Moon Impacts (general)
Moon impacts refer to the collisions between meteors and the lunar surface, resulting in the formation of craters. Since the dawn of the solar system, the moon has accumulated evidence of these impacts, boasting over thirty thousand craters, while Earth has significantly fewer due to its dynamic atmosphere and geological activity that erodes such features. Craters can vary in size and shape, classified into types such as simple craters, complex craters, impact basins, and degraded craters, with their characteristics influenced by the size and speed of the impacting object.
The moon's surface has remained largely unchanged for billions of years, allowing scientists to study these craters to understand historical impacts better. Unlike Earth, where the vast majority of the surface is relatively young, the moon's craters provide insight into early solar system history. Research efforts, such as those conducted by NASA, monitor both natural meteor impacts on the moon and controlled experiments, like the deliberate crash of spacecraft, to gain a deeper understanding of impact processes. Observations of flashes from these impacts can even be made from Earth, enhancing public interest and scientific inquiry into lunar geology and the broader dynamics of our solar system.
Moon Impacts (general)
FIELDS OF STUDY: Astronomy; Observational Astronomy; Sub-planet Astronomy
ABSTRACT: Moon impacts are the result of meteors striking the moon’s surface. Impacts generally create flashes of light, some of which are visible from Earth, and leave behind craters. Scientists study moon impacts because they provide important information about the age of the moon and the types and sizes of meteors near Earth.
Craters
Since the beginning of the solar system, about 4.6 billion years ago, every planet and orbiting body in the solar system, including Earth, has been struck countless times by meteors and the debris of comets. Wind, precipitation, surface water, and plant growth have obscured the effects of these impacts on Earth. However, the moon has retained the evidence of billions of years of space collisions in the form of more than thirty thousand craters.
A crater is formed by the high-speed impact of a smaller space object into a larger one. Although meteors are rarely spherical, craters are nearly always roughly circular. This is because shock waves cause the displaced matter to fly out equally in all directions from the point of impact. The force of the impact is so great that the impactor is often shattered and sometimes even vaporized. The size of the crater is determined by the impactor’s speed and trajectory before the collision.
Craters have several parts. The floor, or bottom, is generally below the surface level. A central peak can form in the center of a large crater when a large amount of displaced matter causes the crater’s edges to collapse back in on itself. The walls, or the sides of the crater bowl, can be steep and sometimes irregular as gravity pulls on the loose material. The rim, or top edge of the crater, is usually pushed higher than the surrounding surface by the impact. Ejecta is the rocky debris thrown clear of the crater. Rays are brighter streaks of ejecta that can sometimes be tossed far from the impact.
Moon Impact Crater Types
Scientists classify impact craters into several groups. Categories include simple craters, complex craters, impact basins, multi-ring basins, irregular craters, and degraded craters. In most cases, the size and shape of the crater is affected by the size, mass, and speed of the impactor. An object that is larger or moving faster creates a bigger crater. Sometimes craters are twenty times the width of the impactor.
Simple craters are small with relatively smooth walls. They resemble round bowls. Complex craters are bigger and have peaks, terraces or multiple layers to the floor, and several rings. The large, dark areas of the moon that are visible from Earth are impact basins that are at least 298 kilometers (about 185 miles) across and at least 11 kilometers (7 miles) deep. Multi-ring basins have a number of mountains circling a large impact basin. Irregular craters result when several meteors hit at the same time, creating multiple impact craters. Meteors with a low trajectory sometimes cause irregular craters. Degraded craters are those that have been affected by weathering, additional impacts, or the sliding of material due to gravity.
History of Lunar Craters and Earth Craters
The moon has more than thirty thousand impact craters, while Earth has fewer than two hundred. More than 99 percent of the lunar surface is three billion years old or older. About 80 percent of Earth’s surface is less than two hundred million years old. The moon’s surface is much older than Earth’s because the moon has no water, atmosphere, or tectonic activity, such as earthquakes and volcanoes. These forces have eroded Earth’s surface. Thus, many of Earth’s craters have worn away over time.
Unlike Earth, the moon’s surface has not been worn away by erosion. This is why its surface shows many more impact craters than Earth’s surface. Larger lunar craters were left by larger impactors early in the moon’s history. As time went on, the impactors became smaller, and the craters they left became smaller as well.
Studying Moon Impacts
The larger bodies in the solar system have settled into consistent orbits, so the larger collisions that pocked the moon and planets have become less frequent. Meteors approach Earth on a daily basis, but they are generally small and burn up in the atmosphere before reaching the surface. Scientists who want to study meteors turn to the moon. The moon lacks an atmosphere to deflect or destroy meteors before impact.
The National Aeronautics and Space Administration (NASA) has several efforts in place to monitor meteors, which are known as near-Earth objects (NEOs) when they are close to Earth. NASA also tracks and observes meteors that impact the moon. In April 2014, NASA allowed its Lunar Atmosphere and Dust Environment Explorer (LADEE) satellite to crash on the moon’s surface after it had completed its mission. LADEE crashed at a relatively low speed of 1,699 meters per second (3,800 miles per hour). It created a crater about ten feet wide. In October 2014 another NASA craft, the Lunar Reconnaissance Orbiter (LRO), was able to identify the crater among all the existing craters on the moon. Studying the results of this planned crash helps scientists better understand the craters that occur naturally on the moon and elsewhere.
The flashes from many moon impacts are visible from Earth with a telescope. In February 2014, a flash caused by a meteor the size of a small car was even visible without any special equipment. Observers also saw a flash during a lunar eclipse in January 2019 when a meterorite crashed into the moon's surface at 38,000 miles (61,000 kilometers) per hour.
Principal Terms
- crater: a bowl-shaped depression on the surface of a planet, moon, or other astronomical body, usually caused by either volcanic activity or a high-speed impact with another, smaller body.
- trajectory: the curved path followed by an object as it moves through space.
Bibliography
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