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From CreationWiki, the encyclopedia of creation science

Lunar impact craters are craters on any celestial body that result from the impact of a meteoroid or comet. Their origin and method of formation have provoked one of the most important controversies between uniformitarian and creationistic astronomy, and also among evolutionism, young earth creationism, and old earth creationism.

Impact craters are known to exist on nearly all celestial bodies in the solar system, except for the Sun and the gas giants. The number of craters on any given body is known to vary at least somewhat with the presence or absence of an atmosphere and with the geologic activity of the body in question. Mercury is the most heavily cratered of all the terrestrial planets; Venus and Mars have relatively few craters. That Venus and Mars have atmospheres while Mercury has none might possibly explain the uneven distribution of craters, but in fact Mars has areas of heavy cratering among nearly crater-free areas.^[1]

Earth has very few well-preserved craters, the largest being the Arizona Meteor Crater. But Earth also has multiple features, called astroblemes, that most geologist accept as crater remnants.^[2] These objects are found throughout the geologic column, even in Precambrian layers.

Contents 1 Characteristics 1.1 Distribution 1.2 Geology and Structure 2 Uniformitarian model 2.1 Problems 3 Young solar system models 3.1 The interstellar asteroid cloud model 3.2 The exploded planet model 3.3 The directed giant comet model 3.4 The hydroplate model 4 References 5 See Also

Characteristics

Distribution

Virtually every moon is cratered to some degree. Even Titan, the one moon with a thick atmosphere, has craters. But some moons have far fewer craters than others. This has led most astronomers to describe certain moons, such as Io, as "geologically young" because they have relatively few craters.

The Moon's cratering is remarkable for another reason: its distribution is markedly uneven. The moon has two distinct types of land area that are relevant to the discussion of impact craters. The highlands show the heaviest distribution of craters. The lowlands, however, are covered with basalt and other igneous rocks, and most planetary scientists assume that these lowlands, called maria (literally, "seas"), are in fact seas of lava. Even more remarkably, however, the far side of the Moon (the side away from Earth) consists almost entirely of highlands, with only one lunar mare, Mare Moscoviensis, on that side. All the other maria, including the three largest (Oceanus Procellarum, Mare Imbrium, and Mare Tranquillitatis), are on the near side or at least visible, at one time or another, from Earth. This uneven distribution has proved very difficult to explain. Some astronomers have assumed that the Earth somehow shielded the Moon's near side from most of the bombardments. But the semi-major axis of the Moon's orbit is 384,400 kilometers, while the diameter of the earth is about 13,000 kilometers. At such a distance, the earth would not have been able to produce much of a shield from a cloud of meteoroids bombarding the Moon from all sides.

Geology and Structure

The shape of craters varies with their size. Small craters with diameters of less than 6 miles (10 kilometers) have relatively simple bowl shapes. Slightly larger craters cannot maintain a bowl shape because the crater wall is too steep. Material falls inward from the wall to the floor. As a result, the walls become scalloped and the floor becomes flat. Still larger craters have terraced walls and central peaks. Terraces inside the rim descend like stairsteps to the floor. The same process that creates wall scalloping is responsible for terraces. The central peaks almost certainly form as did the central peaks of impact craters on Earth. Studies of the peaks on Earth show that they result from a deformation of the ground. The impact compresses the ground, which then rebounds, creating the peaks. Material in the central peaks of lunar craters may come from depths as great as 12 miles (19 kilometers).

Surrounding the craters is rough, mountainous material -- crushed and broken rocks that were ripped out of the crater cavity by shock pressure. This material, called the crater ejecta blanket, can extend about 60 miles (100 kilometers) from the crater. Farther out are patches of debris and, in many cases, irregular secondary craters, also known as secondaries. Those craters come in a range of shapes and sizes, and they are often clustered in groups or aligned in rows. Secondaries form when material thrown out of the primary (original) crater strikes the surface. This material consists of large blocks, clumps of loosely joined rocks, and fine sprays of ground-up rock. The material may travel thousands of miles or kilometers.^[3]

Crater rays are light, wispy deposits of powder that can extend thousands of miles or kilometers from the crater. Rays slowly vanish as micrometeoroid bombardment mixes the powder into the upper surface layer. Thus, craters that still have visible rays must be among the youngest craters on the moon.

Basins are craters that are 190 miles (300 kilometers) or more across. The smaller basins have only a single inner ring of peaks, but the larger ones typically have multiple rings. The rings are concentric -- that is, they all have the same center, like the rings of a dartboard. The spectacular, multiple-ringed basin called the Eastern Sea (Mare Orientale) is almost 600 miles (1,000 kilometers) across. Other basins can be more than 1,200 miles (2,000 kilometers) in diameter -- as large as the entire western United States. Basins occur equally on the near side and far side of Earth's moon. Most basins have little or no fill of basalt, particularly those on the far side. The difference in filling may be related to variations in the thickness of the crust. The far side has a thicker crust, so it is more difficult for molten rock to reach the surface there.

