A Lunar Dichotomy

This LROC Narrow Angle Camera (NAC) frame exposes two seemingly different lithologies; one dark with several craters, and one light with few. What are we observing? LROC NAC observation M1113062041RE, spacecraft orbit 16278, January 17, 2013; 70 cm resolution of a field of view 700 meters across [NASA/GSFC/Arizona State University].

Drew Enns
LROC News System

Today's Featured Image shows a very interesting dichotomy on the Moon. One half of the image displays a dark cratered surface, while the other half is lighter in tone with less craters and a different texture. What could cause the difference in brightness, cratering, and texture? What sort of geologic contact are we observing?

Perhaps the bright material is younger, and as a result has fewer craters. Young lunar craters and their ejecta have a higher reflectance than older ones, and their ejecta blankets are only lightly cratered. The darker material could be older lunar terrain that the ejecta has overlain, and we are looking at the contact between these two surface types.

We could also be observing the interior of a crater. The texture of the brighter material is consistent with textures we see on sloped surfaces such as a crater wall. The difference in crater density can be explained as a result of the sloped wall of the crater. The darker half might then be the impact melt at the bottom of the crater floor.

Another hypothesis might be that we are observing the geologic contact between mare basalts and the lunar highlands. The mare are darker than the highlands but should have fewer craters. But that is not what we observe, unless the lighter material is on a slope (such as in our crater interior hypothesis). Which one of these hypotheses is correct? Let's look at a contextual view from the LROC WAC for help.

Context LROC Wide Angle Camera (WAC) 100 km-wide field of view designating the location shown at high resolution in the LROC Featured Image, released February 12, 2013. Northeast Mare Ingenii, just beyond the nearly buried rim of Thompson crater on the lunar far side. The contact coordinates are 31.069°S, 168.657°E. This Image width is 100 km [NASA/GSFC/Arizona State University].

The WAC mosaic provides the means to test our hypotheses instantly! The Featured Image is observing the geologic contact between mare basalts in Mare Ingenii and the surrounding highland material. This contact is important for the Moon since the mare and highlands are the two major rock lithologies present on the lunar surface. In fact, we can see this contact while standing outside at night and looking up at the Moon with the naked eye! While Mare Ingenii is on the farside of the Moon, the nearside maria are easy to distinguish against the lighter backdrop of the lunar highlands.

Apollo 15, beginning its 14th orbit, 1971, reemerging into day high to the northwest, delivered this beautiful oblique context photograph of the area of interest, Thompson crater and the surrounding quadrant of northeast Mare Ingenii (the contact is near center in this full resolution inset from AS15-87-11724) in the stark relief of the lengthening shadows of late afternoon [NASA/JSC].

The full frame of the Apollo 15 frame of Mare Ingenii taken from a point nearly antipodal to the expedition's landing site at Hadley Rille. Catch the high-resolution reproduction at the Apollo Lunar Surface Journal HERE [NASA/JSC/ALSJ].

Explore more of the Mare Ingenii and the nearby highlands massifs in the full LROC NAC, HERE

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Nearly forty years later, Japan's unmanned lunar orbiter SELENE-1 (Kaguya) captured Mare Ingenii in breathtaking HDTV, including the ancient impact basin's distinctive albedo swirl fields (left) precipitated over anomalous local magnetic fields. It's what most planetary scientists think of when the Sea of Ingenuity is under discussion. The area highlighted in the LROC Featured Image is near the Ingenii basin's northwestern extremes but also just beyond the huge Thompson ghost crater (center. See inset below) [JAXA/NHK/SELENE].

Inset from the oblique HDTV still of Mare Ingenii, the location near the field of view captured in the LROC Featured Image designated with an arrow. The swirls of Mare Ingenii draw the eye in the full-size 16:9 still, though far beyond the Ingenii basin rim, hugging the horizon, is most of the oblong, equally intriguing interior of the Van de Graaff basin. All of these scenes, and beyond, are a part of the widespread South Pole-Aitken basin [JAXA/NHK/SELENE].