What is Diamondback Rille?

What is Diamondback Rille?

This recent entry at Lunar Photo of the Day caught my eye. It is an image of Maskelyne Rille (also called Diamondback Rille) taken by Jérôme Grenier on September 9, 2004. The Rille is the long, sinuous structure weaving from the center of the image towards the top left portion.
The explanation of the image given on LPOD is

Lunar sinuous rilles are lava channels. They did not cut down into pre-existing surfaces, rather they transported the lava that formed the surrounding surfaces. Sinuous rilles were once rivers of molten lava, now they are elusive channels of emptiness.

I found this explanation to be wholly unsatisfying. So, I did some looking around on Google and I found a few more images of lava channels, like this one of the Pu'u O'o lava channel (click on the following images for much higher-resolution versions):
The Pu'u O'o lava channel is only 4 meters wide. Then there's this photo of an inactive lava channel on Kilauea, taken by J. Dixon:
The levees on the sides of a lava channel form when lava fills the channel to overflowing, and then the overflow cools. Such levees are built up over several successive eruptions, as can be seen in the second and in particular the third picture in this post. The scale of the vegetation growing in this inactive channel shows that the channel is also fairly narrow. Here's one more photo, of the 'A'a lava channel:This lava channel is between 20 meters and 50 meters wide.

So, does it make sense that the Diamondback Rille is also a lava channel? Well, it would if the Rille was only a few meters to a few tens of meters wide. However, this is definitely not the case. Such a resolution simply isn't possible with earth-based cameras. So, let's have another look at the Rille, from a photo taken by the Apollo 11 astronauts during the descent phase; I cropped this image from a much higher resolution image taken by the astronauts, which you can see by clicking on the image:
This picture is rotated clockwise by 90 degrees relative to the picture at the top of this post. The large crater at the bottom right of the image is Maskelyne A, and the smaller crater above that is Maskelyne B. Maskelyne A and B are about 24 km and 9 km across, respectively. A richly-labelled (by Markus Mehring) version of the above photo is here.

Using the known diameters of the Maskelyne A and Maskelyne B craters, we can estimate the width of Diamondback Rille. The Rille is approximately 400 meters across as it passes the Maskelyne X double crater (at the top left of the above image). That is between 10 and 100 times the size of lava channels on the earth.

So, does it make sense that Diamondback Rille is a lava channel? If so, why is it so very much wider than lava channels on earth? And where is the large pool of black lava rock downstream from the rille? Why is there no color difference between the downstream area and the surrounding plain? Perhaps one of my readers has the answers to these questions.

Technorati Tags: Space, Moon