10
$\begingroup$

The surface of the Moon looks very different in orbiter images taken at local noon than it does in images from other times. Here are two examples of the same places in Lalande Crater. In each image, the top section was taken near local sunset (sun incidence angle 68o) and the bottom section taken near local noon (sun incidence angle 12o):

fresh impact crater in central peak of Lalande Crater

The above are images M1096551351LC and M1103624254LC (which is flipped horizontally in the online display). The crater shown is about 80 m across. Coordinates about 4.47oS by 8.6oW.

large chunks of debris on south rim of Lalande Crater

The above are images M1154271821RC (which is upside down in the online display), and again M1103624254LC. The white boulder debris field is about 120 m across. It is at coordinates about 4.85oS by 8.6oW.

All of these images were taken with the narrow-angle camera of the Lunar Reconnaissance Orbiter, so they are of the visible-light spectrum only (but recorded in black and white). I have been trying to understand how the big changes in apparent tone work. This blog post from the Moon Zoo project says that fresh impacts and their ejecta are bright because

their newly exposed and broken surfaces are clean and shiny and have a relatively high albedo in comparison to the mature, darker mare material they lie on top of

Does this basically mean that particle surfaces are smoother at the microscopic level? Is there a chemical difference too? I have read about the Opposition Effect, but that doesn't seem relevant. And none of this explains the black stuff - what is that?

Here are a couple of other images of the area, the first also from the LRO, showing normalized surface temperature variations, and the second from Clementine, showing optical maturity, imaged in the UV spectrum.

Average surface temperature from Diviner instrument Optical maturity from Clementine

$\endgroup$
8
  • $\begingroup$ It's interesting to compare the NAC pairs to the images. There are noticeable differences, but they aren't quite as pronounced. $\endgroup$ – PearsonArtPhoto Sep 7 '15 at 21:32
  • $\begingroup$ @PearsonArtPhoto do you mean the C and E versions? I have thought about asking about those pairs in a separate question. I haven't found an explanation of what the difference is. There isn't any reference to there being any filters on the NAC. I did slightly sharpen the top of the first image, otherwise the colors haven't been touched. And i stretched the bottom of the second image to get it to the same scale. $\endgroup$ – kim holder Sep 7 '15 at 22:05
  • $\begingroup$ What are the exposure times on these images? $\endgroup$ – called2voyage Sep 8 '15 at 19:10
  • $\begingroup$ @called2voyage the orbiter scans the ground as it moves, so the exposure time here applies to each line of the image. The frame taken close to local noon that appears in both examples lists a line exposure duration of 0.00051 s. The one taken near sunset in the top one has a line exposure time of 0.00069 s. The one from near sunset in the bottom one was 0.00068. $\endgroup$ – kim holder Sep 8 '15 at 21:02
  • 1
    $\begingroup$ @kimholder I wonder if the darker spots have a greater presence of the iron spherules mentioned here on page 1668. At sunset there is low contrast between highly reflective regions and highly absorptive regions, but at noon the reflective regions shine brightly and the absorptive regions contrast darkly with the rest of the regolith. $\endgroup$ – called2voyage Sep 9 '15 at 14:52
2
$\begingroup$

The two images mainly differ in their dynamic range between the darkest and brightest parts of the image. Here are a few numbers to get started:

  • the albedo of the Moon varies between 0.1 and 0.3. I.e. the brightest spots are three times brighter than the darkest, given an identical illumination.
  • photographic cameras do have a range of at least 10 orders of magnitude between a fully dark and a fully saturated pixel / grain on film.
  • the human eye can distinguish up to 20 orders of magnitude.

While I wasn't able to find precise numbers for the illumination of shadows inside Moons' craters, the difference to areas directly lit by the Sun is many orders of magnitude - in fact there are statements how difficult it was for astronauts to see any details inside shadows.

Now let's look at the images. The lower ones taken at noon don't have (m)any shadows, so the difference between dark and bright spots is given by albedo and maybe one order of magnitude. The upper ones show very pronounced shadows and therefore several orders of magnitude dynamic range. The small changes in albedo are simply too small to be visible.

In photography it is common to not show the raw image "exactly as" captured on film or sensor. There's always (and always has been, even in the very beginning of analog photography) a development step that allows to adjust brightness and contrast.

The pairs of images are post-processed individually to use the full range of brightness available for the images from white to black and therefore can't be used to give a direct impression of the actual contrast.

We can't retrieve the original data from the processed images, but we can try to make them more similar using the albedo numbers above. With a similar amount of contrast in both of them (i.e. one step in brightness in the image corresponds to the same step in brightness in reality), the comparison might look like this:

image comparison with similar contrast

$\endgroup$
3
  • $\begingroup$ Interesting, I think I follow this, however a) could you clarify your phrase "two images are post-processed" - do you mean the pairs of images in the OP? That does make sense its just not explicitly stated. Also b) I don't quite get what you've done in the last step where you say "With a similar amount of contrast in both of them" - could you explain this a little more please in conjunction with the new images? $\endgroup$ – Puffin Mar 13 at 13:56
  • $\begingroup$ @Puffin Is it better now? $\endgroup$ – asdfex Mar 13 at 14:04
  • $\begingroup$ Yes - that made sense as soon as I read it. Especially once I saw the last image. There are versions around that look pretty similar, I just never connected the dots that these images were an attempt to make the features more visible by increasing the contrast, not an accurate portrayal of what a person would see. I do think adding the paragraph on brightness and contrast makes it clearer, those are things a lot of people have experience with. $\endgroup$ – kim holder Mar 13 at 15:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.