Fun Fact - Keeping Cool on Venus and Mercury

I was thinking about a small house on Venus, but the heat is a huge problem. On an outer planet you can always generate heat, by radioactivity if need be, but you can't create cool, so what to do? Let's say you have a source of energy, maybe a nuclear reactor, so that's not a problem. Plenty of power to cool your house. Thing is, an air conditioner works by pumping heat out of your house and into coils in the back yard that lose their heat to the outside air. That means the coils have to be hotter than the outside air; otherwise the heat can't flow "downhill". The air conditioner on Venus has to heat the external tubing to more than 500 C (900 F) to work. With only modest changes in pressure as per the compressor, we need a coolant that is liquid / gass in a temperature range from 0 to 500 C. I don't know any substance like that, do you? I can't imagine how to build an air conditioner that would work on Venus, even in theory. I guess nothing will be landing on Venus any time soon.

In the 1960's and 70's, Russia successfully landed Venera probes on Venus, which sent back pictures of the surface. With no air conditioning, the probes lasted about an hour, more or less. Snap a few pictures, send them back to earth, then succumb to the heat. That's a lot of money, time, and effort for not much data. In contrast, Opportunity has been tooling around Mars for 13 years. One hour versus 13 years. Well we had to see the surface of Venus at least once, didn't we. To make sure the Three Stooges weren't there. So thank you, Russia, for doing that. Yes, there was a movie, Have Rocket Will Travel, released in 1959, wherein the Three Stooges went to Venus, but that's another story.

How about a house on Mercury? Surprisingly, air conditioning can be accomplished, and for not much energy. Daytime temperatures climb to 427 C, 800 F, but at night the temperature falls to -173 C, -280 F. construct a large, subterranean tank of water next to the house. At night, pipes carry pressurized ammonia (or some other liquid) through this reservoir, and out to tubes that are connected to radiator fins. Heat vents into space, whereupon the supercooled ammonia returns to the water, which then freezes into ice. During the day, the ammonia circulates between the tank of ice and your house. The ice melts, and keeps your house cool. You're taking advantage of the day / night cycle, averaging out the temperatures through the thermal inertia of the water. You need a big tank however, because a day on Mercury is 176 earth days long. That's a lot of heat to capture, and then radiate back into space. This trick doesn't work on Venus, because its thick atmosphere keeps the entire planet the same temperature, day and night, all year round.

NASA's Messenger probe used a similar cooling strategy, on a much smaller scale, as it orbited Mercury for four years. Despite a sunshade, it absorbed heat from the sun while it was on the day side of the planet, then it radiated that heat away when it was on the night side. An elliptical orbit gave it more time on the night side to cool off.

Let's say you have a nice little house on Mercury, equipped with the aforementioned air conditioning. As you look out the window at the blazing sun, perhaps you recall the words of Isaiah 38:7-8.

“This is the Lord's sign to you that the Lord will do what he has promised - I will make the shadow cast by the sun go back the ten steps it has gone down on the stairway of Ahaz.” So the sunlight went back the ten steps it had gone down.

This seems unlikely in a literal sense, the amount of energy needed to stop and reverse Earth's rotation is almost unimaginable, but, maybe the prophets were imagining life on Mercury. As mentioned earlier, a day on Mercury is 176 earth days. In other words, Mercury spins very slowly on its axis. That's 88 days of intense sunlight as the sun creeps across the sky, and 88 days of darkness. But watch what happens around noon. The sun has passed the zenith moving west, then it slows to a stop, backs up, now traveling west to east, and passes over your house once again. This happens for 8 earth days, then it stops and resumes its normal east to west trajectory, passing over your house once again. From here the day proceeds normally, as the sun creeps to the west and down to the horizon, where it quietly sets.

Now how can that be? The orbit of Mercury is elliptical, which means it zips around the sun faster when it is close to the sun, and slower when it is farthest from the sun. At its closest, it moves around the sun faster than it spins on its axis. The apparent eastern motion from revolution exceeds the apparent western motion from rotation. From the perspective of your house, the sun seems to back up. It doesn't go forward (westward) again until you are well passed perihelion. If your house is 90 degrees around, you will see the sun set in the west, then rise in the west, just for a couple earth days, then set in the west again. I think that's pretty cool, though I'm not planning to move to Mercury any time soon.

Further Reading

The Venera and Messenger space probes.
Day and night on Mercury.