The last couple of days have been busy as readers have been driving some interesting discussions. On one page there was an unexpectedly interesting discussion about the Moon. It started with an error on my part that was corrected by a reader. It had to do with the predicted and actual surface temperature of the Moon. The impact of this could be significant, but that I simply don’t know yet, but the idea is an interesting one that will require some more information.
The predicted (blackbody) temperature of the Moon is 270K (-3°C). That is a warmer predicted temperature than the Earth has. This is because the Moon reflects away less energy than the Earth does. This predicted temperature is not the actual temperature of the Moon though. The Earth has a predicted temperature 254K (-19°C). It is warmer than that by 33 °C and the Greenhouse Effect is the common answer as to why the Earth is warmer than this. Since there is no atmosphere on the Moon, there cannot be a Greenhouse Effect, but the Moon is not the temperature that is predicted by the Stefen-Boltzmann equations. Most interesting is that it was much warmer during the night and cooler during the day than it was predicted to be.
The reason for this is the ground underneath the surface absorbs energy from the surface. The lunar surface is not a very strong thermally conductive material, but the surface heats up to about 370K during the day. That very high temperature causes the surface underneath to warm up just like what happens on the Earth. When the sun sets the temperature rapidly drops off. This is because the surface is radiating energy like a blackbody at a high temperature. As the temperature drops, the amount of energy radiated away from the surface decreases. Once about 90K is reached the temperature stops dropping and the surface is now colder than the subsurface. The energy flow then reverses and the surface of the Moon is kept warmer by the energy that is now flowing up from the subsurface and then into space. The “daily” cycle looks like this.
One particular item is that the subsurface loses temperature at a constant rate during the night. At 85-90K the Stefen-Boltzmann for a blackbody is ~3.3 W/m2. That would indicate that the subsurface is transferring energy to the surface at about that rate. The total energy is small, but because the rate of heat loss is also low, it is enough to keep the surface temperature from dropping further.
The difference between the theoretical and the actual measured temperature is significant.
The energy that is lost to the subsurface during the day prevents the surface from reaching the predicted maximum temperature. That energy is temporarily stored in the subsurface until the Sun sets. Then the surface cools and once the subsurface is warmer than the surface, the energy is then transferred to the surface and keeps the temperature from dropping as low as it would otherwise. The same cycle repeats each lunar day. The time lag in the observed temperature is evident in the delay between predicted and actual peak temperature. The surface also is delayed in warming because the subsurface initially is absorbing back the energy that was lost during the night.
According to NASA the maximum depth at which the diurnal temperature change can be noticed is 50cm. This small region of the Moons surface that stores and releases energy is enough to prevent the Stefan-Boltzmann equation from being an accurate predictor of the surface temperature of the Moon. If that is enough to prevent the Stefan-Boltzmann from working for the Moon, then how much more error is introduced into the estimated size of the Greenhouse Effect on the Earth by using the Stefan-Boltzmann equation. The Earth is vastly more complicated than the Moon. That small region of subsurface is also enough to cause mimic to a small degree the effects that are often attributed to the Greenhouse Effect. This could be evidence that the stated Greenhouse Effect of 33 °C is inaccurate and that the actual impact of the atmosphere is in fact lower.