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Multi-ring Impact Basins on the Moon

PSI Logo Multi-ring impact basins on the moon


Text by William K. Hartmann


In the summer of 1961 I arrived at the University of Arizona's “Lunar and Planetary LaboratorBill Hartmann 1961y” (LPL), with a Penn State degree in physics, to do graduate work in astronomy and planetary geology. LPL had been recently founded by Gerard P. Kuiper. My graduate assistantship led to work on two projects. (1) Various students and hired assistants, under the direction of David Arthur, measured diameters of lunar craters (on Earth-based telescopic photos) for the monumental LPL/Arthur catalog of lunar craters. (2) I was also assigned to photographic work on the “Rectified Lunar Atlas.” Kuiper had collected the best Earth-based photos and set up a 3-foot white globe on which various assistants projected the photos. We could then walk around to the side of the globe and view various formations from “overhead” – a view not accessible from Earth, since the moon keeps one face toward our planet.

My work on those projects led to my interest in the size vs. frequency distribution of the lunar craters, and the first recognition of the systematics and general phenomenon of what became known as “multi-ring impact basins” – basically the largest impact features on the moon – and my interest in the size of the largest bodies ever to hit the Earth and moon.

This web site makes available those early, now-obscure first papers about lunar basins, first published in Kuiper’s laboratory journal series, and in an international journal, The Moon, which is now defunct. These features, somewhat controversial at first, have now been recognized on most other large planetary worlds, from Mars and Mercury to various satellites.

On this web site I post four significant early papers, now hard to find. The paper by Hartmann and Kuiper (1962), marked the discovery of the Orientale Basin and first recognition of the general multi-ring phenomenon. Orientale (pronounced Or-ee-en-TALL-lay) is so named for lying on the eastern edge of the moon as seen in the sky from Earth. My 1963 and 1964 papers discussed the radial patterns around these basins. The fourth paper, by Hartmann and Wood (1971), summarized crater-count estimates of basin ages, aging phenomena, “peak-ring craters,” and early recognition of pre-mare lunar volcanics masked by a veneer of basin ejecta.

Parts of the following material are based on my M.S. dissertation (Hartmann, 1964a) and a later study about the history and gestalt ramifications of multi-ring basin recognition (Hartman, 1981).

[Thanks to the Barringer Crater Company for a 2010 gift to support impact cratering work at PSI.]


Remember that in 1961-62, a viable argument still existed about whether lunar craters were due to asteroid impacts or some type of lunar volcanism. Proponents of volcanism asked why, if craters were due to impacts, Earth lacked similar numbers of craters. The answer, of course, is that Earth’s surface is profoundly reshaped, not only by erosion but also by plate tectonics, a process that was not fully accepted by geologists until 1968. Multikilometer topographic features on Earth typically survive only tens or hundreds of million years. At the same time, in 1961-62, new results were being published from Canada, revealing that many of the eroded, circular lakes in the oldest regions of Canada were ancient impact craters. Kuiper’s lab, and U.S.Geological Survey groups associated with Gene Shoemaker, were leaders in recognizing that lunar craters were impact features, but these insights were still emerging.

Pioneers such as Grove Karl Gilbert in 1893 and Ralph Baldwin in the 1940s had suggested that lava, mountain, and radial striation features around Mare Imbrium were associated with a giant impact, but because the whole idea of craters’ impact origin was controversial in those days, their ideas had not gained much traction in the mid 1900s. Thus in 1961-62 the lunar “mare” basins, i.e. the dark-colored plains, were considered to be primarily lava-covered, irregular topographic lows on the moon, possibly of volcano-tectonic history. [The latin name mare, or sea, went back to the 1600s, when early telescopic observers thought the dark flat patches might be oceans.] Mare formations were rarely discussed as lava flows on the floors of giant impact scars. What are now seen as radial striations related to basins’ impact ejecta were sometimes discussed as “lineament patterns”. For example, British geologist Gilbert Fielder, whose book The Structure of the Lunar Surface excited us graduate students when it appeared in 1961, acknowledged Baldwin’s discussion of a radial pattern among the Imbrium radial striations, but argued that “a collision could not account for certain members” the system. Fielder described the radial striations more as part of a global “grid system” of tectonic lineaments.

