Featured Image - 04/08/2008
Vallis Schröteri: The Lunar "Grand Canyon"
The extrordinary Vallis Schröteri is approximately 160 km long, up to 11 km wide, and almost half a kilometer deep. It is so large that it has been called a Vallis (or valley), one of only a handful of features large enough to receive this name. For comparison, the Grand Canyon of the American Southwest is approximately 450 km long, up to 29 km wide, and is 1.6 km deep in some places.
This week's image (Figure 1) is taken from Apollo 15 metric frame AS15-M-2087, which was acquired over the Aristarchus plateau (See the 3/11/2008 Featured Image for more information about the Aristarchus region). Figure 2 is a 3D (red-blue anaglyph) image of a portion of the same region.
Figure 1: Vallis Schröteri from Apollo Metric Image AS15-M-2087. The valley is approximately 10 km wide in this region. [NASA/JSC/Arizona State University]
Due to the low sun angle, the floor of the valley is cast in shadow, but as we process other Apollo panoramic and Hasselblad camera images, the floor of this valley will be revealed. As those images become available, we will see that the floor of this valley is relatively flat and has been resurfaced by volcanic flows. There is also a smaller rille that winds its way through the valley floor.
How did this valley form?
Unlike the Grand Canyon of the American Southwest, Vallis Schröteri was not carved by wind and water, but by lava. Vallis Schröteri begins at a circular feature which is often called the "Cobra Head" because of its superficial resemblance to the head of a snake. This feature is generally considered to be the source vent for the volcanic flows that carved the awesome Vallis Schröteri, resurfaced the valley floor, and carved the smaller inner rille. As lunar lavas flow over older materials, sometimes they can thermally (by melting) and frictionally erode their way through the surface over which they flow, eventually carving channels and valleys, which on the Moon are called sinuous rilles.
Lunar scientists are eager to learn more about this fascinating feature because it can tell us about the volcanic history of the Moon and is a testament to how powerful lunar lavas can be. Future explorers on the surface of the Moon will certainly want to experience the awesome scale of this feature for themselves. As these explorers descend to the valley's floor, they will study the layers of lava flows exposed in the walls of the valley to unravel the complex geologic history of this region. Trapped between the lava flows in the valley's walls, there may even be remnants of ancient regolith that once lay on the surface of the Moon exposed to the solar wind. The solar wind contains particles expelled from the Sun's atmosphere, and the samples returned by the Apollo astronauts showed that the lunar regolith collects these solar wind particles. By studying the ancient particles trapped in the pockets of regolith preserved beneath the lunar surface, we will better understand the composition of the Sun as it was billions of years ago.
These future lunar explorers will benefit from new images acquired by LROC next year. LROC will provide images with 50 cm/pixel resolution in the Vallis Schröteri region and will be the basis for maps used by future human explorers of this lunar "Grand Canyon".
Figure 2: This 3D (red-blue anaglyph) image of Vallis Schröteri highlights the grand scale of the Moon's largest sinuous rille. The Valley is almost a half a kilometer deep. You will need red-blue 3D glasses to view the image correctly. Please note that the vertical exaggeration of this anaglyph is not to scale.
(Apollo Image AS15-M-2087 [NASA/JSC/Arizona State University])
References for further reading:
A concise and recent review of Vallis Schröteri's formation is provided by Garry et al. (2008) Lunar and Planetary Science Conference XXXIX, Abstract 2261.