10 keV Electron Irradiation of Methane Ices at Ocean World Surface Temperatures

The surface environmental conditions of many of the icy ocean worlds in the Solar System are outside of the stability field of methane ice. However, mechanisms may exist that entrain or trap methane within water ice that arrives at the surface via plume eruptions or resurfacing events. At the surface, the ice grains may experience charged particle irradiation that could lead to the production of new complex hydrocarbons. We investigated the irradiation products of 12CH4 versus 13CH4, the temperature dependence of CH4 radiation processing and product stability, and the temperature stability of CH4 when mixed with H2O in a cryogenic vacuum environment. Significantly, methane ice experimentally encased in water ice was irradiated at 100 K in an ultra-high vacuum environment and produced similar radiation-formed secondary phases as pure methane ice irradiated at 20 K. The methane radiation products produced outside of the methane ice stability field, while encased in a water ice film, were most notably 13CO2 and a candidate feature of C3H6. Evidence for complex hydrocarbons, such as C2H6, were observed during warming and sublimation of the sample.

太阳系内诸多冰封海洋天体的表面环境条件，均处于甲烷冰（methane ice）的稳定区间之外。不过，仍可能存在相关机制，可通过羽流喷发（plume eruptions）或表面重塑事件（resurfacing events）将甲烷裹挟或捕获于抵达表面的水冰（water ice）之中。在天体表面，冰颗粒可能会经受带电粒子辐照（charged particle irradiation），进而生成新型复杂烃类（complex hydrocarbons）。本研究针对12CH4与13CH4的辐照产物、甲烷辐射加工的温度依赖性及其产物稳定性，以及低温真空环境（cryogenic vacuum environment）下CH4与H2O混合时的温度稳定性展开了系统探究。值得注意的是，本实验中将被水冰包裹的甲烷冰置于超高真空（ultra-high vacuum）环境下于100 K进行辐照，其生成的辐射次生相与20 K下辐照的纯甲烷冰所生成的辐射次生相基本一致。当被包裹于水冰薄膜中时，在甲烷冰稳定区间之外生成的甲烷辐射产物中，最具代表性的为13CO2以及一种疑似C3H6的特征信号。在样品升温和升华（sublimation）过程中，研究人员观测到了诸如C2H6这类复杂烃类的存在证据。