An alternative, impact-based discussion of “Mare Imbrium” was by Harold Urey (1952), a geochemist who won a Nobel Prize in 1934 for his discovery of the heavy isotopic form of hydrogen, “deuterium”, and the existence of the deuterium-based form of water, known as “heavy water”. Urey, along with Kuiper, was one of the literal handful of scientists publishing on the evolution of the solar system in the 1950s. In his 1952 book, The Planets, which I read as a fledgling graduate student, Urey discussed the geologic structure of the Imbrium area. He considered that the Imbrium basin, bordered on the southeast by the arc of the Apennine mountains, was formed by impact, but he saw it in a curious way. He perceived it not with circular symmetry, but with bilateral symmetry, along a northwest to southeast axis. He thought the impactor hit the moon at a low angle on the northwest side, perhaps forming Sinus Iridum. The Apennines, then, were heaved up on the southeast, and the ejecta was sprayed beyond the Apennines even farther to the southeast.

Urey was right about Imbrium being a giant impact site, (or so we believe today), but he was wrong on an important aspect. He considered that the Mare Imbrium lavas were an immediate melt-product of the impact. This seemed ruled out even in 1961, because a population of large lava-flooded craters exists in the Imbrium basin, formed on the basin floor after the impact, but before the lava flows that form the current mare surface. A sizeable time interval was needed to let these impacts accumulate before the lava surface formed. Indeed, Apollo astronauts’ samples indicate Imbrium formed about 3.85 Gy ago, but the lava surfaces date from around 3.38 Gy ago (an average of 6 ages of volcanic materials reported by Stöffler and Ryder, 2001, Table V, p. 33).

I emphasize this last point because mare lavas, particularly in the 1960s-90s, were often confused with “impact melts” produced by the thermal energy input of the impact. This is ruled out, since the visible flows post-date the basins by hundreds of My.



Hartmann, W. K. and G. P. Kuiper 1962. Concentric Structures Surrounding Lunar basins  Comm. Lunar and Planetary Lab., 1, 51-66.


  • Discovery of Orientale Multi-Ring impact basin on the moon
  • Identification of multiple rings and /2 spacing as a common property of impact basins
  • Possible identification of outlying rings of the South Polar Aitken Basin on the moon’s far side.

A crucial week in our Rectified Atlas program occurred some time during the 1961-62 lunar globe photography when someone in our group first noted, merely as a curiosity, concentric ring structures on the east limb of the moon, near a little-known dark patch known as Mare Orientale (the “Sea of the East”). Kuiper had hired Harold Spradley, a photographer from the U.S. Air Force mapping center in St. Louis, to head the photography of the globe. Spradley and I both took 4x5 inch photos of the globe, and I was tasked with making prints of them, for the forthcoming “Rectified Lunar Atlas,” in the LPL darkroom. Spradley, I, or someone else may have been first actually to notice the odd formation. Mare Orientale had previously been noted only as a very obscure, irregular “mare” patch on the east limb with some adjacent mountain ridges, such as the “Rook Mountains.”

Most others assigned to the Rectified Atlas project were focussed on photographing the globe, not in planetary research per se. Since I’d been reading arguments among Baldwin, Urey, Kuiper, and others, about the origin of arcuate mountains, such as the Apennines around part of Mare Imbrium, and the Altai Scarp around part of Mare Nectaris, I immediately recognized significance of a clear bull’s-eye pattern surrounding as much as could be seen of Mare Orientale.Orientale showed us what a clearer (fresher?) impact system looked like.

Once I saw the multi-ring structure as the fundamental impact geometry of an enormous, 1000-km scale impact crater, I was able in the next days and weeks to go back to our rectified “overhead” photography, and show that many other maria also had similar, barely visible, concentric ring patterns, and that the ratio of the ring radii was near /2 in all cases. It was now easy to see the Altai Scarp, curving perhaps 90E around Mare Nectaris, the Apennines part-way around Mare Imbrium, and faint mountain arcs around Mare Humorum, as variously-preserved examples of the same phenomenon. In most cases, the elements (such as ridge segments and isolated peaks), had been mapped, but not perceived as part of a larger pattern of concentric rings.


