Shock Synthesis of Organics from Simple Ice Mixtures?

Introduction: Several recent announcements of: i) the discovery of glycine on the comet 81P/Wild-2/P [1, 2]; ii) the successful shock synthesis of prebiotic compounds from liquid targets [3] and, iii) the results of molecular dynamics simulations demonstrating that amino acids could be created via shock synthesis of ices [4], have prompted a sequence of impact experiments using a light gas gun (LGG) at the Univ. of Kent [5]. The purpose of these experiments is to attempt to synthesise organic compounds from a mixture of simple ices (CO2, NH3 and H2O). NH3 compounds, CO2 and H2O ices in the Saturnian system: According to observations made by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) instrument, Enceladus’ surface is composed mostly of nearly pure water ice except near its south pole, where there are light organics, CO2, and amorphous and crystalline water ice. The absorptions near 3.44 and 3.53 μm could be due to short-chain organics, but other features in the spectrum are still unidentified [5]. Remote IR acquired new high-resolution spectra of Iapetus, Tethys, Enceladus and Rhea that show the absorption feature of ammonia hydrate [6]. It thus seems probable that there are conditions on the surfaces of bodies in the Saturnian system where ammonium compounds, CO2 and water ice co-exist in a solid form. Impact of a bolide traveling with sufficiently high velocity onto such a surface, should impart enough energy to promote shock synthesis of more complex organic compounds, including amino acids, from these ices. Laboratory experiments: Targets were prepared as follows: CO2 ice (commercially purchased from BOC Ltd., stored in a freezer at a temperature of -130 °C) was repeatedly passed through a clean domestic ice crusher until the fragments’ largest dimension was no bigger than ~3 mm. The crushed CO2 ice was then placed in a lidded (135 mm x 95 mm x 110 mm) polystyrene box. 150 ml of previously chilled (-40° C) aqueous ammonia solution (Sigma-Aldrich, Cat. # 32, 014-5) was added, and the mix was shaken until the ammonia started to freeze, creating a well-mixed solid target in the box. The box was then returned to the low temperature freezer and cooled to -130° C. The targets remained in the freezer until the LGG was ready to fire, and then they were removed and placed in the target chamber which was then evacuated to ~30 mbar. The time taken from removal from the freezer to impact was approx. 20 minutes. In order to reduce target contamination from carbon-bearing gun debris during the shot, the (impact) target was wrapped in a layer of cling film (~10 μm thick). A blank control target was also placed in the target chamber (out of the direct line of impact) and left uncovered for comparitive analysis. Table 1. LGG shot parameters. Shot ID Projectile Dens. (g cm) Dia. (mm) Vel. (km s) G161009#1 SS 304 8.00 1.5 5.75