Ceres’ opposition effect observed by the Dawn framing camera
Ceres’ opposition effect observed by the Dawn framing camera
复制标题
黎明分幅相机观测到的谷神星对立效应
DOI：
10.1051/0004-6361/201833596
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发表时间：
2018
影响因子：
6.5
通讯作者：
C. Russell
中科院分区：
文献类型：
--
作者：
S. Schröder;Jian;M. Rayman;S. Joy;C. Polanskey;U. Carsenty;J. Castillo‐Rogez;M. Ciarniello;R. Jaumann;A. Longobardo;L. McFadden;S. Mottola;M. Sykes;C. Raymond;C. Russell
语境。行星风化层的表面反射率可能会在接近零相位角时急剧增加，这种现象称为对立效应（OE）。阴影隐藏 (SH) 和相干反向散射 (CB) 被认为是造成亮度激增的主要因素的两个物理过程。行星风化层中阴影隐藏的发生是不言而喻的，但事实证明，从遥感观测中明确证明CB是很困难的。 CB 理论的一项预测是 OE 角宽度与波长的相关性。
目标。黎明号航天器观测到了矮行星谷神星表面的 OE。我们的目标是表征解析表面上的 OE，包括明亮的 Cerealia Facula，并寻找 SH 和/或 CB 的证据。目前尚不清楚后者是否能够对像谷神星这样黑暗的表面的 OE 做出重大贡献。
方法。我们通过相位曲线的光度建模来分析 Dawn 分幅相机的图像。
结果。我们发现大多数研究表面的 OE 具有非常相似的特征，增强因子为 1.4，半高全宽为 3°（“宽 OE”）。一个值得注意的例外是阿萨卡陨石坑的新鲜喷射物，其显示出非常窄的亮度增强，仅限于相角<0.5°（“窄OE”）；暗示性地，这在 CB 被认为占主导地位的范围内。我们没有发现宽 OE 宽度的波长依赖性，并且缺乏数据来研究窄 OE 的依赖性。与波长相关的 CB 宽度的预测相当模糊，我们建议使用基于物理的理论对黎明观测进行专门建模，以更好地理解谷神星 OE。零相位观测使我们能够将谷神星的可见几何反照率确定为 pV = 0.094 ± 0.005。与其他小行星的比较表明，谷神星广泛的 OE 是其光谱类型小行星的典型特征，其特征主要与表面反照率相关。
结论。我们的分析表明，CB 可能以高度局部化的方式出现在谷神星的黑暗表面。虽然结果尚无定论，但它们为行星表面的 OE 之谜提供了一部分。
Context. The surface reflectance of planetary regoliths may increase dramatically towards zero phase angle, a phenomenon known as the opposition effect (OE). Two physical processes that are thought to be the dominant contributors to the brightness surge are shadow hiding (SH) and coherent backscatter (CB). The occurrence of shadow hiding in planetary regoliths is self-evident, but it has proved difficult to unambiguously demonstrate CB from remote sensing observations. One prediction of CB theory is the wavelength dependence of the OE angular width.
Aims. The Dawn spacecraft observed the OE on the surface of dwarf planet Ceres. We aim to characterize the OE over the resolved surface, including the bright Cerealia Facula, and to find evidence for SH and/or CB. It is presently not clear if the latter can contribute substantially to the OE for surfaces as dark as that of Ceres.
Methods. We analyze images of the Dawn framing camera by means of photometric modeling of the phase curve.
Results. We find that the OE of most of the investigated surface has very similar characteristics, with an enhancement factor of 1.4 and a full width at half maximum of 3° (“broad OE”). A notable exception are the fresh ejecta of the Azacca crater, which display a very narrow brightness enhancement that is restricted to phase angles <0.5° (“narrow OE”); suggestively, this is in the range in which CB is thought to dominate. We do not find a wavelength dependence for the width of the broad OE, and lack the data to investigate the dependence for the narrow OE. The prediction of a wavelength-dependent CB width is rather ambiguous, and we suggest that dedicated modeling of the Dawn observations with a physically based theory is necessary to better understand the Ceres OE. The zero-phase observations allow us to determine Ceres’ visible geometric albedo as pV = 0.094 ± 0.005. A comparison with other asteroids suggests that Ceres’ broad OE is typical for an asteroid of its spectral type, with characteristics that are primarily linked to surface albedo.
Conclusions. Our analysis suggests that CB may occur on the dark surface of Ceres in a highly localized fashion. While the results are inconclusive, they provide a piece to the puzzle that is the OE of planetary surfaces.