This is the online edition of In the Beginning: Compelling Evidence for Creation and the Flood
(7th Edition) by Dr. Walt Brown. The online version of the book is designed to be read online.
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Gravity: How and Why Most Things Move
Figure 143: Near and Far Sides of the Moon. The same side of the Moon always faces Earth during the Moon’s monthly orbit. Surprisingly, the near and far sides of the Moon are quite different. Almost all deep moonquakes are on the near side.10 The surface of the far side is rougher, while the near side has most of the Moon’s volcanic features, lava flows, dome complexes, and giant, multiringed basins. Lava flows (darker regions) have smoothed over many craters on the near side.11
Some have proposed that the Moon’s crust must be thinner on the near side, so lava can squirt out more easily on the near side than on the far side. However, no seismic, gravity, or heat flow measurements support that hypothesis, and the deeper lunar interior is cold and solid. The Moon’s density throughout is almost as uniform as that of a billiard ball,12 showing that little distinctive crust exists. Not only did large impacts form the giant basins, but much of their impact energy melted rock and generated lava flows. This is why the lava flows came after the craters formed. These impacts appear to have happened recently. [See “Hot Moon” on page 37.]
Contemporaries of Galileo misnamed these lava flows “maria” (MAHR-ee-uh), or “seas,” because these dark areas looked smooth and filled low-lying regions. Maria give the Moon its “man-in-the-moon” appearance. Of the Moon’s 31 giant basins, only 11 are on the far side.13 (See if you can flip 31 coins and get 11 or fewer tails. Not too likely. It happens only about 7% of the time.) Why should the near side have so many more giant impact features, almost all the maria, and almost all deep moonquakes?14 Opposite sides of Mars and Mercury are also different.15
If the impacts that produced these volcanic features occurred slowly from any or all directions other than Earth, both near and far sides would be equally hit. If the impacts occurred rapidly (within a few weeks), large impact features would not be concentrated on the near side unless the projectiles came from Earth. Evidently, the impactors came from Earth. Of course, large impacts would kick up millions of smaller rocks that would themselves create impacts or go into orbit around the Moon and later create other impacts—even on Earth. Today, both sides of the Moon are saturated with smaller craters. Can we test this conclusion that the large lunar impactors came from Earth?
Yes. The Moon as a whole has relatively few volatile elements, including nitrogen, hydrogen, and the noble gases. Surprisingly, lunar soil is rich in these elements, which implies their extralunar origin. Furthermore, the relative abundances of isotopes of these elements in lunar soils correspond not to the solar wind but to what is found on Earth.16 This further supports the conclusion that most impactor mass came from Earth. If large impactors came from Earth recently, most moonquakes should be on the near side, and they should still be occurring. They are.
Gravity pulls us toward Earth’s surface. This produces friction, a force affecting and slowing every movement we make. Since we were babies, we have assumed that everything behaves this way. Indeed, none of us could have taken our first steps without friction and the downward pull of gravity. Even liquids (such as water) and gases (such as air) create a type of friction called drag, because gravity also pulls liquids and gases toward Earth’s solid surface.
In space, things are different. If we were orbiting Earth, its gravity would still act on us, but we would not feel it. We might think we were “floating” when, in fact, we would be falling. In a circular orbit, our velocity would carry us away from Earth as fast as we fell.
As another example, in 1965 astronaut James McDivitt tried to catch up (rendezvous) with an object orbiting far ahead of him. He instinctively increased his speed. However, this added speed moved his orbit higher and farther from Earth where gravity is weaker and orbital velocities are slower. Thus, he fell farther behind his target. Had he temporarily slowed down, he would have changed his orbit, lost altitude, sped up, and traveled a shorter route. Only by slowing down could he catch up—essentially taking a “shortcut.”
All particles attract each other gravitationally. The more massive and the closer any two particles are to each other, the greater their mutual attraction. To determine the gravitational pull of a large body, one must add the effects of all its tiniest components. This seems a daunting task. Fortunately, the gravitational pull of a distant body behaves almost as if all its mass were concentrated at its center of mass—as our intuition tells us.
But what if we were inside a “body,” such as the universe, a galaxy, or Earth? Intuition fails. For example, if Earth were a hollow sphere and we were inside, we would “float” ! The pull from the side of the spherical shell nearest us would be great because it is close, but more mass would pull us in the opposite direction. In 1687, Isaac Newton showed that these pulls always balance.17
Tides. A water droplet in an ocean tide feels a stronger gravitational pull from the Sun than from the Moon. This is because the Sun’s huge mass (27 million times greater than that of the Moon) more than makes up for the Sun’s greater distance. However, ocean tides are caused primarily by the Moon, not the Sun. This is because the Sun pulls the droplet and the center of the Earth toward itself almost equally, while the much closer Moon pulls relatively more on either the droplet or the center of the Earth (whichever is nearer). We best see this effect in tides, because the many ocean droplets slip and slide so easily over each other. (To learn more about what causes tides, see page 412.)
