From Contraction to Condensation

In 1755, the great German philosopher Immanuel Kant (1724–1804) theorized that the solar system had begun as a nebula—a cloud of dust and gas—that slowly rotated, gradually contracting until it became flattened into a spinning disk that variously coalesced into the sun and planets.
Later, in 1796, the French astronomer and mathematician Pierre-Simon Laplace (1749–1827) suggested a similar hypothesis, though he thought the planets formed before the sun.

What Do We Really Know About the Solar System?

In a very real sense, then, we do have—in meteorites and moon rocks—“witnesses” to the creation of the solar system. These geological remnants are relatively unchanged from the time the solar system was born. But how do we make up for an absence of precedents from which to draw potentially illuminating analogies? Why can’t we just go find another planetary system forming (around a star younger than the sun) and draw our analogies from it? Well, that has been one of the main goals of astronomers in the past decade or so. In fact, NASA has defined one of its primary missions in terms of this search, called the Origins program. We are just now starting to see the results of these searches. The Hubble Space Telescope, in particular, has given us tantalizing clues about the formation of planetary systems. Around the star Beta Pictoris, astonomers have imaged a disk of material larger than the orbit of the most distant planet in our solar system, Pluto. Are the inner reaches of this disk even now taking shape as planets around that star?
The truth is, even with the best instruments that we have today, we can still learn a lot more about how our solar system formed by looking closer to home. There are a number of fundamental things that we know about our solar system, and any explanation that we come up with must, at the very least, account for what we observe. Here are some undeniable facts that the last 300 years of planetary exploration have given us:

• Most of the planets in the solar system rotate on their axis in the same direction as they orbit the sun (counterclockwise as seen from the North Pole of Earth), and their moons orbit around them in the same direction.
• The planets in the inner reaches of the solar system are rocky and bunched together, and those in the outer part are gaseous and widely spaced.
• Most of the planets (with the exception of Pluto) orbit the sun in elliptical paths that are very nearly circles.
• Except for the innermost planet (Mercury) and the outermost (Pluto), the planets orbit in approximately the same plane (near the ecliptic), and they all orbit in the same direction.
• Asteroids and comets are very old, and are located in particular places in the solar system. Comets are found in the Kuiper Belt and Oort Cloud, and asteroids in the asteroid belt between Mars and Jupiter.

In addition, it is clear that the asteroids we have examined are some of the oldest unchanged objects in the solar system, and that comets travel in highly elliptical orbits, originating in the far reaches of the solar system. The most important conclusion we can draw from these observations is that the solar system appears to be fundamentally orderly rather than random. It doesn’t appear that the sun formed first, and then gradually captured its nine planets from surrounding space.
Although there are important exceptions, the counterclockwise”
(as viewed from above the North Pole of the Earth) aspect of so many properties in the solar system suggests that the planets fragmented and formed from a large rotating cloud of material. That the orbits of the remaining planets are very nearly circular suggests that the solar system has settled down, as it were. Any planets or planetesimals that were on highly elliptical orbits have been cleared out in the last 4.6 billion years. The inclined, markedly elliptical orbit of Pluto is one of the arguments for its being an escaped moon from an outer planet. The physical differences in planets that are related to their distances from the sun suggests that the sun influenced the formation of the planets; that is, the sun must have formed first.

The Biggest Problem: We Weren’t There

Of course, there was no one around to record the series of events that created the solar system. But there are a few fragments that survive from those early moments, like the years-old crumbs from behind the sofa, that give us clues to how the planets took shape around the youthful sun. The most important clues to the origin of the solar system are to be found not in the sun and planets, but in those untouched smaller fragments: the asteroids, meteoroids, and some of the planetary moons (including our own). These objects make up the incidental matter, the debris of the solar system, if you will.
This solar system debris, though rocky, is not mute. As the English Romantic poet William Wordsworth (1770–1850) wrote of hearing “sermons in stones,” so modern astronomers have extracted eloquent wisdom of a different kind from meteorites as well as “moon rocks.”
Atoms are made up of three basic particles, protons and neutrons in the nucleus, orbited by electrons. Most elements exist in different atomic forms, which, while identical in their chemical properties, differ in the number of neutral particles (neutrons) in the nucleus. Deuterium and Tritium are famous radioactive isotopes of the more familiar hydrogen.
For a single element, these atoms are called isotopes. Through a natural process called radioactive decay, a specific isotope of one atom is converted into another isotope at a constant and known rate, often over many millions of years, depending on the element and isotope involved.
Using a device called a mass spectrometer, scientists can identify the “daughter” atoms formed from the “parent” atoms in a sample, such as a meteorite. If the decay rate of the elements in the sample is known, then the ratio of daughter atoms to parent atoms (called isotopic ratios), as revealed by the instrument, betrays the age of the sample. The results of this dating process have been remarkably consistent. Most meteors and moon rocks (which are the only “bits” of the solar system other than the earth we have been able to study exhaustively ) are from between 4.4 and 4.6 billion years old, which has led scientists to conclude with a high degree of confidence that the age of the solar system is about 4.6 billion years.

