MGTS24 wrote:Surely if this image was created using thermal imaging, the various tones correspond to the temperature of the different materials, and not 'light and dark' in repect to the visible spectrum. Or is the correspondence between tone and temperature always very close with lithic materials?
Your understanding is a little off, although to be fair, most people don't realize that "infrared" is a very, very broad description of a type of light that can be interpreted to include anything with a longer wavelength than red (infra = beyond...beyond red), although it's usually considered more limited than that.
Measuring in nanometers (nm = billionths of a meter), the deepest red a human can perceive is around 750-790 nm (can vary person-to-person).
Beyond that, is the so called near-infrared (790 nm to 1400 nm) band. Our eyes aren't sensitive electromagnetic radiation at these wavelengths, but generally, it behaves very similar to visible light (reflects off most objects...only very hot objects radiate these wavelengths...things that are just barely not hot enough to visibly glow).
There's two bands of mid-infrared - from 1400nm to 3000nm, to which the atmosphere is opaque, because water vapor in the atmosphere acts absorbs them, acting like a smoke screen to these wavelengths, and from 3000nm to 8000nm. This latter band is getting into what the type of infrared you're thinking of known as thermal infrared - where objects are visible based on their radiation of heat, but typically only if they're hotter than room temperature.
Finally you have thermal infrared, from 8,000nm to 15,000nm (above 15,000nm, most electromagnetic radiation is again absorbed by water vapor until you get down to the microwave radio wavelengths), or about 10-20 times the wavelength of visible light. Objects at room temperature radiate strongly enough in the thermal infrared spectrum to be detectable.
Anyways, although the name of the camera is the "Thermal EMission Imaging System" (THEMIS), it actually has the ability to detect light from all of the above bands, including visible light.
The image from this APOD was taken in a visible or near visible wavelength, so it is reflections of incoming visible or infrared light, just like taking a picture with a camera.
However, your original premise is correct - if this really were an image of emitted thermal radiation, the brightness would be heavily affected by temperature.
"Any man whose errors take ten years to correct is quite a man." ~J. Robert Oppenheimer (speaking about Albert Einstein)