Friday, December 18, 2009

Really, Really Close-Up on the Alpine Valley

Just how old is the incredible Vallis Alpes, the "Alpine Valley" bisecting the Montes Alps on the northeast "rim" of Mare Imbrium? Through a telescope the valley appears to be a crack radiant from the Imbrium impact (~3.8 billion years). Among other clues the arc of mountains it transects seems out of "sync" with where the eye traces out an apparent circumference for Mare Imbrium. Lunar Orbiter IV obtained excellent shots of the area, and in 2008, Japan's space agency JAXA released a 3D Grand Tour "fly-over" of the valley from data obtained by Kaguya. And last summer, soon after LRO arrived in lunar orbit, its narrow-angle camera (NAC) under the direction of Mark Robinson's LROC team at Arizona State University, LRO flew over the "mouth" of the valley, crossing where its inner channel seems almost to briefly run uphill, over the highest hills along its 134 km length [JAXA/SELENE].

LRO is in a polar orbit, of course, but Vallis Alpes runs southwest to northeast (or vice versa). LRO crossed the valley very much as diagrammed in the Kaguya Terrain Camera image up above. It was necessary to back away a bit to get the entire 8.5 kilometer strip from orbit 562 inside the constraints of this blog's template. Along the whole of the valley's length a sinuous rille meanders through its middle plain. At this location, however, the wider valley has narrowed to a "bottleneck," and the channel continues through the higher mountains not far from the edge of Mare Imbrium [NASA/GSFC/ASU].

Closing in on the bottle neck, using Arizona State University's "Zoomify" Image Browser (which also hosts Apollo's orbital metric and panorama photography), and what is easily the best image ever of Vallis Alpes inner channel comes into view. An island can be seen where the channel of molten material changed course long ago, and strong hints are seen of the channel's true age. "Elephant Skin" mottling, typical of highland hills throughout the Moon, traces down along a hillside that long ago collapsed or weathered down into the channel, burying its north rim [NASA/GSFC/ASU].

As close as it gets thus far in LRO's mission and the east side of the "island" seems to sit in a gently flowing river on Earth, complete with "river gravel." The image is perhaps a kilometer wide at this scale, however, and its depth is ~1.6 meters per pixel, so those stones are really boulders. The landscape presents another mystery, too. Where are the intermediate-sized and secondary craters? The Moon's surface, it is thought, is superficially "gardened" every two million years or so. Is this terrain telling us of Moonquakes and relatively recent landslides?

"A bottleneck at the start of the lunar sinuous rille within Vallis Alpes formed several morphologic features including (from left to right) a lava pond, a breached dam, and an island in the rille." [NASA/GSFC/Arizona State University].

Brent Garry
LROC News System

"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 frames; 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 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."

View the full discussion, the images and diagrams,
at LROC's website

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