PSCI 1414 GENERAL ASTRONOMY LECTURE 12 THE MOON

PSCI 1414 GENERAL ASTRONOMY LECTURE 12: THE MOON ALEXANDER C. SPAHN

THE MOON The average distance from Earth to the Moon is just 384, 400 km (238, 900 mi). The diameter of the Moon is 3476 km (2160 mi), just 27% of Earth’s diameter.

THE MOON Although the Moon has a small mass (just 1. 23% of Earth’s mass), Newton’s third law tells us that the Moon exerts just as much gravitational force on Earth as Earth does on the Moon. Hence, it is not strictly correct to say that the Moon orbits Earth; rather, they both orbit around a point between their centers called the center of mass of the Earth-Moon system. Because Earth is so much more massive than the Moon, the center of mass is close to Earth’s center: Indeed, it actually lies within Earth’s interior.

THE MOON

THE MOON If you observe the Moon over many nights, you will see the Moon go through its phases as the dividing line between the illuminated and darkened portion of the Moon (the terminator) shifts. But you will always see the same pattern of dark and light areas, because the Moon keeps the same side turned toward Earth; you never see the far side (or back side) of the Moon. This is the result of the Moon’s synchronous rotation: It takes precisely the same amount of time for the Moon to rotate on its axis (one lunar “day”) as it does to complete one orbit around Earth.

THE MOON The Moon appears to wobble slightly over the course of an orbit. It seems to rock back and forth around its north-south axis and to nod up and down in a north-south direction. This apparent periodic wobbling, called libration, permits us to view 59% of our satellite’s surface.

THE MOON Clouds are never seen on the Moon since the Moon is too small a world to have an atmosphere. Its surface gravity is low, only about one-sixth as great as that of Earth, and thus any gas molecules can easily escape into space. The absence of an atmosphere means that the Moon can have no liquid water on its surface. Here on Earth, water molecules are kept in the liquid state by the pressure of the atmosphere pushing down on them.

THE MOON Since the Moon has no atmosphere to obscure its surface, you can get a clear view of several different types of lunar features with a small telescope or binoculars. These include craters, the dark-colored maria, and the light-colored lunar highlands or terrae.

THE MOON With an Earth-based telescope, some 30, 000 craters are visible, with diameters ranging from 1 km to several hundred km. Virtually all lunar craters, both large and small, are the result of the Moon’s having been bombarded by meteoroids and asteroids. For this reason, they are also called impact craters – a circular depression on a planet or satellite caused by the impact of a meteoroid.

THE MARIA The large dark areas on the lunar surface are called maria. The singular form of maria is mare (which means “sea” in Latin). The maria are the remains of huge lava flows. The maria get their distinctive appearance from the dark color of the solidified lava. On the whole, the maria have far fewer craters than the surrounding terrain. Craters are caused by meteoritic bombardment, so the maria have not been exposed to that bombardment for as long as the surrounding terrain.

THE MARIA Hence, the maria must be relatively young, and the lava flows that formed them must have occurred at a later stage in the Moon’s geologic history. Although they are much larger than craters, maria also tend to be circular in shape. This suggests that, like craters, these depressions in the lunar surface were also created by large impacts. It is thought that very large asteroids some tens of kilometers across struck the Moon, forming basins.

THE MARIA These depressions subsequently flooded with lava that flowed out from the Moon’s interior through cracks in the lunar crust. The solidified lava formed the maria that we see today. An impact large enough to create a mare basin must have thrust upward tremendous amounts of material around the impact site. This explains the mountain ranges that curve in circular arcs around mare basins. http: //www. whfreeman. com/Brain. Honey/Resource/6714/Sitebuilder. Uploads/animations/ 10030 -03. htm

THE LUNAR HIGHLANDS The light-colored terrain that surrounds the maria is called terrae, from the Latin for “lands. ” The maria on the Moon’s Earth-facing side are 2 to 5 km below the average lunar elevation, while the terrae extend to several kilometers above the average elevation. For this reason terrae are often called lunar highlands. Since the lunar highlands are more heavily cratered than the maria, we can conclude that the highlands are older.

THE LUNAR HIGHLANDS Remarkably, when the Moon’s far side was photographed for the first time by the Soviet. Luna 3 spacecraft in 1959, scientists were surprised to find almost no maria there. Lunar highlands predominate on the side of the Moon that faces away from Earth. The crust is likely thicker on the far side, so even the most massive impacts were unable to crack through the crust and let lava flood onto the surface.

THE MOON By correlating the ages of Moon rocks with the density of craters at the sites where the rocks were collected, geologists have found that the rate of impacts on the Moon has changed over the ages. The ancient, heavily cratered lunar highlands are evidence of an intense bombardment that dominated the Moon’s early history. For around half a billion years, rocky debris left over from the formation of the planets rained down upon the Moon’s young surface.

