Color image from lunar orbit.<br>This photo was used in Figure 18-15 of the <i>Apollo 14 Preliminary Science Report</i> (SP-272), which has the following caption:<br><dl><dd>Floor of Alphonsus Crater showing a smooth-rimmed dark-halo crater with dark interior walls at point 1 and a typical bright-walled impact crater at point 2 (north toward right).</dd></dl><p>The text of the <i>Report</i> refers to the figure as follows:<br></p>
<dl><dd>The origin of dark-halo craters has been a longstanding problem since the early days of telescopic geologic mapping, when all such craters were necessarily considered to be maars (ref. 18-16).  The Ranger and Lunar Orbiter mission results subsequently suggested that at least two varieties of dark-halo craters exist on the Moon. The mapping of Alphonsus Crater at the 1:250,000 scale (ref. 18-17) showed that most of the dark-halo craters on its floor were structurally controlled. The mapping of the northeastern part of the floor of Alphonsus Crater at the 1:50,000 scale (ref. 18-18) indicated that the dark-halo craters, particularly the Alphonsus MD Crater, were morphologically distinct from the dark-halo craters present on the ejecta blankets of Theophilus (Beaumont L, for example) and Copernicus craters. The Alphonsus-type craters are gently convex upward from the edge of the recognizable dark blanket inward to the crater lip, whereas the latter are pronouncedly concave upward. In addition, Earth-based full-Moon photography indicated that the Alphonsus-type crater has dark interior walls as opposed to the bright interiors of the Copernicus- and Theophilus-type craters.  The relatively poor resolution of the Earth-based photography precluded the development of a strong observational case for these differences in the reflectivity of the crater interiors. An excellent sequence of near-down-Sun oblique pictures of the Alphonsus dark-halo craters (fig. 18-15) are provided in photographs AS14-73-10090 to 10097. In successive photographs, the Sun-facing interior slopes of Alphonsus KC Crater are distinctly seen to be nearly as dark as the surrounding rim deposits. This observation suggests that the same type of material drapes both surfaces, as is the case with well-preserved terrestrial volcanic craters. Apollo 14 photography has thus strengthened the hypothesis that at least two basic varieties of dark craters are indeed present on the Moon.  Certain impact events penetrate and excavate intrinsically dark subjacent materials that are deposited mostly within the ejecta blankets; the steeper, more brecciated and unstable interior walls brighten quickly by mass wasting, thus exposing blocks and in some cases the uppermost layers of bedrock. The lower interior slopes are talus covered. Certain volcanic craters bring up material of uniformly dark albedo and distribute this tephra more or less uniformly over the interior walls of the vent and in the surrounding dark blanket. It appears that, as these volcanic craters age by meteoritic churning of the surface, the surrounding blankets and interior walls become lighter and less distinguishable from the surrounding terrain (on the basis of albedo), but they generally retain the convex upward profiles. These two classes of craters are obviously distinguishable only by use of a variety of both high- and low-Sun-illuminated photographs in addition to the high-resolution obliques.</dd></dl>
NASA
1971-02
Lunar and Planetary Institute, Apollo Image Atlas, 70mm Hasselblad Image Catalog, Apollo 14, Magazine M, <a rel="nofollow" class="external text" href="https://www.lpi.usra.edu/resources/apollo/catalog/70mm/magazine/?73">Images AS14-73-10040 to AS14-73-10204</a>