by Carl Sagan
Impact craters are not restricted to the Moon. We find them throughout the inner solar system—from Mercury, closest to the Sun, to cloud-covered Venus to Mars and its tiny moons, Phobos and Deimos. These are the terrestrial planets, our family of worlds, the planets more or less like the Earth. They have solid surfaces, interiors made of rock and iron, and atmospheres ranging from near-vacuum to pressures ninety times higher than the Earth’s. They huddle around the Sun, the source of light and heat, like campers around a fire. The planets are all about 4.6 billion years old. Like the Moon, they all bear witness to an age of impact catastrophism in the early history of the solar system.
As we move out past Mars we enter a very different regime—the realm of Jupiter and the other giant or jovian planets. These are great worlds, composed largely of hydrogen and helium, with smaller amounts of hydrogen-rich gases such as methane, ammonia and water. We do not see solid surfaces here, only the atmosphere and the multicolored clouds. These are serious planets, not fragmentary worldlets like the Earth. A thousand Earths could fit inside Jupiter. If a comet or an asteroid dropped into the atmosphere of Jupiter, we would not expect a visible crater, only a momentary break in the clouds. Nevertheless, we know there has been a many-billion-year history of collisions in the outer solar system as well—because Jupiter has a great system of more than a dozen moons, five of which were examined close up by the Voyager spacecraft. Here again we find evidence of past catastrophes. When the solar system is all explored, we will probably have evidence for impact catastrophism on all nine worlds, from Mercury to Pluto, and on all the smaller moons, comets and asteroids.
There are about 10,000 craters on the near side of the Moon, visible to telescopes on Earth. Most of them are in the ancient lunar highlands and date from the time of the final accretion of the Moon from interplanetary debris. There are about a thousand craters larger than a kilometer across in the maria (Latin for “seas”), the lowland regions that were flooded, perhaps by lava, shortly after the formation of the Moon, covering over the pre-existing craters. Thus, very roughly, craters on the Moon should be formed today at the rate of about 109 years/104 craters, = 105 years/crater, a hundred thousand years between cratering events. Since there may have been more interplanetary debris a few billion years ago than there is today, we might have to wait even longer than a hundred thousand years to see a crater form on the Moon. Because the Earth has a larger area than the Moon, we might have to wait something like ten thousand years between collisions that would make craters as big as a kilometer across on our planet. And since Meteor Crater, Arizona, an impact crater about a kilometer across, has been found to be twenty or thirty thousand years old, the observations on the Earth are in agreement with such crude calculations.
The actual impact of a small comet or asteroid with the Moon might make a momentary explosion sufficiently bright to be visible from the Earth. We can imagine our ancestors gazing idly up on some night a hundred thousand years ago and noting a strange cloud arising from the unilluminated part of the Moon, suddenly struck by the Sun’s rays. But we would not expect such an event to have happened in historical times. The odds against it must be something like a hundred to one. Nevertheless, there is an historical account which may in fact describe an impact on the Moon seen from Earth with the naked eye: On the evening of June 25, 1178, five British monks reported something extraordinary, which was later recorded in the chronicle of Gervase of Canterbury, generally considered a reliable reporter on the political and cultural events of his time, after he had interviewed the eyewitnesses who asserted, under oath, the truth of their story. The chronicle reads:
There was a bright New Moon, and as usual in that phase its horns were tilted towards the east. Suddenly, the upper horn split in two. From the midpoint of the division, a flaming torch sprang up, spewing out fire, hot coals, and sparks.
The astronomers Derral Mulholland and Odile Calame have calculated that a lunar impact would produce a dust cloud rising off the surface of the Moon with an appearance corresponding rather closely to the report of the Canterbury monks.
