【作者】 肖智勇；

【导师】 曾佐勋； Robert G. Strom；

【作者基本信息】 中国地质大学 ， 行星地质与化学， 2013， 博士

【摘要】 月球和水星是内太阳系天体中大小相近、表面形态相似的两个天体。由于二者的基本物理特征相似(如均无大气、均具有硅酸质壳层),前人常类比月球和水星上的相似地质过程,以更彻底的了解其基本物理规律。厘定月球表面地质单元的地层年代具有岩石样品年龄作为约束,最年轻的月球地层年代是哥白尼纪,起始于-800Ma前的哥白尼撞击事件。月球上具有明亮溅射纹的撞击坑大多形成于哥白尼纪。相比之下,水星表面地质单元的地层年代是引用月球地层年代的分类原则建立的。由于没有水星样品作为约束,每个水星地层的绝对起止时间存在极大的不确定性。柯伊伯纪被认为是水星表面最年轻的地层年代,以水星表面的柯伊伯撞击坑命名。柯伊伯纪与月球上的哥白尼纪相对应,前人一般认为柯伊伯纪起始于-800Ma。水星上具有溅射纹的撞击坑形成于该时期。以往对月球与水星地质演化历史的研究认为：自大约800Ma以来,水星和月球表面的内生型地质活动已基本停止,外来天体的撞击作用基本上是水星和月球表面最主要的地质过程。近年来成功入轨的月球勘测轨道飞行器(Lunar Reconnaissance Orbiter; LRO)和水星信使号飞船(MErcury Surface, Space ENviroment, GEochemistry, and Ranging; MESSENGER)获取了价值巨大的探测数据,极大的增进了行星地质学界对这两个天体的认识。更重要的是,通过对比月球与水星以及其他天体上的相似地质过程,对了解太阳系天体表面地质演化的规律提供了极为重要的窗口。利用以往探测器(尤其是LRO和MESSENGER)获取的影像、光谱、重力场和激光高度计数据,本文在月球和水星表面发现了非常年轻的地质活动。其中,有些发现改变了前人对月球和水星热演化史和地质演化史的认识,例如水星表面柯伊伯纪的爆发型火山活动、水星壳层挥发分活动、月球全球发育的哥白尼纪坡面块体运动等；有些发现则加深了对天体表面(包括地球)的地质过程的基本规律的认识,例如通过类比月球哥白尼纪和水星柯伊伯纪的撞击熔融内发育的冷凝裂隙,为研究天体表面柱状节理的成因机制提供了另一扇窗口。总体而言,本文以水星柯伊伯纪和月球哥白尼纪的各种表面地质活动为研究对象,分别分析了其地质过程及其指示意义。本文的具体研究内容及结论包括：(1)更新了水星曼苏尔纪以来的地层年代的起止时间。首先统计了水星全球具有溅射纹的撞击坑以及中低纬度的形态学第一类撞击坑(包括具有溅射纹的撞击坑)。在验证数据库的完整性之后,利用撞击坑统计技术,避开相关干扰因素,采用最新获取的水星撞击坑产生方程和年代方程,计算了水星形态学第一类撞击坑坑群所代表的绝对模式年龄,大约为1.26Ga,这些撞击坑形成于曼苏尔纪；水星上所有具有溅射纹的撞击坑的绝对模式年龄大约为159Ma,其中具有明亮溅射纹的撞击坑的绝对模式年龄大约为40-60Ma。因此可将水星柯伊伯纪的起始时间划在-159Ma前。(2)本文在月球哥白尼撞击坑南东侧的连续溅射毯上发现一处由数十条小型地堑组成的复杂地堑系统,并以此为出发点研究了月球表面的年轻构造与岩浆活动的可能性。这些地堑从地层切割的角度证实尽管月球在哥白尼纪处于全球收缩的挤压背景下,月球表面依然可形成伸展构造。这套地堑系统的独特之处在于,其位于一个局部的地形隆起上,周围未见明显的与之相关的挤压构造,且该区域具有巨大的自由空气与布格重力异常。通过分析这些地堑的形态和几何特征,本文分析了形成其拉伸应力的可能来源,重点讨论了最近提出的月球哥白尼纪浅层岩浆侵入的假说。岩浆侵入的理论模型计算和类比研究证明：这套地堑系统不太可能是由于浅层岩浆侵入造成局部地势隆起而形成的。由于月球持续全球收缩,隐伏逆冲断层活动以及月震更可能是形成这些地堑的原因。(3)利用LRO获取的高分辨率影像数据,论文发现月球全球正在发生大量形式多样的块体运动。虽然月球表面缺乏流水或大气,这些块体运动的形貌与地球上的极其相似,有些具有低粘度的流动特征。基于300多个月球表面块体运动的样例,建立了每个样例的形貌、尺度、所在地质单元的坡度和年龄的数据库。结果发现坡面块体运动是改造月球表面局部地貌的重要地质过程。块体运动通过削高补低,最终夷平月面的地势差。块体运动与撞击作用是决定月面区域尺度的月壤厚度的最重要因素,对区域的撞击坑密度存在一定的影响。酒海纪和前酒海纪的地貌代表了月球表面地貌演化的最终阶段,只有月壤蠕移在其中形成。(4)在水星上发现了几处曼苏尔纪的小范围表面岩浆事件形成的平原物质。这些平原物质覆盖了周围形态学第一类撞击坑的溅射物,表明水星表面的岩浆活动至少持续到了1.26Ga。另外,水星上有多处柯伊伯纪的爆发型火山口,其产生的火成碎屑沉积物覆盖了附近具有溅射纹的撞击坑。表明水星幔部的熔融物在柯伊伯纪依然具有极高的挥发分含量,且水星内部的热活跃程度和壳层厚度尚不足以阻碍表面火山活动的形成。这些发现与以往对水星演化的认识截然不同。另外,水星上大部分的爆发型火山活动形成的火山口和火成碎屑沉积物的反照率比水星的全球平均反照率高,在紫外到近红外波段的反照率光谱曲线斜率相对较陡。与之相反,一些柯伊伯纪的火成碎屑沉积物和火山口的反照率比水星的平均反照率低,表明其物质成分或物理性质不同。这对了解水星内部物质的成分演化具有重要的指示意义。(5)前人一般认为水星自晚期大轰击结束后,也即大约3.8Ga左右,表面的挤压构造单元不再形成。本文在水星表面发现了一些切割形态学第一类撞击坑的、长达数十千米的叶片状悬崖,有些大型叶片状悬崖也可能切割具有溅射纹的柯伊伯纪的撞击坑。另外,在水星表面发现了一些较小的叶片状悬崖切割柯伊伯纪的小型撞击坑,表明水星全球收缩在柯伊伯纪依然存在。对比水星上大型平原物质和大型叶片状悬崖的相对年龄,可发现由于水星持续全球收缩导致壳层增厚,阻碍了水星幔部的熔融物大量上涌,水星表面的大范围岩浆事件在大约1.26-3.8Ga之间停止。(6)在水星上发现了大量与挥发分活动有关的地貌单元：白晕凹陷和暗斑。本文分析了其形貌、大小、全球分布特征、光谱特征、地层年龄、可能的物质成分以及可能的成因机制。白晕凹陷与暗斑发育在除高反照率平坦平原物质之外的所有地貌上。白晕凹陷是形态不规则的、无隆起边缘的、较浅的凹陷,周缘常由高反照率的晕状物质环绕,其反照率大于水星溅射纹的反照率。高分辨率影像数据揭示这些晕状物质上是由高反照率的小型凹陷连接形成的。内部和周围反照率均比背景地质单元高的白晕凹陷可能依然处于活跃状态,而较老的白晕凹陷则不具有高的反照率。暗斑是围绕白晕凹陷发育的薄层状低反照率物质,其反照率是目前水星上最低的。每个暗斑中心都发育有白晕凹陷,相反,并非每个水星白晕凹陷周围都有暗斑。暗斑可能是通过剧烈的富含挥发分的物质以100米/秒左右的速度喷出以脱气作用形成的。与此同时,挥发分的出口形成一个原始的白晕凹陷,后凹陷逐渐、缓慢地侧向和垂向生长出现白晕。白晕凹陷也是壳层内富含挥发分的物质不断散失而生长扩张的,但其生长速度远小于暗斑的形成速度。暗斑物质极不稳定,其存活时间小于水星表面撞击溅射纹的存活时间,因而所有观察到的水星暗斑可能都形成于柯伊伯纪。形成暗斑与白晕凹陷的挥发分物质都富硫,造成二者反照率的差异可能是其物质成分和/或颗粒物理性质不同。(7)月球哥白尼纪和水星柯伊伯纪的撞击坑底形成了大量新鲜的撞击熔融。这些撞击熔融是通过热辐射方式冷却的,冷凝过程中形成的拉张应力造成岩石破裂形成冷凝裂隙。通过分析月球和水星撞击熔融中的冷凝裂隙的形态、大小和组合样式,本文发现撞击熔融的深度、固体碎屑物在撞击熔融中的含量以及撞击熔融冷凝过程中的垂直沉降量控制了裂隙的发育。将月球和水星上的冷凝裂隙与火星和地球上的柱状节理相比,可发现热辐射造成的冷凝速率可能不足以形成柱状节理,热对流和/或热传导是形成柱状节理的主要散热方式,挥发分可能在所有天体表面的柱状节理的形成过程中必不可少。(8)月球和水星上的平均撞击速度和表面重力加速度不同。对比月球和水星上的撞击坑,可分析撞击过程中的主控因素。本文测量并对比了水星和月球上一些形态学第一类复杂撞击坑的外部沉积物(包括连续溅射沉积物和连续二次撞击坑相)的形态与大小,研究了撞击挖掘阶段的主控因素。与前人的研究结果相似,本文证实重力是复杂撞击坑形成过程的挖掘阶段的一个主控因素。另外,本文发现撞击速度在撞击挖掘过程中也起到不可忽视的作用。不同于典型的月球二次撞击坑,有些水星撞击坑形成了大量分布相对离散的圆形二次撞击坑。分析其原因表明水星局部区域和层位的物质具有独特的物性,影响了撞击挖掘过程中的溅射角度。该物质特性很可能与水星壳层内部富含挥发分的低反照率物质有关。(9)在太阳系天体表面,撞击坑内的中央凹陷一般归因于被撞击体中的水冰等挥发分对撞击过程的影响。由于月球和水星的壳层内相对缺乏挥发分,前人一般认为水星和月球撞击坑中不存在中央凹陷。本文在一些月球撞击坑和水星撞击坑内(包括哥白尼纪和柯伊伯纪的撞击坑)发现了中央凹陷。通过建立其形态、大小、年龄、和分布位置的数据库,本文对比了水星和月球与其他天体上的撞击坑内的中央凹陷,并提出形成月球和水星撞击坑的中央凹陷不需要挥发分,而与撞击过程自身有关。更多还原