In the highlands, the overlying ejecta blankets of the basins make up most of the upper few miles or kilometers of material. Much of this material is a large, thick layer of shattered and crushed rock known as breccia (BREHCH ee uh). Scientists can learn about the original crust by studying tiny fragments of breccia.^[3]

Uniformitarian model

The uniformitarian model of the solar system assumes that all solid-surfaced bodies in the solar system suffered from meteoric bombardment, from random directions, at rates varying from a very heavy bombardment in the era of the solar system's formation to a very light bombardment continuing today.^[4] Uniformitarians assume that the heavy distribution of craters in the lunar highlands is the result of what they call the early heavy bombardment of the solar system. They further assume that the volcanic overflows that produced the maria in the lowlands resulted from very high-energy impacts that fractured the lunar crust. This, they say, was the late heavy bombardment, and the processes associated with it are supposed to have obliterated most of the craters that the early bombardment had left in those regions.^[4][1]

Problems

The problems with the uniformitarian cratering model are many. To begin with, it assumes uniform constants and weather/erosion trends. Recent research into crater impacts on the moon has led to a greater understanding of their formation and age, suggesting that space weathering "takes place very rapidly on the Moon," according to Bonnie Buratti of NASA's Jet Propulsion Laboratory.^[5] This in turn has led scientists to make older date estimations by using inaccurate assumptions on space weathering. Other scientists, such as Peter Brown of the University of Western Ontario, insist that there is no absolute criteria for the dating of craters.^[6]

But the worst problem with this model is the presence of ghost craters on the Moon. These are craters within the maria that show evidence of near-total filling by the basalt and other igneous rocks that form the surfaces of the maria. In the deep time scale required by uniformitarianism, the ghost craters are the result of impacts that occurred as much as half a gigayear before the crust-fracturing impacts that caused the lava flows.^[4] This is not a reasonable assumption, because any impact strong enough to crack the crust should have obliterated any craters previously extant. Danny Faulkner has concluded from this that the impacts that formed the ghost craters, and those that let loose the lava flows that formed the maria, occurred within days of one another, not the millions of years the uniformitarianism demands.^[7]

Young solar system models

Three models for the meteoric bombardment that produced the impact craters and the maria on Earth's Moon are currently under discussion.

The interstellar asteroid cloud model

In this model, a large formation of interstellar comets and/or asteroids passed through the solar system, bombarding all bodies uniformly. No evidence of such large formations of high-momentum objects has ever been adduced.^[4]

The exploded planet model

Henrich von Olbers first proposed that the presently observed asteroid belt contains the remains of a planet occupying the Titius-Bode Law "slot" between Mars and Jupiter, a planet that somehow destroyed itself, leaving large quantities of objects. Some of these objects remained in the old orbit of that planet as the asteroid belt, and the rest participated in the bombardments. But planets do not explode spontaneously, and in any event very little of the debris would actually strike any celestial bodies, too little to produce the tremendous bombardments observed.^[4]

The directed giant comet model

Faulkner has lately proposed that two episodes of bombardment occurred, the one occurring incident to the fall of man and the other occurring incident to the global flood. The latter episode produced the astroblemes found on earth and also accounts for the markedly uneven distribution of low- and high-energy impacts on the Moon.^[1][4] The observed bombardment of the other planetary satellites, according to this model, came from the first bombardment episode and not the second.^[1]

Wayne Spencer disputes Faulkner on the matter of two bombardment episodes.^[8] Spencer points out that the impact that produced the South Pole/Aitken Basin^[9] cannot have been a light impact. (Neither could the impact that produced Mare Moscoviensis.) Furthermore, the Aitken Basin is technically on the far side of the Moon (as is Mare Moscoviensis). Spencer also faults Faulkner for assuming that the planets (other than Earth) and their moons all formed by rapid accretion on Day Four of Creation. Differentiated bodies can only form by melting and re-freezing, and Spencer questions whether this could have occurred in the short time between Creation and the Flood.^[8]

The hydroplate model

Main Article: Hydroplate theory

Strangely, the Hydroplate Theory of Walt Brown is largely ignored. This model, alone among catastrophic-bombardment models, suggests that comets, asteroids, and meteoroids all came from Earth and are the result of the Flood event, not its cause. This model would explain the marked differences between the Moon's far and near sides. If all the bombarding objects came from the earth, then the highest-energy impacts ought to have involved the Moon and ought to have involved its near side in preference to the far.^[10]

References

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See Also

• Young solar system evidence
• Nebula hypothesis