Class 1:通常是較大的撞擊坑,直徑基本在50~300km,撞擊坑底部(floor)較深,中央峰(centrak peak)地形複雜,有延伸的撞擊坑壁(extensive wall terraces)。撞擊坑底部通常有徑向和/或同心圓狀的裂隙(fractures),坑壁的斜坡帶(scarp)通常有新月形的月海沉積物(mare deposits)。代表撞擊坑有Humboldt, Atlas, Schülter和Cardanus,下圖ABCD分別為Humboldt撞擊坑(坐標27.2° S, 80.9° E, 直徑207 km)的等高線圖,地質示意圖(B圖中那些線性結構就是fractures),LOLA格網地形圖,白線對應的地形剖面圖,最後一張是Class 1的撞擊坑在月球上的分布圖
Class 2:中等尺度的撞擊坑,通常直徑13-75km,坑壁非常明顯,坑底上凸,有中央峰,有明顯的同心圓狀的fractures,代表撞擊坑有Vitello,Encke和Davy,下圖ABCD分別為Vitello撞擊坑(坐標30.4° S, 37.5° W, 直徑44 km),最後一張是Class 2的撞擊坑在月球上的分布圖
Class 3:撞擊坑尺度範圍較大,直徑在12-170km都有,大部分直徑約30-60km,底部較平坦(有些甚至沒有中央峰),坑壁邊緣有寬而明顯的溝壑(moat),fractures通常為多邊形和/或經向分布但不會延伸進溝壑區域,部分撞擊坑內部有月海物質,代表撞擊坑有Gassendi, Taruntius和Lavoisier,Haldane和Runge撞擊坑有典型的很寬的溝壑,下圖ABCD分別為Gassendi撞擊坑(坐標17.5° S, 39.9° W, 直徑110 km),最後一張是Class 3的撞擊坑在月球上的分布圖,幾乎所有Class 3的FFCs撞擊坑都位於盆地(basin)的內部或邊緣。題主給出的Komarov撞擊坑也屬於這一類(紅框部分)。
Class 4:特徵為具有V型的溝壑(V-shaped moat),有些具有經向和/或同心圓狀的fractures,根據溝壑V型的程度可進一步分為三類Class 4a, 4b, 4c三類。下圖為Class 4a的代表撞擊坑Bohnenberger以及三類撞擊坑的分布。
Class 5:通常較古老,直徑在12-177km,撞擊坑邊緣已退化模糊,內部有明顯的徑向、同心圓和/或多邊形的fractures,這些撞擊坑內部通常為古老的高地物質,代表撞擊坑有Von Braun,Repsold和 Alphonsus。
Class 6:除了特別大和特別小的撞擊坑之外,內部完全被月海物質填充,同心圓狀的fractures全部緊貼撞擊坑壁,部分撞擊坑有中央峰和經向fractures。代表撞擊坑有Pitatus和Fracastorius。
【FFCs地形的形成機制】
對FFCs地形的形成機制,主要有兩種猜想: 1)粘彈性鬆弛(viscous relaxation) (Masursky, 1964; Dane?, 1965; Cathles, 1975; Hall et al., 1981) 2)岩漿侵入(magmatic intrusion) (Brennan, 1975; Schultz, 1976; Wichman and Schultz, 1995; Dombard and Gillis, 2001; Jozwiak et al.,2012, 2015)
粘彈性鬆弛假說(Lunar floor-fractured craters: Evidence for viscous relaxation of crater topography)的理論基礎是:新鮮撞擊坑引起的地形起伏隨著時間推移會慢慢回彈趨於平坦(總體趨勢是低處變高高處變低),但長波(大尺度)地形比短波地形鬆弛速度更快,所以大尺度地形起伏隨著時間推移更快的恢復平坦,而短波地形則變化較慢相對保持原樣,FFCs是這種差異性的回彈形成的線性fractures。
↓ 粘彈性鬆弛的過程示意圖,A為新鮮撞擊坑形態,B為經過一段時間後的撞擊坑形態
但後來的諸多數值模擬的結果都無法支持粘彈性回彈假說(Testing the viability of topographic relaxation as a mechanism for the formation of lunar floor-fractured craters),新的更高解析度的月球高程數據LOLA和影像數據LROC也更支持岩漿侵入假說。岩漿侵入假說的理論基礎是:撞擊坑形成之後發生岩漿侵入,岩牆(dike)向地表方向不斷延伸,延伸最多的OP1類岩牆直接穿透地表引起熔岩流噴出,延伸最少的OP3直接在地下較深處固化,而中等程度的OP2類侵入到地殼淺層附近形成岩床(sill),這類岩漿侵入就可能形成FFCs地形。
下圖展示了岩漿侵入假說認為的FFCs形成過程:A. 岩牆(dike)向月球地殼方向侵入→ B. 岩牆侵入殼層,被撞擊坑底部地下的角礫岩阻擋於是停在了這裡 → C. 壓力累積使得岩牆開始橫向侵入形成岩床(sill) → D. 岩漿繼續累積使得岩床逐漸轉變為穹頂狀的岩蓋(laccolith)或者E. 更厚的岩床,FFCs地形就是岩床/岩蓋向上擠壓撞擊坑底部形成的裂隙。
Schultz, P. H. (1976). Floor-fractured lunar craters. The Moon, 15(3-4), 241-273.
Hall, J. L., Solomon, S. C., Head, J. W. (1981). Lunar floor‐fractured craters: Evidence for viscous relaxation of crater topography. Journal of Geophysical Research: Solid Earth, 86(B10), 9537-9552.
