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The large rock in the centre right position (cut off by the image's right edge) of this photo seems to include the left part of a (small) natural arch:

Apollo 16's Lunar Surface Magnetometer

Is this indeed a rock arch? Are complete pictures or descriptions of it or any other lunar arches?

If it isn't a proper arch, then what is it?

For reference, compare this rock arch from Mars.

Annotated detail showing a small arch with sunlight visible below:

annotated detail

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    $\begingroup$ I couldn't see it either, so I added something makeshift. $\endgroup$
    – uhoh
    Oct 3, 2021 at 22:45
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    $\begingroup$ Does the "this rock arch from Mars" have a name? That way we can find a source for the image that can be imported into Stack Exchange's imgur so that it becomes a permanent part of the question and doesn't depend on offsite images that could disappear. $\endgroup$
    – uhoh
    Oct 3, 2021 at 22:47
  • $\begingroup$ Not an arch, a bulge on the rock that is casting the shadow, so the top of the bulge is illuminated. $\endgroup$
    – antlersoft
    Oct 4, 2021 at 2:20
  • $\begingroup$ @uhoh Permanent image. $\endgroup$
    – Adám
    Oct 4, 2021 at 9:05
  • $\begingroup$ An alien child from a giant species of moon dwellers carved that in her free time. Obviously. $\endgroup$
    – TCooper
    Oct 5, 2021 at 23:05

3 Answers 3

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There's a saying, "ask 4 geologists about a geological feature and you'll get 5 or 6 answers, maybe more".

My interpretation of what you have highlighted is not a rock arch - an arched rock formation created by erosion or other means. Instead, it is a rock that has been ejected from a crater during a meteorite impact.

During the impact phase, the rock has fractured in a way to give a curved surface to one of its sides and a flat surface on the other side. After ejection from the crater, the rock flew and possibly rolled, but landed with the flat side up and the curved side down.

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    $\begingroup$ It's a fairly small rock and there's a bunch of footprints next to it. Is it possible that rather than being caused by an impact, the astronauts picked it up (possibly considering it for a sample return), and then just tossed it and it landed in that position? (Small as it is, it might've been too big to bring back.) $\endgroup$
    – Darrel Hoffman
    Oct 4, 2021 at 13:39
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    $\begingroup$ @DarrelHoffman, it's only small in relative terms. The equipment in the foreground of the picture is the Lunar Surface Magnetometer, each arm of which is three feet long. Based on that, I'd say the rock is about two to three feet long, and probably weighs fifty pounds or so. As a point of reference, Apollo 16 returned 210 pounds of samples in total. $\endgroup$
    – Mark
    Oct 4, 2021 at 21:06
  • $\begingroup$ @Mark Maybe 50 lbs. on Earth - on the Moon that'd only be about 8-9 lbs. Easy enough to lift, I'd think? $\endgroup$
    – Darrel Hoffman
    Oct 4, 2021 at 21:15
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    $\begingroup$ @DarrelHoffman, the concern isn't lifting it, the concern is that it occupies a quarter of the available sample-return mass, and is much, much too large to fit in a sample bag. $\endgroup$
    – Mark
    Oct 4, 2021 at 21:27
  • $\begingroup$ @Mark Fair enough. Didn't they also do some core samples on some of the trips? That could be reason to move some rocks around even if they had no intention of bringing them back. $\endgroup$
    – Darrel Hoffman
    Oct 5, 2021 at 13:16
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If it helps, following the citation trail leads to the back side of the same rock formation being visible on three other photographs on the same reel of film:

https://history.nasa.gov/alsj/a16/AS16-113-18370HR.jpg
https://history.nasa.gov/alsj/a16/AS16-113-18350HR.jpg
https://history.nasa.gov/alsj/a16/AS16-113-18349HR.jpg

Here are cropped views of these:

arch 1 arch 2 arch 3

A blurry but slightly different angle is also visible on https://history.nasa.gov/alsj/a16/AS16-114-18388HR.jpg

Cropped:

arch 4

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    $\begingroup$ Nice finds, but still looks like an arch to me! $\endgroup$
    – Adám
    Oct 4, 2021 at 14:36
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    $\begingroup$ @Adám It's clearly the entrance to a gnome home. $\endgroup$
    – JimmyJames
    Oct 4, 2021 at 15:02
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    $\begingroup$ Obviously a fossilized femur of some large moon pig. $\endgroup$
    – Pete Kirkham
    Oct 4, 2021 at 20:25
  • $\begingroup$ c'mon guys, clearly an alien skull, geesh! $\endgroup$
    – coblr
    Oct 5, 2021 at 22:38
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There are two rocks one big one in the back ground and a smaller one slightly closer. Something is casting a very deep shadow across to bottom of both. The deep shadow makes it impossible to see the definition on the edge of the bottom of the smaller rock, so all we can see is it's illuminated top. Because the top of the small rock looks similar to the background regolith the big rock looks like it has a small arch at its base.

That said there may well be strangely curved rocks and even vaguely Stonehenge like formations where by change rocks were piled up resting on other rocks with a gap, but I suspect they are very rare and I don't think this is one of them. Such a structure would undoubtedly have attracted attention and comment. In other words its a trick of the harsh lighting conditions.

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    $\begingroup$ @ojdo, all Apollo landings were done shortly after sunrise, so that rocks were more easily visible during the landing. $\endgroup$
    – prl
    Oct 5, 2021 at 23:16
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    $\begingroup$ With the newer multi-photo answer, I have changed my mind. Now I see the arc, too. $\endgroup$
    – ojdo
    Oct 6, 2021 at 7:34
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    $\begingroup$ @prl That's why I love the moon. Where else have humans traveled where "shortly after sunrise" is a day or two later? $\endgroup$
    – dotancohen
    Oct 6, 2021 at 9:23
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    $\begingroup$ @dotancohen The Earth's poles, if you go at the right time of year. (Sorry.) $\endgroup$
    – user3067860
    Oct 6, 2021 at 12:00
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    $\begingroup$ @user3067860 Oh, definitely! I've been north of the Arctic Circle in winter, I should have thought about that! We do live on a diverse, interesting planet. $\endgroup$
    – dotancohen
    Oct 6, 2021 at 12:29

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