Tidal effects act everywhere on everything: gases, liquids, solids—and comets. When a comet passes near a large planet or the Sun, the planet or Sun’s gravity pulls the near side of the comet with a greater force than the far side. This difference in “pulls” stretches the comet and sometimes tears it apart. If a comet passes very near a large body, it can be pulled apart many times; that is, pieces of pieces of pieces of comets are torn apart as shown in Figure 144.
Figure 144: Weak Comets. Tidal effects often tear comets apart, showing that comets have almost no strength. Two humans could pull apart a comet nucleus several miles in diameter. In comparison, the strength of an equally large snowball would be gigantic. In 1992, tidal forces dramatically tore comet Shoemaker-Levy 9 into 23 pieces as it passed near Jupiter. Two years later, the fragments, resembling a “flying string of pearls” strung over 180,000,000 miles, returned and collided with Jupiter. A typical high-velocity piece released about 5,000 hydrogen bombs’ worth of energy and became a dark spot, larger than Earth, visibly drifting for days in Jupiter’s atmosphere. We will see that Jupiter, with its huge gravity and tidal effects, is a comet killer.
Spheres of Influence. The Apollo 13 astronauts, while traveling to the Moon, dumped waste material overboard. As the discarded material, traveling at nearly the same velocity as the spacecraft, moved slowly away, the spacecraft’s gravity pulled the material back. To everyone’s surprise, it orbited the spacecraft all the way to the Moon.18 When the spacecraft was on Earth, Earth’s gravity dominated things near the spacecraft. However, when the spacecraft was far from Earth, the spacecraft’s gravity dominated things near it. The region around a spacecraft, or any other body in space, where its gravity can hold an object in an orbit, is called its sphere of influence.
An object’s sphere of influence expands enormously as it moves farther from massive bodies. If, for many days, rocks and droplets of muddy water were expelled from Earth in a supersonic jet, the spheres of influence of the rocks and water would grow dramatically. The more the spheres of influence grew, the more mass they would capture, so the more they would grow, etc.19
A droplet engulfed in a growing sphere of influence of a rock or another droplet with a similar velocity might be captured by it. However, a droplet entering a body’s fixed sphere of influence with even a small relative velocity would seldom be captured.20 This is because it would gain enough speed as it fell toward that body to escape from the sphere of influence at about the same speed it entered.
Earth’s sphere of influence has a radius of about 600,000 miles. A rock inside that sphere is influenced more by Earth’s gravity than the Sun’s. A rock entering Earth’s sphere of influence at only a few feet per second would accelerate toward Earth. It could reach a speed of almost 7 miles per second, depending on how close it came to Earth. Assuming no collision, gravity would whip the rock partway around Earth so fast it would exit Earth’s sphere of influence about as fast as it entered—a few feet per second. It would then be influenced more by the Sun and would enter a new orbit about the Sun.21
Exiting a sphere of influence is more difficult if it contains a gas, such as an atmosphere or water vapor. Any gas, especially a dense gas, slows an invading particle, perhaps enough to capture it. Atmospheres are often relied upon to slow and capture spacecraft. This technique, called aerobraking, generates much heat. However, if the “spacecraft” is a liquid droplet, evaporation cools the droplet, makes the atmosphere denser, and makes capture even easier.
A swarm of mutually captured particles will orbit their common center of mass. If the swarm were moving away from Earth, the swarm’s sphere of influence would grow, so fewer particles would escape by chance interactions with other particles. Particles in the swarm, colliding with gas molecules, would gently settle toward the swarm’s center of mass. How gently? More softly than large snowflakes settling onto a windless, snow-covered field. More softly, because the swarm’s gravity is much weaker than Earth’s gravity. Eventually, most particles in this swarm would become a rotating clump of fluffy ice particles with almost no strength. The entire clump would stick together, resembling a comet’s nucleus in strength, size, density, spin, composition, texture, and orbit. The pressure in the center of a comet nucleus 3 miles in diameter is about what you would feel under a blanket here on Earth.
In contrast, spheres of influence hardly change for particles in nearly circular orbits about a planet or the Sun. Even on rare occasions when particles pass very near each other, capture does not occur. This is because they seldom collide and stick together, their relative velocities almost always allow them to escape each other’s sphere of influence, their spheres of influence rarely expand, and gases are not inside these spheres to assist in capture. Forming stars, planets, or moons by capturing22 smaller orbiting bodies is far more difficult than most people realize.23