The Moon is an ancient body. Its surface is dominated by the ancient highland crust peppered with huge basins, some of which are filled by mare deposits. These mare basalt deposits range in age from 2-4 billion years old. Many millions of years of bombardment by planetary bodies of all sizes have created a thick soil layer (called the "regolith" by lunar scientists) that has been broken up, churned, and homogenized vertically all across the lunar surface. To get past this material and see what's underneath, you have to look for a 'fresh' surface on the Moon---perhaps a steep slope or a spot that has been uncovered by a relatively recent impact crater formation event. Such fresh material may hold the best answers to fundamental questions about the geology of the Moon.
To look for answers to these questions, scientists study 'fresh' craters -- those with a crisp rim and an obvious ejecta deposit -- such as the one shown this image, situated within the thick, dark, pyroclastic blanket near the Rima Bode II rille. With the sun nearly directly overhead in this view (solar incidence is ~11 degrees, where 0 degrees is 'noon'), shadows are few. This crater, which is 230 meters across, and excavated from 10 to 20 m down, has not penetrated through the pyroclastic deposit, so the pyroclastic materials are at least 10 m (or more) thick in this area. The rubble and boulders visible in the crater floor suggest that the crater has excavated a solid, rocky surface beneath the mantle. The presence of a very small (~1 m across), bright crater in the floor may indicate that the underlying highlands material is not far below the floor of this crater.
The Rima Bode region (13 degrees N, 356 degrees E) is (in part) a region of interest because of the presence of this thick layer of extremely dark volcanic material. Known as the Rima Bode pyroclastic deposit, it overlies and mantles an extensive portion of the highlands between the mare basalts in Sinus Aestuum and Mare Vaporum, south of the Apollo 15 landing site. As noted by scientists shortly after the Apollo missions prematurely ended, there are several such 'black spots' on the Moon, including the material sampled by the Apollo 17 astronauts during their adventure in the Taurus-Littrow Valley. These extremely dark volcanic deposits were thought to be young (possibly within the last several hundred millions of years) prior to the Apollo 17 mission based on photogeologic mapping. However, photogeology can only get you so far. Eventually, you need to send human explorers to fully understand these sorts of problems. Subsequent radiometric age dating of the returned Apollo 17 pyroclastic samples indicated a far more ancient age of ~3.5 billion years. The 'black spot' deposits all have rock-free surfaces with remarkably similar properties and are believed to contain the same kind of orange glass and black crystallized spheres as those sampled by the Apollo 17 crew. These particles are the lunar version of ash or cinder, and they are thought to be explosively emplaced on the Moon just as they are on Earth. Rather than forming cinder cones, the lower gravity and near-vacuum of the Moon's environment allows the particles to travel farther up and out from a vent, depositing an extensive blanket of glassy material. The beads have trapped hydrogen and Helium-3 from the solar wind, and enrichments of volatile elements such as sulfur and fluorine have been measured on their surfaces. The pyroclastic materials are also rich in iron and titanium, making them an immensely important as resources for future lunar explorers.
Explore the full resolution NAC image.
Credit: NASA/GSFC/Arizona State University