analytical perspectives

GEOMOR-04852; No of Pages 10 Geomorphology xxx (2014) xxx–xxx

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Planetary geomorphology: Some historical/analytical perspectives V.R. Baker ⁎ Department of Planetary Sciences, University of Arizona, Tucson, AZ 85721, United States

a r t i c l e

i n f o

Article history: Received 2 December 2013 Received in revised form 5 July 2014 Accepted 7 July 2014 Available online xxxx Keywords: Planetary geomorphology Mars Percival Lowell History Lunar craters Scientific reasoning

a b s t r a c t Three broad themes from the history of planetary geomorphology provide lessons in regard to the logic (valid reasoning processes) for the doing of that science. The long controversy over the origin of lunar craters, which was dominated for three centuries by the volcanic hypothesis, provides examples of reasoning on the basis of authority and a priori presumptions. Percival Lowell's controversy with geologists over the nature of linear markings on the surface of Mars illustrates the role of tenacity in regard to the beliefs of some individual scientists. Finally, modern controversies over the role of water in shaping the surface of Mars illustrate how the a priori method, i.e., belief produced according to reason, can seductively cloud the scientific openness to the importance of brute facts that deviate from a prevailing paradigm. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Ever since scholarly historical writings began in ancient Greece controversy occurred as to the proper goal for historical scholarship. The first substantive historical account, Herodotus's The Histories, is meticulous in its detail. It documents the fifth-century B.C.E. conflicts between the Persian Empire and the Greek city states, and it also describes much of the world during that time period, including aspects of its geography and geomorphology. Herodotus was not a participant in the historical events that he documented. He merely recorded details, based on his extensive travels, interviews, and recordings of stories, many of which were probably exaggerated by their tellers. Herodotus's stated purpose was to prevent the fading from memory of the wondrous deeds and glories of the times that he recorded. His detachment from the history portrayed is a forerunner to the view of historiography that steps aside from philosophical commentary as to meaning. The Histories is exemplary for its great storytelling and for its celebration of great accomplishments. In contrast to the approach of The Histories, the other great classic of early historical scholarship is the work of an “insider”, that is, an active participant in regard to the events described. Thucydides, who lived from c. 460 to c. 395 B.C.E, had been an Athenian general during a long period of warfare between Sparta and Athens. His book, The History

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of the Peloponnesian War, is a marvel for its detailed coverage of a war that was tragic for all its participants, revealing of noble and cowardly human actions, and immensely complex for its political and military strategies. Thucydides' History emphasizes the analysis of past events, seeking meaning and explanation. A quotation attributed to Thucydides conveys his different view historiography from that of Herodotus: “History is philosophy teaching by example.” It is more this view that guides the present study. A complete and detailed history of planetary geomorphology cannot be condensed into a short journal article. Very great differences exist between the various periods of planetary surface studies, extending from the earliest telescopic observation, beginning 1608 or 1609, to the era of spacecraft exploration, extending from 1962 to present. The latter period, which began with a flyby of Venus by the Mariner 2 spacecraft on December 14, 1962, has involved about 80 successful missions from many nations (Carr, 2013). The accelerating pace of discoveries from these missions is a wonder and a challenge for the analytical approach to the topic. Nevertheless, it is possible to pursue a few major themes and perhaps to learn something from whatever lessons they might contain. The first of these themes, on the origin of lunar craters, traces back to the early 17th century, when telescopic observations were first made of extraterrestrial planetary surfaces. The second theme concerns the famous Mars canals controversy centered on the eccentric astronomer, Percival Lowell. The third and final theme is also concerned with Mars, but it emphasizes modern research during the present era of spacecraft exploration. As the most Earthlike of the presently known planets, Mars continues to be a source of scientific controversy, particularly in regard to the role of water as an agent for the shaping its surface.

http://dx.doi.org/10.1016/j.geomorph.2014.07.016 0169-555X/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Baker, V.R., Planetary geomorphology: Some historical/analytical perspectives, Geomorphology (2014), http:// dx.doi.org/10.1016/j.geomorph.2014.07.016

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2. Creating craters: lunacy or logic? One of the great scientific controversies for astronomy and geology began in 1608–1609 when Galileo Galilei pointed a crude 3.8-cm diameter telescope at the moon to observe the peculiar circular “spots” on its surface. Galileo was even able to make geomorphological observations: that the “spots” were actually topographic depressions, that many of them had central mountains, and that some were floored with dark material. The controversy over the origin of these circular depressions raged over the next three and a half centuries. Though this is a controversy that has received considerable historical attention (Baldwin, 1949, 1963; Shoemaker, 1962; Green, 1965; Marvin, 1986; Hoyt, 1987; Schultz, 1998), it is worth recounting for some of its more salient episodes that illustrate important points about the nature of geomorphological reasoning. What were, according to Davies (1969), perhaps the first geomorphological experiments took place in 1665. The experimenter was Robert Hooke, a scholar of immense breadth of interests, who made many fundamental discoveries (Drake, 1996). Hooke, who also seems to have delivered the first scientific lectures in Britain on geomorphology (Davies, 1969), had made telescopic observations of the moon, publishing excellent drawings of the cratered lunar landscape in his 1665 book Microgarphia. Using analogical reasoning from experimental observations, Hooke inferred two hypotheses for the origin of the circular lunar depressions. From his observations of pits forming on the cooled surface crust of boiled gypsum Hooke inferred that internal heat (volcanism) could cause similar-appearing pits on the moon. Hooke further observed that somewhat similar-appearing pits could also be produced by impact from the dropping of pellets of clay or musket balls on to a muddy target material. To distinguish between the two hypothesized origins for lunar craters, Hooke invoked the prevailing theory held by the astronomers of his day: that interplanetary space was completely empty and, therefore, could not contain the impactors that would be necessary for the second hypothesis to be true. Thus, Hooke rejected his second hypothesis, that of impact, on the basis of logical inference from a theory that largely derived from a priori presumptions about the natural world. In the cosmology of the Middle Ages the heavens were presumed to be perfect. The planets and the sun, of course, existed but no other objects could interfere with the mathematical perfection that described the movement of various heavenly spheres. Moreover, the mathematical certainties that explained these movements, so famously derived by Hooke's great intellectual competitor, Sir Isaac Newton, seemed to require this perfection. The circularity of this argument continued to elude much of the scholarly world, leading to the dismissal of sightings of “rocks falling from the sky” as “peasants' fables”. For lunar studies after Hooke until the 20th century, nearly all astronomers, emphasized the volcanic origin for lunar craters. Such notable astronomers of the late 18th century as William Hershel and Johann Hieronymus Schröter strongly advocated this position, and it was also the conclusion of numerous astronomical observing campaigns, including the extensive and highly authoritative project of the Paris observatory at the end of the 19th century (Loewy and Puiseux, 1897). Some minority support for an impact origin of lunar craters was offered in the 19th century (e.g., Proctor, 1873), but the volcanic hypothesis continued to prevail. In retrospect, the emphasis on volcanism derived from inadequate understanding of the impacting process and from methodological issues. But progress was also severely impeded by the imposition of authority. W. Pickering, director of the Harvard Observatory and widely regarded as the authority on lunar astronomy, was strongly against the impact hypothesis for lunar craters (Pickering, 1903). Pickering's Harvard colleague and founder of that university's geology program, Nathanial Southgate Shaler, also argued strongly against the impact origin (Shaler, 1903), favoring instead a volcanic origin.