GEOLOGICAL ACTIVITY No evidence for plate tectonics can be found on the Moon. It appears that the Moon is a one -plate world: Its entire lithosphere is a single, solid plate. There are no rifts where plates are moving apart, no mountain ranges of the sort formed by collisions between plates, and no subduction zones. Geologically, the Moon is a nearly dead world. Without an atmosphere to cause erosion and without plate tectonics, the surface of the Moon changes only very slowly. That is why the Moon’s craters, many of which are now known to be billions of years old, remain visible today.

GEOLOGICAL ACTIVITY In 2010, evidence for some surprisingly recent geological activity on the Moon was found. The Moon has long cliffs called scarps that develop as the entire Moon cools and shrinks. A good analogy for the formation of these scarps is the wrinkling seen on a fruit’s skin as the fruit dries out and contracts.

GEOLOGICAL ACTIVITY It had been thought that the Moon cooled quickly after formation, and that all scarps were billions of years old. However, the craters disturbed by a scarp can indicate the scarp’s age and suggest some of these geologic features might be as young as a few hundred million years old. The Moon has probably shrunk about 600 feet in diameter since it first formed, and this new evidence suggests that it might still be shrinking today.

HUMAN EXPLORATION OF THE MOON Politics had much to do with the motivation for going to the Moon. But in fact, there were excellent scientific reasons to do so. Because Earth is a geologically active planet, all traces of Earth’s origins have been erased. By contrast, rocks on the geologically inactive surface of the Moon have been essentially undisturbed for billions of years. Samples of lunar rocks have helped answer many questions about the Moon and have shed light on the origin of Earth and the birth of the entire solar system.

HUMAN EXPLORATION OF THE MOON Lunar missions began in 1959, when the Soviet Union sent three small spacecraft—Lunas 1, 2, and 3—toward the Moon. American attempts to reach the Moon began in the early 1960 s with Project Ranger. The Ranger spacecraft transmitted close-up views of the lunar surface taken in the few minutes before it crashed onto the Moon. In 1966 and 1967, five Lunar Orbiter spacecraft were placed in orbit around the Moon and made a high-resolution photographic survey of 99. 5% of the lunar surface, helping NASA scientists choose suitable landing sites for the astronauts.

HUMAN EXPLORATION OF THE MOON The purpose of the Surveyor program was to land unmanned spacecraft on the Moon. Between June 1966 and January 1968, five Surveyors successfully touched down on the Moon, sending back pictures and data directly from the lunar surface. These missions demonstrated convincingly that the Moon’s surface was as solid as that of Earth and safe to land on.

HUMAN EXPLORATION OF THE MOON The first of six manned lunar landings took place on July 20, 1969, when the Apollo 11 lunar module Eagle set down in Mare Tranquillitatis (Sea of Tranquility). Astronauts Neil Armstrong and Edwin “Buzz” Aldrin were the first humans to set foot on the Moon. The first era of human exploration of the Moon culminated in December 1972, when Apollo 17 landed in a rugged region of the lunar highlands.

HUMAN EXPLORATION OF THE MOON Contemplation of a manned lunar base and the resources required to maintain it has added extra importance to the question, “Is there water on the moon? ” To help answer this question, in 1994 a small NASA spacecraft named Clementine spent more than two months observing the Moon from lunar orbit. Different types of materials on the lunar surface reflect different wavelengths, so analysis of Clementine images has revealed the composition of large areas of the Moon’s surface.

HUMAN EXPLORATION OF THE MOON One remarkable result of the Clementine mission was evidence for a patch of water-ice tens or hundreds of kilometers across at the Moon’s southern pole, in locations where sunlight never reaches and where temperatures are low enough to prevent ice from evaporating. Then, data from the Lunar Prospector spacecraft, which went into orbit around the Moon in 1998, reinforced the Clementine findings and indicate that even more water-ice might lie in deep craters at the Moon’s northern pole.

HUMAN EXPLORATION OF THE MOON Designed specifically for impact and analysis, the Lunar Crater Observation and Sensing Satellite (LCROSS) tried again to detect water directly. The impact target was suspected to contain water in a permanently shadowed region of a crater only 100 meters from the lunar south pole, and should have been cold enough to retain water-ice for billions of years. In 2009, a spent fuel canister weighing more than 2 tons was used as a man-made meteoroid to crash into the suspected ice. Traveling at about 8 times the speed of sound, the impact created a large plume of debris.

HUMAN EXPLORATION OF THE MOON Then, a few minutes later the probe itself traveled through the plume, analyzing the debris, before smashing into the surface. The results were spectacular and lunar water was finally confirmed! The characteristic absorption features due to H 2 O were found in the plume’s infrared spectrum. Ammonia and methane were also found, matching the abundances expected for comets, indicating that comets are the most likely source of the water.

