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50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 3046.pdf 360 YEARS OF LUNAR SCIENCE: FROM GALILEO TO APOLLO 10. C. A. Wood, Planetary Science In- stitute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395; [email protected] Introduction: In 1609 Galileo became the first from formation of meter-scale craters-real changes! scientist to investigate the Moon. He discovered de- How Did Craters Form? The origin of craters pressions and smooth dark plains that he called maria, was a lunar question that most people through the and concluded that the Moon, like the Earth, is a varie- 1950s considered to be solved: Craters were volcanoes. gated world, not a perfect heavenly sphere. He saw no Volcanism was a familiar process on Earth and pre- sign of oceans and realized that any life on the Moon sumably occurred on the Moon. Some observers from would be different from than that on Earth. Robert Hooke on had proposed that cosmic collisions Over the next 360 years a few hundred others re- could create craters, but there were three strong argu- ported observations and speculated beyond their data ments against that. First, no projectiles in space were or knowledge about the alien landscapes they saw. A known to collide with the Moon. Second, if that did series of questions developed about the origins of the occur, impacts would occur at all angles and thus many Moon, its features and conditions. The Apollo program craters would be elongated, but all were circular. sought to answer many of those questions. Third, no impact craters were known on Earth. Is the Moon Inhabited? Long before the tele- In 1893, the American geologist G. K. Gilbert con- scope’s invention philosophers postulated that other sidered impact the most likely crater origin, even sug- bodies in the sky were inhabited. Once Galileo re- gesting that Mare Imbrium, with its radial sculptures, vealed the Moon to be a place, the speculations in- was the largest impact crater, but the idea gained little creased, with nearly everyone agreeing that it was in- ground. To explain why craters were circular rather habited. But as early as 1666 the Frenchman Auzout than elliptical Gilbert suggested that circular craters commented that during the 50 years since Galileo, lu- would be produced if the projectiles were in lunar orbit nar observers had not reported weather, seasonal rather than coming from further away in space. This changes or civilization; Auzout concluded that those led him to propose that the Moon and its craters phenomena did not occur on the Moon. He was ig- formed from a ring of projectiles in Earth orbit. nored and many observers through the mid-1800s as- The discoveries in the early 1800s that meteorites sumed that putative changes resulted from agriculture did come from space, and of asteroids, weakened the and city building. This led to searches for ever smaller lack of projectiles argument. But craters were still objects, while large features such as impact basins round. In 1916, the Estonian astronomer Öpik calculat- were overlooked. Few observers suggested that chang- ed that at the extreme velocity of space projectiles, es compared to earlier maps might be due to improved craters would not be formed by mechanical gouging telescopes or differences in sketching abilities. but from tremendous explosions as kinetic energy was Does the Moon Change? By the mid-1800s it was absorbed by collision. Eight years later the New Zea- generally accepted that the Moon’s atmosphere was land astronomer Gifford, independently came to the very tenuous if it existed at all, and thus advanced life same conclusion. Unfortunately, both Öpik and Gif- was unlikely. From then until today (TLPs – transient ford published in journals not read by astronomers, and lunar phenomena) reported changes were thought to be it wasn’t until the 1940s that the energy of collision due to geological activities such as volcanism creating was accepted as the reason for circular craters. small craters, and degassing periodically concealing In 1906, Earth’s first identified impact crater was them. In the 1830s Mädler, a careful lunar mapper us- confirmed by iron meteorites surrounding it; this ing only a 95 mm aperture refractor, discounted claims recognition was commemorated by the name change of change. However, in 1866 the professional astrono- from Coon Butte to Meteor Crater. Associated meteor- mer Schmidt reported the disappearance of the crater ites also identified the next two discovered impact cra- Linné, and other more minor changes continued to be ters (Odessa and Campo del Cielo) in 1928. In the reported, including some in the 1960s being ascribed to 1930s, geologically older structures called cryptovol- insects and vegetation. As recently as 2015 a galactic canoes were proposed to be of impact origin, as were astronomer tried and failed to discover and document various circular lakes in