Although many of the important observations related to crater morphologies were made by advocates for both the impacting and volcanism origins (Schultz, 1998), there long remained a lack of appreciation for terrestrial examples of impact cratering. In contrast, the many available examples of volcanic phenomena had long been an inspiration for lunar hypotheses by geologists, such as Dana (1846). The other major impediment was the lack of physical understanding for high-velocity impact processes. Even when this began to be understood in the early 20th century (Ives, 1919; Gifford, 1924, 1930), however, those ideas continued to be held with suspicion by most astronomers until the modern era. Many of the problems for understanding the geomorphology of impact cratering are encapsulated in the experience of Grove Karl Gilbert, arguably the greatest geomorphologist of his day. Gilbert's lunar observations were made in 1892, using the 67.31-cm refracting telescope of the U.S. Naval Observatory. He also repeated the experimental approach employed by Hooke by propelling balls of clay and mud into various target materials (El-Baz, 1980; Pyne, 1980), concluding that only an impacting process could produce the kinds of detailed morphological features that he had meticulously described from his telescopic observations (Gilbert, 1893). Similar results were obtained by other experiments around this same time period. Even Alfred Wegner, more famous for his role in the continental drift controversy, was involved (Greene, 1998), concluding that impacting had to be the causative process for lunar craters. Gilbert's lunar studies and those of several other prominent advocates for impacting failed to convince the astronomers who favored volcanic origins. It was a fact of observational astronomy that nearly all lunar craters are circular in outline. Circular craters were presumed by the currently prevailing theory to be only possible if all the impacting objects came from a vertical direction. This would seem to be highly unlikely for the moon, since the expected random approach directions of impacting objects would surely generate a great many oblique impacts. Moreover, even the experiments of Gilbert and others showed that oblique impacts produce elliptical craters, thereby confirming that the impact direction must be vertical to produce a circular crater. Gilbert tried to modify his impact hypothesis to account for this problem by postulating that the lunar-cratergenerating objects derived from a temporary circular orbit around Earth. These objects were then subsequently perturbed to fall vertically to the surface of the moon. Of course, this had the appearance of modifying a hypothesis that had failed experimental testing. In hindsight, of course, the problem here was the presumption that the lowvelocity impacting conditions that were assumed by the theory and demonstrated the testing were indeed the conditions actually applicable to nature. More modern studies of the high-energy, high-velocity nature of the impacting process have shown that oblique impacts also produce circular craters, but this discovery did not come until later in the 20th century. In a subsequent study, Gilbert also addressed the important issue of terrestrial analogues for lunar craters. He did this with an investigation of Coon Butte in northern Arizona. In a paper that makes important statements about the role of analogical reasoning in geology, Gilbert (1896) came to the conclusion that Coon Butte, known today as Meteor Crater, was the product of a steam explosion. The latter seemed consistent with the proximity of the site to an area of extensive volcanic craters. Gilbert's analysis carefully considered an impact hypothesis for the feature, but rejected it, in part because of the lack of understanding of how relatively small hypervelocity projectiles are able to generate the phenomenal energies that drive the impacting process. Even despite his lack of access to this modern understanding, however, Gilbert was, nevertheless, critical of his own results. Toward the end of his paper he refers to some remaining anomalous phenomena, and notes that these illustrate what we now know as a principle of fallibilism that underlies all science, and especially planetary geomorphology (Gilbert, 1896, p. 12):

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This illustrates the tentative nature, not only of the hypotheses of Science, but of what Science calls its results. The method of hypotheses, and that method is the method of Science, founds its explanations of Nature wholly on observed facts, and its results are ever subject to the limitations imposed by imperfect observation. However grand, however widely accepted, however useful in its conclusion, none is so sure that it can not be called into question by a newly discovered fact. In the domain of the world's knowledge there is no infallibility. One of the ironies of the lunar cratering controversy is the fact that Gilbert's fallibilist conclusion in regard to his result was generally ignored by those who chose to cite his steam explosion hypothesis as part of the authoritative basis for ascribing lunar cratering to volcanic processes. For many years the mining engineer D. M. Barringer campaigned against the steam explosion origin for Coon Butte, arguing instead with his associate B.C. Tilghman for its impact origin (Barringer, 1906; Tilghman, 1906). As described in detail by Hoyt (1987), the ensuing debate over three decades pitted the authority of the U.S. Geological Survey, which defended the conclusion of Gilbert (1896), against the impact hypothesis of Barringer and Tilghman. Though it took a long time to recognize Coon Butte as an impact crater, once other terrestrial impact craters were recognized in the 1930s (Spencer, 1933; Boon and Albritton, 1937) there became available an ever-increasing number of terrestrial analogues for lunar impact craters (e.g., Dietz, 1946a, 1946b). But even these discoveries did not cut into the authority that was invested in the volcanism theory. The physicist Ralph B. Baldwin did extensive and detailed work on the morphologies of lunar craters, showing how they compared favorably to bomb craters and known terrestrial impact craters, but unfavorably with terrestrial volcanic calderas. Despite the importance of this work, however, Baldwin had great difficulty getting it published in the prominent astronomical journals of his day (Baldwin, 1978). His results were eventually published in a classic 1949 book (Baldwin, 1949), The Face of the Moon. It was not until the science of cratering was significantly advanced by the advent of nuclear weapons testing that Eugene Shoemaker, David Roddy, and others established the principles whereby the hypervelocity impact cratering process could be understood. The “smoking gun” (Cleland, 2002) for the impact hypothesis at Coon Butte came when coesite, a high-pressure form of silica, was found in the shock melts at the site (Chao et al., 1960). The crater and these melts formed when an object about 25 m in diameter struck with an amount of energy equivalent to a 2-megaton thermonuclear blast (French, 1977). Nevertheless, even this evidence failed to convince some advocates of the volcanism hypothesis. Green's extensive work on volcanic calderas and associated features, such as ray-like tuff patterns (Green, 1970), was used to argue that even the classic lunar impact crater Copernicus was of volcanic origin (Green, 1971). Though the genesis controversy for lunar craters is essentially finished, there continue to be issues with crater forms on other planetary surfaces. Michalski and Bleacher (2013) have recently used many of the same morphological arguments that were applied to lunar craters to propose that some of the highly degraded crater forms of the Martian highlands actually represent processes associated with ancient supervolcanoes.

3. Misinterpreting Mars: canals or channels? In 1610 Galileo Galilei pointed his little telescope at the planet Mars, noting its patterns of light and dark terrains. It was not until 1659, however, that improved telescope technology allowed the Dutch physicist Christian Huygens to make the first informative map of Mars. These observations showed that Mars was indeed another world, one that naturally led to comparisons with our own.

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The phenomenal telescopes constructed by Sir William Herschel in the late 18th century produced amazing discoveries. In 1794 Herschel announced that the Martian day lasted about 24 and a half hours and that Mars, like Earth, had a tilted axis of rotation. Even more exciting for those interested in Mars–Earth comparisons was Herschel's observation that Mars had seasons that coincided with the growth and decay of its polar caps, which presumably consisted of snow and ice. Moreover, the Martian seasons followed the pattern of cold and warmth that should derive from its axial tilt relative to incoming solar radiation. By the early 19th century these observations led most astronomers to infer that Mars had an Earthlike landscape, water bodies (seas), polar ice caps, and a cloudy atmosphere. It was not a great speculative leap for Herschel and many of his contemporary scientists to infer that such a habitable environment would surely have led to the development of living organisms, perhaps even to creatures like ourselves. By the later 19th century many excellent telescopes were available to astronomers for making Mars observations. Nevertheless, it was also recognized that the orbital relationship of Mars and Earth only allowed for limited periods of close approach of the two planets, approximately every two years. Moreover, variations in the Martian atmosphere, now known to be seasonal dust storms, could severely limit opportunities for favorable astronomical viewing conditions. It was in an 1869 period of “favorable seeing” (apparent clarity of view through the atmospheres of Earth and Mars) that Father Pietro Angelo Secchi noted that Mars had white clouds that formed repeatedly over the same regions of Mars. Even more intriguing was his observation that linear patterns could be discerned on the surface of the planet. It was Secchi who introduced the Italian word canale (“channel” in English) to refer to these lines. During the favorable viewing conditions of 1877 and 1879 Giovanni Schiaparelli, Director of the Milan Observatory, made especially important Mars observations, making detailed maps of the various bright and dark areas on the surface of the planet (Fig. 1). The bright areas were interpreted as land, and the dark areas as water, which he designated as mare (oceans and seas). Schiaparelli also gave the Martian surface a marvelous series of names derived from classical mythology: Amazonis, Argyre, Chryse, Elysium, Hellas, Tempe, Tharsis, and many others. This terminology became widely accepted by other astronomers, and is still in use today. Schiaparelli also confirmed Father Secchi's observations, including the linear patterns on the surface that he mapped as canali (Italian for “channels”). Schiaparelli was initially guarded as to the origin of the canali. He recognized that these patterns were very difficult to see, but, nevertheless, his mapping implied that the canali were natural water-filled channels that divided the various landmasses on Mars and connected the mare, in much same way that the English Channel divides Britain from Europe and connects the North Sea to the Atlantic Ocean on Earth. Starting in 1893–1894, after achieving popular fame with books on Japan, Korea, and their people, the prominent Bostonian, Percival Lowell, made use of his considerable personal fortune and his family and business connections to found and build an astronomical observatory in northern Arizona. For the next 15 years Lowell focused his astronomical efforts primarily on observations of Mars, and specifically on his own interpretation of the canali, which he considered to be non-natural “canals”, connecting “oases” at their intersections. Lowell conceived a vision of Mars as an arid, dying planet, irrigated by a network of canals that had been constructed by an evolutionally advanced race of intelligent beings (Lowell, 1906). His methodology for achieving these conclusions, outlined in the first of his Mars books, is revealing (Lowell, 1895, p. 6): …[scientific] proof is nothing but preponderance of probability. All deduction rests ultimately upon the data derived from experience. This is the tortoise that supports our conception of the cosmos. For us, therefore, the point at issue in any theory is not whether there be a possibility of its being false, but whether there be a probability