Figure 2a   Figure 2b

1961/2 discovery photos of the Orientale Basin from the “Rectified Atlas” project.
Left: Image under low light shows rugged bulls-eye arcs of the surrounding rings of cliffs.
Right: Image with somewhat more of the surface under mid-day lighting, showing dark, arcuate mare lava ponds along the cliffs.

Figure 3

1967 Orbiter IV photograph of the Orientale multi-ring basin system from space.
The left side of this image extends onto the back side of the moon, never seen from Earth. (NASA).

To me, a striking aspect of these discoveries was that while observers in the previous four centuries had pushed to detect ever-smaller features of the moon with ever-larger telescopes, these largest-of-all geologic structures had gone virtually unrecognized! As I discussed in a 1981 paper, this was a remarkable example of gestalt psychology, which involves perceptions of whole patterns from disconnected elements (Hartmann, 1981). We record data, but our perceptions of connections among the data lies beyond mere measuring or recording.

Each new map of the moon, from the 1700s on, was an exercise in plotting smaller and smaller details. Observers had studied the lunar landscapes carefully, but, as if on an Escher drawing not “fully perceived,” the individual features had never “popped” into a larger-scale pattern, in this case concentric circles and radial striations. One must remember, however, that many of the features were degraded by later impact craters and some were foreshortened into concentric ovals, due to locations near the limb.

I assembled the photographic evidence and took it to Kuiper, proposing a paper describing the Orientale rings and the related features. As I remember the conversation, Kuiper immediately perceived the importance of the Orientale rings and the associated multi-ring basin patterns.

Many other senior scientists at this point would have taken over the project and assumed the role of first author on the discovery paper. Kuiper, however, graciously allowed me, an unknown graduate student, to be first author. The resulting paper (Hartmann and Kuiper, 1962) is the first one reproduced here. It marks the first proof that virtually all lunar impact scars larger than roughly 400 km diameter form not as single-rimmed craters, but as bull’s-eye like complexes. To these, we gave the name “multi-ring basins,” or just “basins,” to distinguish them from the smaller, simpler craters.

Origin of the rings: Our paper speculated that the rings and their regular spacing had to do with fracturing of the crust, initiated by the impact. As pointed out by Hartmann and Wood (1971, also posted on this web site, see below), there is an uncommon type of crater that makes a transition between “ordinary” impact craters and the basins. This is a form Wood and I named “peak-ring” craters. If you arrange craters by size, you see that small bowl-shaped craters give way to craters with central mountain peaks. Among the rare craters in the diameter range roughly 300-400 km, we found examples where the central peak widens into a ring of peaks on the flat floor of the craters. It is at somewhat larger sizes that this “peak ring” fades from prominence and multi-ring basins appear at diameter above roughly 400 km. In my opinion, as of this posting in 2011, we still lack adequate understanding of the exact mechanical processes that produce the rings.


Our paper was accepted enthusiastically in most of the then-small, but growing planetary community, particularly the workers such as Jack McCauley, Gene Shoemaker, and Hal Masursky who were key founders of the U.S.Geological Survey’s “Astrogeology” group, establishing headquarters in Flagstaff, Arizona. The general concepts of multi-ring impact basins, usually associated with post-impact lava eruptions and radially striated were adopted into the U.S.G.S. stratigraphic mapping programs, gearing up in the 1960s in preparation for the lunar landings.

Amusingly, I was excited a few weeks later to find in my mailbox an envelope from a Nobel Prize winner – a letter about our paper. It turned out to be not very encouraging for a young graduate-student scientist who had just published his first paper! Its first lines said something like this (as I remember from a traumatic moment): “From an older scientist to a younger scientist, I want to tell you that this is not a good way to start your scientific career.” As I recall, Urey had two major complaints. First, he couldn’t give up his idea that the Imbrium basin showed bilateral, not concentric, symmetry. He said he looked at our photographs and could not see the circular features of Imbrium or most other basin systems. That statement that still shocks me, since virtually everyone else could see them. It seems to testify the dangers of brilliant, aging scientists becoming overly fixated on their own earlier views – a sin to which, of course, no one in our generation would ever succumb.

