The Clementine Mission
Multispectral Mosaic of the Aristarchus Crater and Plateau
The Aristarchus region is one of the most diverse and interesting areas on the Moon. About 500 Clementine images acquired through three spectral filters (415, 750, and 1000 nm) have been processed and combined into a multispectral mosaic of this region (latitude 18deg 32 min, longitude 42 deg 57 min W). Shown here is a color-ratio composite, in which the 750/415 ratio controls the red-channel brightness, its inverse (415/750) controls the blue, and the 750/1000 ratio controls the green. Color ratios serve to cancel out the dominant brightness variations in the scene, which are caused by albedo variations and topographic shading, thus isolating the color differences related to composition or mineralogy.
The Aristarchus plateau is a rectangular, elevated crustal block about 200 km across, surrounded by the vast mare lava plains of Oceanus Procellarum. Clementine altimetry shows that the plateau is a tilted slab sloping down to the northwest, that rises more than 2 km above Oceanus Procellarum on its southeastern margin. The plateau was probably uplifted, tilted, and fractured by the Imbrium basin impact, which also deposited hummocky ejecta on the plateau surface.
The plateau has experienced intense volcanic activity, both effusive and explosive. It includes the densest concentration of lunar sinuous rilles, including the largest known, Vallis Schroteri, which is about 160 km long, up to 11 km wide, and 1 km deep. The rilles in this area begin at "cobra-head" craters, which are the apparent vents for low-viscosity lavas that formed the rilles. These and other volcanic craters may have been the vents for a "dark mantling" deposit covering the plateau and nearby areas to the north and east. This dark mantling deposit probably consists primarily of iron-rich glass spheres (pyroclastics or cinders), and has a deep red color on this image. Rather than forming cinder cones as on Earth, the lower gravity and vacuum of the Moon allows the pyroclastics to travel much greater heights and distances, thus depositing an extensive regional blanket.
The Aristarchus impact occurred relatively recently in geologic time, after the Copernicus impact but before the Tycho impact. The 42 km diameter crater and its ejecta are especially interesting because of its location on the uplifted southeastern corner of the Aristarchus plateau. As a result, the crater ejecta reveal two different stratigraphic sequences: that of the plateau to the northwest, and that of a portion of Oceanus Procellarum to the southeast. This asymmetry is apparent in the colors of the ejecta as seen in this image, which is reddish to the southeast, dominated by excavated mare lava, and bluish to the northwest, caused by the excavation of highlands materials in the plateau. The extent of the continuous ejecta blanket also appears asymmetric: it extends about twice as far to the north and east than in other directions, approximately following the plateau margins. These ejecta lobes could be caused by an oblique impact from the southeast, or it may reflect the presence of the plateau during ejecta emplacement.
The Clementine multispectral data will enable lunar geologists to reconstruct the three-dimensional composition and geologic history of this region. In this color-ratio composite, fresh highlands materials are blue, fresh mare materials are yellowish, and mature mare soils are purplish or reddish. The subsurface compositions, buried beneath a few meters or tens of meters of pyroclastics or Aristarchus ejecta, are revealed by craters which penetrated the surface layers, and by steep slopes such as those along the walls of the rilles. From this mosaic, we can see that the plateau is composed of a complex mixture of materials, but that the rilles formed primarily in lavas, except for the cobra-head crater of Vallis Schroteri, which formed in highland materials.
The following movie shows the illumination of the south polar region of the Moon over the course of one month ( one lunar day). The Moon's spin axis is nearly perpendicular to its plane of orbit around the Sun. As a result, an observer in the polar area always sees the sun just slightly above or below the horizon. Irregularities such as mountain peaks or crater floors may be in either permanent sunlight or permanent darkness. This movie was made to search for areas of both types. Permanently lit areas could provide a landed spacecraft with solar power to survive the long (14 Earth day) lunar night. Permanent dark areas could contain cometary ice deposits, a valuable resource for use on the Moon. The south pole is located just inside the rim crest of the circular crater at center at about the 10 o'clock position. As you watch the illumination move 360 degrees around the pole, you will note several areas that seem to be permanently dark; these regions may contain ice. A few very small areas appear to be permanently, or nearly so, illuminated. Work on understanding the lunar polar environment continues with analysis of images from Clementine.