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Fig. 1. Schiaparelli's map of Mars based on his observations of 1877–1888. (From NASA Special Publication 337).

of its being true…The test of a theory is, first, that it shall not be directly contradicted by any facts and secondly, that the probabilities in its favor be sufficiently great. In the last of his Mars books, Mars as an Abode of Life, Lowell (1908) incorporates what he claims to be a considerable body of geological knowledge in formulating a six-stage theory of progressive planetary evolution: (1) an initial solar stage of tremendous heat, (2) a molten stage, (3) a solidifying stage of forming a solid surface, an atmosphere, and metamorphic rocks, (4) a terraqueous stage of forming sedimentary rocks, (5) a terrestrial stage during which oceans have disappeared, and (6) a dead stage in which the atmosphere is gone. The planets of the solar system fit into this scheme as follows: the gas giants, Jupiter and Saturn are in stage 2, Venus with its cloud cover is transitioning from stage 3 or 4, Earth is at stage 4, Mars is at stage 5, and Mercury and the Moon are at stage 6. Lowell's evolutionary views show the influence of the Darwinian revolution, but like some other scholars during this time period, he applied Darwin's insights far beyond what had been intended by their originator. In doing so Lowell believed he was originating the new science that he had earlier labeled “planetology,” a field that continues to the present day, though not in the form originally intended by he who first named it. Lowell's canal theory did not sit well with many of the geologists of his day, but this merely confirmed his personal view that geology was a greatly inferior science relative to astronomy. Lowell believed astronomy to be a science of “cosmic laws” that presided over the unfolding of planetary history, including that of Earth. Geology was merely a local science, dealing only with events that were already prescribed by the cosmic laws, which were the province of astronomy, a subject naturally higher in the hierarchy of generating understanding of the natural world. Also, there probably was for Lowell, a member of a very prominent Boston family, a similarity to the hierarchy of social status that he, an easterner, felt toward those in the western U.S. Lowell invoked all these views during the course of a lecture on “The Revelation of Evolution.” Responding in this lecture to a criticism of his 1908 book, Lowell (1909, p. 181) expressed his disdain, as an eastern aristocrat, for westerners, and, as an astronomer, for geologists (who would merely

“accumulate facts”), while at the same time drawing attention to comparisons between injuries done to himself and those done to Charles Darwin, with whom he claimed intellectual affinity (apparently not knowing of Darwin's background as a geologist): [Darwin was]…the abjured, not only of the ignorant, but of that lower class of scientists who conceive of science to be limited to the accumulation of facts…Even as this essay stood between pen and print a geologist out West, in a long letter to Science, has repeated, in reference to the facts here set forth, the old attacks on Darwin for daring to synthesize the facts… The “geologist from out West” was the University of the Wisconsin's Eliot Blackwelder, who had written in an April 23, 1909, letter to Science magazine (Blackwelder, 1909, p. 659–661): It is not surprising that Mr. Lowell, an astronomer, should have only a layman's knowledge of geology; but that he should attempt to discuss critically the more difficult problems of that science, without, as his words show, any understanding of the great recent progress in geology, is astonishing and disastrous…Mr. Lowell coolly states that “wherever geologists have studied them, the strata tell the same tale,” viz., the land has spread, the oceans shrunk…No competent geologist would admit a word of this…Lowell can not be censured for advancing avowed theories, however fanciful they are…But… [he] stamps his theories as facts; says they are proven, when they have almost no supporting data; and declares that certain things are well known, which are not even admitted to consideration by those best qualified to judge. Censure can hardly be too severe upon a man who so unscrupulously deceives the educated public, merely in order to gain a certain notoriety and a brief, but undeserved, credence for his pet theories. Lowell's “canals” and other observations of linear patterns on Mars continued to be a scientific issue until the modern era of spacecraft observations. Some, like E.W. Maunder, attributed these observations to psychological factors. In 1913 Maunder performed simple psychological experiments on 200 English schoolboys to demonstrate that eye/mind

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interactions will tend to produce imagined linear connections for random arrangements of discontinuous blotches and streaks. This conclusion was supported by the work of E. M. Antoniadi, who employed the largest refracting telescope in Europe during especially favorable viewing conditions to demonstrate that irregular arrays of light and dark spots and splotches characterized the Martian surface, and that these could result in a an illusion of linearity when viewed under poorer conditions (Fig. 2). Other observers noted that the dark patterns changed with the Martian seasons. Even some colors hinted that the observed “seasonal wave of darkening” had tinges of green. This pattern, now known to result from eolian transport of fine dust with volatile ices of water and carbon dioxide, was as late as the 1950s thought to be consistent with seasonal variation in vegetation. Also many who accepted that the linear Mars markings were real ascribed them to such natural causes as crustal fracture zones (Fielder, 1963), linear sand dunes (Gifford, 1964), and rift zones (Sagan and Pollack, 1966). Especially insightful was a series of papers by the astronomer Dean B. McLaughlin, who ascribed the variable dark markings on Mars to combinations of eolian and volcanic processes (see Veverka and Sagan, 1974). 4. More Martian maladies: hydrophobia or hydrophilia? Percival Lowell's Martian canal controversy marks a key point (some might say a low point) in what became, over the next hundred years, oscillating changes in scientific thinking about the geological history of water on Mars. These oscillations reflect changes in collective views, which I will here label as “conceptual positions”, held by that portion of the scientific community most intensively involved in Mars studies. One way to interpret such changes in viewpoint by a scientific community was proposed by the physicist-turned-historian/philosopher, Thomas Kuhn. Kuhn's, 1962 book The Structure of Scientific Revolutions made the still-controversial claim that scientific communities ascribe to certain ways of thinking, or worldviews, that predispose them to see things in a such a collective way that that they become blinded to alternative possibilities. Kuhn (1962) introduced the term “paradigm” to describe this kind of collective thinking in science. Kuhn (1962) applied his paradigm concept to broad historical periods of what he termed “normal science.” Paradigms only come into question when anomalous discoveries build up to such a degree that failure by the currently prevailing view to explain them leads to a revolution in thinking, as exemplified by the development of the heliocentric Copernican view of the solar system. Although the various oscillations in scientific views about water on Mars have been much less grand in scale and intensity than what Kuhn (1962) envisioned in his controversial model of scientific progress and theory change, they have, nevertheless, involved prevailing conceptual positions and the buildup of anomalies in regard to those positions. Moreover, the changes in

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conceptual positions about Mars have occurred much more rapidly than implied by Kuhn's model of historical scientific progress. This is because of the incredible pace of discoveries made during the later part of the 20th century, when a succession of improved spacecraft observations replaced the problematic telescopic views that had plagued Earth-bound observers like Schiaparelli and Antoniadi. Another point of difference in regard to Kuhn's model is that the conceptual positions concerning Mars initially oscillated between two end members in regard to the role of water on the surface of the planet. These are positions that can be termed “hydrophilic” (water loving, or water friendly) and “hydrophobic” (water fearing, or water avoiding). The variations through time for these positions (Fig. 3) can be thought of as kind of experiment in the social epistemology of science, showing how theory and observations continually interact to reinforce or transform conceptual positions, though usually much more slowly than has occurred for the recent hydrophobic/ hydrophilic Mars controversies. The astronomical views about Mars in the late 1800s began, following the extensive observations by Schiaparelli and his contemporaries, were broadly hydrophilic in their conceptual position. The prominent polar caps and their seasonal changes seemed consistent with the dark mare that were then interpreted to be bodies of water. This all suggested that water was being cycled on Mars in much the same manner that was known from Earth. Unfortunately for further progress along these lines, however, before the necessary refinements became available to bolster its observational basis, the Mars hydrophilic conceptual position was tainted by the work of Lowell, who defended his views with extreme tenacity. Lowell's emphasis on “canals” as a part of his broad theory of planetary evolution was pursued with an unfounded dogmatism in regard to intelligent life on that planet. The seemingly obvious reaction to this dogmatism was skepticism in regard to the entire hydrophilic position. Philosophical skepticism infers from the lack of certainty about particular phenomena associated with the general beliefs of the dogmatist that the skeptic can dismiss all those beliefs, despite the fact that some of them may have arisen from a well-reasoned synthesis of many lines of evidence. In contrast to dogmatism and skepticism both, however, a more appropriately scientific attitude holds to open-mindedness in regard to new evidence. In the spirit of Gilbert (1896), such a view is fallibilist without being overly skeptical. Fallibilism, in its more modern formulation, is the holding of well-supported scientific ideas (theories) to be probably true, while at the same time admitting a degree of uncertainty in, and, thus, a need to test further, any particular instance or consequence of those ideas (see Haack, 2003). Following Lowell's work in early 20th century the conceptual position for the Mars science community continued to oscillate, but with a gradual shift toward the hydrophobic end member. On November 28, 1964, the era of Mars studies limited solely to Earth-based observations

Fig. 2. Comparison of E. M. Antioniadi's mapping of Mars markings (left) with a “canal” interpretation. (From NASA Special Publication 337).