Solar System History

Historians of human events generally enjoy two advantages over would-be historians of the solar system. Those who chronicle human history often have records, even eyewitness reports, and they have the availability of precedent events and subsequent events. For example, a historian of the American Civil War not only has a wealth of eyewitness accounts to draw on, but may also look to civil wars both before and after 1861 to 1865 to help explain the War between the States. Comparing and contrasting the American Civil War to the English civil wars of the seventeenth century or the Russian Civil War of the twentieth may help illuminate analysis and make explanations clearer.
The solar system historian lacks both witnesses and precedents. But she has several advantages as well. Human history is complicated by the infinite depths of the human mind. But if we understand the fundamental laws of physics, and make good observations of our solar system today, a recounting of the early history of the solar system should be within our grasp.

And What’s Inside the Moon?

Geologically, the moon is apparently as dead as it is biologically. Astronauts have left seismic instruments on the lunar surface, which have recorded only the slightest seismic activity, barely perceptible, in contrast to the exciting (and sometimes terribly destructive) seismic activity common on Earth and some other bodies in the solar system. It is believed, then, that the interior of the moon is uniformly dense, poor in heavy elements (such as iron) but high in silicates. The core of the moon, about 250 miles (402 km) in diameter, may be partially molten. Around this core is probably an inner mantle, perhaps 300 miles (483 km) thick, consisting of semisolid rock, and around this layer, a solid outer mantle some 550 miles (885 km) thick. The lunar crust is of variable thickness, ranging from 40 to 90 miles (64 to 145 km) or so.
The moon is responsible for everything from the earth’s tides to the length of our day, and perhaps the presence of seasons. Most astronomers think that the moon is with us today because of a gigantic collision early in the life of the solar system.
The moon’s gravity pulls tides across the earth’s surface, and its presence has slowed the rotation of the earth from a frenetic 6 hours to our current 24. Think of that next time you see the moon shining peacefully over your head.

Pocked Face Moon

Look at the moon through even the most modest of telescopes—as Galileo did—and you are impressed first and foremost by the craters that pock its surface. Most craters are the result of asteroid and meteoroid impacts. Only about a hundred craters have been identified on Earth, but the moon has thousands, great and small. Was the moon just unlucky? No. Many meteoroids that approach Earth burn up in our atmosphere before they strike ground. And the traces of those that do strike the ground are gradually covered by the effects of water and wind erosion as well as by plate tectonics. Without an atmosphere, the moon has been vulnerable to whatever comes its way, and preserves a nearly perfect record of every impact it has ever suffered.
Meteoroid collisions release terrific amounts of energy. Upon impact, heat is generated,
melting and deforming the surface rock, while pushing rock up and out and creating an ejecta blanket of debris, including large boulders and dust. It is this ejected material that covers the lunar surface.
It is believed that the rate of meteoroid impact with the moon (and with other objects in the solar system) was once much higher than it is now. The rate dropped sharply about 3.9 billion years ago— at the end of the period in which it is believed that the planets of the solar system were formed—and, some time later, lunar volcanic activity filled the largest craters with lava, giving many of them a smooth-floored appearance.

Moon, The Green Cheese?

