Dark Terrain on Ganymede: Geological Mapping and Interpretation of Galileo Regio at High Resolution☆
Dark Terrain on Ganymede: Geological Mapping and Interpretation of Galileo Regio at High Resolution☆
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木卫三上的黑暗地形：伽利略区高分辨率地质测绘和解释☆
DOI：
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发表时间：
1998
期刊：
影响因子：
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通讯作者：
M. Belton
中科院分区：
文献类型：
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作者：
L. Prockter;J. Head;R. Pappalardo;D. Senske;G. Neukum;R. Wagner;U. Wolf;J. Oberst;B. Giese;J. Moore;C. Chapman;P. Helfenstein;R. Greeley;H. Breneman;M. Belton
在与木卫三的前两次相遇中，伽利略号航天器以高分辨率（76-86米/像素）获得了伽利略区16500公里2部分的图像，这是一大片黑暗的地形。通过对伽利略地区G1和G2目标地点的测绘，我们能够根据地质单元的形态和相对反照率来表征地质单元。我们发现了三个普遍较低的反照率单元：中等反照率平原单元、较低反照率平原单元和最低反照率单元，这些反照率单元位于沟底和火山口底。我们还发现了高反照率单位，包括陨石坑边缘、沟边缘、孤立的隆起和地块。其他特征包括一个被解释为重写层的中间反照率瓣状特征和一个被解释为撞击喷出物的丘状单元。几个过程被解释为发生在伽利略区域内。这些变化包括构造变形、物质消耗、升华、撞击喷射物重新形成地表，以及可能的冰火山作用和均衡调整。我们观察到伽利略地区NW-SE走向的沟槽（Lakhmu Fossae）已经退化，并被较年轻的N-S走向的沟槽（Zu Fossae）所横切。我们还发现了一些其他的构造特征，这些构造特征可能是与这些系统中的一个或另一个有关的小断层或裂缝。通过测绘和陨石坑大小-频率分布，我们能够为伽利略地区的目标地点提出地层。该地区最古老的特征是高反照率旋钮和地块，它们被解释为早期撞击相关特征和沟缘的残余。它们可能与中、低反照率平原单元和沟系大致同时形成。沟槽底的最低反照率单位可能随后通过升华和质量损耗而演变。目标区域东北部的大部分地区随后被最年轻的一个单位所掩盖，这个单位是北部撞击产生的喷出物。我们利用伽利略区域的高分辨率图像来评估暗地形形成的三种端元模型：(1)地壳整体是暗的；(2)表面的物质是低反照率冰冻火山层覆盖高反照率地壳的结果；(3)暗物质在整个地壳中少量分布，地质作用使低反照率物质集中在表面。虽然在黑暗的地形中可能存在不止一种模型的元素，但我们发现第三种模型，即低反照率物质的薄层模型，最符合伽利略地区的观测结果
Abstract During its first two encounters with Ganymede, the Galileo spacecraft obtained images of a 16,500 km 2 portion of Galileo Regio, a large expanse of dark terrain, at high resolution (76–86 m/pixel). Through mapping of the G1 and G2 target sites within Galileo Regio, we are able to characterize geological units based on their morphology and relative albedo. We find three generally low albedo units: an intermediate albedo plains unit, a lower albedo plains unit, and the lowest albedo unit which is found on furrow and crater floors. We also find high albedo units which include crater rims, furrow rims, and isolated knobs and massifs. Other features include an intermediate albedo lobate feature interpreted to be a palimpsest and a hummocky unit interpreted to be impact ejecta. Several processes are interpreted to have occurred within Galileo Regio. These include tectonic deformation, mass wasting, sublimation, resurfacing by impact ejecta, and possibly cryovolcanism and isostatic adjustment. We observe that the NW–SE trending furrows (Lakhmu Fossae) in Galileo Regio are degraded and are crosscut by the younger N–S trending furrows (Zu Fossae). We also find several other tectonic features which may be minor faults or fractures related to one or other of these systems. Through mapping and crater size–frequency distributions, we are able to propose a stratigraphy for the Galileo Regio target site. The oldest features in the area are high albedo knobs and massifs, which are interpreted to be remnants of early impact-related features and furrow rims. These may have formed at approximately the same time as the intermediate and low albedo plains units and the furrow systems. The lowest albedo unit of furrow floors probably subsequently evolved through sublimation and mass wasting. Much of the northeast portion of the target area was subsequently obscured by one of the youngest units, ejecta from an impact just to the north. We use our mapping of the high-resolution images of Galileo Regio to evaluate three end-member models for the formation of dark terrain: (1) the crust is dark throughout, (2) material on the surface is the result of a low albedo cryovolcanic layer over a higher albedo crust, and (3) dark material is distributed in small quantities throughout the crust, and geological processes have acted to concentrate low albedo material on the surface. Although it is possible that elements of more than one of these models are present within the dark terrain, we find that the third model, that of a thin veneer of low albedo material, best fits observations of Galileo Regio