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Fig. 3. A generalized interpretation of oscillations between hydophilia and hydrophobia for the Mars science community between 1890 and 2012.

ended when the flyby of the Mariner 4 spacecraft returned 22 lowresolution images covering a small portion of the Martian cratered uplands. The images (Fig. 4) revealed lunar-like craters that apparently had been little modified over 4 billion years by erosive processes. The pictures seemed to confirm the hydrophobic view that the Martian environment had always been extremely cold and dry. Moreover, this new Mars conceptual position provided the hydrophobic antidote to the Lowell's hydrophilic excesses. An influential model for the cycling of carbon dioxide on Mars (Leighton and Murray, 1966) conceived of the polar caps as cold traps for dry ice, not water. Mars was considered to experience some very long-acting wind erosion of its cold, dry surface through the action of its carbon dioxide atmosphere, but the planet probably never had significant amounts of water. Moreover, the latter

Fig. 4. Image of Martian heavily cratered terrain returned from the Mariner 4 mission in 1964.

conclusion was consistent with an authoritative interpretation of the probable abundance of Martian volatiles based on the presumed implications of noble gas abundances in the atmosphere (Anders and Owen, 1977). Mars has an uncanny tendency to reveal startling new facts in the course of space missions that pose anomalies for prevailing conceptual positions generated to explain phenomena discovered by earlier missions. The hydrophobia that followed the 1964 Mariner 4 flyby very soon came under scrutiny starting in 1972. A global dust storm that had obscured imaging of the planet by the orbiting Mariner 9 spacecraft eventually cleared, allowing a program of imaging the entire planet. The resulting images were subsequently improved upon, starting in 1976, by images obtained by the two Viking orbiter spacecraft. These revealed an array of channels and valleys that were clearly formed by a flowing, free-surface fluid. The obvious candidate for this fluid, based on well- documented studies on Earth, was water (Baker, 1982). Nevertheless, this working hypothesis was contrary to the then-prevailing hydrophobic conceptual position, much of which had developed from methodological approaches used in physics and chemistry. Built on the long scientific tradition of experimental methods, these disciplines employed the understanding of first principles (the established laws in chemistry and physics) that were combined with initial conditions and observational constraints, including assumptions about local and specific causes of phenomena, to deduce logically (that is, to “predict” from models or formulae) various outcomes, such as the presence or absence of water-related features on the surface of the planet. The discovery of the deduced (predicted) features could then confirm (or at least corroborate) whatever model system was under consideration, while the lack of the inferred outcomes could falsify the relevant model system. Though much philosophy of science of the twentieth century supported this view of justification for scientific knowledge, more recent work, much of it unbeknownst to most planetary scientists, has revealed logical flaws with the implied rigor of this methodology (see, for example, Godfrey-Smith, 2003; Cleland, 2013), which is commonly assumed without question to be the very paragon of what it is to be “scientific.” Alternatively, the discovery process can also be viewed as part of a methodology more associated with geology and geomorphology. By

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this I do not mean what Lowell criticized as the “assembly of facts” about Earth or its surface (the “geo” and “morph” parts for the words). Rather, the emphasis here is on the reasoning employed (the “logical” part). In contrast to the methodology outlined above for physics, geology commonly employs reasoning in an inverse manner. Beginning with the observation of the outcomes of various processes, such as the action of rivers, glaciers, or mass movements, the geologist looks for features indicative of those formative processes. This is the process described by Gilbert (1896) in his Coon Bluff study. Of course, as well exemplified by the conclusion of Gilbert's study, the process is inherently fallible. As Gilbert noted in another paper (Gilbert, 1886), the geologist is primarily an investigator rather than a theorist. In much the same way that a master detective searches a crime scene for critical evidence, the experienced geologist is attracted to newly discovered, anomalous phenomena, and inspired to formulate hypotheses for their explanation. The Viking era of the late 1970s was followed by a succession of spacecraft failures and programmatic decisions that produced a period of relative drought in regard to new observational data bearing on the general problem of water on Mars. During the 1980s and 1990s, interesting ideas were generated about large-scale, ancient inundation of the northern plains of the planet (Parker et al., 1989, 1993) and the presence of numerous smaller-scale paleolakes, mainly occupying impact craters (Cabrol and Grin, 1999). These hypotheses, however, were mainly tied to morphological evidence for which alternative explanations could be offered. Moreover, the theoretical work on the early Martian atmosphere seemed to show that the warming conditions implied by active hydrological cycling on the planet (e.g., Baker et al., 1991), cannot be achieved by reasonable levels of greenhouse gases (Kasting, 1991), leading Carr (1996) to conclude that, “…current climate models cannot be satisfactorily reconciled with the concept of an early, warm and wet Mars.” The morphological indications of widespread aqueous activity seemed incongruous with hydrophobic theories that held Mars to be, and generally to have been, extremely cold and dry throughout its history (at least since about 4 billion years ago). A number of nonaqueous hypotheses were posed to explain many of the individual components of what had been interpreted to be a water-related assemblage of landforms. One extreme theoretical construct during this hydrophobic renaissance was the Hoffman (2000) “White Mars” hypothesis that the planet had always was so cold and dry that water had never been liquid on its surface. Hoffman (2000) explained the various fluid-related features on the surface of Mars to have resulted from phase transitions and flow properties of carbon dioxide. “White Mars” was invoked to explain what was at the time a lack of detection of carbonates on Martian surface, which models for a warm, wet atmosphere (Pollack et al., 1987) had predicted. Thus, the deductive logic of physics and chemistry seemed to falsify an expected consequence of the hydrophilic view and to justify hydrophobic ideas like “White Mars.” Moreover, some of the infrared spectral data generated by the Mars Global Surveyor in the late 1990s revealed unaltered mafic minerals over large areas of the planetary surface, and a warm, wet Mars would have been expected to alter any such minerals on the surface by aqueous weathering. In hindsight it can be seen that “White Mars” and other non-aqueous hypotheses were motivated by too much reliance on inadequate data sets and incomplete attention to the full range of interrelated Martian phenomena. Moreover, a lack of understanding also exists by many in the planetary science community of how geological and geomorphological inferences are made, particularly in how these inferences properly rely not solely on simple one-on-one analogies from Martian to terrestrial landforms, but also on a kind of holist reasoning that employs consistency, coherence, and consilience as means for testing those inferences (see Baker, 2014, for a detailed discussion). To be a reasonable alternative to the aqueous explanations, the “White Mars” hypothesis would need to account not simply for individual landforms. It would have to explain whole assemblages of landforms displaying temporal and spatial relationships (as revealed by geological mapping) that can clearly develop from a nexus of interrelated and connected chains of aqueous causation

