火星大沙波纹特征及其形成机制

doi:10.11867/j.issn.1001-8166.2020.080

随着高分率影像数据的获取，火星大沙波纹进入研究视野。火星大沙波纹是一类间距为米级的风成床面形态，其特殊性反映在形态、格局、流动性及其形成过程等几个方面，然而，受限于探测资料特别是缺乏高分辨率影像资料，对其研究的范围和深度都极其有限，导致这些特殊性被长期忽视，研究者们简单地认为其与地球上常见的沙波纹相类似。遵循地貌学思路，基于已有的有限研究介绍了大沙波纹的形态、组成物质及其形成机制，讨论了其潜在的研究意义。与普通沙波纹相比，火星大沙波纹尺度较大，脊线蜿蜒、尖锐，横截面不对称，背风坡坡度明显大于迎风坡，且背风坡具有明显的滑落或滑塌面；具有较强的移动性，以脊线的纵向延伸为主，侧向移动微弱；大沙波纹脊线具有不同走向，构成网格状格局，能够用来反演较长时段的复杂风况；具有沙波纹、大沙波纹和沙丘3级尺度风成床面形态并存的独特现象；初步探测发现，大沙波纹沉积物组成为分选良好的中细沙。目前关于火星大沙波纹的形成机制有3种假说：沙丘假说、冲击假说和流体拖拽假说，其中流体拖拽假说具有较强的说服力。火星大沙波纹不同于常态沙波纹，其独特性对总体上认识火星风沙地貌、火星环境特征及其演化历史具有重要意义，值得深入研究。

关键词: 火星风沙地貌, 大沙波纹, 形态学特征, 形成假说

Large Ripples （LRs） on Mars come into the subject of researchers with the acquisition of high resolution image data. LRs are a kind of aeolian bedforms with meter-scale wavelength， and outstanding features of their morphology， bedform patterns， mobility and formation processes. However， due to the limited exploration data， especially the lack of high resolution image data， the research scope and depth are extremely limited， which leads to the long-term neglect of their uniqueness， therefore， researches simply consider them as common sand ripples on Earth. Following the geomorphology law and based on the existing limited research， this paper introduces the morphology， sedimentology and formation mechanism of LRs， and discusses their potential research significance. Compared with ordinary sand ripples， LRs are larger with sinuous and sharp crest lines， asymmetric topographic profiles， the downwind slope angle is much bigger than that of upwind， and slip faces are marked by the presence of grainflows and grainfalls. LRs have strong mobility with obvious longitudinal extension of ridges， and the lateral migration is very small. They have various orientations and network patterns which can be used to inverse the complex wind regimes over a long period of time. Three hierarchical order aeolian bedforms of sand ripples， large ripples， and sand dunes can co-exist. Preliminary explorations show that the sedimentology of LRs is the well sorted fine-to-medium sand. Three formation hypotheses were proposed for LRs： dune hypothesis， impacting hypothesis and fluid-drag hypothesis， with more evidences supporting the fluid-drag hypothesis. LRs are different from normal sand ripples， and their uniqueness is of great significance to comprehend Martian aeolian geomorphology， environmental characteristics， and revolution history， and therefore， they are worth making an intensive study of.

Key words: Martian aeolian geomorphology, Large ripples, Morphological characteristics, Formation hypothesis

中图分类号:

P185.3

图1 HiRISE高分辨率影像显示的火星大沙波纹（据参考文献［ 1 ］修改）

Fig.1 Typical large ripples and mixed square patterns in Bagnold dunes of Gale Crater（HiRISE image）(modified after reference [ 1 ])

图2 “好奇号”火星车显示的火星另一个床面形态概率分布峰值—大沙波纹（据参考文献［ 3 ］修改）

Fig.2 Distinct modes of Martian aeolian ripples revealed by Curiosity Rover (modified after reference [ 3 ])

图3 拜格诺沙地山丘上大沙波纹的波长变化（据参考文献［ 5 ］修改）

Fig.3 Variation of wavelength of large ripples over a dune in the Bagnold Dunefield (modified after reference [ 5 ])

图4 栏杆相机照片显示的大沙波纹及其上叠置的沙波纹（据参考文献［ 3 ］修改）

Fig.4 Large ripples and their superimposed ripples revealed by Mastcam camera (modified after reference [ 3 ])

