The Moon as it never was

In 1874, James Nasmyth and James Carpenter published a remarkable book: The Moon: Considered as a Planet, a World, and a Satellite. Lavishly illustrated with spectacular images of lunar features based on their own observations and exquisite plaster models, it served as a platform for the authors to advance their views on the nature and evolution of the lunar surface.

In a book review from the same year, British astronomer Sir Joseph Norman Lockyer wrote: “The illustrations to this book are so admirable, so far beyond those one generally gets of a celestial phenomenon, that one is tempted to refer to them first. No more truthful or striking representations of natural objects than those here presented have ever been laid before his readers by any student of Science; and may I add that, rarely, if ever, have equal pains been taken to insure such truthfulness.”

New models

Carpenter was a British astronomer at the Royal Observatory in Greenwich, England. Nasmyth was a prominent Scottish engineer and inventor, as well as an amateur astronomer, philosopher, and artist. Among other things, he invented the steam hammer and the Nasmyth-Cassegrain telescope design. Both men were later honored by having lunar craters named after them.

But what made their book so extraordinary? A major scientific question at the time centered on the nature and origin of the Moon’s craters. The authors were strong advocates of the widely favored theory that craters were formed through volcanism, which drew parallels with the Campanian volcanic arc that includes Mount Vesuvius in the Gulf of Naples. This theory had been introduced in Robert Hooke’s 1665 Micrographia as one of two possible ways lunar craters might have formed. The other was the impact theory of crater formation, which was not formalized until 1892 by geologist Grove Karl Gilbert — and not universally accepted until well into the 20th century. It finally gained real traction in 1960, when Eugene Shoemaker showed that — in sharp contrast to the surrounding volcanic terrain in northern Arizona — the famed Meteor Crater (or Barringer Crater) was created by a large extraterrestrial impactor.

Working together, Nasmyth and Carpenter put forth an ingenious mechanism for the putative formation of craters through volcanism, called the fountain model. It explained how many of the most prominent and relatively young lunar craters, such as Tycho and Copernicus, had formed their protruding walls and central peaks. It could theoretically encompass almost any size crater, including lava-filled ones.

They proposed that as the Moon gradually cooled and shrank after its formation, its outer casing was breached, and underlying molten lava was ejected through the surface, much as in volcanoes on Earth. Moreover, since the Moon lacks an atmosphere and oceans, this must have been be a global phenomenon to create the widely cratered surface we see today. In the authors’ own words: “When the molten substratum had burst its confines, ejected its superfluous matter, and produced the resulting volcanic features, it would, after final solidification, resume the normal process of contraction upon cooling.” Eventually, “the skin, so to term the outer stratum of solid matter, becomes shriveled up into alternate ridges and depressions or wrinkles.”

Re-creating the Moon

Apart from promoting their theories on lunar surface formation, Nasmyth and Carpenter were also eager to produce Moon charts more accurate than hitherto hand-drawn maps, as well as to provide a level of three-dimensional realism that existing charts lacked. Photography, though helpful, was still in its infancy and incapable of resolving the finer surface details readily visible through moderate-sized telescopes.

Consequently, Nasmyth embarked on an ambitious but appealing method of producing far superior representations of selected regions of the Moon. Based on his extensive collection of exquisite drawings made at the telescope, he created plaster models and illuminated them obliquely to match the sunrise and sunset shadow angles corresponding to his observations. These were then photographed and reproduced as woodcuts for the book. The results, in the words of Lockyer, “were perfect; far more perfect than any enlargement of photographs could possibly have been.”

Though exquisite works of art, Nasmyth’s images are not perfect in one vital aspect: Their vertical elevation is greatly overestimated. In their book Epic Moon, William Sheehan and Thomas Dobbins note that because Nasmyth used his models and not actual measurements of shadow angles to quantify elevations, they portray lunar features taller and more jagged than they are.