【Abstract】 The Moon and Mercury are the two similar-sized bodies in the inner Solar System that resemble each other in the appearance. The two bodies share many common physical similarities, for examples both of them have silicate crust and airless surfaces, so that previous studies usually compare similar geological activity on the Moon and Mercury to better understand the basic mechanical rules of the geological activity.The lunar stratigraphic ages are well-constrained with the help of returned samples. The most recent geological epoch on the Moon is the Copernican age that started～800Ma represented by the Copernicus impact. All lunar rayed craters (i.e., craters with impact rays) formed during the Copernican age. On the contrary, the stratigraphic ages of Mercury were referred from those of the Moon and their absolute ages were not well established due to the lack of samples. The Kuiperian age is regarded as the youngest stratigraphic age on Mercury which was named after the Kuiper crater. The Kuiperian age corresponds to the Copernican age on the Moon and previous studies assigned an age of～800Ma for the Kuiperian age. All rayed craters on Mercury formed during the Kuiperian age. It is generally accepted that after～800Ma, endogenic geological activity has ceased on both the Moon and Mercury, and meteor impacts have been the dominant surface geological activity.Recently, the success insertion of the Lunar Reconnaissance Orbiter (LRO) to the orbit about the Moon and the MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) spacecraft to the orbit about Mercury has obtained huge amount of valuable scientific data, which have greatly promoted our understanding of the surface geological evolution of these two bodies. More importantly, analog studies of geologic activity on the Moon and Mercury provide new windows for understanding surface geological evolution across Solar System bodies. Using imagery, topography, gravity and reflectance spectral data returned from various spacecrafts (especially from LRO and MESSENGER), we find that some very recent geological activity has been occurring on both the Moon and Mercury. Among these discoveries, some have changed previous understandings about the thermal evolution and surface geological evolution of both the Moon and Mercury, such the Kuiperian explosive volcanism, activity of crustal volatile on Mercury, and widely-occurred Copernican-aged mass wasting features. Some others have improved our understanding of the basic mechanical rules for each geological activity, for example, analog studies for cooling fractures developed in Copernican-aged impact melt on the Moon and Kuiperian-aged impact melt on Mercury shed light on the development of columnar joints on different planetary bodies.In general, this dissertation targets the Copernican-aged and Kuiperian-aged geological activity on the Moon and Mercury, and