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that are generally well understood from extensive study of their approximate equivalents on Earth. To have a complete mimicking of aqueous phenomena by carbon dioxide processes, by molten rock (e.g., Leverington, 2011), or by some other non-aqueous media would be a rather amazing discovery, with implications far beyond the study of Mars. The lack of detailed confirmation in the holistic sense noted above, however, makes these non-aqueous hypotheses for Mars geomorphology seem highly unlikely from the geological point of view. The most recent major transition from hydrophobic to hydrophilic conceptual positions has come about in the new millennium, and once again it was discoveries of surprising new phenomena that led to changes in the conceptual position. Initially this arose on the authority of nuclear physics, specifically the neutron and gamma ray detections of the Mars Odyssey spacecraft (Boynton et al., 2002). These data document the presence of high concentrations of water ice in the near-surface regolith at high Martian latitudes. The ice at northern latitudes was subsequently measured and imaged directly by the Phoenix lander (Smith et al., 2009). The radar sounding instrument on Mars Reconnaissance Orbiter spacecraft revealed much more water ice, in this case comprising the polar caps (Phillips and 27 others, 2008) and debriscovered glaciers that would have had to be replenished by cyclic water transfer through the atmosphere (Holt et al., 2008; Plaut et al., 2009). Though Mars science at the time of this writing is no longer hydrophobic, many questions remain about the role and activity of water during the geological evolution of the planet. The improved resolution of orbital images has led to the discovery of much more evidence pointing to a rainfall/ runoff origin for the valley networks that formed on the heavily cratered terrain of the planet (Craddock and Howard, 2002; Mangold et al., 2004; Luo and Stepinski, 2009; Hynek et al., 2010) mostly during a particularly intense phase of hydrological activity about 3.8 billion years ago (Howard et al., 2005; Irwin et al., 2005; Mangold et al., 2012). Multiple studies indicate that the formation of these ancient valleys required prolonged periods of rainfall in amounts comparable to that occurring in arid or semi-arid regions on Earth(Howard, 2007; Barnhart et al., 2009; Hoke et al., 2011; Irwin et al., 2011; Matsubara et al., 2013). Associated lakes, deltas, and alluvial fans show complex histories of fluctuating water and sediment discharges (Malin and Edgett, 2003; Moore and Howard, 2005; Di Achille et al., 2006; Fassett and Head, 2005, 2008; Di Achille and Hynek, 2010; Grant et al., 2011; Hoke et al., 2014;), and these also imply prolonged periods of precipitation and runoff (Moore et al., 2003; Jerolmack et al., 2004; Matsubara et al., 2011). Other observations, possibly consistent with the geomorphological evidence for a warm, wet epoch on early Mars, derive from recent geochemical and mineralogical discoveries. These include the hyperspectral detection from obit of clay minerals and hydrated salts (Bibring et al., 2006). These have also been detected in situ on the Martian surface, along with other evidence for liquid water in water-laid sediments, by the Mars Exploration Rover and Mars Science Laboratory Missions (Grotzinger et al., 2005, 2006, 2014; Arvidson et al., 2014). Though many of the complex assemblages of clay minerals on Mars may have resulted from subsurface hydrothermal processes (Ehlmann et al, 2011), other associations are more similar to surface weathering profiles (Le Deit et al., 2012; Loizeau et al., 2012), and analogies to terrestrial weathering processes suggest prolonged periods of precipitation to achieve the necessary leaching (Gaudin et al., 2011). In contrast to these discoveries, however, advances in theoretical modeling have generally not produced any change to general conclusion reached by Kasting (1991) that a CO2–H2O atmosphere, which involves the most parsimonious extrapolation to make from current Mars conditions to those of the ancient past, would not have been able to bring the global mean surface temperature of Mars to near the freezing point of water at or before about 3.8 billion years ago. Advanced 3-D modeling (e.g., Forget et al., 2013; Wordsworth et al., 2013) has yielded the same conclusion: that a CO2–H2O Martian atmosphere cannot

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generate enough precipitation to produce the observed Martian valleys. The quandry posed by geomorphological, sedimentological, and geochemical (clay-mineral) evidence, all indicating liquid water processes on a warm, wet early Mars (about 3.8 billion years ago,) versus climate physics seeming to prohibit exactly this circumstance has led to the idea of a cold early Mars that was, nevertheless, capable of producing the observed liquid water phenomena. In one variant on this cold, icy early Mars conceptual position, impact-induced heating of the waterice-rich Martian crust lead to local and regional epochs of precipitation (Segura et al., 2002, 2008, 2012; Toon et al., 2010), though the amounts of precipitation are far less than what is estimated to be required by Hoke et al. (2011). In another variant, a cold Mars is able experience prolonged periods of liquid water activity on its surface because of highly saline and acidic conditions (Fairen, 2010). Yet another variant on the cold Mars scenario involves global cycling of ice to the subzero southern highlands of Mars, but basal melting of ice caps and local volcanic or impact heating produce scattered liquid water phenomena (Fastook et al., 2012; Wordsworth et al., 2013). A cold climate scenario for explaining the observed clay-mineral stratigraphies on Mars is that of chemical weathering of either eolian loess or airfall tephra in the presence of snow/ice and volcanogenic acidic aerosols (Michalski et al., 2013). In this model small pockets of localized melting generate highly acidic fluids that produce sulfates in the upper parts of these profiles, while clays form in the lower parts. Leaching is generated by the slow downward migration of the acidic fluids. The other response to the climate physics-versus-Mars morphology/ chemistry quandary is to ignore the usual a priori, parsimonious assumptions that are made in physical modeling, in this case, the need to begin from secure knowledge about the current nature of Martian climate that is dominated by a CO2 atmosphere that varies in its water and carbon dioxide content. By rejecting this constraint one can impose forcing mechanisms for climate modeling that are far different from those of today. The reason for rejecting logical parsimony is the goal of making the models fit the warm, wet interpretation of the geomorphological and geochemical indicators. The latter become, in effect, the inspiration for a working hypothesis, and the modeling acts as the means for elucidating the consequences of this working hypothesis. This methodology contrasts with the more common use of various kinds of geological data as the means for testing models that derive from parsimonious presumptions. By adding 5–20 % H2 to the CO2–H2O atmosphere thought to characterize early Mars Ramirez et al. (2014) employ the above modeling strategy and find that this change will raise the mean surface temperature of early Mars above the freezing point of water. In another example, Mischna et al. (2013) envision combinations of three driving factors for promoting transient warm/wet conditions on early Mars: (1) an insolation effect, mainly driven by changes in Mars' obliquity; (2) a trigger effect, mainly as it promotes a water-rich greenhouse effect; and (3) an albedo effect involving relatively dark portions of the Martian surface. The insolation effect results in periods of increased solar heating at various latitudes. The albedo effect can arise either (1) from dark, dust-free exposures of basalt bedrock, or (2) from the temporary presence of relatively low albedo, ponded water, notably the northern plains “ocean” inferred for early Mars (Parker et al., 1989; Baker et al, 1991; Clifford and Parker, 2001). Finally, a trigger effect can be provided by the massive, short-term volcanic injection into the atmosphere of particularly potent greenhouse gases, such as sulfur dioxide and/or methane. Though such gases are generally short-lived in the atmosphere, their temporary warming effect can provide a trigger to get large quantities of water vapor into the atmosphere, and that water will contribute to more prolonged greenhouse warming. The controversies over water on Mars continue. Forty years of geomorphological study of the Martian surface from high-resolution orbital imagery (e.g., Milton, 1973; Baker and Milton, 1974) has confirmed that Mars, like Earth, is indeed a water planet (Baker, 1982), but the exact meaning of that discovery is still to be worked out.

5. Discussion and Conclusions What have these three themes shown us in regard to how the science community comes to accept as most workable various hypotheses and theories? The modes for generating belief, i.e., theory acceptance, were described in a famous 1877 paper by America's greatest logician/philosopher, Charles Sander Peirce. The paper, titled “The Fixation of Belief” (Peirce, 1877), was part of a series of six papers on the logic of science that both derived from some understanding of the Earth sciences, and may have also influenced the late 19th century methodological writings of T.C. Chamberlin, G.K. Gilbert, and W.M. Davis (Baker, 1996). Peirce (1877) distinguishes four means by which the inquirer moves from a state of unease and dissatisfaction with one's ideas to a state of satisfaction and greatly reduced doubt. The first of Peirce's means of fixing belief is that of tenacity. This is well illustrated by the reaction of Percival Lowell to his critics. Lowell was so sure of the truth of his general theory of planetary evolution that he dismissed all evidence against it. Tenacity is generally associated with individuals, who, like Lowell, can ascribe to ideas that are peculiar to their own worldview, and who are, thereby, insolated from the problems posed by brute facts inconsistent with those ideas. Tenacity becomes unstable in a community of inquirers, who will be irritated by such facts, such that some members of that community will question the prevailing view. The instability of tenacity as a mode of reasoning in a community leads to the method of authority as a unifying factor for the fixing of belief. The long persistence of the volcanic hypothesis as the dominating explanation for lunar craters provides excellent illustrations of the role of authority, which gets combined with other modes of reasoning relative to inadequate understanding of the relevant processes and inadequate data in regard to comparable phenomena that can be more easily studied. Peirce's third mode of fixing belief has many advantages over tenacity and authority. The quality of being agreeable to reason, what Peirce termed the “a prior method,” involves an individual or community seeking those beliefs that have a consistency with the rest of the beliefs of that individual or group. Because these are beliefs that generally have proven to be consistent with their practical consequences, they have the appearances of truth. In scientific work, this would mean that they generally accord with the current worldview or paradigm, i.e., “normal science” in Kuhn's (1962) model. Whereas Peirce noted many positive aspects of the a priori method, he also argued that the force of experience will inevitably drive the inquirer to a method that recognizes the priority of that experience to teach us that things are not always as we think them to be, and that it is wise to take such indications very seriously (DeWaal, 2001). For the theme of modern Mars science and the role of water in shaping the Martian landscape, the very strange sense in which Mars seems to have been hiding from each successive planetary mission its best evidence for later discovery by a subsequent mission almost seems to have been anticipated by an observation Peirce made during a 1903 Harvard lecture: In all the works of pedagogy that ever I read—and they have been many, big, and heavy—I don't remember that any one has advocated a system of teaching by practical jokes, mostly cruel. That, however, describes the method of our great teacher Experience. (Peirce Edition Project, 1998, p. 154.) Planetary surfaces seem to be teaching us scientists that we are not as clever as we sometimes think we are, and that we would do well to stay open-minded to the lessons of our great teacher.