STRUCTURE OF THE MOON The Moon landings gave scientists an unprecedented opportunity to explore an alien world. The Apollo spacecraft were therefore packed with an assortment of apparatus that the astronauts deployed around the landing sites. For example, all the missions carried magnetometers, which confirmed that the present-day Moon has no global magnetic field. However, careful magnetic measurements of lunar rocks returned by the Apollo astronauts indicated that the Moon did have a weak magnetic field when the rocks solidified billions of years ago. Hence, the Moon must originally have had a small, molten, iron-rich core.

STRUCTURE OF THE MOON The Apollo astronauts also set up four seismometers on the Moon’s surface, which made it possible for scientists to investigate the Moon’s interior using seismic waves. It was found that the Moon exhibits far less seismic activity than Earth. Roughly 3000 moonquakes were detected per year, whereas a similar seismometer on Earth would record hundreds of thousands of earthquakes per year. Furthermore, typical moonquakes are very weak. A major terrestrial earthquake is in the range from 6 to 8 on the Richter scale, while a major moonquake measures 0. 5 to 1. 5 on that scale and would go unnoticed by a person standing near the epicenter.

STRUCTURE OF THE MOON Analysis of moonquakes reveals that the Moon has a thick mantle surrounding an iron-rich core. The Moon also has a crust with an average thickness of about 60 km on the Earth-facing side and up to 100 km on the far side. For comparison, the thickness of Earth’s crust ranges from 5 km under the oceans to about 70 km under major mountain ranges on the continents.

STRUCTURE OF THE MOON More than 30 years after the original Apollo data revealed a molten core. Like Earth, the Moon’s iron-rich core has a solid inner core, surrounded by a liquid outer core. However, the liquid core is too small for the convection that is needed to generate a global magnetic field. Plate tectonics requires that there be fluid motion just below a planet’s surface, so it is not surprising that there is no evidence for plate tectonics on the lunar surface.

STRUCTURE OF THE MOON Earthquakes tend to occur at the boundaries between tectonic plates, where plates move past each other, collide, or undergo subduction. But there are no plate motions on the Moon. What, then, causes moonquakes? The answer turns out to be Earth’s tidal forces. Just as the tidal forces of the Sun and Moon deform Earth’s oceans and give rise to ocean tides, Earth’s tidal forces deform the solid body of the Moon. As a result, the body of the Moon flexes slightly, triggering moonquakes.

THE RECEDING MOON The Moon’s tidal forces elongate the oceans into a football shape. However, the long axis of this “football” does not point precisely at the Moon. Earth spins on its axis more rapidly than the Moon revolves around Earth, and this rapid rotation carries the tidal bulge about 10° ahead of a line connecting Earth and the Moon. This misaligned bulge produces a small but steady gravitational force that tugs the Moon forward, adding energy to the Moon’s orbit. Increasing the energy of an orbit increases its semimajor axis. Therefore, Earth’s tidal bulge tugs the Moon into an ever larger orbit.

THE RECEDING MOON Friction between the oceans and the body of Earth is gradually slowing Earth’s rotation. The length of Earth’s day is therefore increasing, by approximately 0. 002 seconds per century. In fact, 400 million years ago, the length of Earth’s day was only 21 hours!

THE MOON’S ORIGIN These observations mean that in the distant past, Earth was spinning faster than it is now and the Moon was much closer. Darwin theorized that the early Earth may have been spinning so fast that a large fraction of its mass tore away, and this fraction coalesced to form the Moon. This is called the fission theory of the Moon’s origin. Prior to the Apollo program, two other theories were in competition with the fission theory. The capture theory posits that the Moon was formed elsewhere in the solar system and then drawn into orbit about Earth by gravitational forces. By contrast, the co-creation theory proposes that Earth and the Moon were formed at the same time but separately.

THE MOON’S ORIGIN The present scientific consensus is that none of these three theories—fission, capture, and co -creation—is correct. Instead, the evidence points toward an idea proposed in 1975. In this collisional ejection theory, the proto-Earth was struck off-center by a Mars-sized object, and this collision ejected debris from which the Moon formed. The collisional ejection theory proposes that a Mars-sized protoplanet struck the proto-Earth about 4. 5 billion years ago.

THE MOON’S ORIGIN

THE MOON’S ORIGIN The collisional ejection theory also agrees with many properties of lunar rocks and of the Moon as a whole. For example, rock vaporized by the impact would have been depleted of volatile elements and water, leaving the Moon rocks we now know. If the off-center collision took place after chemical differentiation had occurred on Earth, so that our planet’s iron had sunk to its center, then relatively little iron would have been ejected to become part of the Moon. The lack of iron on Earth’s surface explains the Moon’s low density and the small size of its iron core.

FOR NEXT TIME… • Study! • Homework 12: Will be posted later today online • Exam 2: in 1 week on Wednesday March 2 nd (chapters 7 -10)