温度和光漂白对类Europa条件下辐照氯化钠的影响
The Influence of Temperature and Photobleaching on Irradiated Sodium Chloride at Europa-like Conditions
摘要：
木卫二的主要半球混沌区域在450 nm处有一个光谱特征，这是由于被辐照的氯化钠中的晶体缺陷吸收，称为F中心。实验数据与观测数据之间存在一些差异，包括F心带中心波长的$\sim$ 10 nm的偏移，以及木卫二上缺少M心的720 nm吸收，这是由F心对的合并造成的。在这里，我们对氯化钠进行辐照实验，试图了解这些差异。我们表明，仔细控制样品的温度在120 K的温度下产生F-中心的吸收波长与欧罗巴。此外，我们还测量了光漂白的效果-光子对F中心的破坏-并表明，在欧罗巴上的高能粒子和光子通量下，将达到一个平衡，只有适度的F中心吸收。F心的密度永远不会达到足够高的值来产生次级M心。我们的实验预测，在没有光漂白的情况下，欧罗巴上的F中心在夜间生长，然后在白天部分衰减。我们展示了哈勃太空望远镜的观测结果与这一预测一致。木卫二上450 nm F中心的所有观测结果现在都与实验室测量的氯化钠一致，证实了木卫二上存在这种盐。
英文摘要：
Europa's leading hemisphere chaos regions have a spectral feature at 450 nm that has been attributed to absorption by crystal defects in irradiated sodium chloride, known as F-centers. Some discrepancies exist between the laboratory data of irradiated sodium chloride and the observations, including a $\sim$10 nm shift in central wavelength of the F-center band and the lack of the prominent 720 nm absorption on Europa from M-centers, which result from the coalescence of pairs of F-centers. Here, we perform irradiation experiments on sodium chloride in an attempt to understand these discrepancies. We show that careful control of the temperature of the sample at a temperature of 120 K yields F-centers with an absorption wavelength comparable to that of Europa. In addition, we measure the effect of photobleaching -- the destruction of F-centers by photons -- and show that at the energetic particle and photon flux on Europa, an equilibrium will be reached where only a modest F-center absorption develops. The density of F-centers never reaches high enough values for the creation of secondary M-centers. Our experiments predict that F-centers grow during the night on Europa in the absence of photobleaching and then partially decay during the daytime. We show observations from the Hubble Space Telescope consistent with this prediction. All observations of the 450 nm F-center on Europa are now consistent with laboratory measurements of sodium chloride, confirming the presence of this salt on Europa.
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