彗木两次碰撞过程分析

Ｓｈｏｅｍａｋｅｒ－Ｌｅｖｙ９号彗星与木星碰撞，是人类有史以来第一次有预报的天体大碰撞。有关观测的初步结果，秀快通过新闻报道和电子网络传遍了全世界。本文通过对其１４号、１１号核撞击木星的大量报道的综合分析，对彗木碰掸的物理过程、彗木碰撞时刻的确定等重要问题进行了探讨。     

彗木碰撞时刻及位置的确定

Ｓｈｏｅｍａｋｅｒ－Ｌｅｖｙ０号彗星于１９９４年７月与木星碰撞已成为事实。本文依据众多碰撞时刻的观测报道，并结合碰撞预报，对其进行了系统研究。分析了用不同观测技术和观测手段所给出的碰撞时刻的报道，强调了碰撞物理过程的重要性。最后，按照统一的物理模式初步确定了各个碎核的碰撞时刻。并归算出木星南纬碰撞带区的自转速度约为９ｈ５５ｍｉｎ一周，说明它没有受到彗核连续撞击的影响。本文还将各彗核撞击点在木面上的     

彗木碰撞时刻及位置的确定

Shoemaker－Levy9号童星于1994年7月与木星碰撞己成为事实．本文依据众多碰撞时刻的观测报道，并结合碰撞预报，对其进行了系统研究．分析了用不同观测技术和观测手段所给出的碰撞时刻的报道，强调了碰撞物理过程的重要性．最后，按照统一的物理模式初步确定了各个碎核的碰撞时刻．并归算出木星南纬碰撞带区的自转速度约为9h55min-周，说明它没有受到营核连续撞击的影响．本文还将各管核撞击点在水面上的位置与预报做了比较．     

1．56m望远镜观测彗－木相撞概况

用上海天文台佘山观测站的１．５６ｍ望远镜和ＣＣＤ照相机，观测到彗－木碰撞的六次事件。本文给出由撞击引起的木卫闪光Ｋ＝１２的光变曲线和一些碰撞后木星的照片。     

1.56m望远镜观测彗—木相撞概况

用上海天文台佘山观测站的１．５６ｍ望远镜和ＣＣＤ照相机，观测到彗－木碰撞的六次事件。本文给出由撞击引起的木卫闪光Ｋ＝１２的光变曲线和一些碰撞后木星的照片。     

心事浩渺连广宇──记彗木碰撞标志征集活动

心事浩渺连广宇──记彗木碰撞标志征集活动今年春天，苏梅克一利维９号慧星即将于今年７月份撞击木星的事件在我们北京天文台一批年轻天文学者当中引起了很大反响，我们决心以团委名义组织一项活动以纪念这一历史性天文事件，同时希望借此普及天文知识，宏扬天文文化，增...     

彗木相撞中的惯性引力波

苏梅克-列维9号彗星(SL9)与木星相撞后，在木星上观测到的以常速度(～450m／s)向外扩展的圆环意味着这是碰撞在木星大气中引起的线性波动．我们选取：非旋转、无粘性、密度分层、不可压缩的木星大气模型，而且木星大气以水平速度U=b az运动；给出初始扰动压力P(r；0)作为碰撞的初始条件，用流体力学方程组求解了彗木相撞中的惯性引力波．结果表明：当木星大气以速度U=U0(～170m／s)运动时，彗星碎片的大部分能量都用来产生内波，同时还得到彗星碎片的撞击深度H与水平相速Vp的关系式．当木星大气以速度为U=b az运动时，木星大气的扰动能量不再是在动能和势能间均分。     

我国对苏梅克—列维9号彗星撞击木星事件的观测

１９９４年７月１６日－２２日，苏梅克－列维９号周期彗星的２１颗碎片连续撞击木星事件是一次极为罕见的天文现象。文中概括介绍了我国天文学家对彗木相撞事件的光学和射电观测网点、课题设置以及所取得的主要观测结果。     

地球也曾受连珠炮式的撞击

地球也曾受连珠炮式的撞击林文祝苏梅克一利维９号慧星连续撞击木星，所产生的景观引人注目。观察这一次慧一木碰撞不仅可以获得慧星“杂质”和木星较深处的物质成分，而且还可以提供我们有关行星演化史上的撞击事件的信息。物体相互间碰撞是太阳系行星演化的基本过程之一...     

