealed that Titan’s thick brownish haze is initiated high in the atmosphere at an altitude of about 1,000u2009km, before a slow transportation down to the surface. Close to the surface, at altitudes below 130u2009km, the Huygens probe provided information on the chemical composition of the haze. So far, we have not had insights into the possible photochemical evolution of the aerosols making up the haze during their descent. Here, we address this atmospheric aerosol aging process, simulating in the laboratory how solar vacuum ultraviolet irradiation affects the aerosol optical properties as probed by infrared spectroscopy. An important evolution was found that could explain the apparent contradiction between the nitrogen-poor infrared spectroscopic signature observed by Cassini below 600u2009km of altitude in Titan’s atmosphere and a high nitrogen content as measured by the aerosol collector and pyrolyser of the Huygens probe at the surface of Titan.How does Titan’s thick brownish haze chemically evolve as it is transported from the upper atmosphere observed by Cassini to the lower regions sampled by Huygens? Laboratory vacuum ultraviolet experiments may explain the observed changes in nitrogen chemistry.

Photoinduced Reversible Electron Transfer Between the Benzhydryl Radical and Benzhydryl Cation in Amorphous Water–Ice

The benzhydryl radical is generated in high yields by flash-vacuum thermolysis of 1,1,2,2-tetraphenylethane with subsequent trapping of the product in argon or amorphous water at 3-4 K. Photoionization of the radical with various UV lights and electron sources produces the benzhydryl cation, which was identified by IR and UV-vis spectroscopy. In solid argon, the formation of the benzhydryl cation is irreversible, whereas in amorphous water-ice the electron transfer is reversible, and irradiation into the major absorption band at 443 nm of the cation leads back to the radical by electron attachment. Applications of ionization of organic matter trapped in water-ice to icy environments in astrophysics and planetary sciences, including Earth, are discussed.