Does Earth or Moon emmit/reflect anyother form of radiation than IR into space? I am especially curious if they emmit/reflect UV radiation.

And if they do, where could I find an irradiance spectrum or something like that?


I am especially curious if they emmit/reflect UV radiation.

Except for near ultraviolet, the Earth's ultraviolet spectrum is very dark. Most of the ultraviolet B and shorter light emitted by the Sun is absorbed by the Earth's atmosphere. The ozone layer strongly absorbs ultra violet B and C. Even higher frequencies are absorbed even higher in the atmosphere.

The absorption isn't perfect, however. My first job out of college involved working with an experiment on a NASA satellite that assessed the amount of ozone in the stratosphere by looking at backscattered ultraviolet light. This was a follow-on to the Backscatter Ultraviolet (BUV) experiment flown long, long ago on Nimbus 4. That original experiment wouldn't have worked if earthshine had no ultraviolet component whatsoever. That experiment did work; it has been replicated many times over, including the instrument with which I worked. Looking at backscattered ultraviolet is one of the principal techniques for determining the amount of ozone in the stratosphere.

Here's a spectrogram of a narrow portion of the UV spectrum of earthshine from a more recent experiment, the Global Ozone Monitoring Experiment on the European Remote Sensing (ERS-2) Satellite:

Note how the earthshine spectrum is multiplied by a factor of 200. The ozone layer strongly depletes ultraviolet with wavelengths shorter than 300 nanometers, but doesn't do much at all to sunlight with longer wavelengths (near ultraviolet, visible light, etc.) Near ultraviolet is even more strongly subject to Rayleigh scattering than is blue light. Earthshine contains a hefty amount of near ultraviolet. Here's a broader spectrum, without that factor of 200:

Image source

This spectrum comes from the Lunar Crater Observation and Sensing Satellite (LCROSS). LCROSS was, per its name, was intended to be used to observe the Moon. It pointed its sensors at the Earth to calibrate those sensors. This spectrum shows that there is very little to earthshine at wavelengths shorter than 290 nanometers.

With regard to moonshine, while the Moon does have an atmosphere, it is so extremely tenuous that it does almost nothing to absorb or scatter light. Except for specific absorption bands, the Moon's surface is close to "white" in terms of absorption. (Actually, it's much closer to black than white. The Moon may look bright, but it's not. It's very close to black asphalt in terms of reflectance.) The tiny portion of light reflected by the Moon has very close to the same spectrum as the light source. The Moon will reflect ultraviolet.

  • $\begingroup$ Aside: The ozone hole was first discovered by ground-based equipment. It should have been discovered years earlier by the SBUV/TOMS experiment. So why wasn't it? NASA investigated, and they quickly found that some character named "David Hammen" authored the code that prevented that from happening. I received some nasty phone calls from high up in NASA. I was just out of college at the time I wrote that code, and I did exactly what the PI asked me to do. He didn't trust the data at extreme solar angles, so he asked me to write code to reject that data. So I did. $\endgroup$ – David Hammen Nov 11 '14 at 16:44
  • $\begingroup$ Very interesting! I am curious about the second graph you posted, just want to be sure I understand what you're talking about. It shows amounts of UV light, based on its wavelength, reflected by Earth into space, I mean these are the values(more or less) at AM0? And regarding Moon, it doesnt (or small amounts compared to Earth) reflect any UV light? $\endgroup$ – user1806687 Nov 11 '14 at 20:41
  • $\begingroup$ Yes, that's the spectrum at AM0. LCROSS was well above the Earth's atmosphere at the time that calibration event was performed. With regard to the Moon, I couldn't find any high frequency analyses of it's spectrum. If the Moon is a grey body as assumed in my answer, it should have a good deal more high UV in its spectrum than does the Earth. The Earth "reflects" (scatters is a better word) about 0.5% of the incoming UVB and UVC. If the Moon is a grey body, it reflects about 12% of the incoming ultraviolet. $\endgroup$ – David Hammen Nov 11 '14 at 22:36
  • $\begingroup$ Hi, do you know, what percentage of UVA(~200nm-400nm) does Earth and Moon reflect? I am a bit lost in your second graph. $\endgroup$ – user1806687 Nov 18 '14 at 16:54

There will be the reflected spectrum from the sun, which also has an ultra violet component.

Here is a plot:

black body from sun

The black body radiation emitted by the moon and earth also will have an ultraviolet tail as seen in the formula of Planck's law:.

planck's law

At the temperature of the earth black body radiation in the UV is already is very suppressed , at 255K, and even more from the night side so for the moon. (The day side is can go to 400K). So the reflected UV will be dominant.

Here is a measured reflected spectrum

earthshine spectrum

It stops at the ultraviolet limit, as different techniques are needed, but it is reasonable to assume that there exists plenty of reflected ultraviolet, at a first approximation as much as the blue.

  • $\begingroup$ How can I figure out from the second graph, amount of reflected UV between 250nm to 400nm in W/m2? $\endgroup$ – user1806687 Nov 11 '14 at 14:15
  • $\begingroup$ 5000 angstrom are 500 nanometers, that is why I am talking of estimate, since the plot ends at the beginning of ultra violet. Since the earth is more or less in equilibrium ( we are not boilint or freezing) with the incoming radiation, I would take the watts/m**2 from the first plot as in the correct ball park, one might check the percentage absorption of UV and correct further. earthobservatory.nasa.gov/Features/UVB/uvb_radiation3.php . some UV is completely reflected so the estimate from the original sun curve might not be off $\endgroup$ – anna v Nov 11 '14 at 14:42
  • $\begingroup$ It's not a good idea to extrapolate past the end of a graph. There is very, very little reflected/refracted light beyond the near ultraviolet. $\endgroup$ – David Hammen Nov 11 '14 at 16:30
  • $\begingroup$ @DavidHammen from the link in the comment above it is true that most of the ground level turns into infrared, but there is a lot that is reflected from the troposphere . Usually changes are smooth, the graph you show shows flatness. I think to within an order of magnitude it should be ok to extrapolate the curve. It depends what one aims to do with the number. $\endgroup$ – anna v Nov 11 '14 at 17:20
  • $\begingroup$ Changes aren't always smooth. Think of the photoelectric effect. Much the same happens with the ozone layer. Wavelengths longer than some cutoff just pass through. Below that cutoff, hardly anything passes through. $\endgroup$ – David Hammen Nov 11 '14 at 18:13

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