The Earth's Moon. The Earth's Moon FAK II NW

Transcription

1 S D I Sprachen & Dolmetscher Institut München The Earth's Moon FAK II NW The Earth's Moon The Moon is the nearest body to us in the Solar System, and as a consequence of the Apollo missions is the only extra-terrestrial object that has yet been explored directly by humans. As a consequence of that exploration by both manned and unmanned spacecraft, we now know a great deal about our nearest celestial neighbor.

2 Lunar Tides Consider a water molecule in the ocean. It is attracted gravitationally by the Earth, but it also experiences a much smaller gravitational attraction from the Moon. (Much smaller because the Moon is much further away and much less massive than the Earth). But this gravitational attraction of the Moon is not limited to the water molecules; in fact, the Moon exerts a gravitational force on every object on and in the Earth. Tides occur because the Earth is a body of finite extent and these forces are not uniform: some parts of the Earth are closer to the Moon than other parts, and since the gravitational force drops off as the inverse square distance, those parts experience a larger gravitational pull from the Moon than parts that are further away. Differential Forces We say that differential forces act on the body (the Earth in this example). The effect of differential forces on a body is to distort the body. The body of the Earth is rather rigid, so such distortion effects are small. The fluid in the Earth's oceans, however, is much more easily deformed and this leads to significant tidal effects.

3 A Simple Tidal Model We may illustrate the basic idea with a simple model of a planet completely covered by an ocean of uniform depth, with negligible friction between the ocean and the underlying planet, as illustrated in the figure below. The gravitational attraction of the Moon produces two tidal bulges on opposite sides of the Earth. A Simple Tidal Model Without getting too much into the technical details, there are two bulges because of the differential gravitational forces. The liquid at point A is closer to the Moon and experiences a larger gravitational force than the Earth at point B or the ocean at point C. Because it experiences a larger attraction, it is pulled away from the Earth, toward the Moon, thus producing the bulge on the right side. Loosely, we may think of the bulge on the left side as arising because the Earth is pulled away from the water on that side because the gravitational force exerted by the Moon at point B is larger than that exerted at point C. Then, as the Earth rotates under these bulges, a given point on the surface will experience two high and two low tides for each rotation of the planet.

4 Spring Tides and Neap Tides A complication of a realistic model is that not only the Moon, but other objects in the Solar System, influence the Earth's tides. Their tidal forces are negligible on Earth, but the differential gravitational force of the Sun does influence our tides to some degree. The effect of the Sun on terrestrial tides is less than half that of the Moon. Spring Tides and Neap Tides

5 Spring Tides and Neap Tides For example, particularly large tides are experienced in the Earth's oceans when the Sun and the Moon are lined up with the Earth at new and full phases of the Moon. These are called spring tides the name is not associated with the season of Spring The amount of enhancement in Earth's tides is about the same whether the Sun and Moon are lined up on opposite sides of the Earth (full lunar phase) or on the same side (new lunar phase). Conversely, when the Moon is at first quarter or last quarter phase (meaning that it is located at right angles to the Earth- Sun line), the Sun and Moon interfere with each other in producing tidal bulges and tides are generally weaker. these are called neap tides Surface Properties of the Moon The surface of the Moon has two hemispheres with rather asymmetric properties. As a consequence, the nature of the lunar surface that we can see from the Earth is substantially different from the surface that is always hidden from the Earth.

6 The Near Side The face of the Moon turned toward us is termed the near side. It is divided into light areas called the Lunar Highlands and darker areas called Maria (literally, "seas"; the singular is Mare). The Maria are lower in altitude than the Highlands, but there is no water on the Moon so they are not literally seas. The dark material filling the Maria is actually dark, solidified lava from earlier periods of Lunar volcanism. Both the Maria and the Highlands exhibit large craters that are the result of meteor impacts. There are many more such impact craters in the Highlands. The Far Side The side of the Moon unseen from the Earth is called the far side. One of the discoveries of the first Lunar orbiters is that the far side has a very different appearance than the near side. In particular, there are almost no Maria on the far side, as illustrated in the image shown to the left of a portion of the far side surface. In this figure a number of meteor impact craters are visible.

