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Apollo 15 Journal

Glass on the Station 2 Boulder

Corrected Transcript and Commentary Copyright © 2014 by Eric M. Jones.
All rights reserved.
Last revised 24 August 2014.

During the Apollo 15 EVA-1 traverse, Dave and Jim drove up onto the lower slopes of Mt. Hadley Delta and stopped near a meter-sized boulder that was lying on the surface, just upslope (south) of a small, fresh crater that was probably created when the boulder came in at a shallow angle from the north or northwest. There are no other sizeable rocks in the vicinity. As Dave noticed when he first examined the boulder, much of the surface was coated with 'bubbly' glass that had not been significantly abraded. This 'bubbly' glass had the appearance of molten material that contained a significant number of gas bubbles and that, after the glass came into contact with the rock, the glass was still plastic enough that the gas bubbles erupted and each left behind a rimmed impression the bubble. Later, when they tipped the boulder over (toward the west)  onto its side with the now-exposed bottom facing the Sun, they saw a significant number of glass bubbles adhering to the lower, uphill portion of the bottom surface.  The bubbles were intact and roughly spherical.

Detail from AS15-86-11554 showing the 'bubbly glass' described by Dave Scott. These appear to be raised-rim imprints left by the eruption of gas bubbles from the glass that splashed on this part of the boulder during ballistic flight.

Detail from AS15-86-11565 showing glass bubbles on the bottom of the Station 2 boulder after Dave and Jim tipped it on its side.  Each bubble produces a reflected image of the Sun, which is off-camera to the right.  There is also a glass droplet hanging from the roof of a small alcove in the rock face.  The droplet casts a shadow on the alcove back wall.  Click on the image for a larger version.

The glass on both surfaces was undoubtedly produced in a large, violent impact. Briefly, impact of a projectile hitting the Moon at interplanetary speeds (20 to 45 km/s) transfers all of it energy of motion to material at the impact site, generating an intense shock wave that vaporizes a mass of lunar rock equal to several times the mass of the projectile and, as the shock wave expands and weakens, melts about 100 times the projectile mass. Farther out, material is shattered and is displaced outward. At first, the result is a growing cavity but, as more and more material is thrown out on ballistic trajectories, the cavity starts to look more and more like a crater. The fact that glass is found of opposite sides of the one rock indicates that the coatings were not splashed onto one side of a passive surface rock at some distance from the impact crater. Instead, glass was deposited on the boulder either in the chaos inside the growing cavity/crater or later, when boulder and blobs of still-molten glass came into contact while the boulder was tumbling in ballistic flight.

This discussion is based, in part, on sections 4.1.2 ("The Cratering Process") and 6.4.3 ("Glassy Melt Breccias and Impact Glass") in the Lunar Sourcebook and on the Apollo 15 Preliminary Science Report.