Secondly, Urey complained that our paper appeared not in a peer-reviewed journal, but in a “grey-literature” series started by Kuiper himself. This was valid point. Kuiper had followed a long tradition of European observatories and laboratories. Such institutions often produced catalogs and long reports of multi-year, data-gathering studies that were too bulky to publish in the common journals. Such institutions started their own occasional publications, which were typically distributed to other laboratories. In this spirit, Kuiper started the “Communications of the Lunar and Planetary Laboratory” (of which our paper was No. 12). Urey complained correctly that such papers had little if any outside review, and thus set a bad precedent.

There is one more aspect to the story. Urey and Kuiper had maintained a rather notorious feud for many years, over aspects of science and probably scientific philosophy. It’s hard not to see some aspects of that feud in Urey’s letter. (I kept the letter for many years and showed it to historian Stephen Brush during his work on history of lunar and planetary studies, but I apparently did not get it back into its proper file. It may exist in some archive of Urey’s papers.)


Hartmann, W. K. 1963. Radial Structures Surrounding Lunar Basins, I: The Imbrium System  Comm. Lunar and Planetary Lab., 2, 1-16.

Hartmann, W. K. 1964. Radial Structures Surrounding Lunar Basins, II: Orientale and Other Systems; Conclusion  Comm. Lunar and Planetary Lab., 2, 175-192.


      • 1963 paper reviews early work on Imbrium radial system and provides new rectified imagery.
      • 1964 paper describes Orientale radial system and other systems.
      • Together, the papers establish radial striations, ridges, and grooves as a common property of multi-ring impact basins.

      These papers were follow-on to the 1962 paper on concentric rings. Simultaneous with the recognition of the concentric, multi-ring structures of basins, our rectified photos gave excellent data on the patterns of radial ridges and grooves, which had been seen by Fielder (1961) and others forming a global tectonic “grid” pattern. Actually, they form separate radial patterns projecting outward from the better-preserved impact basins.
      The most famous example of these ridges lies in the center of the moon’s face seen from Earth, radiating from the Imbrium basin. The earliest literature discussed them as local patterns. German moon-mappers Wilhelm Beer and Johann Madler, for example, had discussed them as a system of “mountain ridges” as early as 1837.
      A crucial advance came in 1893 when the noted American geologist, Grove Karl Gilbert (1893), addressed the Philosophical Society of Washington (D.C.) as its retiring president. Far ahead of his time, he defended an impact origin for lunar craters, and noted that the above mentioned “ridges,” “furrows,” and “sculpture” formed a provocative “system in their arrangement,” and remarked that this aspect of their arrangement had apparently never been described before. Namely, he said they converge “near the middle of the plain called Mare Imbrium.” He then correctly suggested (for the first time?) that a huge impact at that spot was the cause. The ridges and furrows indicate, he said, “that a collision of exceptional importance occurred in the Mare Imbrium, and that one of its results was the violent dispersion in all directions of a deluge of material....” which had sculpted the radial system.
      Ironically, Gilbert in 1891 had studied Meteor Crater, Arizona, and mistakenly concluded that it was a volcanic explosion pit, and that the iron meteorites strewn about its rim were there merely by coincidence! Gilbert’s suggestions about an Imbrium impact, however, were basically correct, although they were attacked and he himself is said to have later doubted some his own ideas about impacts. For these various reasons, Gilbert’s pioneering ideas were mostly ignored in following decades.
      The impact origin of craters was notably taken up again by Ralph Baldwin in the 1940s, based partly on studies of World War II bomb craters. Baldwin, an astrophysicist and businessman, summarized his work in his 1949 book,

