Does rock varnish accurately record ancient desert wetness

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Does rock varnish accurately record ancient desert wetness?

Viewpoint: Yes, desert varnish (rock varnish) may be an accurate indicator of ancient desert wetness.

Viewpoint: No, rock varnish does not accurately record ancient desert wetness because it cannot be dated effectively and its mineral composition cannot exclusively be attributed to climate change.

The subject of desert varnish, or rock varnish, as we shall see, carries with it a controversy concerning the relative levels of wetness that once existed in what are today desert environments. But this particular topic also illustrates several persistent themes in the earth sciences as well. Among these are the long periods of time usually required to bring about any noticeable change in geologic features (that is, aside from dramatic instances of tectonic activity, such as volcanism). There is also the matter of the gradualism that characterizes the transport of solid-earth material from one place to another, and its accretion in one place. Also interesting, from the perspective of the geologic sciences as a whole, is the combination of physical, chemical, and biological processes, discussed in the essays that follow, which bring about the "varnishing" of rocks.

Large outcroppings of rock do not tend to be all of one color, even if the rock itself is of the same mineral makeup. The reason relates to rock varnish, a type of coating that accumulates when rock is exposed to the elements for millions of years. The term rock varnish applies to the dark coloration that coats rocks on wet cliffs, on the walls of caves, and in a variety of other locales. One particular kind of rock varnish is known as desert varnish, a term for a dark coating that gathers on the surfaces of rocks in arid regions.

The layer of varnish on desert rocks is extremely thin: usually no more than 200 microns, or 0.00787 inches. Also known as a micrometer, a micron (represented by the Greek letter μ) is equal to one-millionth of a meter, or 0.00004 inches. Despite the fact that this constitutes a membrane-thin coating, desert varnish is easily visible to the naked eye, in part because of the minerals of which it is made.

Desert varnish is composed of various iron oxides, which are reddish, and varieties of manganese oxide, which tends toward slate-gray or black. Also contributing to the coating are various clays, which further the darkening of the rock surface, assuming that the surface remains undisturbed. Naturally, not all surfaces of a rock are exposed for the same amounts of time: one particular face may be much more susceptible to weathering than another, for instance, or heavy winds and rains may remove a chunk of rock from one side of an outcropping, exposing virgin material to new weathering.

Ancient Native Americans and others in dry regions around the planet used desert and rock varnishes as a surface in which to carve out images known as petroglyphs, or rock carvings. Using sharp stones to cut into the darkened rock surface, they carved out light-colored images—using the type of contrast seen in a photographic negative today—to represent hunters, animals, and other scenes. But these man-made pictures may not be the only stories contained in the mineral-varnished surfaces of desert rocks.

There is a body of scientific opinion to the effect that desert varnish on rocks provides evidence of wetness in deserts prior to the beginning of the present geologic epoch, the Holocene, which began at the end of the last ice age about 11,000 years ago. As shown by the essays that follow, there is considerable evidence on either side of the argument, and indeed, the same experts can be cited as proponents of either theory. Thus, both essays make favorable note of a study published in the August 2001 GSA Today by Wallace S. Broecker and Tanzhou Liu of Columbia University's Lamont Doherty Earth Observatory.

Critical to understanding the information contained in the desert-varnish record—and the information contained in these essays—is an appreciation of radiometric and other forms of dating used by geologists. The degree to which a given rock is varnished by iron oxides, manganese oxide, clays, and other substances is a form of relative dating, which indicates the age of a rock or other object in relation to other items. Absolute dating, by contrast, involves the determination of age in actual years or millions of years.

Absolute dating methods usually center around the idea that over time, a particular substance converts to another, mirror substance. By comparing the ratios between them, it is possible to arrive at some estimate as to the amount of time that has elapsed since the organism died. Many varieties of absolute dating are radiometric, involving the decay of radioactive isotopes, or radioisotopes, which eventually become stable isotopes.

Each chemical element is distinguished by the number of protons (positively charged particles) in its atomic nucleus, but atoms of a particular element may have differing numbers of neutrons, or neutrally charged particles, in their nuclei. Such atoms are referred to as isotopes. Certain isotopes are stable, whereas others are radioactive, meaning that they are likely to eject high-energy particles from the nucleus over time, until they eventually become stable.