彗木大碰撞的联想

彗木大碰撞的联想余新惠悲哉？壮哉？９４之夏的“替木大碰撞”为人类展现了一幕惊心动魄的宇宙奇观。地球人不约而同地把关注的目光投向浩瀚宇宙间的木星。木星，这颗太阳系中最大的行星，在深这的茫茫夜空中在古闪烁，令人高深莫测，有史以来，一直受到人类的顶礼膜拜，...     

The age of the Saltpan impact crater,South Africa

Abstract— The 1.13-km-diameter Pretoria Saltpan impact crater is located about 40 km NNW of Pretoria, South Africa. The crater is situated in 2.05 Ga old Nebo granite of the Bushveld Complex that is locally intruded by about 1.3 Ga old volcanic rocks. In 1988, a borehole was drilled in the center of the crater. At depths >90 m, breccias were found that contained minerals with characteristic shock-metamorphic features, thus confirming the impact origin of the crater. Fragments of impact glass were recovered from the melt breccias and several hundred sub-millimeter-sized glass fragments were subjected to fission track analysis. The measurements were complicated by the inhomogeneous composition of the impact glasses, but analysis of a large number of tracks yielded an age of 220 ± 52 ka for the Saltpan crater.     

Geology and impact features of Riachão structure,northern Brazil

Riachão, located at S7°42′/W46°38′ in Maranhão State, northeastern Brazil, is a complex impact structure of about 4.1 km diameter, formed in Pennsylvanian to Permian sedimentary rocks of the Parnaíba Basin sequence. Although its impact origin was already proposed in the 1970s, information on its geology and shock features is still scarce in the literature. We present here the main geomorphological and geological characteristics of the Riachão impact structure obtained by integrated geophysical and remote sensing analysis, as well as geological field work and petrographic analysis. The identified lithostratigraphic units consist of different levels of the Pedra de Fogo Formation and, possibly, the Piauí Formation. Our petrographic analysis confirms the presence of shock‐diagnostic planar microdeformation structures in quartz grains of sandstone from the central uplift as evidence for an impact origin of the Riachão structure. The absence of crater‐filling impact breccias and melt rocks, shatter cones, as well as the restricted occurrence of microscopic shock effects, suggests that intense and relatively deep erosion has occurred since crater formation.     

Energy loss and impact cratering in aerogels: theory and experiment

Aerogel collectors have been deployed in low-Earth orbit to collect orbital debris and micrometeorites. An array of silica aerogel collectors is currently en-route back to Earth following an encounter with the Comet Wild-2 on board the Stardust spacecraft. Stardust is returning, for laboratory analysis, cometary and interstellar dust grains which impacted into the aerogel collectors at hypervelocities. While the morphology of impact craters in aerogels has been studied empirically, a theoretical understanding of the physical mechanisms responsible for the formation of impact craters in these solids is lacking. Here we propose and test a model of compaction driven impact cratering in aerogels. Our model derives impact crater dimensions directly from energy and momentum deposition.     

Numerical simulation of impact cratering on granular material

A new numerical code based on the Distinct Element Method (DEM) is developed to study the impact cratering processes on granular material. This code has a potential advantage to simulate the cratering process on granular material, since the movement of discrete particles can be treated. To show the physical plausibility of this code, we conduct 3-D numerical simulations of vertical impact into granular material targets that consist of 384,000 particles, and compare the results with those from experimental studies. It is shown that the excavation stage of cratering derived from experimental studies is represented well by our simulation: the size of the crater cavity, and the ejecta velocity and angle distributions are consistent with those obtained in laboratory experiments. The impact simulation code developed in this study is thus suggested to be useful for the analysis of the impact cratering process on granular material.     

Impact solutions of Asteroid 2007 WD5 with Mars

A method for computing impact probabilities between asteroids and the planet Mars is presented that uses impact clones and validation analysis based on a normal distribution of computed errors. This method uses OrbFit software, and we present a calculation of the impact probabilities between Asteroid 2007 WD₅ and Mars, which passed within about 20,000 km of the martian surface on January 30, 2008. This method can be generalized for computing impact probabilities between asteroids and other planets including Earth. Presented method applies in principal the same technique already in use for years at the JPL NASA and by the group of researchers at the University of Pisa [Milani, A., Chesley, S.R., Sansaturio, M.E., Tommei, G., Valsecchi, G.B., 2005a. Icarus 173, 362].     

