AAQ Solar System Section

Lunar Observing Programme - Crater Moltke

This article describes Crater Moltke, the first feature targeted by the AAQ Lunar Observing Programme. This feature was selected for the observing programme due to it being an excellent example of a recent simple crater and its proximity to Statio Tranquillitatis, the Apollo 11 landing site. (Note that the Apollo 11 astronauts did not visit this crater).

Crater Moltke was named after Helmut Karl von Moltke (1800 – 1891AD), a Prussian Field Marshall who secured the publication of a map of the Moon produced by the German selenographer, Johann Schmidt. Figure 1 illustrates the location of Crater Moltke. The crater is positioned near a small embayment on the south-southwest margin of Mare Tranquillitatis, the Sea Of Tranquillity. Despite its small size the crater is relatively easy to find, particularly near Full Moon, due to the contrast of the bright ejecta blanket against the dark background of the mare. Sunrise occurs approximately 5.5 days after New Moon and sunset occurs approximately 5.5 days after Full Moon. High magnification (e.g. > 200 x) is required to observe crater detail.

Facts

Location:

Diameter:

Depth:

Age:

Sunrise:

Sunset:

0.6°S, 24.2°E

6.5 km

1.3 km

Copernican

~ Day 5.5

~ Day 20.5

Figure 1: Location Of Crater Moltke
Full Moon image courtesy of Russell Croman, © 2005 Russell Croman, www.rc-astro.com
Inset image courtesy of Brent Joyce

Figure 2, taken during the Apollo 10 mission, shows that Crater Moltke is an archetypal simple impact crater with a raised sharp circular rim 6.5 km in diameter, a smooth bowl shaped interior depression 1.3 km deep and a bright undisturbed relatively circular ejecta blanket surrounding the crater.

Figure 2: Crater Moltke
Image courtesy of NASA (AS10-29-4324)

Close to the crater rim the ejecta forms a continuous blanket which is hummocky in profile. As the radial distance increases from the crater the ejecta blanket becomes discontinuous and exists as a series of low hummocks and radial and tangential ridges. Numerous smaller craters surrounding the main crater are also evident in Figure 2. These smaller craters are called secondary craters and are produced when large fragments of ejecta from the main crater impact the surrounding area. Secondary craters are often noncircular due to the low velocities (typically 2 - 3 km/s) and low inclination trajectories during impact. Clusters or chains of secondary craters are visible in the image.

Whilst both the interior and immediate exterior of the crater appear smooth in Figure 2, it is likely that both surfaces would be quite rough and uneven over distances of tens of metres. The probable small scale roughness at Crater Moltke is likely to be similar to that shown in Figure 3, an Apollo 17 mission photograph of the interior of Shorty Crater.

Figure 3: Interior Of A Simple Crater - Shorty
Image courtesy of NASA (AS17-137-20992)

Information on the formation processes of simple impact craters such as Moltke can be found in Heiken et al (1991) and Taylor (1982) or in my article “Simple Impact Crater Formation” published in the AAQ December 1999 Newsletter. During impact some of the rock would be vapourised or melted or undergo shock metamorphism, some glass would be produced and the underlying rock would be intensely fractured. Much of this altered material is ejected from the crater and forms the surrounding ejecta blanket. However some of the melted, shocked and fragmented rock slump back into the crater and line the crater floor with a new rock type called breccia that consists of angular fragments of rock, minerals and glass surrounded by a fine grained matrix of pulverised and melted rock. Breccia composition can vary widely from site to site.

The circular outline of the crater rim and surrounding ejecta blanket suggests the meteoroid, which was probably 250 – 450 m in diameter, struck the lunar surface on a high angle and perhaps subvertical trajectory. As the crater is located on the surface of Mare Tranquillitatis, it must be younger than the Imbrian Period (radiometric dating of basalt rock samples returned by the Apollo 11 mission indicate that the mare is 3570 - 3860 million years old). The sharp rim and bright ejecta blanket, that is free of degradation, suggests Crater Moltke formed during the Copernican Period, making it is less than 1000 million years old. The stratigraphy at the Apollo 11 landing site and hence the Crater Moltke area is interpreted as an approximately 5 m thick surface regolith layer overlying mare basalt that in turn overlies ejected material from the Imbrium, Serenitatis and Nectaris impact basins. The mare basalt layer is believed to be less than 500 m thick in this area due to its proximity to the Tranquillitatis Basin margin. Consequently Crater Moltke has probably excavated through the mare basalts into the underlying Imbrium Basin impact ejecta. Some of the feldspathic rock samples that originated from the lunar highlands and were recovered from the regolith by the Apollo 11 astronauts at the landing site may have been distributed there by the Moltke impact.

Other nearby features of interest, some of which are shown in Figure 4, include:
• Rimae Hypatia -a network of graben (rift valley) rilles 180 km long and approximately 3 km wide that have been offset, possibly by faulting, at a few locations;
• Craters Armstrong (4.6 km diameter), Aldrin (3.4 km) and Collins (2.4 km) named after the Apollo 11 astronauts; and
• Wrinkle ridges running radial to Mare Tranquillitatis associated the “ghost crater” Lamont.

Figure 4: Crater Moltke Area
Image courtesy of NASA (AS11-37-5447)

An interesting exercise is to compare Crater Moltke with other larger simple craters such as Crater Hortensius (14.6 km), Crater Birt (17.0 km) and Crater Rosse (12.0 km) that are located near other features targeted by the AAQ Lunar Observing Programme. As these larger craters are nearing the transition from simple to complex craters their interiors do not have the same classical smooth bowl shaped depression of Crater Moltke. What other morphological differences can you observe with these craters?