If such an impact were made only 800 years ago, the crater should still be visible. Erosion on the Moon is so inefficient, because of the absence of air and water, that even small craters a few billion years old are still comparatively well preserved. From the description recorded by Gervase, it is possible to pinpoint the sector of the Moon to which the observations refer. Impacts produce rays, linear trails of fine powder spewed out during the explosion. Such rays are associated with the very youngest craters on the Moon—for example, those named after Aristarchus and Copernicus and Kepler. But while the craters may withstand erosion on the Moon, the rays, being exceptionally thin, do not. As time goes on, even the arrival of micrometeorites—fine dust from space—stirs up and covers over the rays, and they gradually disappear. Thus rays are a signature of a recent impact.
The meteoriticist Jack Hartung has pointed out that a very recent, very fresh-looking small crater with a prominent ray system lies exactly in the region of the Moon referred to by the Canterbury monks. It is called Giordano Bruno after the sixteenth-century Roman Catholic scholar who held that there are an infinity of worlds and that many are inhabited. For this and other crimes he was burned at the stake in the year 1600.
Another line of evidence consistent with this interpretation has been provided by Calame and Mulholland. When an object impacts the Moon at high speed, it sets the Moon slightly wobbling. Eventually the vibrations die down but not in so short a period as eight hundred years. Such a quivering can be studied by laser reflection techniques. The Apollo astronauts emplaced in several locales on the Moon special mirrors called laser retroreflectors. When a laser beam from Earth strikes the mirror and bounces back, the round-trip travel time can be measured with remarkable precision. This time multiplied by the speed of light gives us the distance to the Moon at that moment to equally remarkable precision. Such measurements, performed over a period of years, reveal the Moon to be librating, or quivering with a period (about three years) and amplitude (about three meters), consistent with the idea that the crater Giordano Bruno was gouged out less than a thousand years ago.
All this evidence is inferential and indirect. The odds, as I have said, are against such an event happening in historical times. But the evidence is at least suggestive. As the Tunguska Event and Meteor Crater, Arizona, also remind us, not all impact catastrophes occurred in the early history of the solar system. But the fact that only a few of the lunar craters have extensive ray systems also reminds us that, even on the Moon, some erosion occurs.* By noting which craters overlap which and other signs of lunar stratigraphy, we can reconstruct the sequence of impact and flooding events of which the production of crater Bruno is perhaps the most recent example. On this page is an attempt to visualize the events that made the surface of the lunar hemisphere we see from Earth.
The Earth is very near the Moon. If the Moon is so severely cratered by impacts, how has the Earth avoided them? Why is Meteor Crater so rare? Do the comets and asteroids think it inadvisable to impact an inhabited planet? This is an unlikely forbearance. The only possible explanation is that impact craters are formed at very similar rates on both the Earth and the Moon, but that on the airless, waterless Moon they are preserved for immense periods of time, while on the Earth slow erosion wipes them out or fills them in. Running water, windblown sand and mountain-building are very slow processes. But over millions or billions of years, they are capable of utterly erasing even very large impact scars.
On the surface of any moon or planet, there will be external processes, such as impacts from space, and internal processes, such as earthquakes; there will be fast, catastrophic events, such as volcanic explosions, and processes of excruciating slowness, such as the pitting of a surface by tiny airborne sand grains. There is no general answer to the question of which processes dominate, the outside ones or the inside ones; the rare but violent events, or the common and inconspicuous occurrence
s. On the Moon, the outside, catastrophic events hold sway; on Earth, the inside, slow processes dominate. Mars is an intermediate case.
Between the orbits of Mars and Jupiter are countless asteroids, tiny terrestrial planets. The largest are a few hundred kilometers across. Many have oblong shapes and are tumbling through space. In some cases there seem to be two or more asteroids in tight mutual orbits. Collisions among the asteroids happen frequently, and occasionally a piece is chipped off and accidentally intercepts the Earth, falling to the ground as a meteorite. In the exhibits, on the shelves of our museums are the fragments of distant worlds. The asteroid belt is a great grinding mill, producing smaller and smaller pieces down to motes of dust. The bigger asteroidal pieces, along with the comets, are mainly responsible for the recent craters on planetary surfaces. The asteroid belt may be a place where a planet was once prevented from forming because of the gravitational tides of the giant nearby planet Jupiter; or it may be the shattered remains of a planet that blew itself up. This seems improbable because no scientist on Earth knows how a planet might blow itself up, which is probably just as well.
