The "Face on Mars" photograph taken by Viking 1 in 1976 of the Cydonia region on Mars has multitudinal similar sized small dots in a seemingly random pattern all over it. Are all those dots impact craters and if not what are they?
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1$\begingroup$ I thought there was already an answer about this image referring to salt and pepper noise. There is this answer but it's not related to this particular image. I remember inverting this actual image a while ago just to see it, but can't find any trace of that now. Hmm... $\endgroup$– uhohJul 4, 2021 at 13:03
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2$\begingroup$ Bingo! astronomy.stackexchange.com/a/42110/7982 Also, somewhat related here in Space; What caused Cydonia mesas, such as “The Face on Mars”, to be created? and Why is this “grainy footage” of a Martian (sky) from Curiosity so grainy? Cloud snaps by Opportunity look great! and Which Viking-1 orbiter photo is this? When was it taken and what part of Mars is shown? $\endgroup$– uhohJul 4, 2021 at 13:05
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2$\begingroup$ @uhoh The standard approach with regard to imagery errors traceable to the pixel level is to give the suspect pixels an off-scale low or off-scale high numerical value (which appear as black or white) and mark them as suspect or missing in the metadata. JPEG doesn't read the metadata, and it smears to reduce the image size. $\endgroup$– David HammenJul 4, 2021 at 14:09
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Are all those dots impact craters and if not what are they?
They are artifacts of some sort.
They could be salt and pepper noise, but I suspect not. If it was salt and pepper noise the image would also have lots of white dots, which it does not. These artifacts are instead missing data. The standard standard approach with regard to imagery errors traceable to the pixel level is to give the suspect pixels an off-scale low or off-scale high numerical value (which appear as black or white) and mark them as suspect or missing in the metadata. Quoting from photojournal.jpl.nasa.gov,
The speckled appearance of the image is due to missing data, called bit errors, caused by problems in transmission of the photographic data from Mars to Earth. Bit errors comprise part of one of the 'eyes' and 'nostrils' on the eroded rock that resembles a human face near the center of the image.
answered Jul 5, 2021 at 14:17
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1$\begingroup$ I think the image does have roughly the same number of black and white isolated pixels. We don't see the white ones as readily as the black ones in the normal image because our vision system doesn't "top out at 255" when looking at displays, but both are easier to discern in the raw (and inverted) image shown in this answer, which cites a few sources calling it "salt and pepper noise". $\endgroup$– uhohJul 6, 2021 at 1:06
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1$\begingroup$ @uhoh You recently asked How is stacking oranges in 24 dimensions related to receiving and decoding signals from the Voyagers?. Transmissions are typically encoded so that some small level of bit errors are correctable and a slightly larger level of bit errors are detectable (but not correctable). The standard treatment for bad data is to give those bad data values an off scale low value, resulting in a black pixel. This is not salt and pepper noise. It's data transmission errors detected with an error correcting / error detecting encoding. $\endgroup$ Jul 8, 2021 at 5:35
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$\begingroup$ I'm beginning to see what you mean. I checked the GIF again (from here) It has values from 0 to 127 and there are many pixels that deviate from all their adjacent pixels by large amounts, but the deviations seem to have many values, not just 0 and 127. i.stack.imgur.com/6s9A7.png $\endgroup$– uhohJul 10, 2021 at 10:57
