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From Dust to Life
The Origin and Evolution of Our Solar System
By JOHN CHAMBERS PRINCETON UNIVERSITY PRESS
Copyright © 2014 John Chambers and Jacqueline Mitton
All rights reserved.
ISBN: 978-0-691-17570-6
CHAPTER 1
COSMIC ARCHAEOLOGY
A FASCINATION WITH THE PAST
The temple at Karnak on the River Nile is one of the most magnificent monuments to survive from ancient Egypt. Construction of the vast temple complex began 3,000 years ago, and 30 different pharaohs developed and extended the site for a millennium afterward. Everywhere at Karnak, the stone walls and columns of the temple precincts are inscribed with historical texts, prayers, and accounts of religious rituals. Today, guides routinely explain to tourists the meaning of the symbols incised in stone and the significance of this immense monument. Yet for 1,500 years no one in the world could make sense of the writing, and much of ancient Egyptian civilization was a mystery.
The inscriptions at Karnak are composed of hieroglyphics, one of the oldest written languages in the world. The ancient Egyptians used this pictorial script for formal and sacred documents, but its use declined after Egypt became a Roman province in 30 BC. When Egypt became Christian in the 4th century AD, all memory of hieroglyphics was lost. Over the following centuries, scholars puzzled over the meaning of hieroglyphs but never managed to decode them.
In 1799, a French soldier in Napoleon's army discovered a gray slab of stone built into a fort near the Egyptian town known as Rashid or Rosetta. The stone was inscribed with religious proclamations written in three languages: ancient Greek, hieroglyphics, and a more modern Egyptian script called Demotic. Scholars quickly translated the Greek and Demotic writing and realized the same proclamation was repeated in all three languages. Unfortunately, the top portion of the slab had broken away, leaving only 14 lines of hieroglyphs, but these proved to be enough. A painstaking comparison of the languages and some inspired detective work allowed researchers to decode the hieroglyphics for the first time in more than a millennium. The Rosetta stone became the key to unlocking a priceless treasury of information about ancient Egypt and its people.
The story of the Rosetta stone is a good example of how archaeologists can piece together human history by carefully studying rare artifacts that have survived the rigors of time. Occasionally, evidence of the past is staring us in the face just waiting to be identified, like the stone slabs in Karnak. More often the past is buried under debris accumulated over many centuries, as in the legendary city of Troy in Turkey. The past can even be found hiding in the most unlikely of places, such as the details of human history recorded in our genetic code.
Teasing out this information from a variety of sources and grasping its significance is far from easy. It has taken several centuries to develop the tools and know-how that enable today's scientists to interpret clues from the past and turn them into an account of human history. Breakthroughs in archaeology and other sciences often have to wait for a chance discovery like the Rosetta stone, or the introduction of a new technology, or the unique insights of an imaginative mind. Despite these difficulties, scientists persevere because of a deep fascination within all of us: a desire to know about our origins.
Scientists pondering the history of the solar system are much like archaeologists sifting through the sands of Egypt. They bring different methods and tools to the job, but both strive to glean as much as possible from precious relics from the past, and combine this with information deduced from our current surroundings. The distances and timescales may be different but the big questions are the same. Where do we come from? How did we get here? What was the world like in the past? Deciphering the history of the solar system is archaeology on a grand scale. For human society to arise, our species needed to evolve from those that went before. Prior to this, life had to appear on a suitably habitable planet orbiting a long-lived star. Before any of this could happen, our solar system had to take form from the near nothingness of interstellar space. The story of this transformation and how scientists have pieced it together is the subject of this book.
A SOLAR SYSTEM TO EXPLAIN
We start by taking stock of the solar system we see today. The solar system is dominated by a star, the Sun, which contains more than 99.8 percent of the system's mass. Compared to any of the planets the Sun is huge: roughly 1.4 million km (840,000 miles) across, or 109 times the diameter of Earth. The Sun is a rather ordinary star, but "average" is not quite the right word: it is actually brighter and more massive than 90 percent of the stars in our galaxy. The Sun is roughly in the middle of its 10-billion-year life span, neither young nor old, and it has few noteworthy features. It lacks the variability, unusual composition, or excessive magnetic field of some of its more exotic stellar counterparts. From the point of view of life on Earth, this is a good thing: a stable and predictable star provides a pleasant environment for life to flourish.
