Wednesday, January 10, 2018

How to make your own moon

In the first installment of 2018, our resident experimental PDF discusses retrofitting our planetary simulation cryovacuum chamber to simulate a nearby environment: that found in the permanently shadowed regions of our own moon. An image of our first run can be seen above.

By Dr. Paul Godin

One of the experiments happening at the PVL is called the Aniu Investigation, which has the goal of testing to see if frost could be detected in shadowed regions of the moon using reflected starlight (Lyman-alpha radiation, 121 nm). Unfortunately, the moon is quite far away from York University and expensive to get to, so we’ll need to simulate the moon in the lab.

To build a moon in the lab we’ll need the following “ingredients”:

1.     A stainless-steel vacuum chamber.
2.     A vacuum pump.
3.     Liquid nitrogen.
4.     A “cold finger” heat exchanger
5.     Simulated lunar regolith
6.     A UV lamp.

Once we have all the above we can start building our moon. First, is to attach the vacuum pump to the vacuum chamber. The pump will remove the air from the chamber, allowing us to simulate the vacuum of space. Pumping out the air also has some other benefits from an experimental side; the lack of air in the chamber increases its thermal stability since there’s no longer a medium in which heat can be conducted/convected through the chamber. This means that temperature fluctuations in the lab are unlikely to be felt inside the chamber. A second benefit is air absorbs Lyman-alpha radiation quite strongly, meaning if we left the air inside the chamber the “starlight” would be absorbed before it even hit the surface of our moon.

Now that we’ve simulated the atmospheric pressure of the moon, next we’re going to want to get the surface temperature right. During lunar night, temperatures can typically reach -180 oC. Liquid nitrogen has a temperature of -196 oC, so we can use it to cool our moon down to the desired temperature. The liquid nitrogen is pumped through a “cold finger” as shown in Figure 1. The cold finger has a feed-through to allow it to pass through the vacuum chamber to a copper plate. Copper is chosen as it is an excellent conductor of heat, yet has a high heat capacity making it an excellent thermal capacitor. It’s on this copper plate that we’ll build our lunar surface.

Figure 1: Diagram of Cold Finger and feed through system.

The surface of the moon is covered in rocky dust known as regolith. Unfortunately, lunar regolith samples are restricted here on Earth; luckily though they’ve been studied well enough that it’s possible to produce simulated lunar regolith, which can be purchased. We’ll place some simulated lunar regolith in a copper dish and attach it to the copper plate of the cold finger inside our vacuum chamber. We now have surface with the same pressure, temperature, and texture of the moon right here on Earth!
The last thing to do is to attach a Lyman-alpha light source to the chamber to simulate the starlight and a camera to record our findings. We don’t have a UV camera yet (it’s on the way), in lieu of that I’ve attached a visible light source using a fiber optic connection and have mounted a monochrome camera with a filter. The completely assembled lunar simulation chamber is shown in Figure 2.

Figure 2: Lunar simulation chamber along with liquid nitrogen tank.

Figure 3 shows the first image of our “lunar” surface, taken at -175 oC and a pressure of 1.4x10-4 Torr. The copper dish holding the lunar regolith is subdivided into four different areas: empty (control), dry regolith, wet/frozen regolith, and pure ice. With the visible light/camera not much difference can be seen between the wet/dry regolith. Once the UV camera arrives we expect to be able to see a significant difference between the two.

Figure 3: “Lunar surface” take with monochrome camera and filter at 730nm.

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