木卫二的半球色二分性对非同步自转的限制
Europa's Hemispheric Color Dichotomy as a Constraint on Non-synchronous Rotation
摘要：
木卫二的表面反射率表现出明显的半球二分法，这被假设是由于木星磁层中携带的高能粒子对尾半球的辐射增强而形成的。我们建议，这种模式只能持续，如果变色的时间尺度比木卫二的旋转相对于同步状态短得多，并提供了一种手段，用于约束使用所观察到的颜色模式的旋转速率。通过将旅行者数据中的纵向紫外和可见光颜色变化分解为正弦和余弦项，我们没有发现可检测到的非同步旋转（NSR）特征。同样的结论也可以用两种观察到的变色模型得出：一种代表一个活跃的变色表面，另一种假设表面上的外生变色处于稳定状态。预期签名的震级作为Pwyll陨石坑的年龄的函数，这是用来约束变色的时间尺度。此外，我们开发了变色的物理模型来验证几何模型，产生一致的结果。使用木卫二的半球颜色二分法未能识别NSR的签名放大了应力起源的突出问题，以解释木卫二的普遍构造特征。
英文摘要：
Europa's surface reflectance exhibits a pronounced hemispheric dichotomy, which is hypothesized to form due to enhanced irradiation of the trailing hemisphere by energetic particles entrained in the jovian magnetosphere. We propose that this pattern can only persist if the timescale for discoloration is much shorter than that of Europa's rotation relative to the synchronous state, and provide a means for constraining the rotation rate using the observed color pattern. By decomposing the longitudinal ultraviolet and visible color variations from Voyager data into sine and cosine terms, we find no detectable signature of non-synchronous rotation (NSR). This same conclusion is reached with two observational models of discoloration: one representing an actively discoloring surface, and the other assuming that the present-day exogenic discoloration on the surface is in steady-state. Magnitudes of the expected signature are presented as functions of the age of the crater Pwyll, which is used to constrain the timescale of discoloration. Furthermore, we develop a physical model of discoloration to validate the geometric models, producing consistent results. The failure to identify a signature of NSR using Europa's hemispheric color dichotomy magnifies the outstanding problem of the origin of the stress to explain Europa's pervasive tectonic features.
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