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| Exterior impact melt ponded to the east of high-latitude Anaxagoras crater. A roughly 1.8 km-wide field of view from LROC Narrow Angle Camera (NAC) M185949707L, LRO orbit 12482, March 9, 2012; angle of incidence 73.08° at 1.76 meters resolution from 180 kilometers. View the 1000 x 1000 pixel original LROC Featured Image HERE [NASA/GSFC/Arizona State University]. |
Lillian Ostrach
LROC News System
Impact melt drastically altered the landscape in and surrounding Anaxagoras crater (73.458°N, 349.934°E, ~52 km diameter). Previous Featured Images focused on the interior melt pond and melt-covered mounds, but the exterior ponds are important, too. Impact crater formation is violent: all of the energy associated with a moving bolide is imparted to the target immediately upon impact. Some of the energy may vaporize the impactor, while a substantial amount of the energy is responsible for excavating the crater cavity. In many cases, some of the target rocks are heated to their melting temperatures (~1300°C for basalt, ~1500°C for anorthosite) to form impact melt. Once formed, impact melt may be excavated in a similar manner to unconsolidated ejecta. Some melt remains in the crater interior, pooled on the crater floor and terraces and forming a veneer on the crater walls, while some melt is emplaced on or near the crater rim. Similar to the crater interior, the immediate exterior of the crater may be veneered with impact melt and exhibit melt channels and flows, as well as exterior melt ponds.
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| LROC WAC monochrome (604 nm) mosaic, stitched from 10 sequential passes on March 21, 2011, shows a roughly 80 km-wide view of Anaxagoras and vicinity, itself situated on the rim of Goldschmidt to the east. Image center is 72.4°N, 350.0°E. The the location of the area detailed in the LROC Featured Image released September 12, 2012 is indicated by the yellow arrow [NASA/GSFC/Arizona State University]. |
The opening image by itself might be mistaken for a smooth mare region because there was enough impact melt to coalesce into a pond and evenly bury the underlying surface, but the WAC context image (above) shows this not to be the case everywhere. On the flanks of the eastern rim of Anaxagoras, the melt pond is very smooth in some areas (opening image, left) while relatively rougher elsewhere (opening image, right), which may indicate that this melt pond did not have enough volume to completely obscure the preexisting surface. In the very smooth portion, there are some fractures similar to those observed in other melt ponds. There are also several impact craters with irregular morphologies. These craters, all less than 100 m in diameter, may have these different morphologies for several reasons related to target properties. The 94 m diameter crater in the upper right may be irregular because the crater punched through a thin resistant layer of impact melt rock to the looser unconsolidated preexisting surface. Other nearby craters may have formed in partially molten melt or may have formed much later, after a thin layer of regolith was generated by micrometeorites. Additional study of superposed crater morphology on impact melt, paired with analog laboratory experiments, may help distinguish the difference between impacts into a partially molten material and those into a layered target (previously studied in depth by Drs. Oberbeck and Quaide, among others).
What do you think? Take some time to examine the exterior impact melt ponds in the full LROC NAC image, HERE.
Breached Levee
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| A closer look, at more than twice the resolution, we see one of the more prominent features on the impact melt pond in LROC NAC frame M185949707L, subject of the LROC Featured Image released September 12, 2012. LROC NAC M122252864L, spacecraft orbit 3150, March 3, 2010. The resolution available from only 43 km above during this earlier fly-over was 48 cm per pixel [NASA/GSFC/Arizona State University]. |
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