THE USM SOUTHWORTH PLANETARIUM

207-780-4249     www.usm.maine.edu/planet

70 Falmouth Street     Portland, Maine  04103

43.6667° N                   70.2667° W 
Altitude:   10 feet below sea level

Founded January 1970

Julian date:  2458618.5

      "Let's hope this reboot really reboots."

THE DAILY ASTRONOMER

Wednesday, May 15, 2019
An Inexplicably Hot Moon?

Do you remember when Neil and Buzz hopped around on the lunar surface to the astonishment of all of humanity, (apart, of course, from those skeptical few who believe the entire matter was contrived on a sound stage)?    Did you know that their footprints still remain impressed onto the lunar soil?   Lacking the efficient erosion mechanisms that are constantly polishing Earth, the moon retains its bumps, craters, gashes, dents and foot fall impressions for quite a long time.    However, did you ever wonder why the astronauts had to wear such heavy footwear?    We know they had to bring their pressurized suits and ample oxygen supplies to survive on a vacuous terrain devoid of Earth air.  They needed the footwear to protect themselves from the furiously hot lunar soil, the temperature of which was comparable to that of boiling water.       

Regard the following conversation (#11,489) that occurred in our hobbit hole planetarium:

"I don't understand why the moon is hot on its daylight side."

"Yes, it's about 200 degrees or so."

"That doesn't make any sense, though.  That show just mentioned that planets have atmospheres to trap heat.  I know that Mercury would be hot, anyway, because it is so close to the Sun, but the moon is much farther away from Mercury and it doesn't have any heat-trapping air.    Shouldn't it be really cold, even on its lit side?"

A cool question that involved more than one explanation.

Yes, our atmosphere does retain heat energy, although not as efficiently, thank Heaven, as Venus's choking gas envelope.   However, the atmosphere can and does absorb incoming solar energy.   It is for this reason that we are colder (generally) in the winter than we are in the spring and summer.  The Sun's altitude is lower and therefore the incoming radiation passes through more atmosphere than it does when higher above the horizon.     A planet's surface will sizzle sweetly on its sunlit side if no atmosphere absorbs any of the incoming heat.   However, as the moon lacks any atmosphere, its dark* side cools quickly.  An atmosphere is a coolant during the day and a heat retention shield at night.

It is also a matter of albedo, the ratio of reflected to incidental sunlight.   Earth's mean albedo (the albedo averaged over its entire surface) is about 0.33, meaning that approximately 33% of all the incoming sunlight is reflected back into outer space.   The moon's mean albedo is 0.07!  About 93 percent of all the sunlight is absorbed by the lunar surface.   

Also, there is the matter of rotation:  remember that the moon rotates about once every 27.5 days. Consequently, any point along the lunar terrain will remain exposed to the sunlight for about two whole weeks.   That time represents a great deal of baking!       

"So, it is the lower albedo, the slower rotation and the lack of air that is responsible for the hotter moon?"

"Yes, and the lack of water, too.   Water has the highest known heat capacity of any liquid, meaning that it requires more heat energy to raise its temperature than any other fluid.   Earth's surface is predominantly water and that has a great moderating effect."

Perhaps a hot moon isn't inexplicable, after all.

*"Hey, didn't you at one time say that the moon doesn't have a "dark side?"    

Well, perhaps we did, but we should have clarified the matter.     The side of the moon always facing away from Earth should be rightly dubbed "the far side," Mr. Larson, and not the dark side, as that term wrongly implies that the far side never receives any sunlight.      The moon does actually have a "dark side," as does Earth and the other planets:  the dark side is merely the side of the world that isn't directed toward the Sun at any given moment.