also their indications. The abstracts for the discovered recent geological activity are the following:(1) This dissertation updates the absolute age scale for mercurian straitigraphies younger than Calorian. Global rayed craters and morphological Class1craters in the mid-low latitude regions on Mercury are collected. After testing the completeness of the database, the absolute model ages for these crater populations are calculated using the most updated crater counting technique and avoiding potential problems. The results suggest that the Class1crater population on Mercury has an average model age of～1.26Ga and the rayed crater population has an average model age of159Ma. Mercurian craters that have sharp rays yield a model age of40-60Ma.(2) A complex graben system that is composed of dozens of small graben is found in the southeastern continuous ejecta deposits of the Copernicus crater on the Moon. These graben verify that Copernican-aged extensional structures could form on the Moon although its global stress state has been compressional due to continuous global contraction. The graben system is located on a local high-relief area and no adjacent compressional features are visible associated with the graben. This area is located within a free-air and Bouguer gravity anomaly. After analyzing the morphology, geometry, and assemblage pattern of the graben system, we analyze the possible sources for the extensional stresses that formed the graben, especially about the reliability of the recently proposed shallow igneous intrusion model. Model calculation of the igneous intrusion model suggests that the gravity anomaly is not likely to be associated with the graben, and shallow igneous intrusion is not likely, or necessary in explaining the origin for the graben. The graben are most likely to form by a combined effect of activity of blind thrust faults and moonquakes, which is consistent with late-stage global contraction of the Moon.(3) Global-wide mass wasting movements have been happening on the Moon as seen in high-resolution images returned from the LRO mission. Although no surface water or atmosphere exists on the Moon, the mass wasting features highly resemble those on the Earth and some exhibit characteristics of low viscosity. Based on over300examples of various lunar mass wasting features, the morphology, geometry, slope angles, and ages of these features are included in a database. This dissertation finds that mass wasting is an important surface geological process in shaping regional geomorphology on the Moon. Mass wasting tends to decrease slope gradients over time. Nectarine and Pre-Nectarine terrains represent the final stage of surface topographic evolution on the Moon, where only regolith creeps occur. Mass wasting and impact cratering is the most important geological process in determining the thickness of regolith at regional scales, and mass wasting also affects the density of impact craters on slopes.