The rings of Saturn bear some resemblance to the asteroid belt: trillions of tiny icy moonlets orbiting the planet. They may represent debris prevented by the gravity of Saturn from accreting into a nearby moon, or they may be the remains of a moon that wandered too close and was torn apart by the gravitational tides. Alternatively, they may be the steady state equilibrium between material ejected from a moon of Saturn, such as Titan, and material falling into the atmosphere of the planet. Jupiter and Uranus also have ring systems, discovered only recently, and almost invisible from the Earth. Whether Neptune has a ring is a problem high on the agenda of planetary scientists. Rings may be a typical adornment of Jovian-type planets throughout the cosmos.
Major recent collisions from Saturn to Venus were alleged in a popular book, Worlds in Collision, published in 1950 by a psychiatrist named Immanuel Velikovsky. He proposed that an object of planetary mass, which he called a comet, was somehow generated in the Jupiter system. Some 3,500 years ago, it careered in toward the inner solar system and made repeated encounters with the Earth and Mars, having as incidental consequences the parting of the Red Sea, allowing Moses and the Israelites to escape from Pharaoh, and the stopping of the Earth from rotating on Joshua’s command. It also caused, he said, extensive vulcanism and floods.* Velikovsky imagined the comet, after a complicated game of interplanetary billiards, to settle down into a stable, nearly circular orbit, becoming the planet Venus—which he claimed never existed before then.
As I have discussed at some length elsewhere, these ideas are almost certainly wrong. Astronomers do not object to the idea of major collisions, only to major recent collisions. In any model of the solar system it is impossible to show the sizes of the planets on the same scale as their orbits, because the planets would then be almost too small to see. If the planets were really shown to scale, as grains of dust, we would easily note that the chance of collision of a particular comet with the Earth in a few thousand years is extraordinarily low. Moreover, Venus is a rocky and metallic, hydrogen-poor planet, whereas Jupiter—where Velikovsky supposed it comes from—is made almost entirely of hydrogen. There are no energy sources for comets or planets to be ejected by Jupiter. If one passed by the Earth, it could not “stop” the Earth’s rotation, much less start it up again at twenty-four hours a day. No geological evidence supports the idea of an unusual frequency of vulcanism or floods 3,500 years ago. There are Mesopotamian inscriptions referring to Venus that predate the time when Velikovsky says Venus changed from a comet into a planet.† It is very unlikely that an object in such a highly elliptical orbit could be rapidly moved into the nearly perfectly circular orbit of present-day Venus. And so on.
Many hypotheses proposed by scientists as well as by non-scientists turn out to be wrong. But science is a self-correcting enterprise. To be accepted, all new ideas must survive rigorous standards of evidence. The worst aspect of the Velikovsky affair is not that his hypotheses were wrong or in contradiction to firmly established facts, but that some who called themselves scientists attempted to suppress Velikovsky’s work. Science is generated by and devoted to free inquiry: the idea that any hypothesis, no matter how strange, deserves to be considered on its merits. The suppression of uncomfortable ideas may be common in religion and politics, but it is not the path to knowledge; it has no place in the endeavor of science. We do not know in advance who will discover fundamental new insights.
Venus has almost the same mass,* size, and density as the Earth. As the nearest planet, it has for centuries been thought of as the Earth’s sister. What is our sister planet really like? Might it be a balmy, summer planet, a little warmer than the Earth because it is a little closer to the Sun? Does it have impact craters, or have they all eroded away? Are there volcanoes? Mountains? Oceans? Life?
