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?
[image: hot moon.jpg]
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.