Canada in 1950s and 60s. All changes. No reported change interpreted as a geologic of these interpretations of ancient structures as impact event has ever been confirmed, including red spots craters (astroblemes) were highly disputed until the near Aristarchus and gases escaping Alphonsus. Since 1960 discovery in Meteor Crater glass of coesite, an 2000 A.D. newly formed craters have been imaged by unusual mineral formed only during an impact. LROC and amateur observers have detected flashes The three arguments against impact cratering were 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 3046.pdf overwhelmed by new discoveries, but it wasn’t until sumably due to radioactive melting of mantle rocks. Ralph Baldwin’s The Face of the Moon in 1949 that a The final major discovery in the early 1960s was coherent scientific argument emerged for the impact Bill Hartmann and Kuiper’s recognition that Mare Ori- origin of lunar craters. Baldwin rediscovered Gilbert’s entale was at the center of a bull’s eye pattern of con- Imbrium Sculpture and realized that extraordinary en- centric mountains; Hartmann documented similar con- ergy would be required to propel mountains of rock centric and radial structure at every circular mare. hundreds of kilometers beyond Imbrium’s rim. He How Old is the Lunar Surface? In the months be- introduced major evidence, a graph showing that ex- fore Apollo 11, Shoemaker and Baldwin published age plosion crater depths were proportional to their diame- estimates for lunar maria; Hartmann, a graduate stu- ters from meter scale bomb craters of WW2, to the few dent had published an age earlier. All three accepted known terrestrial impact craters, to 100 km wide lunar that lunar craters formed by impact, with more craters craters. This evidence immediately convinced Gerard per unit area corresponding to an older age. Baldwin Kuiper and Harold Urey, leading planetary scientists of and Shoemaker estimated that maria were only a few the time, and soon most of the developing lunar sci- hundred million years old. Hartmann determined an ence community accepted the impact origin. Strangely, age of 3.6 billion years, exactly the age of Apollo 11 many European and especially Soviet scientists clung mare samples. to volcanic interpretations for another decade – why? How Did the Moon Form? Before Apollo there Baldwin, Kuiper and Urey overlooked an earlier were no good models for the origin of the Moon. The paper that assembled a surprisingly modern under- earliest theory was a version of Laplace’s nebular hy- standing of the Moon. The geologist Robert Dietz pothesis that the Moon accreted from a local rotating showed that lunar craters were morphologically unlike nebula forming the Earth. In the 1870s George Darwin volcanic craters. He accepted Gifford’s argument for proposed that the Moon, which had been discovered to hypervelocity formation of craters, recognized that be receding from Earth, had originally spun off an ear- lunar crater morphology changed with increasing ener- ly rapidly rotating Earth, with the Pacific Ocean being gy (crater diameter), that maria are within the largest the scar. In 1930 Harold Jeffreys calculated that was impact craters, and that subsequent impacts would de- geophysically impossible. grade landforms. He further stated that the random In 1893 Gilbert proposed that the Moon formed by distribution of craters was not consistent with volcan- accretion of small rocky particles in Earth orbit. Dur- ism, that crater rays were likely to be pulverized im- ing the tail end of accretion the craters visible on the pact ejecta, that pervasive impact events would frac- Moon formed. More than 60 years later Urey support- ture the upper crust (megaregolith), that the high ener- ed this view, believing that accretion would be a cold gy of impact would melt rocks (impact melt), and that process and the Moon was made of primitive, 4.5- the small Moon would now be cold and dead. Dietz billion year old chondritic material, with no volcanism, recognized that the maria formed later in lunar history, just impact cratering and associated melting. Apollo and the vast energy released by a cosmic collision later confirmed that the maria were volcanic lava would imelt the lunar crust, producing the maria. Eve- flows, and the Moon had been hot. Urey originated rything he deduced, except that last statement, was true another unlikely hypothesis that the Moon formed and decades ahead of others. Dietz’ work was over- elsewhere and was captured by Earth. looked for it published in a journal astronomers didn’t All the theories ran afoul of one or more of three read, the prestigious Journal of Geology. severe constraints known before Apollo: the Moon is In the late 1950s and early 1960s Eugene Shoe- large, of low density, and the Earth–Moon system has maker developed fundamental understandings of lunar a high angular momentum.

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