The first person to look at Venus through the telescope was Galileo in 1609. He saw an absolutely featureless disc. Galileo noted that it went through phases, like the Moon, from a thin crescent to a full disc, and for the same reason: we are sometimes looking mostly at the night side of Venus and sometimes mostly at the day side, a finding that incidentally reinforced the view that the Earth went around the Sun and not vice versa. As optical telescopes became larger and their resolution (or ability to discriminate fine detail) improved, they were systematically turned toward Venus. But they did no better than Galileo’s. Venus was evidently covered by a dense layer of obscuring cloud. When we look at the planet in the morning or evening skies, we are seeing sunlight reflected off the clouds of Venus. But for centuries after their discovery, the composition of those clouds remained entirely unknown.
The absence of anything to see on Venus led some scientists to the curious conclusion that the surface was a swamp, like the Earth in the Carboniferous Period. The argument—if we can dignify it by such a word—went something like this:
“I can’t see a thing on Venus.”
“Why not?”
“Because it’s totally covered with clouds.”
“What are clouds made of?”
“Water, of course.”
“Then why are the clouds of Venus thicker than the clouds on Earth?”
“Because there’s more water there.”
“But if there is more water in the clouds, there must be more water on the surface. What kind of surfaces are very wet?”
“Swamps.”
And if there are swamps, why not cyacads and dragonflies and perhaps even dinosaurs on Venus? Observation: There was absolutely nothing to see on Venus. Conclusion: It must be covered with life. The featureless clouds of Venus reflected our own predispositions. We are alive, and we resonate with the idea of life elsewhere. But only careful accumulation and assessment of the evidence can tell us whether a given world is inhabited. Venus turns out not to oblige our predispositions.
The first real clue to the nature of Venus came from work with a prism made of glass or a flat surface, called a diffraction grating, covered with fine, regularly spaced, ruled lines. When an intense beam of ordinary white light passes through a narrow slit and then through a prism or grating, it is spread into a rainbow of colors called a spectrum. The spectrum runs from high frequencies* of visible light to low ones—violet, blue, green, yellow, orange and red. Since we see these colors, it is called the spectrum of visible light. But there is far more light than the small segment of the spectrum we can see. At higher frequencies, beyond the violet, is a part of the spectrum called the ultraviolet: a perfectly real kind of light, carrying death to the microbes. It is invisible to us, but readily detectable by bumblebees and photoelectric cells. There is much more to the world than we can see. Beyond the ultraviolet is the X-ray part of the spectrum, and beyond the X-rays are the gamma rays. At lower frequencies, on the other side of red, is the infrared part of the spectrum. It was first discovered by placing a sensitive thermometer in what to our eyes is the dark beyond the re
d. The temperature rose. There was light falling on the thermometer even though it was invisible to our eyes. Rattlesnakes and doped semiconductors detect infrared radiation perfectly well. Beyond the infrared is the vast spectral region of the radio waves. From gamma rays to radio waves, all are equally respectable brands of light. All are useful in astronomy. But because of the limitations of our eyes, we have a prejudice, a bias, toward that tiny rainbow band we call the spectrum of visible light.
In 1844, the philosopher Auguste Comte was searching for an example of a sort of knowledge that would be always hidden. He chose the composition of distant stars and planets. We would never physically visit them, he thought, and with no sample in hand it seemed we would forever be denied knowledge of their composition. But only three years after Comte’s death, it was discovered that a spectrum can be used to determine the chemistry of distant objects. Different molecules and chemical elements absorb different frequencies or colors of light, sometimes in the visible and sometimes elsewhere in the spectrum. In the spectrum of a planetary atmosphere, a single dark line represents an image of the slit in which light is missing, the absorption of sunlight during its brief passage through the air of another world. Each such line is made by a particular kind of molecule or atom. Every substance has its characteristic spectral signature. The gases on Venus can be identified from the Earth, 60 million kilometers away. We can divine the composition of the Sun (in which helium, named after the Greek sun god Helios, was first found); of magnetic A stars rich in europium; of distant galaxies analyzed through the collective light of a hundred billion constituent stars. Astronomical spectroscopy is an almost magical technique. It amazes me still. Auguste Comte picked a particularly unfortunate example.