The average density of the Sun is similar to that of water, but it is largely composed of lighter materials — hydrogen and helium — that are tightly compressed by the Sun's gravity. These two chemical elements make up 98 percent of the Sun's bulk, while all the others contribute the remaining 2 percent, a composition that turns out to be a fair reflection of stars in general. Like other stars, the Sun is made of plasma, an electrically charged gas that reaches temperatures of millions of degrees in the solar interior. Nuclear reactions in the Sun's core provide a continuous source of energy that keeps the Sun shining, and this sunlight provides an important source of heat for Earth and the other planets.
The overwhelming mass of the Sun means that its gravity dominates the motion of all the other members of the solar system. To a good approximation, the Sun lies at the center of the system while every other object revolves around it. Somewhat surprisingly, the Sun accounts for only about 2 percent of the solar system's angular momentum, or rotational inertia. The Sun spins rather slowly, with each rotation taking roughly a month, although the Sun's fluid nature means that different layers in its interior rotate at somewhat different speeds. Most of the rotational energy of the solar system is carried by the planets as they travel around the Sun. This fact has puzzled scientists for a long time and has strongly influenced theories for the origin of the solar system, as we will see in Chapter 3.
The Sun has eight major planets. These follow elliptical orbits around the Sun, all traveling in the same direction — anticlockwise when viewed from above the Sun's north pole. The orbits are almost — but not quite — in the same plane, like concentric hoops lying on a table. With the exception of Mercury and Mars, the orbits are very nearly circular. Mercury and Mars follow more elongated paths — in mathematical terms their orbits are eccentric. The eccentricity of Mars's orbit was an important clue that helped early astronomers understand the motion of all the planets, as we will describe in Chapter 2.
A useful yardstick for measuring distances in the solar system is the astronomical unit, or AU for short. This is the average distance between Earth and the Sun, roughly 150 million km (93 million miles). The realm of the major planets extends out to 30 AU from the Sun, but it is divided into two distinct domains. The four inner planets all orbit within 2 AU of the Sun. These small objects are called the terrestrial (Earth- like) planets since they all have solid surfaces, and their structure and composition resemble those of Earth.
The four outer planets are arranged more spaciously, orbiting between 5 and 30 AU from the Sun. These bodies are giants compared to the terrestrial planets. Jupiter, the largest, is 300 times more massive than Earth. The giant planets are constructed in a very different way than their rocky cousins, consisting of multiple layers of gas and liquid with no solid surface.
Each of the giant planets forms the hub of a system of rings and a considerable family of satellites. Saturn's spectacular rings are made up of countless chunks of almost pure water ice, ranging in size from a few meters (several feet) down to tiny specks of dust. The rings of Jupiter, Uranus, and Neptune are much darker and less extensive by comparison. As we write, astronomers have found 168 moons orbiting the four giant planets, but it seems almost certain that more will be discovered in the future. In marked contrast, the inner planets have only three satellites — our own Moon and Mars's two tiny companions, Phobos and Deimos. None of the terrestrial planets has rings.
Before we move on to asteroids, comets, and the other members of the solar system, we need to take a moment to describe how astronomers classify things. Astronomical bodies can be grouped in many different ways: based on their shape (roughly spherical or irregular), their composition (rocky or icy), their appearance through a telescope (fuzzy like a comet or a single point of light), or the nature of their orbits. When it comes to planets, however, the popular feeling is that size is the most important factor: a planet is something that is smaller than a star but larger than everything else. The question is how large. Billions of objects orbit the Sun, ranging in size from Jupiter, with a diameter 11 times larger than Earth, down to microscopic grains of dust. Nature has no regard for our habit of allocating objects to particular pigeonholes. To a large extent, the dividing line between a major planet and a smaller body is arbitrary, much like the distinction between a river and a stream.
According to the current convention, our solar system has eight major planets. Pluto used to belong to this club, but astronomers recently moved it to a different category based on its similarity to other objects in the outer solar system. This rearrangement didn't please everybody, and Pluto's status remains a topic of debate. With remarkable foresight, astronomer Charles Kowal reflected on the problem of how to define a planet in his 1988 book on asteroids. The largest known asteroid, Ceres, is 952 km (592 miles) in diameter, while Pluto — which was treated as a major planet at the time — is just over 2,300 km (1,400 miles) across. "What will happen if an object is found with a diameter of 1500 km?" Kowal asked. "Will it be called an asteroid or a planet? You can be sure that astronomers will not answer this question until they are forced to!" On this last point he was entirely correct.
