Down the Rille
Credit: Apollo 10 AS10-31-4646
Explanation:
Even when we have a great telescopic view of the Moon we typically
see it as a distant world, without a sense of dimensionality. But the oblique Apollo
images give us a feel for the real three-dimensional Moon. And the high resolution
improves our understanding of the surface geology. This dramatic Apollo 10 view along
the Ariadaeus Rille reveals it as a place rather than a line. Linear rilles such
as the Ariadaeus are graben - places where the ground has slid down between two parallel
bounding faults. We see that the sharp-edged walls slope inward - they are not vertical.
The Ariadaeus Rille is wider where it cuts the ridge near the Silberschlag crater
at the center of this image. This extra width compared to the lower areas allows
the slope (or dip in geological terms) of the rille wall to be estimated as about
55 degrees. If this is correct, rilles have some on the steepest slopes on the Moon.
Many linear rilles have associated volcanics and are probably formed over dikes.
A dike is a narrow vertical sheet of magma that rises buoyantly towards the surface.
The dike forces apart the lunar crust thus making the extensional forces that causes
the parallel faults. Projecting the 55 degree slopes downward suggests that the
base of the Ariadaeus graben faults is at depths of 2 to 3 km. This is the same as
the average thickness of the megaregolith (the fragmental layer formed of ejecta
from craters and basins), suggesting that the discontinuity in rock strength (strong
below and weak above the megaregolith) gives rise to the dikes.
—
Chuck
Wood
Technical Details:
I thank Project Apollo Archive for the image!
http://www.apolloarchive.com/
Related Links:
Rukl Plates 34