The amount of time it takes for half the isotopes in a sample to stabilize is called its half-life. This varies greatly between isotopes, some of which have a half-life that runs into the billions of years. By analyzing the quantity of radioactive isotopes in a given sample that have converted to stable isotopes, it is possible to determine the age of the sample.

Isotopes are usually identified by the chemical or element symbol, with a superscript number before or after the symbol indicating the combined number of protons and neutrons. For example, one isotope mentioned below as an useful indicator for radiometric dating is beryllium-7, designated as ⁷Be. A quick glance at the periodic table of elements reveals that beryllium has an atomic number of 4, meaning that there are four protons in the nucleus; thus, the superscript number 7 indicates the presence of three neutrons in the nucleus of this radioisotope.

—JUDSON KNIGHT

Viewpoint: Yes, desert varnish (rock varnish) may be an accurate indicator of ancient desert wetness.

All desert varnish is rock varnish, but not all rock varnish is desert varnish. The very thin (less than 200 microns-thick), but very visible, coating found so commonly in desert regions is called desert varnish. Similar coating is found on wet cliffs and the walls of some caves that are not all in desert regions. The term rock varnish applies to the dark patina found on some rocks, anywhere.

Desert varnish is of particular interest because it may provide an accurate record of ancient desert wetness. Admittedly the adjective "accurate" seems to be too optimistic a term to describe records of climatic events that occurred more than 30,000 years ago. But, it is used here in the context of being as accurate as the dating of the interglacial periods, the time frame in paleoclimatology where records of events are not readily available from other sources.

Researchers Wallace S. Broecker and Tanzhuo Liu at the Lamont Doherty Earth Observatory of Columbia University spent five years studying desert varnish to evaluate its uses as a recorder of paleo-wetness. The results of their U. S. Department of Energy-funded efforts convinced them that "rock varnish does indeed bear an amazing record of wetness in Earth's desert regions."

The researchers examined tens of thousands of varnished rocks in the field and thousands of thin sections in the lab. What they found explains why others before them were unsuccessful in relating desert varnish to ancient wetness. Random sampling rarely provided a reliable full sequence of chemical events over the time the varnish formed. They got consistent results by using extreme care to select samples from the most stable rock surfaces and looking only at the varnish-filled microbasins that have a width-to-depth ratio of approximately 10. These selected samples contain the most complete varnish microstratigraphy. Element maps and element line profiles were developed using an electron microprobe to establish patterns of concentration relative to depth as indicators of past regional environmental conditions.

Corroborating Evidence

Where the Columbia team used an electron microprobe to study the sedimentary bands in desert varnish that represent the records of micrometers of accumulated elements per millennium, another team used scanning electron microscopy (SEM) to study them. Under the direction of Richard L. Orndorff, Assistant Professor at the University of Nevada, Las Vegas, a team of graduate students used SEM to study laminations that can be identified and energy dispersion spectrometry (EDS) to determine differences in relative abundances of elements which they hypothesized reflect changes in climate. Their samples were taken from boulders along the shorelines left by a pale-olake near Fallon, Nevada. All the samples studied had well-developed desert varnish, some with thickness of 200 microns. The team found layers of greater quantities of elements consistent with wind-blown clay that appear to correspond to dry periods, and layers of dark manganese oxide that are believed correspond to humid periods. The conclusion of the team is that sublayers in the microlaminations reflect climate oscillations in the region studied. Their work was presented at an Annual Meeting of the Geological Society of America in May 2002.

Composition of Rock Varnish

Clay minerals dominate desert varnish. These are composed of Mg-Al-Si (magnesium-aluminum-silicon) oxides, representative of the weathered rock in the local region. The trapped fine dust particles are submicrometer in size and are not resolved in electron probe maps. Oxides of Mn (manganese), Fe (iron), Ba (barium), and Ca (calcium) are also found in the chemical makeup. The calcium and manganese interrelate, but the iron oxide content of varnish is unchanged through the entire record of the varnish chemistry. The color in rock varnish comes from black manganese oxide (the mineral birnessite) and red iron oxide (the mineral hematite), according to researchers at Caltech's Geological and Planetary Sciences division. NASA scientists at the Ames Research Center (ARC) have also recently documented magnetite, another iron oxide but one that is not as completely oxidized as hematite, a curious discovery in that environment.

