I've seen descriptions of satellites orbiting earth which can create maps of continents with elements represented as colors on a map, how can they do this? I understand how a heated black body emits radiation that can be used to identify elements([S.O. elemental composition of mars spectroscopy][1]. I'm also able to convince myself that looking at the spectra from gaseous/liquid substances (in chromatography spectroscopy) works by showing which wavelengths are passed through. How does spectroscopy work for solid objects reflecting light? Is the idea that different elements/materials on the earth's surface reflect a portion of the spectrum from the sun? Notably, reflection is different from absorption in the case of liquid/gas chromatography, right?

If so, I am confused about a few things.

  • If you shine a light at substance (like a chair or a piece of wood) and look at the reflected light in a spectroscope - surely you can't identify the materials in it... I thought you'd need black body radiation, right?

  • A little less related, but if you have two objects that are black but made of different materials (e.g. charcoal vs. black paint) would looking at the spectrum of reflected sunlight help you ID the material?

  • how do these satellites deal with atmospheric interference?

  • In spectroscopy, is it important to differentiate between spectrum absorption versus fluorescent emission, such as when an object absorbs white light from a source, reflects some of it, but also has a fluorescent emission?


1 Answer 1


Many questions here. I'll see how many I can answer.

First, remember that blackbody radiation tells you nothing at all about what the body is made of. It only tells you what its temperature is.

Second, rocky planets and asteroids are made of mineral compounds- molecules that contain metals, oxygen, sometimes hydrogen, carbon, etc. If you shine a white, IR, or UV light at a mineral like this, for some broad classes of minerals you can indeed detect their presence by looking at the reflectance spectrum and comparing it to the spectra of known substances you have studied in the lab.

This includes knowledge of whether or not the minerals and elements in question possess any particular fluorescent properties.

(Luckily, planet surfaces aren't made of wood, which makes the identification processes a lot easier.)

This is true also for frozen ices like those of water, nitrogen, methane, carbon dioxide and so on.

Bot once you know its temperature, then you'll know what chemical elements and compounds might be present as solids, liquids, or gases and tailor the search techniques to account for that.

To account for atmospheric effects, you look at light that has passed through the atmosphere by scooting around the backside of the planet and measuring the spectrum of transmitted light that has passed close enough to the surface of the planet to have gotten "filtered" by the gases there. The gaseous components of the atmosphere, along with their densities, can be readily detected and measured in this way, and if desired you can use that information to subtract out their influences when looking at surface minerals through a thin layer of that atmosphere.


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