Impact Related Features Outside the Second Layer of Martian Double-Layer Ejecta Cra- ters: What they tell us about the Parent Crater

Unlike other types of fluidized ejecta impact craters, Martian double-layered ejecta (DLE) craters rarely have secondary impact craters [1]. In our continued effort to understand DLE and other types of Martian fluidized ejecta craters we have examined recently acquired THEMIS images, and found two exceptionally fresh DLE craters that include well-developed secondary crater fields. One of these craters (crater 1 in Figure 1) is a 12.0 km diameter, 1015 m deep crater located at 54.6o N, 190.7o E. The other crater (crater 2 in Figure 2) is a 13.0 km diameter, 1201 m deep crater located at 73.0o N, 38.3o E. Morphologically, crater 1 is a typical DLE crater, but crater 2 includes an additional thin (~ 5-10 m thick), relatively smooth and extensive outer ejecta deposit (note: technically, making this a MLE crater). It extends continuously to an average of ~ 6-8 R from the crater rim, but, in places, long narrow strands of ejecta extend to ~ 10-12 R. Secondaries from crater 1 excavate the surrounding plains. The Vastitas Borealis Formation (VBF) blankets these plains. Our preliminary data suggest secondaries from crater 1 are absent in areas nearby where the VBF has been eroded away. In contrast, secondaries from crater 2 are found only in its thin outer ejecta layer. The depths of the secondaries from both craters appear to be approximately as deep as the thickness of these weak, easily excavated and eroded surface units compared with the material beneath [1, 2, 3], and as a result would be completely erased if these deposits were removed (or filled) by erosion. Therefore, the new data suggest that weak ejecta blocks are produced by DLE craters, but unless they impact weak materials they break apart upon impact like dirt clods, leaving little trace. There are three likely ways to produce weak ejecta blocks, (1) the target rock is composed of intrinsically of weak and/or fragmented material [4], or (2) the target rock is comminuted by the cratering process, such as when significant water is present in the target materials [5, 6], or (3) when airborne blocks are crushed by the dynamic pressure during the high-velocity outflow of gas-rich ejecta [7]. The latter two require abundant volatiles, with case (2) requiring volatiles in the subsurface, and 3 requiring them either in the subsurface and/or the atmosphere. Consequently, we suggest case (3) that DLE craters require the presence of significant amounts of volatiles in order to produce the observed relationships. The amount of volatiles must have varied considerably in space and/or time during Martian history.