From OttawaRasc
The Two Faces of the Moon
by Simon Hanmer
|
In my two previous lunar presentations, we've looked at the large-scale aspects of the planetary geology of the Moon, and the kinds of surface features we can observe through amateur telescopes that allow us to understand how the surface of the Moon came to look the way it does. To briefly recap, the two principal features we can see, apart from the heavily cratered terrain are the light coloured highlands and the dark coloured lunar Mare, or "Seas". The light coloured material formed when crystals of the lightweight mineral feldspar floated to the top of a Moon-wide magma ocean. The dark material formed when basalt lavas erupted into the giant Basins created by large impacts. But there's something fundamental that's missing from our story.
|
|
On the near-side of the Moon, the side we can see, there are large patches of Mare basalt, some of which are clearly ringed by the Highlands thrown up by forces generated by the giant collisions that formed huge impact Basins some 4 billion years ago. The dark Mare basalts were erupted some 1 billion years later when the heat generated by the breakdown of radioactive elements inside the Moon finally produced widespread melting of the Moon's interior about 3 billion years ago. But what about the far-side of the Moon? Where are the huge "Seas" of 3 billion year old basalt lava? The simple answer is that, except for the South Pole-Aitken Basin, they're not there! So the question is, can lunar geology tell us why the Mare are all on one side of the Moon, and more especially, what are they all doing on our side?
|
Asymmetrical Moon
|
Let's start with a simple observation: not only is the Moon asymmetrical on the surface, but its centre of mass is not at the centre of its shape. Geologically, the reason for the offset of the centre of gravity is because the lightweight crust, or the outer layer of the Moon, is thicker on the far-side than it is on the near-side, while the inner layers, called the mantle and the core, appear to be concentric like the stone in a peach. Generally available planetary geology texts* appear to attribute this difference in crustal thickness to the gravitational influence of the Earth, potentially relating it to the fact that the Moon always presents the same face to us here on Earth.
|
|
How do we know that the crust is thinner on the near side? When satellites such Lunar Orbiter and Clementine systematically orbited the Moon, the different crustal thicknesses influenced the Moon's gravity field, tugging on the satellites as they passed over different parts of the lunar surface. By measuring the effects of gravity on the satellite orbits, planetary geologists can calculate models for the thickness distribution of the lunar crust. What all this means is that if a magma or lava is produced at a given level within the lunar interior (mantle), it has further to travel in order to reach the surface on the far-side, compared with the near-side. Lava cools as it rises, so there's a greater chance that the lava will freeze before it gets to the surface on the far-side. Therefore, there are more lunar mare on the near side.
|
Magma Buoyancy
|
Actually, its just a little more complicated than that! Bubbles and hot-air balloons rise because they are lighter than the stuff they are rising through, whether its air or water, cooking oil or custard! In part lava rises for the same reason: because it's lighter than the stuff it's moving through. And the reason it's lighter, is because it's hotter!
|
|
The lunar crust is about 70 km thick on the far-side. Most importantly, it is denser in the lower part and lighter in the upper part. When we say that the lunar crust is thinner on the near side, this means that much of the upper lighter part is missing. Therefore, lavas only have to rise through the lower, denser part to get very close to the lunar surface. In contrast, on the far-side, the same lava has to rise through a good thickness of light crust before it reaches the surface. If the lava is not hot enough, it will be too heavy to rise through light lunar crust, so it gets trapped underground where it freezes and stops moving. What all this means is that Mare lavas reached the surface of the Moon on the near-side much more easily than on the far-side.
|
An experiment in oil and hone
If you've followed me this far, I have to admit that this idea of light magma rising through dense lunar crust 3 billion years ago ... when the crust was still warm enough to flow and make way for the rising lava can be a bit awkward to take in without seeing it for yourselves. So I've put together a short animation made of everyday materials to simulate what I mean.
|
Many years ago, I built a perspex box in order to reproduce a famous experiment very familiar to many geologists. Into the box I poured liquid honey and really dirty sump oil from a truck engine. The honey was transparent and relatively dense, whereas the sump oil was opaque (black!) and relatively light. After sealing the perspex box, I left it to sit for 24 hours so that all of the light sump oil floated upwards and ended up on top of the dense honey. Then I put the whole thing into the fridge for 6 hours to cool it right down.
|
|
Why did I prepare the experiment this way? Because the honey simulates the dense lower lunar crust, and the light sump oil simulates hot magma, or molten rock. What I was going to do next was to flip the perspex box over so that the light oil (magma) would lie underneath the dense honey (lower lunar crust). The reason for cooling the experiment was to increase the viscosity of the honey and oil so as to slow things down in order to be able to photograph what was about to happen.
|
|
When I flipped the perspex box over, the light magma (you have to get into the spirit of this!) began to collect together in the middle of the base of the box, and to rise. It rose toward the top of the dense lower lunar crust in two large bubbles. As they reached the boundary between the top of the dense lower lunar crust (honey) and the light upper lunar crust (above the perspex box), the magma bubbles spread out horizontally because they could not rise any further. Of course, in the experiment, the magma bubbles were trapped by the rigid perspex lid of my box. In the real Moon, they were prevented from rising further because the light crust was as light as the hot magmas. Remember that hot bubbles only rise because they are lighter than what they are rising through!
|
|
The accompanying picture pretty well sums things up. On the far side of the Moon, lava that has risen through the dense, lower part of the lunar crust gets trapped below the thick, light, upper crust. On the near side, the lava finds itself below a very thin upper crust that it can easily break through and flood the old impact basins. Hence the two faces of the Moon look very different!
|
Lunar History
Here's a quick reminder of what I've tried to show you in these 3 lunar presentations:
- The Moon is almost as old as the Earth (~4.5 billion years!).
- It formed as a the result of an unimaginable collision between a body the size of Mars and the early Earth.
- It condensed from a ring system, similar in form to the rings of Saturn.
- It developed asymmetrically, with two very different looking faces.
- It's crust is the result of light crystals floating to the top of a magma ocean.
- The big dark patches are made of 3 billion year-old lava that filled the interiors of giant impact basins that formed a billion years earlier.
- after all that, impact cratering continued at a much slower rate. For example the impact crater Copernicus formed about 1 billion years ago, and Tycho "just" 100 m.y. ago
Most importantly, our Moon, so easily visible with any amateur telescope, shows us evidence of the very same processes that have affected all of the rocky planets of our Solar System. She may blot out your favourite galaxy tonight, but she's a source of wonder in her own right.
- The New Solar System, 4th Edition, Eds. Beatty, Peterson and Chaikin, Cambridge University Press, 1999; Planetary Landscapes, 2nd Edition.by Greely, Chapman and Hall, 1994; Universe, 6th Edition by Freedman and Kaufmann, Freeman and Company, 2002.
|