7 Cratering Density The amount of cratering is usually an indication of the age of a geological surface: The more craters, the older the surface, because if the surface is young, there hasn't been time for many craters to form. Thus, the Earth has a relatively young surface because it has few craters. This is because the Earth is geologically active, with plate tectonics and erosion having obliterated most craters from an earlier epoch. In contrast the surface of the Moon is much older, with much more cratering. Further, different parts of the surface of the Moon exhibit different amounts of cratering and therefore are of different ages: the Maria are younger than the highlands, because they have fewer craters. Cratering Density The oldest surfaces in the Solar System are characterized by maximum cratering density. This means that one cannot increase the density of craters because there are so many craters that, on average, any new crater that is formed by a meteor impact will obliterate a previous crater, leaving the total number unchanged. Some regions of the moon exhibit near maximum cratering density, indicating that they are very old.

8 Age of Lunar Material The abundance of radioactive elements in rock samples can be used to tell the age of the rock in a process called radioactive dating. The findings: Samples from Mare Imbrium and the Ocean of Storms brought back by Apollo 11 and Apollo 12 are about 3.5 billion years old, which is comparable to the oldest rocks found on the surface of the Earth. The ejecta blanket from the Imbrium Basin (which was formed by a gigantic meteor impact) was returned by Apollo 14 and found to be about 3.9 billion years old. Lunar Highlands rocks returned by Apollo 16 are about 4 billion years old. The oldest Lunar rock found was located by Apollo 17 and appears to be about 4.5 billion years old. Age of Lunar Material Thus, the oldest material from the surface of the Moon is almost as old as we believe the Solar System to be. This is more than a billion years older than the oldest Earth rocks that have been found. Thus, the material brought back from the Moon by the Apollo missions gives us a window on the very early history of our Solar System that would be difficult the find on the Earth, which is geologically active and has consequently has obliterated its early geological history.

9 Lunar Phases The Moon appears to go through a complete set of phases as viewed from Earth because of its motion around the Earth. Perigee and Apogee The greatest separation between the Earth and Moon on its orbit is called apogee and the smallest separation is called perigee.

10 Solar Eclipses One consequence of the Moon's orbit about the Earth is that the Moon can shadow the Sun's light as viewed from the Earth, or the Moon can pass through the shadow cast by the Earth. The former is called a solar eclipse and the latter is called a lunar eclipse. The common perception that eclipses are infrequent is because the observation of a total eclipse from a given point on the surface of the Earth is not a common occurrence. Geometry of Solar Eclipses The geometry associated with solar eclipses is illustrated in the following figure. The shadow cast by the Moon can be divided into the completely shadowed umbra and the partially shadowed penumbra.

11 Types of Solar Eclipses Three general classes of solar eclipses can be defined: Total solar eclipses occur when the umbra of the Moon's shadow touches a region on the surface of the Earth. Partial solar eclipses occur when the penumbra of the Moon's shadow passes over a region on the Earth's surface. Annular solar eclipses occur when a region on the Earth's surface is in line with the umbra, but the distances are such that the tip of the umbra does not reach the Earth's surface. Appearance of a Total Solar Eclipse If you are in the path of totality the eclipse begins with a partial phase in which the Moon gradually covers more and more of the Sun. As totality approaches the sky becomes dark and a twilight begins to descend. In the final instants before totality light shining through valleys in the Moon's surface gives the impression of beads on the periphery of the Moon (a phenomenon called Bailey's Beads). The last flash of light from the surface of the Sun as it disappears from view behind the Moon gives the appearance of a diamond ring and is called, appropriately, the diamond ring effect.

12 Totality of a Solar Eclipse Lunar Eclipses As already noted, the Earth casts a shadow that the Moon can pass through. When this happens we say that a lunar eclipse occurs. Lunar eclipses can be partial or total, depending on whether the light of the Sun is partially or completely blocked from reaching the Moon. The figure above illustrates a total lunar eclipse with the Moon lying in the umbra of the Earth's shadow.

13 S D I Sprachen & Dolmetscher Institut München Questions? The End