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Remnants of the Imbrium Impact
Mare basalts embayed ejecta structures formed by the massive Imbrium impact in LROC NAC image M131501983R. Arrows denote the contact between younger mare basalts and older Imbrium ejecta, image width is 902 m. Credit: NASA/GSFC/Arizona State University
Mare basalts fill most of the large impact basins on the Moon, and in many cases the pre-existing topography was buried by the huge outpourings of lava. However, sometimes pre-existing topography is not completely buried by the mare basalts. When lava flows around a topographic high and does not bury it completely, the resulting landform is called a kipuka, and may be used to tell scientists about the region before the lavas flowed across the landscape. In the case of today's Featured Image, this kipuka in southwestern Lacus Somniorum is probably ejecta from the impact that formed the Imbrium basin. When looking at a regional view, this knob and others form relatively linear chains which can be traced back to the Imbrium basin.
Larger image not available LROC Wide Angle Camera view of southwestern Lacus Somniorum; arrow points to location of today's Featured Image. LROC WAC inage M117339055M. Credit: NASA/GSFC/Arizona State University
Since the mare basalts are embaying this feature, the basalts postdate the formation of the Imbrium basin. Using remotely-sensed data to establish this kind of geologic relationship on the lunar surface helps to clarify the geologic history of the Moon.