I learned that a titan of lunar science passed away last month.
Baldwin, an astronomer by training, noticed the spectacular images of the Moon displayed in Chicago’s Adler Planetarium while giving public lectures there. Those images showed a system of grooves and lineations that seemed to radiate from the center of Mare Imbrium (a large dark region on the lunar near side). Upon investigation of the literature, Baldwin was surprised to find no good explanation for these patterns. In fact, to astronomers, the Moon was an unattractive target for their attentions. The biggest influence the Moon had on their work was that its bright, reflected light shining in the night sky often ruined observations of the faint star field that lay beyond.
The prevailing wisdom in the 1940s held that lunar craters were primarily produced by volcanism and Baldwin quickly discovered that no one was particularly interested in his new observation. However, he persisted wondering what could have produced such a global pattern. He noted that the largest volcanic eruption in recorded history on Earth ( Tambora in 1815) had left a crater a mere 4 km in diameter. He reasoned that such a process – especially on the smaller and presumably cooler Moon – could not possibly have produced the “crater” the size of the Imbrium basin, a depression over 1000 km in diameter. In his mind, the only alternative was that it formed by the impact of a large asteroid or comet.
Today we take the process of impact for granted but it was quite controversial before the space age. Geologists hated the concept of impact because it smacked of catastrophism (the doctrine that singular, large-scale events could create landforms) and as that battle of ideas had been fought for over a hundred years, they resisted the idea that catastrophes may happen. Astronomers ignored the concept completely, feeling that anything dealing with the planets (and certainly the Earth and Moon) was outside their purview. So by championing the impact origin of lunar craters and attributing scientific importance to the Moon, Baldwin gained the distinction of being a double heretic.
Baldwin used his collected observations of the Moon, terrestrial impact craters such as Meteor crater in Arizona, and aerial images of bomb craters (created during the recently ended war) to argue that the vast bulk of lunar craters were formed by the collision of solid objects with the Moon’s surface. He believed that impacts were explosive events that threw enormous amounts of debris across the surface of the Moon and, in the case of Imbrium, created the radial “sculpture” that dominates portions of the near side. His book, The Face of the Moon, published by the University of Chicago Press in 1949 was widely ignored by most but not all.
During a party, Nobel laureate Harold Urey, a chemist who had worked on the Manhattan Project, picked up Baldwin’s book. He consumed it in one sitting. The idea of impact fascinated him and he became convinced the Moon was the critical object to understanding the origin of the Solar System. As Urey’s opinion was considered important, the scientific stature of the Moon rose. Researchers like Eugene Shoemaker, a young geologist with dreams of flying to the Moon, combed the literature for information about it. The Face of the Moon was one of the few geologically insightful works available at that time. Shoemaker went on to become a renowned lunar scientist and cratering expert, instrumental in assuring that astronauts going to the Moon conducted lunar field geology. Although he never made the journey to the Moon in his lifetime (some of his ashes were sent to the Moon on the Lunar Prospector mission in 1998), he trained the Apollo astronauts how to observe and collect samples on the Moon – samples that demonstrated the importance of impact on our history.
Ralph Baldwin (whose day job was Vice President of Oliver Machinery Corporation of Grand Rapids Michigan, the family business) was a gentlemen scholar who studied the Moon simply because he loved it. His two principal books (he wrote an extension and revision of his book in 1963 entitled The Measure of the Moon) got the lunar story correct. Almost all lunar craters were formed by impact. The largest craters on the Moon are the gigantic, multi-ringed basins that contain the dark maria of the Moon. The maria are volcanic lava flows, unrelated to the basins that contain them and erupted on the Moon long after the basins were created by impact. The Moon’s surface features are very old; we don’t find large numbers of craters on the Earth because they formed mostly during the early history of the planets and have been erased from the Earth’s surface by active geological processes. Baldwin had found his niche, achieving an important understanding of the Moon at a time when most of the scientific community was preoccupied with virtually any problem except the Moon.
I was fortunate to meet Ralph Baldwin in 1981 at a special conference on the origin of multi-ring basins (my dissertation topic). I remember standing in front of a wall-sized enlargement of the lunar near side with him, looking at basin rings. He was surprised and pleased to find out that I agreed with him on the existence of a very old, vaguely expressed basin in the central highlands. We talked mostly about the Nectaris basin, a key feature in lunar history. A couple of years later, while I working at the U. S. Geological Survey in Flagstaff, my boss Larry Soderblom gave me a paper by Baldwin on the topography of the Nectaris basin that he had been sent to review. Baldwin believed that basins had relaxed through viscous flow of their topographic relief. Since rocks have finite physical strength, given enough time, they will deform plastically. High peaks or deep holes slowly will be erased as the rocks flow and relax under their own weight. Baldwin had collected data that (he thought) demonstrated this effect occurred in lunar basins. While this is undoubtedly possible, the degree to which it occurs on the Moon is still fiercely debated.
I read Baldwin’s paper in detail and made extensive notes. I collected new measurements of the Nectaris basin using topographic maps that Baldwin apparently did not have. I wrote all this up in a ten-page memo and sent it to Larry for his use in reviewing Baldwin’s paper. To my initial horror, Larry simply sent my comments directly to Baldwin. My fear was that he would be offended by an upstart student, questioning his data and conclusions. To my delight and surprise, Baldwin phoned to thank me for my review efforts and asked for copies of the newly made topographic maps. He later wrote me a very courteous and kind letter, thanking me for all of my review efforts and data collection on his behalf. To me, this episode demonstrated the mark of the true scientist, one who is willing to consider valid criticism from wherever it may come.
As it turned out, Baldwin retained his original hypothesis, regardless of my own (and others) criticisms. It is very human to hold to one’s own ideas. Scientists must strive to discard useless or wrong concepts as new data or insight becomes available (something we regularly do, though sometimes reluctantly). I too have felt the tendency to hold on to an idea, even when overwhelming new evidence shows it to be wrong, or at least, incomplete. As in many other fields of endeavor, scientific research is a very human experience. Scientists get intensely involved in their research and personally invested their ideas – more Vincent Van Gogh than Mr. Spock.
With little background in geology, except that which he taught himself, Baldwin deciphered most of the geological story of the Moon. Moreover, his scientific work was done in his spare time and largely alone, as opportunities to discuss his work with other interested parties was rare. It is astounding to leaf through the 1949 The Face of the Moon today and realize how his insight – much of it completely intuitive – is still pertinent. One of my personal favorites: Baldwin suggests that extinction events in the fossil record might be caused by the impact of large meteorites and comets (page 155). This suggestion, a throw-away line in a chapter about the frequency of large body impact, was proven correct in 1980 with the discovery of a large impact at the end of the Cretaceous, causing the extinction of dinosaurs and many other fossil families.
After all is said and done, I’ll let my own mentor in lunar science, Don Wilhelms, have the last word. In the dedication of his excellent history of lunar science, To a Rocky Moon: A Geologist’s History of Lunar Exploration, he wrote:
“Dedicated to the amazing Ralph Baldwin, who got so much so right so early.”