Ina Caldera – One Of The Moon’s Coolest, Hottest Mysteries

Ina Caldera sits atop a low, broad volcanic dome or shield volcano, where lavas once oozed from the moon’s crust. The darker patches in the photo are blobs of older lunar crust. They’re “domed” and rise higher than the rougher surface between them. Credit: NASA

In the March 2012 issue of Sky and Telescope magazine there’s a fascinating short article about a possible lunar volcanic landform called Ina Caldera. This 2-mile long D-shaped patch looks like nothing I’ve ever seen on the moon.

Blobs of older, crater-pitted lunar crust (darker blobs) rise some 250 feet above the younger, rubbly surface like melted cheese on pizza. Brighter areas on the moon generally indicate younger surface features. Solar and cosmic radiation darkens airless worlds like the moon and asteroids over the long haul of time. Not only is the bumpy region lighter-toned, but it appears to have far fewer craters, another sign of its relative youth.

Ina was first noticed only as recently as 1971 when photographed by the astronauts aboard Apollo 15. Recent photos taken by the low-flying Lunar Reconnaissance Orbiter (LRO) show a level of detail that reveal how unique this curious feature really is. So what’s going on in Ina?

Steam issues from the active volcano Nyiragongo in the Democratic Republic of the Congo in May 2010 as photographed by satellite. Credit: NASA

Theories abound. In a real volcano, calderas are caused by the collapse of material at the volcano’s top after magma from below has drained away following an eruption. Perhaps the top of the low volcanic dome on which Ina sits collapsed unevenly and relatively recently to form the patchwork we see today. Another possibility is that magma from below heated trapped gases like carbon dioxide and water to such high pressures, they violently blasted right through the crust, sending rock and debris flying for miles.

This photo shows the moon at 8 days old, but the lighting should be good on tonight’s 7-day-old moon to look for Ina. Ina is located just south of the arc of the Apennine Mountains in the moon’s northern hemisphere almost midway between the craters Conon and Manilius. Photo: Bob King

Other researchers with the LRO mission agree that Ina’s two terrains are a contrast of young and old but don’t think the brighter areas are as young as assumed. Additional high resolution photos from the orbiter show a good number of craters there (see photo below).  I encourage you to explore Ina yourself by going to the LRO’s ACT-REACT Quick Map and entering its latitude of 18.65 and longitude of 5.31. As you zoom in, you’ll feel as if you’re coming in for a landing.

Extreme closeup of a small portion of Ina’s interior. The smoother areas are rounded, blobby “islands” sitting atop the rougher terrain. Credit: NASA

Whatever happened created an enigmatic moonscape that scientists are still trying to understand. In so many ways, the moon is an undiscovered world. With just six short visits during the Apollo era and 150 lunar meteorites collected on Earth, we’ve barely scraped the lunar regolith.

You can appreciate how small the Ina Caldera is this spectacular closeup photo taken by Alan Friedman. Find the Apennines and Conon crater on the range’s southern flank, then navigate toward Manilius using high magnification. Click to see the complete high resolution image. Copyright Alan Friedman/

One of the constant joys of amateur astronomy is seeing with your own eyes what you read or heard about. And trust me, you can see much more than you might think. Even Ina. While most of you will explore via the ACT-REACT map, if you’ve got an 8-inch or larger telescope and you’re generally familiar with the moon, you can start hunting for Ina this evening. This tiny feature is most easily visible when light strikes it from a low angle as it will on tonight’s first quarter moon. If it’s cloudy tonight, try again over the next few nights. Hope for calm air, use high power and you might just be able to spot this enigma.

17 Responses

  1. Debra

    Bob have you seen Ina. I dont think I’m that experienced in amateur astronomy to spot this yet but it will be fun trying

    1. astrobob

      Hi Debra,
      I’ve been all over the moon but have never seen Ina. I’ll be out tonight looking for it. You can see it’s very small, so not exactly a visual feast. That’s why I provided the Act-React link so most folks could go exploring there.

      1. caralex

        Bob, I can never figure out if Ina is an ‘innie’ or an ‘outie’, that is, is it raised or sunken. It looks sunken in your image, with raised central blobs, but in the LROC image, it looks the opposite. What’s the best way for the brain to resolve the dome/crater problem?

        1. astrobob

          This has always been a conundrum. It happens to me too. What I do is force my mind to see the craters as holes, then the rest usually falls into place. It’s too bad the LROC image wasn’t shot when the lighting was more nuanced. It’s funny you should bring this up. Before posting the blog, I rotated that image around a 1/4 turn at a time just to find what angle the blobs looked like blobs instead of depressions. None was ideal but the orientation I went with seemed to my eye to be easiest to interpret.

          1. caralex

            Yes, it drives me mad too – I often have to stare at a photo for ages before the craters sink into place. I still see Ina as raised, though, and the blobs as depressions!!

          2. astrobob

            Try looking at the blobs closer to the center and top, where they gather together. Now imagine their perimeters sloping down to the rubble below. Does that help?

  2. caralex

    Yes, I see them as rounded ‘blobs’ in your first photo, top. But in your close-up further down your article, I’m afraid they’re still depressions. No amount of squinting, head tilting, or monitor turning will convert them into the convex! Sorry!

      1. RC

        For me it helps to visualize which way the sunlight is coming from. In the upper picuture, the sunlight is coming from the top. The upper side of the blobs are white as the sun reflects off the cliffs, but the bottom side is dark since those cliffs cast shadows. In the up-close picture near the bottom, the sunlight is coming from the right. I hope that helps!

  3. Howard

    This is really weird! After dealing with the optical illusion (I have to keep reminding myself that the smooth areas are higher), I’m wondering what the high albedo areas are at the edge of the area. They’re located along a longitude line of roughly 5.26, ranging from lat 18.63 – 18.65. Then there’s also a very bright crater nearby at 18.69/5.21.


  4. Russ Stern

    To me, Ina caldera looks just like it bubbled up – and the bubbles, domed up, burst or just popped. And what remained were the smooth, flattened areas we see today. They look like lakes that have dried up. Maybe they were the molten regolith, and when the bubble popped.. well, even the remaining ‘dimples’ look like tiny bubbles.

    I will be looking with the 12 inch sct !

    1. astrobob

      Hi Russ,
      Good luck with Ina! I know you’ll be thrilled to finally get a look at it for real in the telescope. While Ina sits atop a low, broad volcanic dome, the many small “dimples” you see really are impact craters on ancient crust despite appearances. Wishing you steady seeing!

      1. Russ Stern

        Ahhh ! Clouds here. The moon is up about 25 degrees and.. clouds. Clouds to the west also. Maybe tomorrow ! After that, I’ll wait until moon is waxing. It is getting late for my old bones. Thank you for the great article and I love your site !

        1. astrobob

          Hi Russ,
          Thank you! Your writing me about Ina reminds me to return to it also. The last time I pointed the scope at it, the seeing was too blurry for a good high power view. It’s on my list again – thanks.

  5. Warren Platts

    Re: the optical illusion: here’s a trick I stumbled upon. If you’re using a laptop, tilt the screen toward you until you are looking at the top edge on. Then slowly tilt it back. The bulges will show up first and then stay bulges. Works every time for me.

    Great site BTW Bob!

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