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arXiv 1506.04673astro-ph.EP
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天王星大卫星上CO2冰的分布及其近表面成分分层的证据

Distribution of CO2 ice on the large moons of Uranus and evidence for compositional stratification of their near-surfaces

Richard J. Cartwright, Joshua P. Emery, Andy S. Rivkin, David E. Trilling, Noemi Pinilla-Alonso

摘要:

大型天王星卫星表面的特征是H2O冰和一种黑暗的、可能富含碳的成分以及CO2冰的混合物。在天王星系的平均日心距离上,天然的二氧化碳冰应该在比太阳系的年龄更短的时间尺度上被移除。因此,检测到的CO2冰可能是主动产生的。类似于木星和土星系统中冰冷卫星的辐射,我们假设天王星磁场中捕获的带电粒子轰击天王星卫星的表面,驱动辐射分解CO2生产循环。为了验证这一假设,我们通过分析这些卫星的近红外(NIR)光谱来研究CO2冰的分布,这些光谱是使用NASA红外望远镜设施(IRTF)(2000 - 2013)的SpeX光谱仪收集的。此外,我们使用斯皮策太空望远镜(2003 - 2005)上的红外阵列相机(IRAC)收集的图像进行分光光度测量。我们发现,检测到的CO2冰主要是在最接近天王星的卫星的尾半球上,与这些卫星的其他观测结果一致。我们的波段参数分析表明,检测到的CO2冰是纯的,从其他成分分离。我们的分光光度分析表明,IRAC是不敏感的CO2冰检测SpeX,可能是因为CO2被保留在一个薄的表面层,主要是水冰,是不透明的光子在IRAC波长。因此,我们结合SpeX和IRAC分析表明,近表面(即,顶部几个100微米)的天王星卫星是成分分层。我们简要地比较了在天王星卫星上检测到的CO2冰的光谱特征,以及其他地方的冰冷卫星,我们还考虑了观测到的CO2冰分布的最可能的驱动因素。

英文摘要:

The surfaces of the large Uranian satellites are characterized by a mixture of H2O ice and a dark, potentially carbon-rich, constituent, along with CO2 ice. At the mean heliocentric distance of the Uranian system, native CO2 ice should be removed on timescales shorter than the age of the Solar System. Consequently, the detected CO2 ice might be actively produced. Analogous to irradiation of icy moons in the Jupiter and Saturn systems, we hypothesize that charged particles caught in Uranus' magnetic field bombard the surfaces of the Uranian satellites, driving a radiolytic CO2 production cycle. To test this hypothesis, we investigated the distribution of CO2 ice by analyzing near-infrared (NIR) spectra of these moons, gathered using the SpeX spectrograph at NASA's Infrared Telescope Facility (IRTF) (2000 - 2013). Additionally, we made spectrophotometric measurements using images gathered by the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope (2003 - 2005). We find that the detected CO2 ice is primarily on the trailing hemispheres of the satellites closest to Uranus, consistent with other observations of these moons. Our band parameter analysis indicates that the detected CO2 ice is pure and segregated from other constituents. Our spectrophotometric analysis indicates that IRAC is not sensitive to the CO2 ice detected by SpeX, potentially because CO2 is retained beneath a thin surface layer dominated by H2O ice that is opaque to photons over IRAC wavelengths. Thus, our combined SpeX and IRAC analyses suggest that the near-surfaces (i.e., top few 100 microns) of the Uranian satellites are compositionally stratified. We briefly compare the spectral characteristics of the CO2 ice detected on the Uranian moons to icy satellites elsewhere, and we also consider the most likely drivers of the observed distribution of CO2 ice.

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