Processing of analogues of plume fallout in cold regions of Enceladus by energetic electrons
Processing of analogues of plume fallout in cold regions of Enceladus by energetic electrons
复制标题
DOI：
10.1051/0004-6361/201423546
复制
发表时间：
2014-10-01
影响因子：
6.5
通讯作者：
Fraser, H. J.
中科院分区：
文献类型：
--
作者：
Bergantini, A.;Pilling, S.;Fraser, H. J.
上下文。土卫二是土星的一颗冰冷的小卫星，是太阳系中最引人注目的天体之一。这颗卫星是一个地质活跃的天体，尽管其大部分表面温度较低，但地热加热的南极地区存在间歇泉，喷出由水（约90%）、二氧化碳、甲烷、氨和甲醇以及其他分子组成的羽流。大多数向上运动的粒子没有足够的速度逃离月球的引力影响而落回月球表面。冰中的分子不断地暴露在电离辐射中，如紫外线和x射线光子、宇宙射线和电子。随着时间的推移，电离辐射促进分子键断裂，破坏和形成分子、自由基和碎片。我们分析了在寒冷的地表地区（北极）用1 keV电子处理类似于土卫二沉降冰的冰混合物。主要目标是寻找尚未在这颗卫星上发现的复杂物种，并确定相关的物理化学参数，如冰中所研究分子的破坏和形成横截面以及半衰期。实验是在20 K的超高真空室内对一种类似土卫二的冰混合物（H2O: CO2: CH4: NH3: CH3OH）进行电子辐照。采用红外光谱法在中红外区域（4000 ~ 800 cm（-1）或2.5 ~ 12.5 μ m）进行分析。测定了在模拟土卫二辐照条件下，亲本分子的绝对解离截面、子代物质的形成截面和亲本物质的半衰期。在产种中，初步检测到CO（一氧化碳）、OCN-（氰酸阴离子）、HCONH2（甲酰胺）和H2CO（甲醛）。
Context. Enceladus, a small icy moon of Saturn, is one of the most remarkable bodies in the solar system. This moon is a geologically active object, and despite the lower temperatures on most of its surface, the geothermally heated south polar region presents geysers that spouts a plume made of water (similar to 90%), carbon dioxide, methane, ammonia, and methanol, among other molecules. Most of the upward-moving particles do not have the velocity to escape from the gravitational influence of the moon and fall back to the surface. The molecules in the ice are continuously exposed to ionizing radiation, such as UV and X-rays photons, cosmic rays, and electrons. Over time, the ionizing radiation promotes molecular bond rupture, destroying and also forming molecules, radicals, and fragments.Aims. We analyse the processing of an ice mixture analogue to the Enceladus fallout ice in cold resurfaced areas (north pole) by 1 keV electrons. The main goal is to search for complex species that have not yet been detected in this moon, and to determine relevant physico-chemical parameters, such as destruction and formation cross-sections and the half-life of the studied molecules in the ice.Methods. The experiment consisted of the electron irradiation of an Enceladus-like ice mixture (H2O: CO2: CH4: NH3: CH3OH) in an ultra-high vacuum chamber at 20 K. The analysis was made by infrared spectrometry in the mid-infrared region (4000-800 cm(-1) or 2.5-12.5 mu m).Results. The absolute dissociation cross-sections of the parent molecules, the formation cross-section of daughter species, and the half-life of the parental species in a simulated Enceladus irradiation scenario were determined. Among the produced species, CO (carbon monoxide), OCN- (cyanate anion), HCONH2 (formamide), and H2CO (formaldehyde) were tentatively detected.