Formation of the large impact crater field in Wyoming, USA: secondary cratering or asteroid breakup?

2018年，在怀俄明州发现了大量直径10-120米的小型撞击坑。在许多这样的陨石坑中发现了冲击效应。这个所谓的道格拉斯撞击坑散布地的大小为12.8 x 6.9公里。所有的陨石坑都在石炭-二叠系卡斯珀组的最上层呈层状分布，年龄一致约为280 Myr。现在的初步工作表明，在相同的地层层位上，在报道的陨石坑场之外还有更多的陨石坑，进一步增加了陨石坑场的规模。陨石坑的大小和年龄都非常不寻常。该提案的目的是调查怀俄明州撞击坑场的整个范围，描述陨石坑的特征，并解释陨石坑场的起源。两个基本假设将被测试：场景1 -怀俄明陨石坑是由一颗或多颗小行星进入大气层时破碎造成的。场景2 -怀俄明陨石坑场的陨石坑代表了次级陨石坑，这些陨石坑是由一个巨大的、尚未未知的主要陨石坑周围的大块物体喷出形成的。如果这一假设被证明是正确的，那么直接的问题就是主要撞击坑的大小和位置。情景1可以通过陨石抛射物的痕迹、平行的撞击轨迹和以散布场椭圆形式分布的陨石坑来验证。另一方面，情景2的特点是没有陨石抛射物的痕迹、径向撞击轨迹和大规模分布的次级陨石坑。主要撞击坑的位置将通过交叉重建的轨迹来确定。为了评估陨石坑场的大小，将使用遥感方法来检查怀俄明州东南部卡斯珀组的地层水平，以寻找额外的陨石坑结构。探测到的陨石坑将在形态、结构和岩石学上进行表征，陨石坑的椭圆性和喷出物分布的不对称性提供了撞击轨迹的信息。冲击效应的检测起着核心作用。同样重要的是利用同位素特征和亲铁元素对潜在抛射痕迹进行地球化学鉴定。将用数值方法对有利的地层情景进行定量分析。流星体破碎（场景1）的模拟采用“煎饼”和“分离碎片”模型。在二次陨石坑形成的情况下（场景2），使用iSALE的数值模拟将首先确定二次陨石坑的冲击能量和速度，然后模拟考虑大气的弹道路径，最后模拟主要陨石坑本身。通过对整个冲击过程的正演模拟，验证了后向建模的正确性。环境干扰，如热辐射的喷出羽流或大气冲击波也将被模拟。

In 2018, a large number of small impact craters, 10-120 m in diameter, was discovered in Wyoming. Shock effects were detected in many of these craters. The size of this so-called Douglas impact crater strewn field is 12.8 x 6.9 km. All craters occur stratiform in the uppermost layer of the Permo-Carboniferous Casper Formation and have a uniform age of about 280 Myr. Preliminary work now shows that additional craters are located outside the reported crater field in the same stratigraphic horizon, further increasing the size of the crater field. The size and age of the crater field are extremely unusual. The objective of this proposal is to survey the entire extent of the Wyoming impact crater field, characterize the craters, and interpret the origin of the crater field. Two basic hypotheses will be tested: Scenario 1 - The Wyoming crater field was caused by fragmentation of one or more asteroids as they entered the atmosphere.Scenario 2 - The craters of the Wyoming crater field represent secondary craters formed by ejection of large blocks from around a large, as yet unknown, primary crater. If this hypothesis proves true, the immediate question is the size and location of the causative primary impact crater. Scenario 1 can be verified by evidence of traces of the meteoritic projectile, parallel impact trajectories, and by a distribution of craters in the form of a strewn field ellipse. Scenario 2, on the other hand, can be characterized by the absence of traces of meteoritic projectile material, radial impact trajectories, and a large-scale distribution of secondary craters. The location of the primary crater would be determined by intersecting the reconstructed trajectories.To assess the size of the crater field, remote sensing methods will be used to examine the stratigraphic horizon of the Casper Formation in southeastern Wyoming for additional crater structures. Detected craters will be characterized morphometrically, structurally, and petrographically, with crater ellipticity and asymmetry of ejecta distribution providing information on impact trajectory. The detection of shock effects plays a central role. Equally important is the geochemical identification of potential projectile traces using isotopic signatures and siderophile elements. The favored formation scenario will be quantitatively analyzed by numerical methods. Simulation of meteoroid fragmentation (Scenario 1) is performed by the "Pancake" and "Separated Fragments" models. In the case of secondary crater formation (Scenario 2), numerical simulation using iSALE will first determine the impact energy and velocity of the secondary craters, then model the ballistic path taking into account the atmosphere, and finally model the primary crater itself. The backward modeling is validated by forward modeling of the entire impact process. Environmental disturbances such as heat radiation from the ejecta plume or the atmospheric shock wave will be also simulated.