Tuesday, February 17, 2009

Does Water Fall More Slowly on the Moon?

Karl wrote:
I was asked by a seven-year-old recently if things fall more slowly on the Moon. But as it happens, the specific example he used was: if you pour a glass of water, will it take longer to reach the ground than if you were to pour it here on Earth?

(This was asked indirectly via his mother, but the example was his. Seven-year-olds are more excited by spilling liquids all over the floor than by carefully dropping stones from the Leaning Tower of Pisa, I guess! He already knew that all objects fall at the same rate here on Earth, and he understands about atmospheric drag.)

So I divided my answer into two parts. For the first part, I said:

"Yes, things fall more slowly on the moon, or in other places with less gravity than Earth, like Mars. So if you drop your shoes on the Moon, they take longer to hit the ground. And it's not just because there's no air to slow them down -- it's really because there's just less gravity."

(This is all paraphrased. Unlike Barack Obama, I don't perfectly remember all the conversations I've ever had. However, when I write my memoirs, I will.)

I went on: "But the specific example you chose is interesting. If you pour out water on the Moon, something special will happen. It won't stay liquid water -- it will break up into a fine ice dust. Because there's no air pressure, it boils at room temperature, in other words, it evaporates right away, because there's no pressure keeping the water inside itself. The water molecules are all bouncing around, and now they are free to bounce away in all directions. But the water also freezes as it boils, because its surface area increases so much (it's just little tiny specks of water now) that it radiates all its heat and turns to ice. So boiling and freezing can happen at the same time, when there's no air."

The rough outlines of my answer are confirmed by


But there's something I'm not sure of:

In the rare-to-nonexistent atmosphere of the Moon (let's assume it's a perfect vacuum, for the sake of discussion), why would water lose any of its heat? In other words, I get that it would boil right away. But would it also freeze? And would it behave differently in a shadowy part of the Moon versus a sun-drenched part?

I told him I'd ask Planetary Astronomer Mike. He's counting on you.
So, In the middle of the day on the moon, we'd expect the water to completely boil away. However, it will cool down in this process, though, because of latent heat.

This has to do with the idea that even though 100° C liquid water and 100° C steam have the same temperature, the steam has way more heat. If we want to raise a gram of water from room temperature (20° C) to its boiling point at 100° C, we need to add 80 calories of heat. If we actually want to boil that gram of water once its at that temperature, though, we need to add significantly more heat: 540 calories. This second injection of heat is what's known as latent heat, since it doesn't go into raising the temperature, but rearranging the molecules.

In its liquid state, water is actually pretty tightly bound. A hydrogen atom from one water molecule has a strong magnetic affinity for the oxygen side of another water molecule, an interaction known as hydrogen bonding. That extra heat for boiling is required to break the hydrogen bonds. This hydrogen bonding gives water most of its unique properties: strong surface tension, ice less dense than water, huge latent heat, etc. A great, albeit technical, website is the "anomalies of water" page.

So in the case of the moon during daytime, we drop the water, it begins to boil it away, but that boiling process will remove enough heat that the water's temperature will drop to the point that it just freezes. Only after our subsequent lump of ice either radiatively absorbs enough sunlight or conductively absorbs enough surface heat can it boil away completely.

On the moon at night, some of the water will again boil (just because of the initial heat in it), freeze, and our subsequently block of ice will lay relatively dormant. Over long time periods, some of that ice will sublimate...just due to random molecular vibrations, occasionally a surface molecule will get enough energy to break its bonds and roam free as a steam particle. That process is highly temperature dependent, though...if I'm recalling correctly, ice needs to get down to around 150K or below to be stable against sublimation.

There have actually been a couple space missions now looking for water ice buried in the North and South Pole craters on the moon, where permanent darkness should be capable of maintaining ice in this stable state. The jury is still out on this (there's another similar mission due to launch later this year), but what's even more weird is that there's been very good radio evidence of extremely thick ice sheets in the craters at the poles of Mercury.

I have no idea how you'll explain this to a seven-year-old.



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