Telescope Reviews: Mare Nectaris: A Sweet Sea

Beautiful Mare Nectaris, the Sea of Nectar, located in the Moon’s southeastern quadrant is of special interest to me as a lunar observer because it so dramatically displays multi-ring impact basin morphology. Despite it’s age and the fact that it is smaller than most of the large basins, this small dark circular mare and its bulls-eye rings of concentric scarps and motes are striking examples of what happens when the big boys come to play.

Generally, Multi-Ringed Basins are where the largest of the last big impacts occurred on the Moon. Striking with a velocity of 10 to 25 km/sec, the asteroid that produced Mare Nectaris was in the range of 30 to 50 km in diameter. That’s actually relatively small as basin forming asteroids are concerned, many nearside basins were formed from collisions with asteroids in the range of 100-150 km in diameter. The kinetic energy from the impact produced expanding seismic shock waves, a tremendous moonquake that distorted the surrounding crust, much as a circular ripple spreads out from the center of the disturbance caused by a pebble thrown into a calm pond. As these waves in the crustal rock thrust the surface up, the highest concentric zones were fractured and remained uplifted as arcuate scarps above lower relief zones. Similar to all multi-ringed structures on the Moon, the ring spacing around the Nectaris basin follows a simple square root of 2 relation. Take any ring diameter and multiply by 1.414 and you get roughly the next ring outward from the center. By far the most dramatic of these is the Rupes Altai or the Altai Scarp, whose ancient and irregular cliffs tower between 3.5 and 4.0 kilometers high forming a semi-circular arc hundreds of kilometers long. In my image, you can trace it from near the Mons Penck massif and, arcing around southward, where it tapers away past Piccolomini. The Mare Nectaris basin is deeper and has a much less extensive mare lava fill than most basins, a fact that as Charles Wood points out, allows us to view the multiple rings relatively free of lava submersion that most near side basins have suffered.

The target area was 70 km thick , heavily impacted highlands, with the Tranquillitatis basin to the north and the Fecunditatis basin to the northeast. Paul Spudis, in his monumental “The Geology of Multi-Ringed Basins” (Cambridge University Press, 2005) suggests that the deficit in highland material due to these two preexisting basins and their encroaching lava flows resulted in the lack of rings to the north and east of Nectaris. In addition he indicates that it is possible that the lack of large craters in the Alphonsus category, to the west of Mare Nectaris in the south eastern mid highlands, may be explained as Nectaris ejecta burying preexisting craters of this size. Undoubtably, the crustal thickness was a major factor in the preservation of the basin ring structure.

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Rich

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