(4) Several Mansurian-aged small-scale smooth plains that have formed from extrusive volcanic activity are found on Mercury. The plain material covers ejecta from adjacent morphological Class1craters, suggesting that extrusive volcanism occurred on Mercury after-1.26Ga. Moreover, some Kuiperian-aged volcanoes and pyroclastic deposits that have formed from explosive volcanism are found on Mercury. The pyroclastic deposits cover impact rays from adjacent craters indicating that melted material caused by partial melting of Mercury’s mantle retains a high content of volatiles during Kuiperian, and the crustal thickness and thermal state of Mercury are still suitable to form surface volcanism. These findings are dramatically different from previous believes. Moreover, normal volcanoes and pyroclastic deposits on Mercury have both a higher reflectance and a larger spectral slope at the UV-VIR wavelengths compared with the global average of the planet. On the contrary, some Kuiperian-aged volcanoes and pyroclastic deposits have reflectances lower than the global average and smaller spectral slopes than the older ones. This suggests that the Kuiperian-aged pyroclastic deposits have different compositions and/or physical properties, which is meaningful to understand the thermal evolutionary history of the interior of Mercury.(5) Previous studies believed that after the end of late heavy bombardment in the inner Solar System (-3.8Ga), compressional tectonic features no longer formed in the crust of Mercury. However, here some giant lobate scarps on Mercury that are dozens of kilometers long are found to have transected both morphological Class1craters and also potentially rayed craters. Some lobate scarps crosscut small Kuiperian-aged craters on Mercury, indicating that compressional features caused by late-stage global contraction of Mercury occurred at Kuiperian. Comparing the relative age between vast areas of smooth plains and giant lobate scarps, it could be certain that due to continuous cooling and global contraction, the thickening of Mercury’s crust have prohibited the extrusion of enormous amounts of melted material from the mantle during1.26-3.8Ga, and vast areas of extrusive volcanism has ceased during that time.(6) Numerous hollows and dark spots that are caused by loss of crustal volatiles are found on Mercury. Their morphology, geometry, global distributions, reflectance spectral, stratigraphic ages, potential compositions, and possible formation mechanisms are discussed. Both hollows and dark spots occur on various terrains on Mercury, except for high-reflectance smooth plains material. Bright haloed hollows are irregular-shaped rimless shallow depressions that are sometimes surrounded by high-reflectance material. The surrounding material has the highest reflectance on Mercury, and high-resolution images reveal that the high-reflectance surroundings are actually composed of numerous small pits that would grow larger to join the parent hollow. Hollows that have both bright interiors and exteriors may be still active and those without high reflectances might have ceased growing. Dark spots are thin surfacial dark material that occurs around hollows. The dark spot material has the lowest reflectance yet identified on the planet. However, not every hollow on Mercury has a surrounding dark spot. Dark spots may have formed from intense outgassing events that feature outgassing velocity exceeds～100m/s. Simultaneously, an embryonic hollow forms with the outgassing event that would steadily grow slowly to form bright haloed hollows. The dark spot material is very unstable on the surface of Mercury and its life time is smaller than that of impact rays on the planet. Therefore, all visible dark spots on Mercury must have formed at late Kuiperian. Materials forming dark spots and hollows are rich in volatiles that might have a high concentration of sulfur. The reason for the dramatically different reflectances between hollows and dark spots might be their different compositions and/or physical properties.