Every now and again, I am strongly reminded that our Moon is really weird.
It has a ton of familiar features on it, so it's easy to get complacent. Giant impact craters, vast relatively smooth plains called maria (MAH-ree-uh) that are filled with dark magmatic basalt (which form the features we can see by eye on the Moon), and everything else a perfunctory examination through a small telescope reveals. Wondrous, yes; beautiful, absolutely; but the sort of stuff that we can mostly explain.
And then there's Ina.
This is a small depression on the Moon inside Lacus Felicitatis, one of a series of small maria tucked in between the much larger maria Imbrium and Serenitatis. Ina is about 3 x 2 kilometers in size, and roughly shaped like the letter D. As for what it is specifically, well, that's anyone's guess.
Seriously. Take a look at it:
I mean, come on. If I told you that was a close-up of a pan I used to cook pancakes*, you'd believe me.
It was first spotted by Apollo astronauts, and since that time several other spots like it have been found. They're always found in the maria, and they're generically called irregular mare patches, a shoulder-shrug of a name if I ever heard one.
The terrain is obviously complicated, with two major components. A smooth, darker one that's at a higher elevation by a few tens of meters, and a lighter, rougher one that's at lower elevation.
[Note: I had a devil of a time seeing that; most of the images I looked at made it look like the rougher lighter surface was higher! This is due to the well known "crater illusion," which can make craters look like mounds and vice versa depending on the lighting. I've rotated the images here as needed so that it looks like the lighting is coming from the top, more or less, which tends to minimize the illusion. Download them and flip them over to knock your brain goofy.]
One thing I noticed right away is that the smooth dark areas have lots of craters, but the rougher lower areas don't. That's usually a dead giveaway; surface that's been around longer accumulates more craters as random impacts pepper the Moon. So the smooth area is older than the rough stuff, right?
Maybe. Maybe not. Crater counts in the darker features give an age of a billion years or so, consistent with the age of Lacus Felicitatis around it. But that's weird. It looks to me (and I'm guessing) like the smooth features are lava that's bubbled up in a depressed area, which would make them younger, but they appear to be older. Very weirdly, the edges of the dark mounds are steep and sharp, which indicates youth; they erode over time due to impacts and solar wind weathering, so this indicates they're only about 50 million years old or so. Other studies also indicate Ina is very young and may have formed as a release of undersurface gas; indeed they posit it may still be changing even today.
While the exact formation isn't known, a team of scientists has an alternate idea. They propose that the rough terrain is actually very old, but it hides the impact craters. If the eruption that formed it had lava with gas in it, it would have lots of voids in it (bubbles is what I'd call them on Earth, but the Moon has no atmosphere, so that sounds a little weird) called vesicles. Pumice is the example you see in volcanoes here on Earth. The rock would be very porous, and material like that doesn't create craters as easily in a small impact. A lot of the energy gets absorbed — this is the same principle as a "crumple zone" in a car frame — so the crater isn't as big.
Another way to measure age is to look at the thickness of the regolith, the layer of crushed rock on the surface. This gets thicker with time as it accumulates. Interestingly, regolith is thinner in the rough lower areas, implying a younger age. How do they explain this?
In their idea, the porous, foamy material would allow finer grains of regolith to sift down. Any nearby small impact generates a bit of seismic shaking, which would act like tapping the side of a colander to let flour sift through. Together, these two features imply the surface there could be much older than it appears.
That's consistent, which is nice, but we don't know if it's the correct explanation or not. It still leaves lots of questions, like why the mounds have sharp edges, and just how this whole thing formed in the first place.
Like I said: The Moon is weird.
Someday I hope we send a probe, or even better, humans to Ina or one of the other irregular patches. There's a lot we still have to learn about our nearest cosmic neighbor, but this mystery is an itch I'd like to get scratched.
*I first heard about Ina from the Lunar Reconnaissance Orbiter site, and the first thing I thought of when I saw the image was the pan analogy… which they use as well on the site! Great astronomical minds etc. etc.