Night on the Moon - where does the heat go?

[piff]

We all know that temperatures on Earth's Moon vary wildly between day and night since each is the equivalent of a fortnight long by Earth standards. Under the glare of the SUn for 2 solid weeks it is easy to see why the rocks heat up during the lunar day, but it has just struck me recently that I do not understand how they cool down at night. How do they lose the heat, and where does it go? In the absence of an atmosphere I assume that heat can neither be conducted nor radiated into the vacuum of space (that is why vacuum flasks work!) Is the heat conducted into sub-surface rocks, or what? 

[AKB]

Absolutely no problem with radiating into a vacuum.  That’s what the sun does, after all.  A vacuum flask limits heat loss by conduction and convection, not radiation.

[AKB]

4 minutes ago, AKB said:

A vacuum flask limits heat loss by conduction and convection, not radiation.

Actually, to be fair, it does cut down on radiation by having reflective coatings, but that’s nothing to do with the vacuum.

[coatesg]

Radiation to space is indeed the answer - remember that the cosmic microwave background is 2.726 Kelvin or so in all directions. Anything warmer than that will lose heat by radiation out into space...

[Charic]

1 hour ago, piff said:

I assume that heat can neither be conducted nor radiated into the vacuum of space (that is why vacuum flasks work!) Is the heat conducted into sub-surface rocks, or what?

Heat as we know it, here on Earth  transfers by one or a combination of the following: convection, conduction & radiation.
On the Moon, heat cannot be convected through the lack of an atmosphere, so that leaves conduction or radiation.
Conduction  by 'contact' and would take ages, therefore as coatesg suggests, your left with radiation, in-fact thermal radiation.

Thermal radiation into space also explains the reason why car windscreens here on Earth freeze the way they do, yet the side windows can remain un-frozen!

[SilverAstro]

4 hours ago, AKB said:

cut down on radiation by having reflective coatings, but that’s nothing to do with the vacuum.

Yep quite so,

you beat me to it on the initial reply (I was still typing  similar!)

but now we could get into entropy !? and the human time scale ducks&runs

[Zakalwe]

8 hours ago, piff said:

 In the absence of an atmosphere I assume that heat can neither be conducted nor radiated into the vacuum of space

Thermal radiation into space (effectively a black body) is faster WITHOUT the insulating layer of atmosphere. One of the Apollo astronauts (it might have been John Young) learned this to his detriment when he placed a rock sample in the shade of the LM for a few minutes. He later touched the rock with his bare hand at re-pressurising the LM. The rock had radiated most of it's heat away and a frost-burnt finger resulted.

 

8 hours ago, piff said:

 Is the heat conducted into sub-surface rocks, or what? 

Some will be, no doubt. But the majority of the heat load will be radiated into space.
The same principle is why telescope lenses and mirrors dew up. You point the scope at a dark sky and as a result you are pointing it at a virtual black-body. The lens can radiate it's heat away very quickly and it's temperature quickly drops below the dewpoint. Point a scope at cloudy skies when the ambient temperature is the same as a clear night and the scope won't dew up as quickly.

[AKB]

4 hours ago, SilverAstro said:

but now we could get into entropy !?

...all I know was learnt from the Flanders and Swann song: First And Second Law Of Thermodynamics.

[Ravenous]

As @Zakalwe has already said, heat can be radiated away on earth too, when exposed to a clear, dark sky.

In the right circumstances the ground can lose heat and cool faster than the air above it.  You can end up with a temperature inversion, where there's a layer of cold air in contact with the (also cold) ground, and warmer air above it.  This is why morning mist can briefly form thin layers around dawn. Quite fascinating stuff we often have no idea is going on around us until we think about it...