The Face of the Moon

      , which had been one of my bibles on my road toward graduate work under Kuiper. Baldwin published at an unfortunate time, when few geologists or astronomers were interested in the moon or planets. (As a young teenager, I had bought this now-classic book off a remainder table in a Pittsburgh bookstore, probably around 1951. It was published at $5.00, but marked down for sale at the princely sum of $1.49). Baldwin, like Gilbert, saw Imbrium as a giant impact site. He described the radial patterns in more detail than Gilbert had, and shared Gilbert’s view that it was associated with ejected debris.
      Most importantly, Baldwin began to recognize repeated patterns among features of large mare lava plains, relating them to the smaller craters. This was an important step in seeing lunar features in terms of related patterns, rather than as a hodge-podge of individual structures. Baldwin, linking Imbrium to impact craters, stated that the radial pattern around Imbrium “ simply an exaggeration of the structure surrounding so many of the newer-appearing craters...”  Neither Gilbert nor Baldwin emphasized bulls-eye-like concentric ring patterns, but Baldwin noted that various of the maria, or lava plains, were associated with circular patterns which he associated with the circular rim of every smaller impact crater. He noted that various peaks protruding from Mare Imbrium formed a circular ring; that Mare Nectaris also had several features that made it the “closest parallel” to Imbrium, that Mare Crisium had had a “mountainous rim” and “radial valleys, ridges, and grooves,” and so on.
      As I noted in my 1981 review, Baldwin’s “impact on the recognition of multi-ring basins as a repeated type of structure was diluted by the sheer scope of his work.” The above observations come from one chapter on “Surface Features,” somewhat buried in the larger attempt to prove that circular features on the moon were similar to bomb craters, and were due to impact explosions rather than volcanic processes of eruption and collapse. He was trying mainly to prove that some big features like Imbrium and Nectaris were related to impact craters, and he recognized radial systems as part of the pattern – but he did not quite make the leap to multi-ring basins as the standard pattern for the largest impact features. What he lacked was the “smoking gun” clarity of the Orientale basin, but his emphasis on repeated patterns was what let me see why the Orientale photos were so important.

Origin of the radial patterns around multi-ring basins

      : The profuse descriptive terminology about the radial patterns – “valleys,” “ridges,” “sculpture,” “grooves,” “lineaments,” “grids” etc. obscures, and by the same token hightlights, the confusion about the precise origin of these features. On the one hand, it seems clear from morphology, not to mention Apollo samples (especially those from Apollo 14 and 16), that these features are associated with massive ejecta blankets dumped by basin-forming impacts on surrounding terrains. On the other hand, some of the features seem to defy the idea of simple debris deposition from the sky. For example, as I point out in these papers, many craters which lie in the striated regions, but also lie on the boundaries of contact between mare lavas and the cratered uplands, have rims that are completely broken into strange, individual ridges that lie radial to the nearby basin. This is not a result of blanketing from above. My best solution is that some of the radial structure is the product of intense, radial fracturing, so that the terrain breaks up along these fractures during volcanic flooding processes.


Hartmann W. K. and C. A. Wood. (1971).  Origin and evolution of multi ring basins  The Moon 3:3 78.


  • Summary of 31 basins with crater counts on (1) the basin structure and (2) the later, superimposed lava-fill plains.
  • Evidence of a wide range of crater densities among the basin structures, much greater than on the typical lava plains (2.4 to 27 times average crater density on maria).
  • Coining of the term “peak-ring” to refer to transitional forms with central peak expanded into a ring of peaks.
  • Hypothesis of pre-basin lava flows, masked by ejecta from craters, which tends to wipe out albedo differences.
  • Comparisons with 1969-70 papers on lunar multi-ring basins.

This paper, published in a now-rare journal, resulted from a project by Charles A. Wood and myself to characterize all the known basins, especially in terms of age measurements from systematic crater counts, in two categories. First, we made crater counts on the basin rim structures and ejecta, to estimate the age of the basin forming impact. Second, we made (easier) crater counts on the smoother lava plains that now fill most various basins. This was intended to clear up the still prevalent idea, from Urey (1952) for example, that the mare lava flows dated from the basin impact. The crater densities show a strong difference between basin impact age and the age of lava surfaces inside the basin.