The record of manganese oxide, MnO2, content relates to wetness periods. The Columbia researchers note there is a trend toward higher content during periods of increased precipitation, although they admit "the Mn content in varnish cannot be used as an absolute paleo-rain gauge." A plot of manganese oxide against annual rainfall for Holocene varnish produces a nonzero intercept. This suggests that manganese enters the varnish from some other source—possibly with dust, aerosols, and dew. Holocene refers to the recent epoch that covers from 11,000 years BP (before the present) to today.

To test their study of manganese oxide as indicators of ancient wetness, in spite of its nonzero intercept, the Columbia research team set up an eight-month experiment in Columbia's Biosphere 2 campus where eight rock samples were studied, four were protected from environmental variables and four were exposed. The varnish was analyzed for the presence of ⁷Be, a 53-day half life radioisotope produced in the atmosphere by cosmic radiation. The rocks exposed to the Biosphere's environment had marked accumulation of the isotope compared to the sheltered rocks. The test suggested to the team that the nonzero intercept for manganese is the result of immediate environmental conditions.

Barium oxide is in much lower concentration in desert varnish than manganese oxide, but it correlates well to the rise and fall of manganese oxide and produces more conspicuous dark layers in the thin section probes that are interpreted by the team to correspond with glacial time. The Columbia team sees this as reinforcing the idea that desert varnish can be considered as an indicator of ancient wetness.

Origin of Rock Varnish

Whereas clay makes up about 70% of desert varnish, it is generally thought the process starts with fine windblown soil deposited as a thin film on the rock, possibly sticking because there is a little dew on it. The composition of the varnish is totally different from the rock under it. Gradually the oxides of manganese and iron that were transported with the soil bind it to the surface so layer upon layer of varnish evolves, and there is evidence that it continues to evolve.

The Columbia team also investigated varnish samples for such things as lead and zinc that have been added to the atmosphere in the last hundred years from the use of leaded gasoline, and from smelters releasing zinc into the environment. Both were found in the outer desert varnish layers. They also tested for nuclides produced during nuclear bomb tests. They were found, too. Lead (a decay product of heavy radioactive elements) was the most convincing. It was found in a concentration ten times higher in outer layers than in records of earlier periods.

There are two scenarios proposed for how rock varnish is formed. The older hypothesis suggests a chemical origin. A newer hypothesis suggested by scientists in the Department of Geosciences at the University of Arizona proposes varnishes are the result of a biogenic deposit produced by mixotrophic bacteria living on rock surfaces. A plot of the rate of varnish production versus moisture, published in 1982 by Dorn and Osborne of the University of Arizona group, indicates semiarid environments are optimal for growth of varnish. Hyper-arid environments lack water for bacterial growth, and varnish growth is indicated as extremely slow or nonexistent on the graph. It has also been suggested that the bacteria live on the energy produced from the oxidation of manganese. The authors of the Columbia University study on the correlation of desert varnish layering features to wet events in ancient times think that enough evidence is not available for them to support the biogenic hypothesis.

There are convincing studies on the presence of bacteria in layered rock varnishes but whether they are the reason for the varnish being there is still in dispute. Studies using transmission electron microscopy (TEM) conducted by geologists David H. Krinsley and Brian G. Rusk at the University of Oregon suggest the presence of bacteria in varnish from images of the proper size and shape for bacteria that are apparently impregnated with Mn and Fe oxides. Their measurements of varnish pH indicate conditions unsuitable for physical and chemical oxidation of manganese, suggesting the bacteria as the cause. Their work was presented at the Second International Conference on Mars Polar Science and Exploration 2000, held August 2000 in Reykjavik, Iceland. Their study also speculated on the possible presence of bacteria in layered rock varnish on Mars.

The Dating Dispute

There has been considerable controversy on the direct dating of rock varnish. Interest in direct dating has been intense among those involved with dating ancient rock art, also called petroglyphs, although dating is also very important to paleo-climatology. Petroglyphs represent a record of the very earliest human activities. Pictures were cut into the dark varnish exposing the lighter underlying rock. Paleoclimatology deals with climate before humans began collecting instrumental measurements.

Since the 1980s there have been attempts to directly date rock varnish using what is called cation-ratio dating. For several years this method was hailed as a great breakthrough in dating rock varnish, but as more research was done by a number of different geochemists the method gave inconsistent results. Rock varnish contains very little carbon, and the source of any carbon that is found is uncertain.

