Vallis Alpes (Alpine Valley) is a spectacular linear valley along the northeastern edge of the Imbrium impact basin. It is easily visible in amateur telescopes. The floor of the valley was flooded by mare basalts that host a sinuous rille which stretches for more than 150 km. High-resolution NAC images reveal small features that not previously resolved in the existing Lunar Orbiter frame (Fig. 2); including an "island" within the rille, a breached dam, and a remnant lava pond. Based on the available data, there are some outstanding geologic questions about this rille that will be addressed by future human exploration. For instance, where is the source for the lavas in the middle of the ejecta blanket? Are these lavas older, younger, or related to lavas in Mare Imbrium and Mare Frigoris? Did a fault or graben create the long valley now occupied by frozen lava? Was the valley formed as a result of the Imbrium impact event or is it younger? For now NAC images, combined with previous maps and data sets (Figs. 3, 4), allow scientists to make observations of the rille's morphology and stratigraphic relationships between the different units to piece together the geologic history of this rille and the surrounding region.
Figure 2. Lunar Orbiter IV image 4115 h3 showing Vallis Alpes and the location of the LROC featured image (arrow) [NASA/USGS/LPI].
Figure 3. A subsection of the geologic map for the region surrounding Vallis Alpes showing the spatial context with northern Mare Imbrium, Mare Frigoris, and the impact crater Plato [M'Gonigle and Schleicher, 1972]. The full geologic map can be downloaded here.
Figure 4. Clementine Near IR 'enhanced' color composite map of Vallis Alpes and the surrounding region. The lower reflectance regions are mare basalt deposits and the higher reflectance materials are predominantly ancient crustal material disrupted by the formation of Imbrium basin. Black sections are data gaps. Grid spacing is 5 degrees, simple cylindrical projection, image from USGS Map-A-Planet [Gaddis et al., 2007].
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