[piff]

Hi Guys

Very grateful for your input on this. You all seem to agree that radiation is the answer but I would be grateful for a little further clarity on this. Sorry to sound dim, but I had the impression that radiation means that particles are actually ejected from the "parent" body. Are you saying that molecules of rock get so excited by the heat energy they have absorbed that they leap off the surface into space? That would explain it, certainly. It just sounds a little counter-intuitive, if you know what I mean. If that is what happens, presumably the molecules of rock would not reach escape velocity and would fall to the surface again, adding to the thin layer of surface dust? 

[AKB]

No physical particles involved.  Heat radiation is simply electromagnetic radiation in the infra-red.  Just the same as radio, light, X-rays, and gamma rays. 

[Stu]

48 minutes ago, piff said:

I had the impression that radiation means that particles are actually ejected from the "parent" body.

Radiation as you are thinking there would be alpha and beta radiation which are indeed particles, but that is not the process involved here.

Plenty on it here:

https://en.m.wikipedia.org/wiki/Radiation

As mentioned above, the mechanism is infrared radiation, not the high energy Gamma radiation for example.

[robin_astro]

My measurement at microwave wavelengths showing the moon radiating its surface heat away

http://www.threehillsobservatory.co.uk/astro/radio_astronomy/radio_astronomy_1.htm

Robin

[SilverAstro]

4 minutes ago, robin_astro said:

My measurement at microwave wavelengths showing the moon radiating its surface heat away

and you say there " Note that the lunar signal at microwave wavelengths is mainly due to the thermal signal from the moon’s surface layer with only a small contribution from the reflected sunlight."

Oh! I had not given it any thought like that before. One tends always to think of heat as IR,  but black body and Boltzmann (sp? ) gets forgotten ? is that the ball park ? Thanks, nice one.

 

[robin_astro]

Yep the Sun, Moon and deep space are all approximately black bodies. The distribution of radiated electromagnetic energy with wavelength depends on the temperature. (The Planck curve) For example the maximum of the Planck curve for the Sun at ~5000K is in the visible region. The radiation from the big bang at 2.7K peaks at microwave wavelengths. 

Cheers

Robin 

[AKB]

10 hours ago, robin_astro said:

My measurement at microwave wavelengths showing the moon radiating its surface heat away

That’s a fabulous demonstration... thanks for sharing.

[Stu]

10 hours ago, robin_astro said:

My measurement at microwave wavelengths showing the moon radiating its surface heat away

http://www.threehillsobservatory.co.uk/astro/radio_astronomy/radio_astronomy_1.htm

Robin

Some fascinating stuff on your site Robin, will have a good look around there when I have time.

So I understand correctly, the signal shows the moon as it passes through the detector, rather than showing a decay curve over time? Would the latter be possible? 

[robin_astro]

2 hours ago, Stu said:

So I understand correctly, the signal shows the moon as it passes through the detector, rather than showing a decay curve over time? Would the latter be possible? 

Yes, measuring an object by allowing it to drift across the field is a standard procedure in radio astronomy. (A radio telescope is effectively a one pixel camera so you have to scan it over the sky to create an "image".) You could track the moon though if you wanted to measure changes with time. I would not expect much over the course of a day, though it might be interesting to compare the signal at full and new moon.  The new moon is definitely a strong radio signal. I attended a weekend practical course at Jodrell Bank a few years ago where we were measuring the signal from a galactic radio source (I forget which one) during the day using the drift transit technique. We saw the expected signal and then another unexpected huge signal appeared. Checking a planetarium program we saw that the new moon was next to  our intended target in the sky ! 

Robin

[Stu]

Thanks Robin. Is this signal related to temperature then or am I getting the wrong end of the stick?

[robin_astro]

Yes though the exact relationship depends on the wavelength where you make the measurement.  As the temperature of a black body increases the total amount of radiation goes up as T^4 and the distribution by wavelength shifts towards the blue. Here is a nice little simulator which shows the effect of temperature on the spectrum of radiated energy.

https://phet.colorado.edu/sims/blackbody-spectrum/blackbody-spectrum_en.html

Cheers

Robin

[Stu]

Thank you