A rocky hill on the continuous ejecta of Ziwei crater revealed by the Chang’e-3 mission

The Chinese Chang’e-3 mission landed close to the eastern rim of the ~450 m diameter Ziwei crater. Regional stratigraphy of the landing site and impact excavation model suggest that the bulk continuous ejecta deposits of the Ziwei crater are composed by Erathothenian-aged mare basalts. Along the traverse of the Yutu rover, the western segment features a gentle topographic uplift(~0.5 m high over ~4 m), which is spatially connected with the structurally-uplifted crater rim. Assuming that this broad topographic uplift has physical properties discontinuous with materials below, we use data returned by the high-frequency lunar penetrating radar onboard the Yutu rover to estimate the possible range of relative permittivity for this topographic uplift. Only when the relative permittivity is ~9 is the observed radar reflection consistent with the observed topography, suggesting that the topographic uplift is composed of basaltic blocks that were excavated by the Ziwei crater. This result is consistent both with the impact excavation model that predicts deeper basaltic materials being deposited closer to the crater rim, and with observation of numerous half-buried boulders on the surface of this hill. We note that this study is the first to use topography and radargram data to estimate the relative permittivity of lunar surface uplifts, an approach that has had many successful applications on Mars. Similar approaches can apply other ground penetrating radar data for the Moon, such as will be available from the ongoing Chang’e-4 mission.

The subsurface structure and stratigraphy of the Chang’E-4 landing site:orbital evidence from small craters on the Von Kármán crater floor

Chang’E-4(CE-4)successfully landed on the floor of the Von Kármán crater within the South Pole-Aitken basin(SPA).One of its scientific objectives is to determine the subsurface structure and the thickness of lunar regolith at the landing site and along the traverse route of the Yutu-2 rover.Using orbital data,we employed small craters(diameters<1 km)on the floor of the Von Kármán crater as probes to investigate the subsurface structure and stratigraphy of the CE-4 landing site.In this study,40 dark-haloed craters that penetrate through the surface Finsen ejecta and excavate underlying mare deposits were identified,and 77 bright ray craters that expose only the underlying fresh materials but do not penetrate through the surface Finsen ejecta were found.The excavation depths of these craters and their distances from the Finsen crater center were calculated,and the thickness distribution of Finsen ejecta on the Von Kármán floor was systematically investigated.The boundary between Finsen ejecta and underlying mare basalt at the CE-4 landing site is constrained to a depth of 18 m.We have proposed the stratigraphy for the CE-4 site and interpreted the origins of different layers and the geological history of the Von Kármán crater.These results provide valuable geological background for interpreting data from the Lunar Penetrating Radar(LPR)and Visible and Near-infrared Imaging Spectrometer(VNIS)on the Yutu-2 rover.The CE-4 landing site could provide a reference point for crater ejecta distribution and mixing with local materials,to test and improve ejecta thickness models according to the in situ measurements of the CE-4 LPR.

Correlation of the largest craters,stratigraphic impact signatures,and extinction events over the past 250 Myr

The six largest known impact craters of the last 250 Myr(≥70 km in diameter),which are capable of causing significant environmental damage,coincide with four times of recognized extinction events at 36(with 2 craters),66,and 145 Myr ago,and possibly with two provisional extinction events at 168 and215 Myr ago.These impact cratering events are accompanied by layers in the geologic record interpreted as impact ejecta.Chance occurrences of impacts and extinctions can be rejected at confidence levels of99.96%(for 4 impact/extinctions)to 99.99%(for 6 impact/extinctions).These results argue that several extinction events over the last 250 Myr may be related to the effects of large-body impacts.

太湖冲击坑溅射物的发现及其意义

太湖冲击成因说由来已久,但始终未成定论。近几年在太湖及周边湖泊的淤泥层中发现了许多形态各异的奇石,经多种方法测试研究,确定为太湖冲击坑的溅射物。根据成份,溅射物分为两大类。一类富铁质,以菱铁矿及其胶结的碎屑为主,包括微小球粒、棍状及各种形态的块状和片状体;另一类贫铁质,以方解石及其胶结的长英质碎屑为主,碎屑为锐角状的石英晶屑及少量粘土和长英质岩屑。溅射物的大小从厘米级块体到毫米级球粒,再至微米级尘粉都有。溅射物外形多具有旋转扭曲形态及熔壳特征,显示了熔融、塑性-半塑性特征。这些特征显示其成因经历了冲击震碎、熔融、挖掘抛射、空中飞行,最后溅落在冲击坑及其周围。溅射物的成分反映了太湖靶岩基岩岩性特征。太湖冲击坑溅射物的发现是继太湖诸岛石英砂岩中石英晶体的冲击变质微结构发现之后,又一重大突破。综合其它特征,可以确定太湖为一冲击坑。

