Lunar Day Eight
Let us begin our Moon walk tonight with a deeper look at the “Sea of Rains” (Figs. 9.1 and 9.2). Our mission is to explore the disclosure of Mare Imbrium, home to Apollo 15. Stretching out 1,123 km over the Moon’s northwest quadrant, Imbrium was formed around 38 million years ago when a huge object impacted the lunar surface creating a gigantic basin. The basin itself is surrounded by three concentric rings of mountains. The most distant ring reaches a diameter of 1,300 km and involves the Montes Carpatus to the south, the Montes Apenninus southwest, and the Caucasus to the east. The central ring is formed by the Montes Alpes, and the innermost has long been lost except for a few low hills which still show their 600 km diameter pattern through the eons of lava flow. Originally, the impact basin was believed to be as much as 100 km deep. So devastating was the event that a Moon-wide series of fault lines appeared as the massive strike shattered the lunar lithosphere. Imbrium is also home to a huge mascon, and images of the far side show areas opposite the basin where seismic waves traveled through the interior and shaped its landscape. The floor of the basin rebounded from the cataclysm and filled in to a depth of around 12 km. Over time, lava flow and regolith added another 5 km of material, yet evidence remains of the ejecta which was flung more than 800 km away, carving long runnels through the landscape.