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Acknowledgments As an “insider” reflecting on the history of planetary geomorphology, I have benefited from four decades of research support through various NASA science programs. The present manuscript was improved by review comments provided by Jim Head and Jim Kasting. My current work on water-related geomorphic processes for early Mars has been supported by a subcontract number 1433898 from the N.A.S.A. Jet Propulsion Laboratory, and my work on Mars cataclysmic flooding channels is supported by N.A.S.A. Agreement Number NNX 13AR11G to Jim Rice of the Planetary Science Institute. References Anders, E.,Owen, T., 1977. Mars and earth: origin and abundance of volatiles. Science 198, 453–465. Arvidson, R.E.,Squyres, S.W.,Bell III, J.F.,Catalano, J.G.,Clark, B.C.,Crumpler, L.S.,de Souza Jr., P.A., Fairen, A.G., Farrand, W.H., Fox, V.K., Gellert, R., Ghosh, A., Golombek, M.P., Grotzinger, J.P., Guinness, E.A., Herkenhoff, K.E., Joliff, B.L., Knoll, A.H., Li, R., McLennan, S.M., Ming, D.W., Mittelfehldt, D.W., Moore, J.M., Morris, R.V., Muchie, S.L., Parker, T.J., Paulsen, G., Rice, J.W., Ruff, S.W., Smith, M.D., Wolff, M.J., 2014. Ancient aqueous environments at Endeavor Crater, Mars. Science 343. http://dx.doi.org/10. 1126/science.1248097. Baker, V.R., 1982. The Channels of Mars. University of Texas Press, Austin, Texas (198 pp.). Baker, V.R., 1996. The pragmatic roots of American Quaternary geology and Geomorphology. Geomorphology 16, 197–215. Baker, V.R., 2014. Terrestrial analogs, planetary geology, and the nature of geological reasoning. Planet. Space Sci. 95, 5–10. Baker, V.R.,Milton, D.J., 1974. Erosion by catastrophic floods on Mars and Earth. Icarus 23, 27–41. Baker, V.R.,Strom, R.G.,Gulick, V.C.,Kargel, J.S.,Komatsu, G.,Kale, V.S., 1991. Ancient oceans, ice sheets and the hydrological cycle on Mars. Nature 352, 589–594. Baldwin, R., 1949. The Face of the Moon. University of Chicago Press, Chicago (232 pp.). Baldwin, R., 1963. The Measure of the Moon. University of Chicago Press, Chicago (474 pp.). Baldwin, R.B., 1978. An overview of impact cratering. Meteroritics 13, 364–379. Barnhart, C.J.,Howard, A.D.,Moore, J.M., 2009. Long-term precipitation and late-stage valley network formation: Landform simulations of Parana Basin, Mars. J. Geophys. Res. 114, E01003. http://dx.doi.org/10.1029/2008JE003122. Barringer, D.M., 1906. Coon mountain and its crater. Proc. Nat. Sci. Soc. Phila. 57, 861–886. Bibring, J.-P., Langevin, Y., Mustard, J.F., Poulet, F., Arvidson, R., Gendrin, A., Gondet, B., Mangold, N., Pinet, P., Forget, F., 2006. the OMEGA Team, 2006. Global mineralogical and aqueous Mars history derived from OMEGA/Mars Express Data. Science 312, 400–404. Blackwelder, E., 1909. Mars as the abode of life. Science 29, 659–661 (n.s.). Boon, J.D., Albritton Jr., C.C., 1937. Meteorite scars in ancient rocks. Field Lab. 5, 53–64. Boynton, W.V., Feldman, W.C., Squyres, S.W., Prettyman, T.H., Brückner, J., Evans, L.G., Reedy, R.C., Starr, R., Arnold, J.R., Drake, D.M., Englert, P.A.J., Metzger, A.E., Mitrofanov, I., Trombka, J.I., d'Uston, C., Wänke, H., Gasnault, O., Hamara, D.K., Janes, D.M., MArcialis, R.L., Maurice, S., Mikheeva, I., Taylor, G.J., Tokar, R., Shinohara, C., 2002. Distribution of hydrogen in the near surface of Mars: evidence for subsurface ice deposits. Science 297, 81–85. Cabrol, N.A., Grin, E.A., 1999. Distribution, classification, and ages of Martian lakes. Icarus 142, 160–172. Carr, M.H., 1996. Water on Mars. Oxford University Press, Oxford (229 pp.). Carr, M.H., 2013. Geologic exploration of the planets: the first 50 years. Eos 94, 29–30. Chao, E.C.T., Shoemaker, E.M., Madsen, B.M., 1960. First natural occurrence of coesite. Science 132, 220–222. Cleland, C.E., 2002. Methodological and epistemic differences between historical science and experimental science. Philos. Sci. 69, 474–496. Cleland, C.E., 2013. Common cause explanation and the search for a smoking gun. In: Baker, V.R. (Ed.), Rethinking the Fabric of Geology. Geological Society of America Special Paper, 502, pp. 1–9. Clifford, S.M.,Parker, T.J., 2001. The evolution of the martian hydrosphere: implications for the fate of a primordial ocean and the current state of the northern plains. Icarus 154, 40–79. Craddock, R.A., Howard, A.D., 2002. The case for rainfall on a warm, wet early Mars. J. Geophys. Res. 107 (E11), 5111. http://dx.doi.org/10.1029/2001JE001505. Dana, J.D., 1846. On the volcanoes of the moon. Am. J. Sci. 11, 335–355. Davies, G.L., 1969. The Earth in Decay: A History of British Geomorphology. MacDonald, pp. 1578–1878 (390 pp.). DeWaal, C., 2001. On Peirce. Wadsworth, Belmont, California (91 pp.). Di Achille, G., Hynek, B.M., 2010. Ancient ocean on Mars supported by global distribution of deltas and valleys. Nat. Geosci. 3, 459–463. Di Achille, G.,Marinanageli, L.,Ori, G.G.,Hauber, E.,Gwinner, K.,Reiss, D.,Neukum, G., 2006. Geological evolution of the Tyras Vallis paleolacustrine system, Mars. J. Geophys. Res. 111 (E4). http://dx.doi.org/10.1029/2005JE002561. Dietz, R.S., 1946a. Geological structures possibly related to lunar craters. Pop. Astron. 54, 465–467. Dietz, R.S., 1946b. The meteorite impact origin of the moon's surface features. Geol. Soc. Am. Bull. 54, 359–375. Drake, E.T., 1996. Restless Genius: Robert Hooke and His Earthly Thoughts. Oxford University Press, Oxford (386 pp.).