Eventually the first scientists to use cation-ratio dating chose to withdraw their conclusions. The end result of the controversy is that no method of direct dating of rock varnish is presently recognized as having much merit by many leading scientists in the field.

The problems of dating came about because the source of carbon used in the cation-ratio dating method (accelerator mass spectrometry dating, AMS ¹⁴C) came from carbon compounds and sources that were inadequately identified. The results first produced with this method could not reliably be reproduced by other researchers. The use of 232Th/²³⁸U, a common isotope pair suitable for radiometric dating, is not possible because the ratio of these two isotopes in rock varnish is near unity.

With the problem that rock varnish cannot be directly dated, how can it provide a useful record of ancient desert wetness? The dates are determined by other features in the environment such as raised shorelines, basalt flows, allu-vial fans, and moraines. In some places algal deposits from periods of ancient wetness can be dated using radiocarbon methods. Cosmogenic isotopes may also be useful. Some high energycosmic radiation is always bombarding the Earth. It can penetrate meters into rock and produce long-lived radionuclides. The concentration of isotopes is extremely small but the build-up of isotopes over ages can provide a way to date the surface of such features as lava flows.

Rock Varnish Microbes on Mars?

The rock varnish record may go beyond the deserts on Earth. No one expects to find petroglyphs carved into rock varnish on Mars, but there is considerable evidence that there are some rocks on Mars that have shiny dark coatings consistent with desert varnish. If NASA's future rovers on Mars corroborate what has been reported by CNN.com (July 2, 2001) that Martian meteorites have high concentrations of manganese, and the information is combined with the known presence of red iron oxides on the surface of Mars (which is why it is called the Red Planet), then there is a real possibility that what appears to be desert varnish is desert varnish. The next speculation is that Martian desert varnish could be formed by microbial actions and that would be firm evidence that there is, or was, life on Mars!

Researchers at NASA-ARC report that magnetite has been found on the surface of Mars, as unfavorable a location to find it as the Earth-bound desert varnish where they also found magnetite. This discovery adds credibility to the idea that there is desert varnish on Martian boulders. Plus, there is the information from the Krinsley team presented at the Second International Conference on Mars Polar Science and Exploration on their observation of bacterial presence in layered rock varnish on Earth that could possibly be an indication of the same being present in rock varnish on Mars.

—M. C. NAGEL

Viewpoint: No, rock varnish does not accurately record ancient desert wetness because it cannot be dated effectively and its mineral composition cannot exclusively be attributed to climate change.

The Miniature Universe of Rock Varnish and Its Relation to Ancient Climate

Rock varnish is a microscopically thin coating of organic and mineral material that develops, over thousands of years, on exposed rock surfaces in many arid and semiarid environments. By examining samples of these ancient, microscopic universes collected from desert regions around the world, scientists hope to determine the age of rock surfaces and, in particular, to reconstruct climate changes in those deserts dating back to the last interglacial period. The ability to accurately determine ancient desert wetness would provide an important "missing link" in the theory of major climate changes on a global scale. However, by the beginning of the twenty-first century, scientific evidence remained inconclusive, at best, that rock varnish contains an accurate record of desert wetness.

Rock varnish is seldom more than 200 microns thick and samples are collected from tiny varnish-filled dimples, or microbasins, no more than.04 in (1 mm) in diameter in the surface of rocks. Thin slices or cores are cut from different areas of selected dimples for research purposes. Wallace S. Broecker and Tanzhuo Liu of the Lamont-Doherty Earth Observatory, Columbia University, in their article in GSA Today, explain that: "Rather than being measured in centimeters of accumulation per millennium, the sedimentary record in varnish is measured in micrometers of accumulation per millennium."

Exactly how rock varnish, often called desert varnish, develops remains uncertain; however, it is composed of layers of clay and deposits rich in minerals that appear to be formed of wind-blown detritus—loose materials such as rock particles and organic debris created by disintegration and destruction. Many researchers hypothesize that rock varnish also may contain a type of manganese-fixing bacteria living on the rock surface that digests organic debris. Other hypothesis relating to its formation also exist.

