Crazy About Concentric Craters

With the Moon riding high this week, what better time to look for its three best-known yet enigmatic "ring" craters?

High Moon, high hopes

As the Moon waxes to gibbous in the coming nights, it rides high in the sky in steadier air, ideal  for hunting concentric craters.
Bob King

We welcome back the waxing Moon this week. It's a chance for many of us to put dark-sky targets on the back burner and give some love to she who lights the night. During fall, the evening Moon "rides low" in the southern sky during its most attractive phases. Not until January does the ecliptic's steepening angle to the horizon finally loft the quarter Moon high in the southern sky, where it's viewed to best advantage.

February continues what January began. Each night, from now until early March, the "high-riding" Moon dares anyone with a telescope to brave the cold and behold an alien world up close. Each night, the terminator advances eastward, revealing thousands of new features, including three of the most remarkable craters you'll ever see — Hesiodus A, Crozier H, and Marth.

All three belong to a weird breed called concentric craters. Concentric craters are generally small, so you have to really look for them. None jumps out like, say, Copernicus or Tycho. There are about 58 all told, with diameters ranging from 3-20 km (average 8 km), each containing an inner ring usually about half as wide as the crater.

Craters and their inner donuts

Our three featured concentric craters. Hesiodus A, at 15 km across, is the largest and easiest to see. Marth and Crozier H are more demanding.
NASA

At first impression, you might think two unrelated impacts occurred precisely atop another, excavating not one but two crater rims. When I first saw Hesiodus A, the best and easiest of the trio, it struck me as artificial. How could something so symmetrical exist on the battered Moon?

Concentric crater pinpointer

Locations of Hesiodus A, Crozier H and Marth. The day number is how many days after New Moon the crater first becomes visible. "First light" for Hesiodus A occurs Friday Feb. 27th; Marth follows on the 28th.
NASA (with annotations by the author)

Some of the rings look like bagels or donuts, others are clearly rings of hills. Some are elliptical like Crozier H, and others more circular like Hesiodus A. Most occur near the margins of the dark lunar seas (or maria) and the crater-scoured highlands where volcanic activity was intense in the distant past. And as we'll learn, their location provides a clue to their origin.

Ringing in our first crater

Hesiodus A lies nearly due west of the distinctive Pitatus, a large crater with a smooth floor and distinctive off-center central peak. South is up.
George Tarsoudis

Hesiodus A, the easiest, can be seen in a 4.5-inch telescope. The others will require a 6-inch or larger scope. Remember, all are relatively small and their inner rings smaller, so don't shy from high magnification once you've found your target. Crank it up to 200x-250x. Unlike some lunar features which disappear in the shadowless sunlight of the full Moon, many concentric craters remain visible, looking like bulls-eyes.

Not too far from our first target, you'll find the 7 km-diameter crater Marth. This one's really small, but the bone-white inner ring pops out in an 8-inch scope. Crozier H is a bit easier to make out than Marth and occupies a busy cratered area along the edge of Mare Fecunditatis in the eastern half of the Moon.

A ring for your pinkie

Marth is located on low volcanic dome in the Palus Epidemiarum region west of Pitatis and north of Capuanus. This small double-ringed crater will take more patience to discern, but it's a gem and not to be missed. Around the time of the full Moon, Marth looks like a bulls-eye. South is up.
Christian Viladrich

A variety of hypotheses have been proposed to explain the origin of concentric craters' unique inner rings, everything from simultaneous impacts to mass wasting (rock and soil moving downslope under the force of gravity) to impacts in dual-layered lunar shield volcanoes better known as domes.

Caught between craters near the Sea of Fertility

Crozier H's inner ring (upper left) is a little easier to make out compared to Marth. To find it, first locate the medium-large crater Goclenius and shoot a line from it through Bellot to Crozier H. South is up.
NASA

But based on the distribution, size, and other characteristics of concentrics, lava welling up through fissures or cracks in the craters' floors appears to be the best fit. Many lunar craters show evidence of lava flooding. Well-known examples include Plato, Archimedes, and the beautiful Bay of Rainbows aka Sinus Iridum, once a huge basin but nearly submerged by lavas that poured like hot oil across the lunar plains between 3.3 and 2.5 billion years ago.

Exactly why the lavas in concentric craters bubbled up to form donut-like rings remains unclear. It may have to do with cracks preferentially forming along the edges of crater floors, or the rate of lava flow as well as its texture. A plug of molten rock beneath the crater may have lifted up its floor, which later deflated in the center like a collapsed soufflé, leaving a ring. It's even possible that dense impact melt on the crater floor resisted uplift from lavas pushing up from below. Whatever the specific reason, most lunar experts agree that concentric craters owe their formation to volcanic processes.

Deflated souffle or thicker lava?

Unnamed 11.5 km-diameter concentric crater in the Apollo Basin on the Moon's farside.
NASA/LRO

If you've tackled our three examples and crave more concentricity, head over to this gold mine for additional challenges. Wishing you a "ringer" on your next night out!


Wondering what to call that crater? Use a Sky & Telescope Moon Globe to guide your explorations of the lunar surface!