Even with the naked eye, the moon doesn’t look particularly green. And Neil Armstrong confirmed that the surface of the moon was more dusty than cheesy. On any night that the moon is visible, the large, dark maria are clearly visible. These are vast plains created by lava spread during a period of the moon’s evolution marked by intense volcanic activity. The lighter areas visible to the naked eye are called highlands.
Generally, the highlands represent the moon’s surface layer, its crust, while the maria consist of much denser rock representative of the moon’s lower layer, its mantle. The surface rock is fine-grained, as was made dramatically apparent by the image of the first human footprint on the moon. The mare resemble terrestrial basalt, created by molten mantle material that, through volcanic activity, swelled through the crust.
The mass of the moon is insufficient for it to have held on to its atmosphere. As the sun heated up the molecules and atoms in whatever thin atmosphere the moon may have once had, they drifted away into space.
With no atmosphere, the moon has no weather, no erosion other than what is caused by asteroid impacts—and no life. While it was thought that the moon had absolutely no water, recent robotic lunar missions have shown that there may be water (in the form of ice) in the permanent shadows of the polar craters.

Earth & Moon: Give and Take

Newton proposed that every object with mass exerts a gravitational pull or force on every other object with mass in the universe. Well, the earth is much more (80more) massive than the moon, which is why the moon orbits us, and not we it. (If you want to get technical, we both actually orbit an imaginary point called the center of mass.) However, the moon is sufficiently massive to make the effects of its gravitational field felt on the earth.
Anyone who lives near the ocean is familiar with tides. Coastal areas experience 2 high and 2 low tides within any 24-hour period. The difference between high and low tides is variable, but, out in the open ocean, the difference is somewhat more than 3 feet. If you’ve ever lifted a large bucket of water, you know how heavy water is. Imagine the forces required to raise the level of an entire ocean 3 or more feet! What force can accomplish this?
The tidal force of gravity exerted by the moon on the earth and its oceans. The moon and the earth mutually pull on each other; the earth’s gravity keeping the moon in its orbit, the moon’s gravity causing a small deformity in the earth’s shape.
This deformity results because the moon does not pull equally on all parts of the earth. It exerts more force on parts of the earth that are closer, and less force on parts of the earth that are farther away. Just as Newton told us: Gravitational forces depend on distance. These differential or tidal forces are the cause of the earth’s slightly distorted shape—it’s ovoid rather than a perfect sphere— and they also make the oceans flow to two locations on the earth: directly below the moon, and on the opposite side. This flow causes the oceans to be deeper at these two locations, which are known as the tidal bulges. The entire Earth is pulled by the moon into a somewhat elongated—football—shape, but the oceans, being less rigid than the earth, undergo the greatest degree of deformity.
Interestingly, the side of the earth farthest from the moon at any given time also exhibits a tidal bulge. This is because the Earth experiences a stronger gravitaional pull than the ocean on top of it, and the Earth is “pulled away” from the ocean on that side. As the Earth rotates beneath the slower-moving moon, the forces exerted on the water cause high and low tides to move across the face of the earth.
The tides of largest range are the spring tides, which occur at new moon, when the moon and the sun are in the same direction, and at full moon, when they are in opposite directions. The tides of smallest range are the neap tides, which occur when the sun and the moon are at 90 degrees to one another in the sky. Tides affect us every day, of course, especially if you happen to be a sailor or a fisherman. But even if you live high and dry in Kansas or Nebraska, say, tides (and the moon) still affect you. Every day, the earth is spinning a little slower on its axis because of the moon. The earth’s rotation is slowing down at a rate that increases the length of a day by approximately 2 milliseconds (2/1,000 of a second) every century. Over millions of years, though, this slowing effect adds up. Five hundred million years ago, a day was a little over 21 hours long, and a year (1 orbit of the sun) was packed with 410 days. When a planetesimal plowed into the earth early in the history of the solar system, it was rotating once every 6 hours. (And you think there aren’t enough hours in the day now!)
How does this happen? Well, let’s think again of why tides occur. The moon’s gravity causes two bulges to form in the earth’s oceans, and the earth rotates (once every 24 hours) beneath that bulge. As the earth spins, friction between it and the oceans tends to pull the high tide ahead, so that the “bulge” actually leads the position of the moon overhead.
With the ocean’s bulge thus slightly ahead of the moon’s position, the moon’s gravity exerts a force that tends to slow rotation. Eventually, the earth’s rotation will slow sufficiently to become synchronized with the orbit of the moon around the earth. When that happens, the moon will always be above the same point on Earth, and the earth’s rotation period will have slowed (billions of years from now) from its present 24 hours to 47 days.
But that is only half of the picture. The earth can’t be slowing down without something else speeding up (as a result of one of the fundamental conservation laws of physics). What’s speeding up? The moon. And what does that mean? That it’s spiraling away slowly, and getting smaller and smaller in the sky.