9

Ehlmann, B.L., et al., 2011. Subsurface water and clay mineral formation during the early history of Mars. Nature 479, 53–60. El-Baz, F., 1980. Gilbert and the moon. In: Yochelson, E.L. (Ed.), The Scientific Ideas of G.K. Gilbert. Geological Society of America Special Paper, 183, pp. 69–80. Fairen, A., 2010. A cold and wet Mars. Icarus 208, 165–175. Fassett, C.I.,Head, J.W., 2005. Fluvial sedimentary deposits on Mars: ancient deltas in crater lake in the Nili Fossae region. Geophys. Res. Lett. 32 (14). http://dx.doi.org/10.1029/ 2005GL023456. Fassett, C.I., Head III, J.W., 2008. Valley network-fed, open-basin lakes on Mars: distribution and implications for Noachian surface and subsurface hydrology. Icarus 198, 37–56. Fastook, J.L.,Head, J.W.,Marchant, D.R.,Forget, F.,Madelaine, J.-B., 2012. Early Mars climate near the Noachian–Hesperian boundary: independent evidence for cold conditions from basal melting of the south polar ice sheet (Dorsa Argentina Formation) and implications for valley network formation. Icarus 219, 25–40. Fielder, G., 1963. On the topography of Mars. Astron. Soc. Pac. Publ. 75, 75–76. Forget, F., Wordsworth, R., Millour, E., Madeleine, J.-B., Kerber, L., Leconte, J., Marcq, E., Haberle, R.M., 2013. 3D modelling of the early martian climate under a denser CO2 atmosphere: temperatures and CO2 ice clouds. Icarus 222, 81–99. French, B.M., 1977. The Moon Book. Penguin, New York (287 pp.). Gaudin, A., Dehouck, E., Mangold, N., 2011. Evidence for weathering on early Mars from a comparison with terrestrial weathering profiles. Icarus 216, 257–268. Gifford, A.C., 1924. The mountains of the moon. N. Z. J. Sci. Technol. 7, 129–142. Gifford, A.C., 1930. The origin of the surface features of the moon. Scientia 48, 69–80. Gifford Jr., F.A., 1964. The Martina canals according to a purely aeolian hypothesis. Icarus 3, 130–135. Gilbert, G.K., 1886. The inculcation of scientific method by example. Am. J. Sci. 31, 284–299. Gilbert, G.K., 1893. The moon's face—a study of the origin of its features. Philos. Soc. Wash. Bull. 12, 241–292. Gilbert, G.K., 1896. The origin of hypotheses illustrated by the discussion of a topographic problem. Science 3, 1–13 (n.s.). Godfrey-Smith, P., 2003. Theory and Reality: An Introduction to the Philosophy of Science. Grant, J.A., Irwin III, R.P., Wilson, S.A., Buczkowski, K., Siebach, K., 2011. A lake in Uzboi Vallis and implictions for Late Noachian–Early Hesperian climate on Mars. Icarus 212, 110–122. Green, J., 1965. Hookes and spurrs in selenology. Ann. N. Y. Acad. Sci. 123, 373–402. Green, J., 1970. Significant morphological features of the moon. Image Dyn. 5, 29–30. Green, J., 1971. Copernicus as a lunar caldera. J. Geophys. Res. 76, 5719–5731. Greene, M.T., 1998. Alfred Wegener and the origin of lunar craters. Earth Sci. Hist. 17, 111–138. Grotzinger, J.P., Arvidson, R.E., Bell III, J.F., Calvin, W., Clark, B.C., Fike, D.A., Golombek, M., Greeley, R., Haldermann, A., Herkenhoff, K.E., Joliff, B.L., Knoll, A.H., McLennan, S.M., Parker, T., Soderblom, L., Sohl-Dickstein, J.N., Squyres, S.W., Tosca, N.J., Waters, W.A., 2005. Stratigraphy and sedimentology of a dry to wet eolian depositional system, Burns Formation, Meridiani Planum, Mars. Earth Planet. Sci. Lett. 240, 11–72. Grotzinger, J.,Bell III, J.F.,Herkenhoff, K.E.,Johnson, J., Knoll, A., McCartney, D.,McLennan, S. M., Metz, J., Moore, J., Squyres, S., Sullivan, R., Ahronson, O., Arvidson, R., Joliff, B., Golombek, M., Lewis, K., Parker, T., Soderblom, J., 2006. Sedimentary textures formed by aqueous processes, Erebus crater, Meridiani Planum, Mars. Geology 34, 1085–1088. Grotzinger, J.P., Summer, D.Y., Kah, L.C., Stack, K., Gupta, S., Edgar, L., Rubin, D., Lewis, K., Scheiber, J., Mangold, N., Milliken, R., Conrad, P.G., DesMarais, D., Farmer, J., Siebach, K., Califf III, F., Hurowitz, J., McLennan, S.M., Ming, D., Vaniman, D., Crisp, J., Vasavada, A., Edgett, K.S., Malin, M., Blake, D., Gellert, R., Mahffy, P., Wiens, R.C., Maurice, S., Grant, J.A., Wilson, S., Anderson, R.C., Geegle, L., Arvidson, R., Hallet, B., Slettern, R.S., Rice, M., Bell III, J., Griffes, J., Ehlmann, B., Anderson, R.B., Bristow, T.F., Dietrich, W.E., Dromart, G.,Eigenbrode, J.,Fraeman, A.,Hardgrove, C.,Herkenhoff, K.,Jandura, L.,Kocurek, G.,Lee, S.,Leshin, L.A.,Leveille, R.,Limonadi, D.,Maki, J.,McCloskey, S.,Meyer, M.,Minittie, M., Newsom, H.,Oehler, D.,Okon, A.,Palucis, M.,Parker, T.,Rowland, S.,Schmidt, M.,Squyres, S., Steele, A.,Stolper, E.,Summons, R.,Treiman, A.,Williams, R.,Yingst, A.,MSL Science Team, 2014. A habitable fluvio-lacustrine environment at Yellowknife Bay, Gale Crater, Mars. Science 343. http://dx.doi.org/10.1126/science.1242777. Haack, S., 2003. Defending Science—Within Reason: Between Scientism and Cynicism. Prometheus Books, Anhertst, N.Y (411 pp.). Hoffman, N., 2000. White Mars: a new model for Mars' surface and atmosphere base on CO2. Icarus 146, 326–342. Hoke, M.R.T.,Hynek, B.M.,Tucker, G.E., 2011. Formation timescales of large martian valley networks. Earth Planet. Sci. Lett. 312, 1–12. Hoke, M.R.T.,Hynek, B.M.,Di Achille, G.,Hutton, E.W.H., 2014. The effects of sediment supply and concentrations on the formation timescale of Martian deltas. Icarus 228, 1–12. Holt, J.W.,Safaeinili, A., Plaut, J.J., Head, J.W., Phillips, R.J., Seu, R., Kempf, S.D., Choudhary, P., Young, D.A., Putzig, N.E.,Biccari, D., Gim, Y., 2008. Radar sounding evidence for buried glaciers in the southern mid-latitudes of Mars. Science 322, 1235–1238. Howard, A.D., 2007. Simulating the development of martian highland landscapes through the interaction of impact cratering, fluvial erosion, and variable hydrologic forcing. Geomorphology 9, 332–363. Howard, A.D., Moore, J.M., Irwin III, R.P., 2005. An intense terminal epoch of widespread fluvial activity on early Mars: 1. Valley network incision and associated deposits. J. Geophys. Res. 110, E12S14. http://dx.doi.org/10.1029/2005JE002459. Hoyt, W.G., 1987. Coon Mountain Controversies. University of Arizona Press, Tucson (442 pp.). Hynek, B.M., Beach, M., Hoke, M.R.T., 2010. Updated global map of Martian valley networks and implications for climate and hydrologic processes. J. Geophys. Res. 115, E09008. http://dx.doi.org/10.1029/2009JE003548. Irwin, R.P.I.I.I., Howard, A.D., Craddock, R.A., Moore, J.M., 2005. An intense terminal epoch of widespread fluvial activity on early Mars: 2. Increased runoff and paleolake development. J. Geophys. Res. 110, E12S15. http://dx.doi.org/10.1029/2005JE002460.