16 thoughts on “Crazy About Concentric Craters

  1. Anthony BarreiroAnthony Barreiro

    Thanks Bob for these interesting lunar challenge objects. I enjoy observing the whole Moon at low power through a small refractor (and sharing the view with passersby), so I probably won’t attempt to see these craters in the near future. One small correction: Sinus Iridum (Bay of Rainbows) not Sinus Iridium (Bay of Silvery White Metal).

    1. Bob KingBob King Post author

      Hi Anthony,
      You are correct! Sorry, a slip. Long ago I called it that and my brain still recalls the time. I encourage you to try 75x-100x on that refractor to at least get a glimpse of Hesiodus A.

    1. Bob KingBob King Post author

      Good luck and if you don’t see it tonight, wait till a fatter gibbous phase, try again and let us know how it went.

      1. rocksnstarsrocksnstars

        I just tried tonight (a day later, Friday the 27th, 9 day old moon) and the bottom of the crater was not lit at all. Oh, I see it says the crater first becomes visible; I read it before as the ring becomes visible. Maybe tomorrow night.? Also, could you please let us know the age of the moon for the photo of Hesiodus? Thanks and thanks for the article! Always neat to hear of something new to look for after 40 years of observing, even if it is on the moon!

        1. rocksnstarsrocksnstars

          Tonight the bottom is a little over half lit and I could see half the concentric circle at 175X in an 8 inch. Tomorrow night should be a full ring!

          1. Bob KingBob King Post author

            Excellent rocknstars – glad you got to see it. Like you said, it creeps out as the Moon becomes more fully illuminated.

        2. Bob KingBob King Post author

          rocknstars,
          I will have to ask the photographer on the date. I should think the inner ring would definitely show on “Day 10” and be easy on Day 11.

  2. rocksnstarsrocksnstars

    Being a galaxy and comet freak, I usually stay inside if the moon is in the sky, but this is something I’ve never heard of and must try, especially Hesiodus A since it is so close to the Straight Wall. I looked up the area on http://www.moon.com.co/atlas/sections/f3.shtml. Based on that photo, if you can see the Straight Wall clearly, you won’t see the ring of Hesiodus A.

  3. Terry-Atwood

    Bob–

    Great article! It reminds me of those great challanges Alika Herring drew of lunar craterlets in the 1950’s-70’s using his 12-inch Astrola. I am going to pass this along to my star gazing class at Worley Observatory here in Shreveport, LA as well as members of the Shreveport-Bossier Astronomical Society. Always interesting sights on the moon for novice amateurs as well as veterans looking for challanges.

    Terry Atwood

    1. hildyrock

      I got setup too late tonight to check out these features; the moon was already tangled in the trees to the west. But the weather is clear all tonight and tomorrow (Saturday) night. That means the scopes will spend the rest of the night and Saturday out on the obs deck, ready to fire up as soon as it starts getting dark. I will then be looking for Hesiodus A and its friends. By the way, we are down to 5*F on the eastern Blue Ridge. I’ve seen a lot colder but for some reason tonight the cold blasted its way thru my heavy glove and I had to break out the exotherm hand warmers. Fear not, I’ll be back out shortly as Saturn gets higher.

  4. Mike-McCabe

    Thanks for posting this interesting challenge Bob! I was out last night (27th) with my 1976 Sears #4451 60mm F15 refractor and gave this a shot just for kicks. I was curious just exactly what I could see with such a small scope. As it turns out, last night, at least during the hours I was out there, wasn’t the night to see the interior. The sun angle was still a bit low, and while Hesiodus A itself was easy to see, the only illuminated parts were the western flanks of the outer rim. Tonight is supposed to clear and steady so I think I’ll give it another go, both with the Sears and a larger scope and see what’s what.

    1. Mike-McCabe

      Just got in from checking out Hesiodus A in two scopes, a 4″ F11 refractor and a 4.5″ F8 reflector. The sun angle was still just a wee bit shallow, but the middle ring was definitely visible. In the refractor at 160x it was visible about halfway around, and in the reflector at 185x it was visible nearly all the way around. There was still some shadowing on the eastern side of the crater, so a higher sun might show the inner ring even better. We’ve got a storm moving in so there’ll be no attempt tomorrow, but it may clear out for Monday night’s 94% phase.

    2. Bob KingBob King Post author

      Mike,
      When I wrote it up I realized it Hesiodus A might be a bit “shady” when first revealed by the advancing terminator on “Day 9” past New. Another couple evenings should bring it into better light.

      1. Mike-McCabe

        I got out last night to try again at the 94% phase. There was definitely more light, and it really emphasized how big of a difference of aperture and magnification will make. I was still working with the 4.5″ reflector and the seeing was pretty good. At 200x I could just make out the inner ring of Hesiodus A, but Marth was just a dot. As you can imagine, navigation was a challenge in the flattened contrast of the high lighting. I didn’t even bother looking for Crozier H. The low contrast didn’t help matters in differentiating the rings either. I was able to use up to 260x without too much breakdown, but this is something that I’ve got to go back to with the 10″ to see what the improved resolution will yield. Fun challenge if you like the moon. Thanks!

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