Thursday, December 26, 2013

La Luna: The Lunar Environment

Note: This program first aired December 21, 2013.

As we wrap up our lunar series with today’s program we’ll take this time to think about what its like on the moon. If we were to go there, what would we experience? Only 12 people can answer that question from first hand knowledge, here’s what one of them had to say: “It suddenly struck me that that tiny pea, pretty and blue, was the Earth. I put up my thumb and shut one eye, and my thumb blotted out the planet Earth. I didn’t feel like a giant. I felt very, very small.” Those are, of course, Neil Armstrong’s words, reflecting what I believe would be the near universal humbling of seeing our home planet from outer space.

If we fast forward forty four years, we find a renewed interest in the moon. China has just successfully landed a landing craft and robotic rover on the surface of the moon, giving the world fresh images and insight on the lunar landscape.

When you watch the images of the Chinese lunar rover leaving the landing craft, or of Neil Armstrong walking around during his moon walk, the first thing you will notice is the dust. It appears that the moon is covered with very fine dust. It results from the impact of millions of micrometeorites, which pulverize the lunar surface rocks. The rocks get so hot on impact that they vaporize, and then condense as tiny glassy spheres, the dust particles we know so well from images of rover tracks and footprints on the moon. Those footprints will be there a while, because the only thing that can make them go away is more impacts from these micrometeorites. There’s no wind or rain on the moon, so there are really no forces of erosion, at least like we have here on Earth. That’s why the moon is still covered with craters. There’s nothing to make those craters disappear. The only factor you can think of as erosive is the ongoing micrometeorite impact issue. It transforms the surface of the moon very slowly,  and not especially dramatically.

And why is there no wind on the moon?  Because for all practical purposes there is no atmosphere on the moon. Wind as we know it is a function of pressure differentials in the atmosphere, air moves from areas of high density (and pressure) to areas if low density (and pressure), at its root it is simple diffusion. If there is no air, or gaseous atmosphere of some other mix, there can be no wind, because there is nothing to move around. To be fair, the moon does have an incredibly diffuse amount of gas at its surface, sometimes called an exosphere. But these gaseous molecules are so few and far between, they don’t behave in ways remotely like an atmosphere.

Another impact the lack of an atmosphere has on the moon is temperature. By now we are all familiar with the concept of the green house effect here on Earth. The Earth’s atmosphere allows in light energy from the sun, and then traps that energy as heat, keeping the planet warm at night. Incidently, the other thing our atmosphere does is NOT let in all the light energy from the sun. A large proportion is reflected back into space, which prevents us from getting too hot! The atmosphere moderates Earth’s temperature in two ways, preventing excess energy from entering, and preventing heat from escaping back into space. You may see where this is going. The moon, lacking an atmosphere, also lacks temperature moderation. So during the lunar day, temperatures can reach 100 C the boiling point of water, or higher.  During the lunar night, temps can then plunge 250 degrees C, dropping to about 150 degrees below zero. 

And why doesn’t the moon have an atmosphere? The short answer is that it isn’t big enough. It is gravity that keeps the Earth’s atmosphere on the Earth, and the moon simply isn’t big enough to have enough gravity to keep gaseous elements around. It’s also no longer tectonically active, so there are no volcanoes to out gas and supply the surface of the moon with gaseous elements.

And lastly, I mentioned the lunar day. You might be surprised but a lunar day is quite a bit longer than an Earth day. An Earth day is 24 hours long, the time it takes for the Earth to complete one full rotation. For the moon to complete a full rotation it takes about 27 Earth days, which is also the same amount of time it takes for the moon for to complete its orbit of the Earth. You might have to let this sit in your brain for a little bit, but the net result of this is that the same side of the moon is always facing the Earth.

So there you have it; if we were to go to the moon right now, we’d take a walk around a dusty, very still, crater pocked vacuum. We would either be incredibly hot and well lit or incredibly cold and in the dark for nearly two weeks, and we would weigh about 1/6 of what we do here on Earth. These conditions may sound appealing to some and uncomfortable to others, but I think we would all benefit from the perspective the view of Earth rise over the moons horizon would bring.


Info about the 12 people to have walked on the moon:

New York Times article about the Chinese lunar mission:

Images of from the Chinese lander and rover:

Science Daily on lunar dust:

Great list serve on all things astronomical

Cool site with lots of interesting lunar info, widgets and apps: