Cave-Men on the Moon

Wrinkles in the Plot 

Lunar rilles seen from my backyard through a Celestron 5” f/12 telescope and a 2X Barlow.

What is common between an old woman, a fox, a three legged toad, a rabbit, Jack and Jill?

All of the above have been characters supposedly plastered on the face of the moon, based on folklore in different countries around the world. Storytellers have used their imagination to explain the light and dark patches that can be seen by naked eye on the moon. We now know that these features called maria - are made of ancient solidified lava that flowed out from the lunar mantle and settled in impact basins eons ago. Humans have given them beautiful names like the “Sea of Tranquility” and “Sea of Serenity”. The other prominent features that can be seen by naked eye on the moon are larger impact craters. These include Copernicus and Tycho with signature rays of ejecta radiating in all directions reminder, of a violent past when meteorites routinely collided with the moon.

Rima Hyginus and Hygins crater seen from the Lunar

Reconnaisance Orbiter cameras. 

[NASA/GSFC/Arizona State University]

Earthbound small telescopes can see more, including the smaller craters, mountains, and a rather unique feature comprising of long sinuous grooves known as rilles, which appear like wrinkles in the moon's skin. On January 17, after struggling with poor seeing and a hopelessly roiling atmosphere while trying to image mars, I settled on the moon. I have imaged lunar craters before - but this was the first time I became aware that lunar rilles could even be seen with backyard telescopes.  

Two specific rilles I focused on are known as Rima Hyginus and Rima Ariadaeus. Both lie to the south of the Sea of Serenity. Rima Ariadaeus the longer of the two is 300 Km long and 5 km wide (compare with the Grand Canyon at 446 Km long and up to 18 Km wide). However this was harder to see and it appeared as a very thin line. Rima Hyginus, though smaller at 220 Km long, on the other hand, appears clear as a shallow "V" shape with the Hyginus crater at its vertex. But there is only so much you can see from a quarter million miles away.

A Nasa spacecraft known as the Lunar Reconnaissance Orbiter (LRO) continuously orbits the moon, one of its missions is to make 3D maps of the surface. Flying merely 30 miles above the moon its cameras yield much more detailed images such as the second one above. One can see what appears as a featureless lines in the first picture are sinuous paths riddled with a chain of closely spaced craters.

Craters of another kind

It turns out these craters are of igneous origins. Unlike impact craters that are abundant on the surface of the moon, these have rimless rounded edges, are not nearly as circular and have no ejecta blanket from scattered debris. The most favored theory is that these are depressions like sink holes, caused by collapsed tops of lava tubes. Geologists believe these form in the same way as lava tubes on earth but in the absence of an atmosphere, are larger.

A single pit from a larger sinuous chain.
Source: LROC, NASA/GFC/ASU

Chain of collapsed pits to
uncollapsed segment.
Source:LROC, NASA/GSFC/ASU 

Lava tubes on earth form when an open channel lava flow develops crusts on the sides. Over time, these expanding crusts meet in the center forming a roof, resulting in a tube with lava still flowing in it. Lava tubes on earth do not get “plugged” when the lava cools. Rather, as the flow diminishes, the liquid level drops forming hollow cavities within them, like caves. Scientists believe lunar lava tubes may be similar, hosting caves within them. 

Nāhuku- Thurston Lava Tube, Hawaii

New Age Cave-Men

In 1962, scientists first proposed that drained lunar lava tubes can be used for sustained human habitat. Unlike on earth, the moon has no atmosphere to protect the human bodies from space hazards such as cancer causing cosmic rays and coronal mass ejections during solar storms. These structures provide natural protection reducing the amount of build required with man made materials. 

The world is in a new race to create sustained human presence on the moon and other planets, extending our species' chance of survival. On the moon, caves may go from being a quirk in planetary geology to the first base of organized human presence, much like it began on earth. If this happens, then at the leading edge of our civilization, will essentially be handful of new age cave men with modern day sticks and stones, forging our future in new worlds. 

Concept of Toyota LUNAR CRUISER, a crewed pressurized rover, expected to launch in 2029

References:

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• From Nasa.gov: Moon Stories from Around the World - Night Sky Network
• From Lunar Reconnaissance Orbiter CameraScience Operations Center, Phoenix, Arizona: 
• Inside Hyginus Crater 
• Sinuous Chain of Depressions
• Team Japan Sets Sights on Space! Update on LUNAR CRUISER Development

Additional Resources:

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Daily Moon Guide  - from Nasa's interactive map of observing the moon each day of the year