At the time of our project, 1969-1971, radiometric ages on Apollo samples were just beginning to appear, mostly dating the mare lava plains. The first two landings, Apollo 11 and 12, were on lava plains in 1969. After Apollo 13's inability to land, Apollo 14 landed on Imbrium ejecta in February 1971. Our wide range of crater densities for the basins (2.4 to 27 x average “mare era” densities) seemed at the time to indicate that many of the basins dated back to early times, such as 4.0, 4.1, 4.3, perhaps even 4.4 Gy, much prior to the “mare era” of about 3.4-3.7 Gy ago.

Note that these ideas more or less predated the emergence of the idea of a lunar cratering “cataclysm” at 3.9 Gy (e.g. Tera et al. 1974). Today, we can see that if a cratering cataclysm happened in the interval of 3.85 (Imbrium impact?) to 4.0 Gy, then the range of crater densities could have piled up in that narrow interval of time. Crater counts alone, in spite of showing a huge range in crater density, thus can’t distinguish between the two models – an extended period of basin formation, versus basin formation concentrated in a 150 My cataclysm, such as favored by Stöffler and Ryder (2001) who assert that many basins have been radiometrically dated, and that the dates prove the second model.  Wood and I also examined the gradual degradation of basins and other features. Subsequent impacts and ejecta not only interrupt and also tend to bury concentric rings and radial ejecta, but also mask albedo differences. We discussed evidence that early mare lava plains were masked by this process.

In my (currently minority) opinion, there is still an issue whether the oldest basins, beaten up by heavy post-basin cratering, date from the earliest history of the moon around 4.3-4.4 Gy ago, or (according to the more widespread current view) date from the beginning of the cataclysm around 4.0 Gy ago. Contrary to Ryder’s (1990) conclusion that negligible impacts occurred between about 4.4 Gy and the putative cataclysm at 3.9 Gy, radiometric dating of apparent major impacts in that interval is now being increasingly reported from fragments in ancient lunar highland breccias. These may refer to early basin-forming events.


Baldwin, Ralph B. (1949).   The Face of the Moon, (Chicago: Univ. Chicago Press).

Gilbert, Grove Karl (1893).   "The Moon’s Face: A Study of the Origin of its Features". (Washington: Philosophical Society of Washington; Bulletin, vol. XII, pp. 241-292).

Hartmann, W. K. and G. P. Kuiper (1962).   "Concentric Structures Surrounding Lunar Basins". Comm. Lunar and Planetary Lab. Number 12, 1, 51-66.

Hartmann, W. K. (1963).   "Radial Structures Surrounding Lunar Basins – I". Comm. Lunar Planetary Lab. Number 24, 2, 1-16.

Hartmann, W. K. (1964).   "Radial Structures Surrounding Lunar Basins – II". Comm. Lunar Planetary Lab. Number 36, 2, 175-192.

Hartmann, W. K. (1964a).   Radial Structures Surrounding Lunar Basins. (M.S. Thesis, Dept. Geology, University of Arizona, Tucson).

Hartmann W. K. and C. A. Wood. (1971).   "Origin and evolution of multi ring basins". The Moon 3:3 78.

Hartmann, W.K. (1981).    "Discovery of multi-ring basins: Gestalt perception in planetary science".  In Multi-Ring Basins, Proc. Lunar and Planetary Sci., 12A, pp. 79-90, ed. P. H. Schultz and R. B. Merrill (N.Y.: Pergamon Press).

Ryder, Graham (1990).   "Lunar samples, lunar accretion, and the early bombardment of the Moon". EOS Trans. AGU, 71: 313.

Stöffler, D., and Ryder, G. (2001).   “Stratigraphy and Isotope Ages of Lunar Geologic Units: Chronological Standard for the Inner Solar System,” in Chronology and Evolution of Mars, Eds. R. Kallenbach, J. Geiss, and W. K. Hartmann. Kluwer Academic Publishers, Netherlands, pp. 105-164.

Tera, F., D.A. Papanastassiou, and G. J. Wasserburg (1974).   "Isotopic evidence for a terminal Lunar cataclysm, Earth Planet". Science lett. 22, 1-21.

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