Dating Rock Varnish

Regardless of mechanism of development, rock varnish forms in layers and, by studying these layers, researchers have attempted to determine the periods during which they developed. In one study, Steven L. Reneau and colleagues of the Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico, collected rock varnish samples from the Cima volcanic field in the Mojave Desert, California. This location was chosen because studies have determined age estimates for a series of lava flows there, and this dating permitted varnish sampling from substrates of similar composition and under similar climatic conditions but of differing ages. In their article in the American Journal of Science, these researchers explained that varnish stratigraphy primarily is composed of concentrations of manganese (Mn), iron (Fe), silicon (Si), and aluminum (Al). In earlier work by other researchers, stratigraphy was assessed under the assumption that ancient environmental changes affected the Mn:Fe ratio. However, Reneau and colleagues found that the Mn:Fe ratios in upper layers of their samples appeared also to be affected by factors other than climate changes. Samples taken from the same location, and therefore presumably accreted (formed) under the same environmental conditions, varied widely in major elemental composition. The study therefore cautioned against correlating variations in stratigraphic composition to environmental and climatic changes.

The thickness of rock varnish was also assumed to indicate its relative age, as it tends to become thicker over time. In research published in Quatenary Research Reneau investigated samples collected from geomorphic surfaces of different ages in the Soda Mountains of the Mojave Desert. He discovered that varnish accretion on surfaces of similar ages and with similar characteristics could vary greatly over a distance as small as 1 km (approximately half a mile). This undermined the earlier assumption that rock varnish does not erode, but remains stable over the course of eons. He therefore recommended further research before varnish thickness can be correlated to ancient climatic conditions.

In their attempt to date rock varnish, Liu and Broecker studied its accumulation rate. They found a large variance between samples taken from the same site and concluded that "varnish thickness does not correlate with the age of the associated geomorphic feature." They also stated in their article in GSA Today : "Based on the examination of tens of thousands of varnished rocks in the field and thousands of thin sections in the lab, it appears to us that randomly sampled varnish surfaces rarely record reliably the full sequence of chemical events that transpired since the geomorphic surface was created."

It is not possible to date rock varnish radio-metrically (radiocarbon dating, or 14C). A collaborative investigation by W. Beck and seven other researchers from four different laboratories in the United States and Switzerland, published in Science, details the failure of this technique in attempts to date rock varnish. They refer to a study by Ronald Dorn, now at Arizona State University, who used this method. He found a mixture of type I (resembling coal) and type II (resembling pyrolyzed wood charcoal fragments) materials that made radiocarbon dating possible. However, type I materials had previously been dated at approximately 28,000 years old, while type II at only about 4,000 years. These huge age differences in the same minute specimens brought Dorn's research into question. A subsequent independent study by Liu, followed by the collaborative effort of Beck and colleagues, tested samples from the same rock fragments studied by Dorn and found no such materials. Dorn's research and dating technique fell into serious disrepute, as has another of his dating techniques, the cation-ratio method proposed by him in 1983.

The inability to date rock varnish directly makes it virtually impossible to determine when its various layers formed. Therefore, although many researchers think that rock varnish record periods of desert wetness, determining when those periods occurred and correlating samples from different parts of the world to determine climatic events on a global scale is not possible. As Broecker and Liu concluded, rock varnish must be virtually precluded as a relative age indicator in geological and anthropological research.

Why Study Rock Varnish?

Scientists believe—but cannot confirm—that certain areas that are now deserts were once much wetter, and some tropical areas much drier, during the glacial periods. Approximately 16,000 years BP (before the present), Earth's climate warmed and created climatic conditions much as we know them today. This warming trend, called the interglacial period, brought an end to the preceding glacial period (ice age) and was followed about 12,000 years BP by another glacial period. During the interglacial period, evidence suggests that Lake Victoria in Africa, dry during the last glacial period, became full; and that Lake Lahontan in the Great Basin of the United States suddenly shrank to ten times smaller than its original size. Scientists are attempting to find solid evidence that climate changes causing these phenomena occurred on a global scale and—because well-dated and detailed records of climate changes in deserts are difficult to find—many look toward the mini-universe of rock varnish for answers.