图5 火星机械臂透镜成像仪照片显示的大沙波纹坡面上的颗粒滑落与滑塌现象（据参考文献［ 3 ］修改）

Fig.5 Grainflow and grainfall on a large ripple slope revealed by MAHLI camera (modified after reference [ 3 ])

图6 火星拜格诺沙地沙丘上大沙波纹的网格状格局（据参考文献［ 5 ］修改）

Fig.6 Network patterns of large ripples on a dune of the Bagnold Dunefield (modified after reference [ 5 ])

图7 火星拜格诺沙地沙丘上大沙波纹的两期照片对比显示的大沙波纹移动量（据参考文献［ 5 ］修改）

Fig.7 Minimum migration axial field computed for the interval between two images (modified after reference [ 5 ])

图8 火星拜格诺沙地沙丘上大沙波纹显示的几种不同风况（据参考文献［ 5 ］修改）

Fig.8 Length-weighted circular distributions of trends of large ripples (modified after reference [ 5 ])

图9 火星维多利亚陨击坑开铺圣玛丽露头显示的埋藏大沙波纹（据参考文献［ 3 ］修改）
(a)火星探测漫游者全景相机在维多利亚陨击坑开铺圣玛丽露头拍摄的照片；(b)图（a）中白色方框区域的分米级交错层理；（c）对图（b）交错层理的深入解析；（d）通过复合床面动力学模型模拟的不能被解释（上）和能被解释（下）的层理结构；黄色和红色线条分别代表沙丘和大沙波纹移动产生的侵蚀面;蓝色线条代表大沙波纹的交错层理;黑色表示不能被解释的层理

Fig.9 Candidate wind-drag ripple stratification on Mars: Mars Exploration Rover Panoramic Camera image (P2441, sol 1212) of Cape St. Mary outcrop, Victoria crater, Mars (modified after reference [ 3 ])
(a) Mars Exploration Rover Panoramic Camera image of Cape St. Mary outcrop, Victoria crater, Mars; (b) Decimeter-scale cross-strata of the white box shows the location of (a); (c) An interpretation of stratal features from (b)；(d) Uninterpreted (top) and interpreted (bottom) stratification produced by kinematic modeling of compound bedforms. Yellow and red lines represent erosional surfaces produced by migration of dunes and wind-drag ripples separately; Blue lines indicate wind-drag ripple cross-stratification; Black lines represent uninterpreted stratification

图10 火星大沙波纹的纵向延伸（据参考文献［ 3 ］修改）
（a）火星盖尔陨击坑拜格诺沙地大沙波纹的纵向移动（T1~T3时间段）;（b）不同时期Y-结点的位置;（c）移动方向与大沙波纹走向对比；（a）和（b）中黄色、红色和蓝色线条分别代表大沙波纹在T1、T2和T3时间段的纵向移动

Fig.10 Longitudinal migration of LRs (modified after reference [ 3 ])
(a) Longitudinal migration of LRs in the Bagnold dunefield, Gale crater, Mars (at T1~T3); (b)The location of Y junction with time; (c) Comparation of longitudinal direction of Y junction and crest orientation of LRs; Yellow, red, and blue lines in figure (a) and (b) represent longitudinal migration of LRs during the period of T1, T2, and T3

图11 美国亚利桑州那切里大峡谷现代河流中的中细沙水下沙波纹（据参考文献［ 3 ］修改）

Fig.11 Subaqueous ripples in fine-to-medium sand, in a modern river near the Canyon de Chelly, Arizona, United States (modified after reference [ 3 ])

图12 流体拖拽沙波纹的尺度比例关系（据参考文献［ 3 ］修改）
火星大沙波纹的波长与流体拖拽沙波纹理论吻合，而火星沙丘与小沙波纹则与之不吻合

Fig.12 Scaling of fluid-drag ripples (modified after reference [ 3 ])
Large Martian ripples match fluid-drag ripple theory, in contrast to Martian dunes and small Martian ripples

图13 火星风力拖拽沙波纹波长随大气密度的变化（据参考文献［ 3 ］修改）

Fig.13 Wavelength of wind-drag ripples on Mars as a function of atmospheric density (modified after reference [ 3 ])

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摘要

Characteristics and Formation Mechanism of Large Ripples on Mars