There is a sequence of three raw images by Mars Curiosity Rover taken during Sol 3066 (22/03/2022, roughly) that show the same features of Mars' landscape and sky. They are separated by 38 seconds' difference each. This is the first one. This is the last one. Interestingly, the middle one shows a black, oval spot in the low sky, that is not present in the other two:


Just in case some one think I have some obscure intentions, I state I am not an UFO fan, conspiracy theory fan, or anything like that. This is a genuine question.

So, what do you think that might be? Could well be camera noise, or perhaps a satellite (there are a few orbiting Mars), or just some storm debris (there are quite a lot of storms in Mars). Any guesses?

Here's a section blown up so that every pixel produced by the JPGEG is now 5x5 pixels simply to make it easier to see the artifact:

manipulated from https://mars.nasa.gov/raw_images/912750/

Update: Ok, it seems there are lots of similar images. For instance, an image from more recent Sol (3067) also shows a black spot in the sky. An image a bit earlier shows a white spot (from comment below). Here there is what it looks like a meteorite with a trail.

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    $\begingroup$ This is on-topic here but just to let you know that questions about images taken by spacecraft cameras on other planets are also answered in Space Exploration SE. CCDs have all kinds of artifacts but usually they are bright. Tracks from cosmic rays are common. But this dark feature is not so easy to dismiss. $\endgroup$
    – uhoh
    Mar 23, 2021 at 1:51
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    $\begingroup$ @uhoh Oh, thanks. Didn't know that site exist! That site might be more appropriate for it, don't know. I imagine mods can make a decision whether to move or keep it. I have no prior. (and thanks for the image close-up, very good idea). $\endgroup$
    – luchonacho
    Mar 23, 2021 at 2:05
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    $\begingroup$ @StuartRobbins dust on the lens would be out of focus; this is of order 1 pixel wide. It has to be something either fairly distant and within the DOF of the lens, or happening very near or in the CCD. $\endgroup$
    – uhoh
    Mar 23, 2021 at 9:58
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    $\begingroup$ could it be Phobos or Dimos at near new moon? The image seems to be looking (nearly) into sunward direction (just left of image). But if the answer should be yes... I will be surprised about the amount of light reduction... Possibly more likely the influence of cosmic radiation on the read-out electronics (which reads-out a number of pixels at once) or in the transmission (though I recon that's unlikely as some error-correcting form will be chosen) $\endgroup$ Mar 23, 2021 at 12:35
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    $\begingroup$ @planetmaker how can a moon outside the atmosphere make the sky in front of it darker? That doesn't make sense, The sky brightness is additive, that's why the shadowed part of the moon during the day is sky blue rather than dark. 1, 2, 3, 4... $\endgroup$
    – uhoh
    Mar 25, 2021 at 2:28


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