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Featured Image - (04/22/2008)
The Complex Lunar Mare

The multifaceted nature of the lunar mare is apparent in this Apollo 15 metric mapping photograph (Figure 1, below) showing a region in Mare Imbrium northwest of Euler crater (diameter: 28 km) under low Sun conditions.  Euler is in the lower right of this image and the ejecta blanket of the crater Diophantus is barely visible in the upper left of the image.  
AS15-M-1702
Figure 1.  Region of Mare Imbrium between Euler and Diophantus craters under low Sun conditions. Numerous secondary craters that postdate the formation of the mare dot the surface. (Apollo Image AS15-M-1702 [NASA/JSC/Arizona State University])
Annotated AS15-M-1702
Figure 2. Region of Mare Imbrium between Euler and Diophantus craters under low Sun conditions with specific lunar geomorphology features highlighted.  A close-up view of the region highlighted with the rectangle is provided as Figure 3. (Apollo Image AS15-M-1702 [NASA/JSC/Arizona State University])
Close-up view of dendritic sinuous rille.
Figure 3.  Close-up view of a lunar sinuous rille in Mare Imbrium northwest of Euler crater.  The dendritic nature of sinuous rilles is evident in this image. (Apollo Image AS15-M-1702 [NASA/JSC/Arizona State University])

Although the lunar mare look uniform to the naked eye from Earth, the low Sun angle in this close-up photograph shows the complex geomorphological features on this mare surface.  Three of the most prominent features are highlighted in Figure 2 (above), and include:
  • Multiple lava flows.  Many lava flows can be distinguished in this image.  The edges of mare lava flows often have lobate morphology and are sometimes referred to as "lobes." The outline of one of these lobes is shown in the upper right of Figure 2.  All of these lava flows point to the northeast (in the approximate direction of the arrow in the upper right of this image), which indicates that the mare materials that filled this part of the Imbrium basin flowed from the lower left to the upper right.
  • Mare ridges, which are caused by the subsidence and contraction of mare lavas.
  • Numerous small sinuous rilles that with dendritic branching that resembles the shape of terrestrial rivers.  These look very similar to terrestrial rivers formed by the erosion of water. However, these sinuous rilles were actually carved into the lunar surface by very hot and thus very fluid flowing lavas that thermally eroded the lunar surface . Figure 3 shows a close up view of one of these sinuous rilles. Notice how the rille weaves and branches off in different directions.
During stays at a lunar outpost, astronauts will make extended exploration traverses across complex lunar terrain like this one.  In order for lunar geoscientists to fully understand the complex geology of regions such as this, we will have to combine the absolutely unsurpassed field geological abilities of highly-trained astronauts, the collection and analysis of lunar samples with known geologic contexts, and the detailed remote sensing data returned by the armada of spacecraft (Kaguya, Chang'e, Chandrayaan-1, and the Lunar Reconnaissance Orbiter) that are going to be orbiting the Moon by the end of 2008.

For more information, please see: Apollo over the Moon: The View from Orbit, H. Masursky, G. Colton, and F. El-Baz, editors, NASA SP-363, United States Government Printing Office, pg. 192.
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