Astronomy Picture of the Day 10-29-02

Explanation: What could cause a long indentation on the Moon? First discovered over 200 years ago with a small telescope, rilles (rhymes with pills) appear all over the Moon. Three types of rilles are now recognized: sinuous rilles, which have many meandering curves, arcuate rilles which form sweeping arcs, and straight rilles, like Ariadaeus Rille pictured above. Long rilles such as Ariadaeus Rille extend for hundreds of kilometers. Sinuous rilles are now thought to be remnants of ancient lava flows, but the origins of arcuate and linear rilles are still a topic of research. The above linear rille was photographed by the Apollo 10 crew in 1969 during their historic approach to only 14-kilometers above the lunar surface. Two months later, Apollo 11, incorporating much knowledge gained from Apollo 10, landed on the Moon.

Astronomy Fun Fact:

The Earth/Moon system is really not a planet/satellite system but a double planet! One definition of a natural satellite ("moon") is that it always "falls toward" its planet during its orbit. Our Moon fails that test! The Moon and Earth orbit a common center of mass, the "barycenter".

Thus: the Moon is not a moon!

This photo shows the Hadley Rille on the southeast edge of Mare Imbrium. It is fairly well known because Apollo 15 landed there (see next image). The rille begins at the curved gash in the bottom left corner, and is clearest in the rectangular, mare-floored valley shown here. In the upper left, it gets much shallower and it slowly fades out of sight in Palus Putredinis. In all, the rille is over 75 miles (120 km) long. It is up to 5000 feet (1500 m) across and is over 950 feet (300 m) deep in places. It formed nearly 3.3 billion years ago . In contrast, lava channels on Hawaii are usually under 6 miles (10 km) long and are only 150 - 300 feet (50-100 m) wide. This contrast in channel size probably reflects (1) differences in the volume of erupted lava and (2) the difference in gravity. Note -- The bright bumps surrounding Hadley are peaks of the Montes Apenninus. These mountains mark the edge of the impact basin holding Mare Imbrium. They rise from 6000 to 15,000 feet (1800 - 4500 m) above the mare. (Apollo 15 image M-1135, arrow marks landing site of Apollo 15. Image taken from NASA SP-469, Geology of the Terrestrial Planets)

Schroter's Valley in the Aristarchus plateau is one of the largest lunar sinuous rilles (width in picture is about 5 km). The valley consists of an arcuate rille (1) that contains a meandering sinuous rille (2). The valley here traverses what appears to be a lava plain embaying low hills in the southern part of the picture. The blocky outcrop ledges, probably lava layers, near the rim (3) and the blocks at the inside base of the slopes (4) are of interest as is the downslope movement of material in the walls, which results in partial burial of the inside rille (5) and shows that the valley is laterally enlarged by mass wasting processes.

The sinuous shape, uniform width, presence of low levees, irregular depressions at the head of this and other rilles (outside the picture), and uniform cratering of floor and surrounding terrain suggest that the feature originated as lava flow channels, or collapsed lava tubes (Greeley, 1971). The rilles in the picture probably formed by the draining of a large lava flow channel and a smaller channel in a somewhat later flow that was confined within the boundaries of the larger channel. Incision by thermal erosion of lava streams with turbulent flow is an alternative explanation for the formation of sinuous rilles such as Schroter's Valley (Hulme, 1973). Other conspicuous features in the pictures are secondary crater clusters from the young crater Aristarchus (6). The secondary clusters cross the rille at (7), and show that the crater Aristarchus is younger than the rille.

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