But this error does not detract from the realistic appearance and positional accuracy of his models. To demonstrate that, we have directly compared some of them with telescopic digital images of the same regions obtained under comparable angles of illumination. All digital images were taken with a Celestron 14-inch (C14) telescope (except where otherwise stated) and various cameras, and processed by Leo Aerts.

Telling a story

One of the book’s most imposing images is the Theophilus, Cyrillus, and Catharina group of craters. Their age differences are made apparent by the progressive erosion of their respective walls and central peaks.

Theophilus, the northernmost crater, sports terraced walls, a flat floor, and a tall central peak, all marking it as the youngest of the trio. To its southwest is Cyrillus with a similar appearance, though less well preserved, only slightly older than Theophilus. Nasmyth and Carpenter found these two craters the perfect examples for their fountain model. Conversely, the largest and flattest crater of the trio, Catharina (to Cyrillus’ southwest) didn’t support their idea of crater formation. It is now clear that Catharina has been modified by several subsequent impacts, particularly its destroyed ancient rim, marking it as the oldest of the three.

Another showpiece trio of craters, from youngest to oldest, are Arzachel, Alphonsus, and Ptolemaeus. Nasmyth and Carpenter saw this grouping as a near-clear example of their fountain model progression of crater formation. Arzachel’s prominent circular wall and central peak indicated the early stage of post-volcanic activity, whereas Alphonsus was in the intermediate stage and Ptolemaeus was in the final stage, with the lowest walls and a central peak completely engulfed by a smooth layer of lava. However, the latter posed quite a challenge for them.

As it lacked a central volcanic peak, or cone as they termed it, Ptolemaeus, with a size of 95 miles (154 kilometers), did not readily fit within the theoretical size range of the fountain model. Like the crater Petavius, with a true size of 110 miles (177 km) in diameter (at the time of the book’s publication, the crater was measured to be 78 miles [125 km] wide), Ptolemaeus had “no central cone” and was, “therefore, not manifestly volcanic as those which possess this feature.”

Nasmyth and Carpenter concluded that the only feasible method to explain why enigmatic basins like Mare Crisium had such expansive smooth floors was a spherical upheaving force below the lunar surface: “[W]e see that an intense but extremely confined explosion … beneath the moon’s crust must disturb a circular area of its surface, if the intervening material be homogeneous.”

Curiously, despite being such strong advocates of large-scale volcanism on the lunar surface, the authors did not consider massive lava flooding of basins such as Ptolemaeus as the most likely mechanism for its appearance. Ample evidence of past lava flooding can be seen in their models of the Archimedes and Aristillus regions, along with the Plato-Montes Alpes complex. Both regions fall near Mare Imbrium, which was formed by a collision with a large impactor about 3.9 billion years ago and was later flooded by basaltic lava. This formed the smooth volcanic surface evident today, which still shows protruding mountaintops and long wrinkled ridges.

Advanced for its time

Nasmyth and Carpenter were far off the mark with their volcanic theory of crater formation. Yet they showed remarkable foresight in proposing that the Moon’s surface contracted as it gradually cooled and shrank since its formation.

Based on moonquake activity recorded by surface seismometers left by Apollo astronauts and more recent Lunar Reconnaissance Orbiter data, NASA scientists have concluded that while the Moon’s exterior is now cold and solid, the interior is still in the process of cooling and contracting. This internal contraction has the global effect of forcing the total surface area to decrease and the crust to shrivel up.

In retrospect, it is easy to see why The Moon: Considered as a Planet, a World, and a Satellite attracted so much attention at the time. It was not universally accepted; Sheehan and Dobbins note in Epic Moon, “Though the volcanic theory flourished in the years after Nasmyth and Carpenter published their fountain theory, it did not go entirely unchallenged.” Scientists eventually surmised that the moons and planets were formed by a process of accretion, which ultimately exposed our airless Moon to bombardment and left it as we see it today, pockmarked by countless craters and solidified lava basins.

Yet while their theories were ultimately inaccurate, Nasmyth and Carpenter were among the first to propose credible mechanisms to account for the Moon’s chaotic-looking topography — and they did so with groundbreaking and unique illustrations.