Algorithms for Lunar Flash Video Search, Measurement, and Archiving

Lunar meteoroid impact flashes provide a method to estimate the flux of the large meteoroid flux and thus their hazard to spacecraft. Although meteoroid impacts on the Moon have been detected using video methods for over a decade, the difficulty of manually searching hours of video for the rare, extremely brief impact flashes has discouraged the technique’s systematic implementation. A prototype has been developed for the purpose of automatically searching lunar video records for impact flashes, eliminating false detections, editing the returned possible flashes, and archiving and documenting the results. Several utilities for measurement, analysis, and location of the flashes on the moon included in the program are demonstrated. Application of the program to a year’s worth of lunar observations is discussed along with examples of impact flashes.     

A new method to determine the direction of impact: Asymmetry of concentric impact craters as observed in the field (Lockne), on Mars,in experiments,and simulations

Most impacts occur at an angle with respect to the horizontal plane. This is primarily reflected in the ejecta distribution, but at very low angle structural asymmetries such as elongation of the crater and nonradial development of the central peak become apparent. Unfortunately, impact craters with pristine ejecta layers are rare on Earth and also in areas with strong past or ongoing surface erosion on other planetary bodies, and the structural analysis of central peaks requires good exposures or even on‐site access to outcrop. However, target properties are known to greatly influence the shape of the crater, especially the relatively common target configuration of a weaker layer covering a more rigid basement. One such effect is the formation of concentric craters, i.e., a nested, deeper, inner crater surrounded by a shallow, outer crater. Here, we show that with decreasing impact angle there is a downrange shift of the outer crater with respect to the nested crater. We use a combination of (1) field observation and published 3‐D numerical simulation of one of the best examples of a terrestrial, concentric impact crater formed in a layered target with preserved ejecta layer: the Lockne crater, Sweden; (2) remote sensing data for three pristine, concentric impact craters on Mars with preserved ejecta layers further constraining the direction of impact; as well as (3) laboratory impact experiments, to develop the offset in crater concentricity into a complementary method to determine the direction of impact for layered‐target craters with poorly preserved ejecta layers.     

Remote sensing of terrestrial impact craters: The TanDEM‐X digital elevation model

With the TanDEM‐X digital elevation model (DEM), the terrestrial solid surface is globally mapped with unprecedented accuracy. TanDEM‐X is a German X‐band radar mission whose two identical satellites have been operated in single‐pass interferometer configuration over several years. The acquired data are processed to yield a global DEM with 12 m independent posting and relative vertical accuracies of better than 2 m and 4 m in moderate and mountainous terrain, respectively. This DEM provides new opportunities for space‐borne remote‐sensing studies of the entire sample of terrestrial impact craters. In addition, it represents an interesting repository to aid in the search for new impact crater candidates. We have used the TanDEM‐X DEM to investigate the current set of confirmed impact structures. For a subsample of the craters, including small, midsized, and large structures, we compared the results with those from other DEMs. This quantitative analysis demonstrates the excellent quality of the TanDEM‐X elevation data. Our findings help to estimate what can be gained by using the TanDEM‐X DEM in impact crater studies. They may also be beneficial in identifying the regions and morphologies where the search for currently unknown impact structures might be most promising.     

Geochemical signals of the late Jurassic,marine Mjølnir impact

Abstract— Of the only seven submarine impact craters that have been found globally, the Mjølnir crater is one of the best preserved and retains crater and ejecta. Geochemical studies (organic pyrolysis using the Rock Eval technique and XRF analysis for major, minor, and trace elements) of the Institute for Petroleum Research (IKU) core 7430/10-U-01 that was taken from a drillhole located ～30 km north-northeast of the crater rim show gradual establishment of anoxic sea floor conditions through the late Jurassic. These poorly ventilated water conditions were overturned due to the Mjølnir impact event. Waves and currents transported impact glass (which is now partly weathered to smectite) into the depositional area where the drillhole is located. The succeeding crater collapse transported impact material (e.g., shocked quartz and Ir) from the crater rim and deeper levels to the core site. Normal marine depositional conditions were established a short time after the crater collapsed.     

Spots on asteroid surfaces: Research opportunities using ground-based instruments

We discuss possible mechanisms for the formation of albedo spots on asteroids. We infer that the most likely mechanisms are impact cratering and related processes. This is confirmed by the reflectance spectra of the asteroids 10 Hygiea, 135 Hertha, and 196 Philomela, the results of a spectral frequency analysis of the sizes of features on the surface of 4 Vesta and 21 Lutetia, and the estimates for the parameters of impact features.