Wichman, R. W., Schultz, P. H. (1995). Floor-fractured craters in Mare Smythii and west of Oceanus Procellarum: Implications of crater modification by viscous relaxation and igneous intrusion models. Journal of geophysical research, 100(E10), 21-201.
Dombard, A. J., Gillis, J. J. (2001). Testing the viability of topographic relaxation as a mechanism for the formation of lunar floor‐fractured craters.Journal of Geophysical Research: Planets, 106(E11), 27901-27909.
Jozwiak, L. M., Head, J. W., Zuber, M. T., Smith, D. E., Neumann, G. A. (2012). Lunar floor‐fractured craters: Classification, distribution, origin and implications for magmatism and shallow crustal structure. Journal of Geophysical Research: Planets, 117(E11).
Jozwiak, L. M., Head, J. W., Wilson, L. (2015). Lunar floor-fractured craters as magmatic intrusions: Geometry, modes of emplacement, associated tectonic and volcanic features, and implications for gravity anomalies. Icarus,248, 424-447.
Komarov is the subject of Ridge Section 096. Komarov is a 78 km wide lunar crater that lies across the southeastern edge of Mare Moscoviense (Section 95), on the northern hemisphere of the far side of the Moon. It is a complex feature with an irregular appearance. The northern rim of Komarov bulges outwards into the mare, giving the crater a pear shape. The regions around the northeastern and southern rims are rugged and uneven, and the crater floor in between has been resurfaced by lava flows that have completely submerged the western third of the interior. This surface is marked by a pattern of multiple rilles that run primarily in a north-south direction, but are cracked like a drying mud bed. Along the interior of the eastern rim is a cleft-like formation that curves along the inner rim. The northwestern rim has an outer rampart where it slopes down to the plain of the neighboring mare. The best image comes from the Lunar Reconnaisance Orbiter with its Wide angle Camera (see LAC 49 WAC link above). New information for this section comes in the form of YouTube videos regarding the controversial images of Titov, which is the crater within Mare Moscoviense at upper right. (Credit: Some of the information used in the section feature descriptions was obtained from wikipedia.org. Section096 and this directory was created by Fran Ridge and Ned Haskin of The Lunascan Project).