Laboratory Study of Extraterrestrial Ices Electrical Properties and Interaction with Irradiation.

摘要

This dissertation describes experimental efforts intended to investigate various physical and chemical processes relevant to the ice coated objects in the outer solar system and interstellar medium. These cold icy surfaces are constantly bombarded by energetic ions, electrons, UV photons and micrometeorites, which alter the physical and chemical properties of the surfaces. Experimental investigations on laboratory analogs are useful to accurately interpret the astronomical observations.;Ultra high vacuum, UV photons/electrons/ions irradiation and low temperature prepared ices are usual ways to simulate the space environment. We present an improved method to better simulate the radiation environment, by maintaining an ambient pressure of relevant molecules during irradiation. This ambient gas mimics the tenuous molecular atmospheres surrounding icy objects, such as the O2 exospheres of Jovian satellites Europa and Ganymede and Saturn's icy Rings and satellite Rhea, formed from sputtering/sublimating of radiolysis and photolysis products near the surface. The coexistence of ambient gas and energetic magnetospheric ion / UV photon irradiation leads to enhanced oxygen adsorption in the nanoporous ice films (T 70K), due to ion induced pore collapse. The high release temperatures of O2 molecules (T > 140K) helps explain the detected solid O2 on Jovian satellites, Europa, Ganymede and Callisto at surface temperatures high enough to sublime solid O2. Besides, we found that the environment analog leads to new mechanisms in molecule synthesis. Oxygen enrichment due to ion-dissociated ambient O2 results in enhanced H2O2 production, while hydrogen enrichment from UV photon-dissociated H2 within ice+H2 mixtures suppresses the synthesis of H2O 2. These radiation chemical processes may help understanding the origin of detected extraterrestrial molecules.;Moreover, we researched on the electrostatic charging/discharging effect of ices due to ion bombardment. Ice films can be charged to a surface potential >200 Volts. Further charging of the ice is limited by the dielectric strength of the ices. The findings help characterizing the surface potential of the icy objects which, if high enough, may reflect low energy magnetospheric ions.