Please cite this article as: Baker, V.R., Planetary geomorphology: Some historical/analytical perspectives, Geomorphology (2014), http:// dx.doi.org/10.1016/j.geomorph.2014.07.016

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Irwin III, R.P., Craddock, R.A., Howard, A.D., Flemming, H.L., 2011. Topographic influences on development of Martian valley networks. J. Geophys. Res. 116, E02005. http://dx.doi. org/10.1029/2010JE003620. Ives, H.E., 1919. Some large-scale experiments imitating the craters on the moon. Ap. J. 50, 245–250. Jerolmack, D.J.,Mohrig, D., Zuber, M.T.,Byrne, S., 2004. A minimum time for the formation of Holden Northeast fan, Mars. Geophys. Res. Lett. 31, L21701. http://dx.doi.org/10. 1029/2004GL021326. Kasting, J.F., 1991. CO2 condensation and the climate of early Mars. Icarus 62, 175–190. Kuhn, T., 1962. The Structure of Scientific Revolutions. University of Chicago Press, Chicago. Le Deit, L.,Flahaut, J.,Quantin, C., Hauber, E.,Mège, D.,Bourgeois, O.,Gurgurewicz, J.,Masse, M.,Jaumann, R., 2012. Extensive surface pedogenic alteration of the Martian Noachian crust suggested by plateau phyllosilicates around Valles Marineris. J. Geophys. Res. 117, E00J05. http://dx.doi.org/10.1029/2011JE003983. Leighton, R.B.,Murray, B.C., 1966. Behavior of carbon dioxide and other volatiles on Mars. Science 153, 136–144. Leverington, D.W., 2011. A volcanic origin for the outflow channels of Mars: key evidence and major implications. Geomorphology 132, 51–75. Loewy, M.,Puiseux, P.H., 1897. Photographic studies of the moon at the Paris Observatory. Astrophys. J. 5, 51–59. Loizeau, D., Werner, S.C., Mangold, N., Bibring, J.-P., Vago, J.L., 2012. Chronology of deposition and alteration in the Mawrth Vallis region, Mars. Planet. Space Sci. 72, 31–43. Lowell, P.L., 1895. Mars. Houghton. Mifflin and Company, New York (228 pp.). Lowell, P.L., 1906. Mars and Its Canals. Macmillan, New York (453 pp.). Lowell, P.L., 1908. Mars as the Abode of Life. Macmillan, New York (338 pp.). Lowell, P.L., 1909. The revelation of evolution: a thought and its thinkers. Atl. Mon. 104, 174–183. Luo, W.,Stepinski, T.F., 2009. Computer-generated global map of valley networks on Mars. J. Geophys. Res. 114, E11010. http://dx.doi.org/10.1029/2009JE003357. Malin, M.C.,Edgett, K.S., 2003. Evidence for persistent flow and aqueous sedimentation on early Mars. Science 302, 1931–1934. Mangold, N., Quantin, C., Ansan, V., Delacourt, C.,Allemand, P., 2004. Evidence for precipitation on Mars from dendritic valleys in the Valles Marineris area. Science 305, 78–81. Mangold, N., Adeli, S., Conway, S., Ansan, V., Langlais, B., 2012. A chronology of early Mars climatic evolution from impact crater degradation. J. Geophys. Res. 117, E04003. http://dx.doi.org/10.1029/2011JE004005. Marvin, U.B., 1986. Meteorites, the moon, and the history of geology. J. Geol. Educ. 34, 140–165. Matsubara, Y.,Howard, A.D.,Drummond, S.A., 2011. Hydrology of early Mars: Lake basins. J. Geophys. Res. 116, E04001 1029/2010JE003739. Matsubara, Y., Howard, A.D., Gochenour, J.P., 2013. Hydrology of early Mars: valley network incision. J. Geophys. Res. 118, 1365–1387 (1029/jgre20081). Michalski, J.R.,Bleacher, J.R., 2013. Supervolcanoes within an ancient volcanic province in Arabia Terra, Mars. Nature 502, 47–52. Michalski, J.R., Niles, P.B., Cuadros, J., Baldridge, A.M., 2013. Multiple working hypotheses for the formation of compositional stratigraphy on Mars: insights from the Mawrth Vallis region. Icarus 226, 816–840. Milton, D.J., 1973. Water and processes of degradation in the Martian landscape. J. Geophys. Res. 78, 4037–4047. Mischna, M., Baker, V., Milliken, R., Richardson, M., Lee, C., 2013. Effects of obliquity and water vapor/trace gas greenhouses in the early Martian climate. J. Geophys. Res. 118, 560–576. http://dx.doi.org/10.1002/jgre.20054. Moore, J.M.,Howard, A.D., 2005. Large alluvial fans on Mars. J. Geophys. Res. 110, E04005. http://dx.doi.org/10.1029/2004JE002352.

Moore, J.M., Howard, A.D., Dietrich, W.E., Schenk, P.M., 2003. Martian layered fluvial deposits: implications for Noachian climate scenarios. Geophys. Res. Lett. 30 (24). http://dx.doi.org/10.1029/2003GL019002. Parker, T.J.,Saunders, R.S.,Schneeberger, D.M., 1989. Transitional morphology in the west Deuteronilus Mensae region of Mars: implications for modification of the lowland/ upland boundary. Icarus 82, 111–145. Parker, T.J., Gorsline, D.S., Saunders, R.S., Pieri, D., Schneeberger, D.M., 1993. Coastal geomorphology of the Martian northern plains. J. Geophys. Res. 98, 11061–11078. Peirce, C.S., 1877. The fixation of belief. Pop. Sci. Mon. 12, l–15. Peirce Edition Project (Ed.), 1998. The Essential Peirce: Selected Philosophical Writings. Indiana University Press, Bloomington. (584 p.). Phillips, R.J.,27 others, 2008. Mars north polar deposits: stratigraphy, age, and geodynamical response. Science 320, 1182–1185. Pickering, W.H., 1903. The Moon. Doubleday, New York. Plaut, J.J., Safaeinili, A., Holt, J.W., Phillips, R.J., Head, J.W., Seu, R., Putzig, E., Frigeri, A., 2009. Radar evidence for ice in lobate debris aprons in the mid-northern latitudes of Mars. Geophys. Res. Lett. 36, L02203. http://dx.doi.org/10.1029/2008GL036379. Pollack, J.B., Kasting, J.F., Richardson, S.M., Poliakoff, K., 1987. The case for a warm wet climate on early Mars. Icarus 71, 203–224. Proctor, R.A., 1873. The Moon. Alfred Brothers, Manchester. Pyne, S.J., 1980. Grove Karl Gilbert: A Great Engine of Research. University of Texas Press, Austin 306 pp. Ramirez, R.M.,Kopparapu, R.,Zugger, M.E.,Robinson, T.D.,Freedman, R., Kasting, J.F., 2014. Warming early Mars with CO2 and H2. Nat. Geosci. 7, 59–63. Sagan, C., Pollack, J.B., 1966. On the nature of the canals on Mars. Nature 212, 117–121. Schultz, P.H., 1998. Shooting the moon: understanding the history of lunar impact theories. Earth Sci. Hist. 17, 92–110. Segura, T.L., Toon, O.B., Colaprete, A., Zahnle, K., 2002. Environmental effects of large impacts on Mars. Science 298, 1977–1980. Segura, T.L., Toon, O.B., Colaprete, A., 2008. Modeling the environmental effects of moderate-sized impacts on Mars. J. Geophys. Res. 113, E11007. Segura, T.L., McKay, C.P., Toon, O.B., 2012. An impact-induced, stable, runaway climate on Mars. Icarus 220, 144–148. Shaler, N.S., 1903. A comparison of the features of the Earth and the Moon. Smithson. Contrib. Knowl. 34, 1. Shoemaker, E.M., 1962. Interpretations of lunar craters. In: Kopal, Z. (Ed.), Physics and Astronomy of the Moon. Academic Press, New York, pp. 283–359. Smith, P.H., Tamppari, L.K., Arvidson, R.E., Ball, D., Blaney, D., Boynton, W.V., Carswell, A., Catling, D.C., Clark, B.C., Duck, T., De Jong, E., Fisher, D., Goetz, W., Gunnlaugsson, H.P., Hecht, M.H., Hipkin, V., Hoffman, J., Hviid, S.F., Keller, H.U., Kaunaves, S.P., Lange, C.F., Lemmon, M.T., Madsen, M.B., Markiewicz, W.J., Marshall, J., McKay, C.P., Mellon, M.T., Ming, D.W., Morris, R.V., Pike, W.T., Renno, N., Staufer, U., Stoker, C., Taylor, P., Whiteway, J.A., Zent, A.P., 2009. H2O at the phoenix landing site. Science 325, 58–61. Spencer, L.J., 1933. Meteorite craters as topographic features on the Earth's surface. Annual Report of the Smithonisan Institution, pp. 307–325. Tilghman, B.C., 1906. Coon Butte, Arizona. Proc. Nat. Sci. Soc. Phila. 57, 887–914. Toon, O.B., Segura, T., Zahnle, K., 2010. The formation of martian river valleys by impacts. Ann. Rev. Earth Planet. Sci. 38, 303–322. Veverka, J., Sagan, C., 1974. McLaughlin and Mars. Am. Sci. 62, 44–53. Wordsworth, R.,Forget, F.,Millour, E.,Head, J.W.,Madeleine, J.-B.,Charnay, B., 2013. Global modeling of the earth martian climate under a denser CO2 atmosphere: water cycle and ice evolution. Icarus 222, 1–19.

Please cite this article as: Baker, V.R., Planetary geomorphology: Some historical/analytical perspectives, Geomorphology (2014), http:// dx.doi.org/10.1016/j.geomorph.2014.07.016