月球典型撞击坑溅射物研究及对月球地质编图的意义

撞击坑是月表最典型的地质单元,其溅射物作为撞击坑的坑外组成部分可分布到距离坑中心10个直径距离之外的区域,因此撞击溅射物也是月球地质编图中最重要的表达要素之一。本文使用月球勘测轨道器(LRO)的激光高度计(LOLA)数据、广角相机(WAC)影像、窄角相机(NAC)影像以及Clementine的UVVIS多光谱数据,研究了哥白尼纪正面月海区直径31km的Kepler撞击坑和背面月陆区直径30km的Necho撞击坑。哥白尼纪撞击坑溅射沉积物可以分为三个相:连续溅射沉积相(CE)、不连续溅射沉积相(DE)和辐射纹(CR)。连续溅射沉积相分布在最大约2.6个半径范围之内,不连续溅射沉积相分布在最大近11个半径范围之内,辐射纹分布在最大近29个半径范围之内。本文强调了多源数据结合在识别撞击坑溅射沉积物中的作用,对Kepler坑和Necho坑溅射沉积物进行了填图,不对称分布的特征表明这两个坑可能形成于倾斜撞击。

“嫦娥三号”降落区溅射物空间分布关系

针对“嫦娥三号”着陆区溅射物与撞击坑间缺乏多尺度耦合关系解译的问题,提出了基于大小、频率和方向的空间分布关系量化表达方法。以“嫦娥三号”降落相机影像为基础,采用人工识别的方式,提取出直径大于0.1 m的撞击坑2564个,以及尺寸大于0.1 m的石块2363个。划分不同直径石块和撞击坑所在区域避免分辨率不均的影响,并通过方位角分区研究溅射物与方向的关系。结果表明,石块、撞击坑的数量随直径的增大而减小;“紫微”撞击坑的溅射是有方向性的;着陆点恰好位于2个溅射区之间,着陆区域主要分布直径0.5 m以下的石块和撞击坑,相比于着陆区周边,该区的地形较为平坦。

嫦娥4号着陆区冯·卡门撞击坑溅射物的识别及年代分析

撞击坑是月球、火星和水星等固态行星表面重要的地貌和构造,其形成时产生的溅射物包含了大量地质信息,是认识撞击坑形成过程和区域地质演化的重要研究对象。本文以冯·卡门撞击坑为研究对象,结合月球勘测轨道飞行器(LRO)影像数据、激光高度计(LOLA)地形数据、Clementine多光谱数据等对冯·卡门撞击坑的溅射物进行综合解析。冯·卡门撞击坑的溅射物后期改造比较严重,只有南部区域保存相对完整。选取连续溅射物中受其他撞击坑溅射物影响较小的区域,并以宽角相机(WAC)数据和高分辨率地形相机(TC)数据为基础,利用撞击坑提取工具CraterTools对均一的区域进行撞击坑统计分析,进而利用月球产率和年代学函数推导出绝对模型年龄,获得了冯·卡门撞击坑的年龄为4.15Ga。本文主要强调了多源数据在识别溅射物时的重要性,以及年代分析时排除周围溅射物影响的必要性。

月球撞击坑溅射堆积与辐射纹蒙特卡洛模拟

撞击坑是月球上最常见的地质构造之一,通常只有较大规模的撞击坑才能形成超长辐射纹.然而最近我们的研究发现,在一些大规模撞击坑的边缘,溅射堆积发育有良好的孔隙结构;在这些多孔的撞击坑边缘区域,即使是小规模撞击事件,也易于形成具有辐射纹的撞击坑.本文建立了包含孔隙结构的撞击抛射模型,并利用蒙特卡洛方法模拟了垂直撞击溅射堆积构造和超长辐射纹,不仅揭示了在多孔结构岩石区域易于形成超长辐射纹的原因,同时为撞击辐射纹的形成提供了一种可行的解释方案.