Flaws in Using Rock Varnish to Assess Ancient Desert Wetness

Reneau believes that, although there are many reasons to suspect that stratigraphic variations in rock varnish chemistry should be, at least in part, related to climatic variations, evidence from his research into this area was inconclusive. Also, while Broecker and Liu firmly think rock varnish has the potential to confirm the theory of global climate change, they explain why available evidence is flawed:

First, scientists hypothesize that the higher the Mn content in varnish layers, the greater the wetness during the period of that particular layer's development. They admit, however, that this hypothesis remains unproven.

Second, precipitation may not be the only ingredient in the mix that causes rock-varnish development. While rainfall is the primary contributor, dust, dew, and aerosols also appear to be influencing factors.

Third, randomly sampled varnish seldom records reliably the full sequence of chemical events that have occurred from the time the surface on which it grows was created.

Fourth, as Reneau discussed, random samples are often adulterated by erosion, peeling, and "solution" events. During the nineteenth and twentieth centuries, a plethora of substances produced by humans have been released into the atmosphere that may change the chemical content and growth rate of rock varnish. For example, while aerosols are suspected to enhance varnish growth, acids threaten to dissolve accumulated varnish. Lack of careful selection of samples, therefore, may yield inaccurate or misleading results.

Fifth, methods of dating rock varnish have proven, at best, unreliable.

Concluding their article, Broecker and Liu ask the question: "Will rock varnish ever become a widely applied proxy for desert wetness?" They answer no—at least, they suspect, for the first decade of the twenty-first century, because too many unanswered questions remain pertaining to its formation and how its chemical composition relates to local environmental changes.

—MARIE L. THOMPSON

KEY TERMS

ACCRETION:

Process of growth or enlargement by gradual build-up.

CATION:

A positively charged ion, an atom with one more proton in the nucleus than electrons in the region around the nucleus.

DETRITUS:

Loose, organic particles that result directly from disintegration.

GEOMORPHIC:

Of or relating to the form of the earth or a celestial body, or its solid surface features.

ISOTOPE:

Any of several atoms of an element having different atomic weights as a result of having different numbers of neutrons in the nucleus; radioisotopes are radioactive isotopes.

NUCLIDE:

A particular isotope with a specific number of neutrons; identified with the atomic weight as a superscript; example ²⁶Al for aluminum, element 13 which has 13 protons in the nucleus as all aluminum atoms do, but also 13 neutrons which not all aluminum atoms have.

PALEO:

Prefix meaning ancient.

PYROLYZED:

A substance in which chemical change occurs due to the action of heat.

STRATIGRAPHY:

Geology that deals with the arrangement of strata (layers).

SUBSTRATE:

Base on which an organism lives; a substance acted upon.

THE MYSTERIES OF PETROGLYPHS

Just about everything related to petroglyphs is a mystery: Are they symbols representing individual accomplishments, or the beliefs of the carver? Or are they merely idle doodles amounting to nothing more than ancient graffiti?

Carvings are found pecked and chiseled into the varnish coating of desert rocks, on the walls of cliffs, and in caves around the world, wherever ancient people lived. Many petroglyphs have been found in the American Southwest. More than 7,000 have been cataloged in Utah alone. Many of the petroglyphs in the Southwest are thought to have been created by the people of three distinct, though related, cultures which emerged between 100 and 500 a.d.: the Hohokam, the Mogollon, and the Anasazi. These ancient people were thought to have disappeared around 1500 a.d., but it is now believed they are the ancestors of present-day Pueblo Indians.

Rock carvings fall into three general classifications. Any type of human figure is known as anthropomorphic, animal figures are called zoomorphics, and all the rest are described as geometric. Carvings of hand-prints are common in the Southwest. Are these doodles, or do they stake a claim to a territory? Scenes of hunters pursuing animals are great finds. If the hunter has a spear, the petroglyphs were probably drawn prior to 200 a.d., since in Arizona bows used for hunting have been found around that time. But dating petroglyphs is uncertain at best and depends greatly on where the drawings are found.

Most carvings of zigzags, spirals, and dots are a mystery, although some appear to have celestial meanings. The Anasazi positioned large stone slabs so sunlight would fall on spiral-shaped petroglyphs. Could this be an ancient calendar? Archeoastronomers have found rock carvings that seem to indicate solstices, and some that seem to record planetary movements. The most unusual theory suggests that some petroglyphs are pictures of extraterrestrial visitors.

—M. C. Nagel

Science in Dispute