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Three holes in a row: but one is a lava-filled lunar basin that we didn't realize was there.

The realization in the late 1940s that the Moon's dark, circular maria exist within very large impact craters (called basins) was profoundly important. Excavated during titanic impacts, these big, deep holes threw their ejecta widely across the Moon, later became filled with mare lava flows, and eventually developed concentric fractures and mare ridges as they subsided. Basins are thus the most important lunar landforms.

When observing the Moon, your eye is drawn to the major lava-filled basins such as Imbrium, Crisium, Humorum, and Nectaris, but numerous smaller ones are often overlooked.

A powerful tool that has helped geophysicists recognize basins is the global gravity map created by NASA's Gravity Recovery and Interior Laboratory (GRAIL) orbiters, twin craft that circled in tandem at low altitude before crashing onto the lunar surface in December 2012 (S&T: June 2015, p. 54). New Zealand amateur Maurice Collins created the color map at lower left that is a mash-up of GRAIL gravity data with a visualization of Lunar Reconnaissance Orbiter digital topography. The area shown, about 1,000 kilometers (650 miles) wide, stretches from west of Copernicus to just east of Manilius.

Colors represent the local gravity field gradients (from an analysis by Jeffrey C. Andrews-Hanna of the Colorado School of Mines), with blue indicating where the field is stronger, and red weaker, than average. These variations result from the amount of mass present near the surface. A lower-mass signature can be caused by a big hole in the ground or by lower-density rocks such as those of the lunar highlands. Higher gravity readings require dense rocks such as lava flows or, more importantly, an upwelling of the lunar mantle beneath the surface.

Basins are usually surrounded by uplifted circular rims of pulverized, overturned rock that create gravity "lows" (notice the red rim of Serenitatis at upper right). The interiors of many basins are pronounced gravity "highs," the famous mass concentrations or mascons discovered in the early days of lunar exploration.

The exceptional spatial resolution of the GRAIL data allows unprecedented delineation of gravity highs and lows, and Collins' mash-up clearly defines three smaller basins southeast of Mare Imbrium. Two of these can be seen by eye in the topography, but the third is nearly invisible.

Mare Vaporum very likely fills a small impact basin even though it isn't ringed with mountains. Instead, notice the modest gravity high caused by dense lava filling a depression. GRAIL data clearly show a curved red rim on the southern side of the mare. The diameter of the Vaporum basin, as defined by the partial red ring, is about 250 km.

At the telescope you can observe that the end of the rille Rima Hyginus abruptly stops when it reaches the southeast edge of Mare Vaporum, showing that those lavas are younger than the rille.

A more pronounced red gravity ring defines the basin of Sinus Aestuum, which retains no remnant of its topographic rim except for the isolated mountain southwest of Eratosthenes. We used to think this little elongated mountain was an isolated plop of Imbrium ejecta, but the gravity data suggest that in reality it's part of the small basin's rim. Based on the red gravity ring, the basin is about 275 km wide.

The Aestuum mascon is more pronounced than Vaporum's, perhaps indicating a thicker lava filling. The ridges that form an inner basin ring at Aestuum were produced by faulting in the mare when the underlying basin subsided.

A red ring and blue mascon "bull's-eye" defines a third small basin on which Copernicus later appeared. This small, 200-km-wide basin displays no convincing evidence of a topographic rim and, unlike Vaporum and Aestuum, wasn't identified previously. Some lunar geologists suspected it was the eastern half of a larger, rather speculative basin called Insularum. However, thanks to the GRAIL data, we now know that no basin underlies Mare Insularum. So, to avoid confusion, I informally call this newly recognized feature "Copernicus-Fauth basin," after the only two named craters within it.

The fact that Copernicus formed directly above the basin's now-absent rim might explain a peculiar compositional aspect of the crater's central peaks. These contain the mineral olivine, thought to be common in the lunar mantle. The rim of the Copernicus-Fauth basin might include olivine excavated from great depth and later brought to the surface as the Copernicus peaks.

Volcanism, which commonly occurred inside large basins, also marks all three of these smaller ones. For example, a small, steep-sided volcanic dome was recently recognized along the north shore of Mare Vaporum.

The Moon's most extensive deposits of pyroclastic (ash) material occur along the eastern and southern shores of Aestuum, and a similar deposit lies along the southeast rim of my Copernicus-Fauth basin.

All of these volcanic features can be observed at the telescope, but it's a bigger challenge to find evidence for the rims of these three small basins.

The Moon * August 2015

Phases

* LAST QUARTER August 7,2:03 UT

* NEW MOON August 14,14:53 UT

* FIRST QUARTER August 22,19:31 UT

* FULL MOON August 29,18:35 UT

Distances

Perigee 225,023 miles

Apogee 252,182 miles

Perigee 222,631 miles

August 2,10h UT diam. 33' 13"

August 18,3h UT diam. 29' 17"

August 30,15h UT diam. 32' 59"

Librations

Pingre (crater)      August 2
Pascal (crater)      August    10
Mare Humboldtianum   August 17
Eichstadt (crater)   August 30
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Title Annotation:OBSERVING: Exploring the Moon
Author:Wood, Charles A.
Publication:Sky & Telescope
Date:Aug 1, 2015
Words:896
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