(7) Impact melt vastly occur on floors and ejecta blankets of Copernican-aged lunar impact craters and Kuiperian-aged mercurian impact craters. Impact melt deposits on the Moon and Mercury mainly cool by thermal emission and the cooling process can form large enough thermal stresses creating extensional fractures. The morphology, geometry, and assemblage pattern of cooling fractures in impact melt deposits on the Moon and Mercury are studied and compared. The results suggest that the combined effect of depths of impact melt deposits, amounts of entrained solid debris in impact melt, and degree of vertical subsidence formed during the cooling process controls the development of cooling fractures. By comparing the cooling efficiency of impact melt and lava between the Moon and Mercury, it could be ascertain cooling rate caused solely by thermal radiation is not large enough to form columnar joints, and thermal convection and/or conduction is more important. Volatiles may be a necessary element in forming columnar joints on planetary bodies.(8) The median impact velocity of projectiles and surface gravity on the Moon and Mercury are different. We study the morphology and geometry of crater exterior structures for similar-sized fresh complex craters on the Moon and Mercury (including continuous ejecta deposits and continuous secondaries facies), the controlling factors during the impact excavation stage is analyzed using impact cratering scaling laws and comparative studies. This dissertation confirms a previous finding that gravity is a dominant controlling factor during the impact excavation stage of forming complex craters. We also find that impact velocity plays an equivalent role. Some craters on Mercury have more circular and isolated secondaries on the continuous secondaries facies than typical lunar secondaries. The most possible reason is that at some layers and positions on Mercury, the crustal material has special properties that have affected the impact excavation stage causing larger ejection angles and more circular secondaries. These target properties might be associated with low-reflectance material that may be caused by a higher content of volatiles compared with the average of the planet.(9) Central pits in impact craters on planetary bodies are supposed to be caused by the effect of target volatiles on cratering processes. Previous studies suggested that central pits would not form in impact craters on the Moon and Mercury due to the assumed low concentration of crustal volatiles. Here some impact craters on the Moon and Mercury are found to have both floor pits and summit pits (occur on central peaks) that are morphologically similar with those on Mars and icy satellites. Based on the database for the morphology, geometry, global distributions, and age of the central pit craters on the Moon and Mercury, central pit craters on different planetary bodies are compared. It is suggested that the central pits in impact craters on the Moon and Mercury do not need crustal volatiles to form, and forming central pits are related to an unknown mechanical process related to the cratering event.更多还原