The day of reckoning came in 2003 when astronomers discovered four large objects orbiting beyond Neptune. Three of these, Makemake, Haumea, and Sedna, appear to be about 1,500 km (900 miles) in diameter. The fourth, Eris, is roughly the same size as Pluto but about 27 percent more massive. If Pluto is called a planet, then surely Eris should be as well. Should we classify the other three new objects as planets too? What will happen when more large objects are discovered? Will there soon be 20 planets, or 50, or 1,000? It was time for a reappraisal. In a controversial decision, the International Astronomical Union (IAU) voted to create a new class called "dwarf planets," with Pluto, Eris, and asteroid Ceres as founder members. Pluto, formerly a major planet, was redesignated minor planet number 124340, reducing the number of major planets to eight.
As of 2012, only five objects have been added to the list of dwarf planets. That still leaves many thousands of known objects that are not planets, dwarf planets, or moons. According to the IAU, these are "small solar system bodies," a category that is divided into "comets," icy bodies that sometimes develop a fuzzy coma and a tail, and "minor planets," rocky objects that always look like points of light when seen from Earth. Few people actually use the term "minor planet" in practice, and small rocky objects are almost always called "asteroids" instead.
A major belt of asteroids lies between the terrestrial and giant planets. Astronomers have found over 300,000 asteroids so far, mostly concentrated between 2.1 and 3.3 AU from the Sun. Hundreds more are discovered every month. Close-up pictures show that asteroids look very different from planets: they are often elongated or have irregular shapes, and their surfaces are covered in ridges, boulders, and craters. Despite their great number, the asteroids contain relatively little mass in total. If all the known asteroids were combined into a single object, it would be smaller than Earth's Moon.
The vast majority of asteroids lie in this main belt between Mars and Jupiter, but some venture farther afield. Asteroid Eros crosses the orbit of Mars, and in 1931 it came within 23 million km (14 million miles) of Earth — about half the minimum distance to Venus. Another asteroid, Hidalgo, moves on a highly elliptical orbit that takes it out beyond Saturn. Some asteroids even cross Earth's orbit, and a small fraction of these will eventually collide with our planet. Two large groups of asteroids, called Trojans, share an orbit with Jupiter, traveling in lockstep around the Sun 60 degrees ahead of the planet or 60 degrees behind it. Astronomers have recently found similar Trojan asteroids that share orbits with Mars and Neptune.
Another belt of small bodies orbits the Sun just beyond Neptune. This region, called the Kuiper belt, is home to Pluto, Eris, and hundreds of other objects found within the past two decades. These discoveries are probably just the tip of the iceberg, and the Kuiper belt probably contains far more mass than the main asteroid belt. Astronomers usually refer to bodies orbiting beyond Neptune as Kuiper belt objects or trans-Neptunian objects to distinguish them from "asteroids," a term that has come to mean small bodies in the inner part of the solar system.
Only a handful of comets have been viewed at close range. These typically look rather like asteroids, although they contain large amounts of ice as well as rocky dust. Comets remain inert as long as they stay cold. However, if a comet comes within a few AU of the Sun, its ices begin to vaporize, releasing gas that blows dust grains off the surface. This gas and dust accumulates around the solid nucleus, forming a huge diffuse cloud called a coma, and streaming away into space to form tenuous tails (one of gas, one of dust) that can extend for millions of kilometers (millions of miles).
Asteroids orbit within a few AU of the Sun, and astronomers had long assumed they were free of ice. In 1996, asteroid Elst-Pizarro surprised many people by developing a tail like a comet as it passed the point in its orbit closest to the Sun (Figure 1.2). In 2001 and 2007, the same thing happened again. Elst-Pizarro is now classed as both a comet and an asteroid. Several other objects in the outer parts of the asteroid belt display this dual personality. These bodies must harbor reservoirs of ice that partially vaporize when the temperature becomes high enough. Icy deposits have recently been detected on the surface of Themis, one of the largest asteroids in the main belt. It may be that other asteroids contain ice in their interior, protected from sunlight by a layer of rocky dust on the surface. Clearly, the boundary between asteroids and comets is not as sharp as astronomers once believed.
Most comets follow highly elongated orbits, arriving in the inner solar system from beyond Neptune and then making the return journey. A few hundred comets have become trapped on smaller orbits by the pull of Jupiter's gravity, and these rarely travel much beyond the giant planet's orbit. Typically, these "Jupiter family comets" have traveled around the Sun many times, losing much of their former glory over time. Most comets move on much larger orbits by comparison, taking thousands or even millions of years to travel around the Sun. Tracing the motion of these "long-period" comets backward in time along their orbits shows that they come from a vast reservoir of icy bodies far from the Sun. This spherical swarm of comets, known as the Oort cloud, is concentrated between 20,000 and 50,000 AU from the Sun, and it marks the true outer boundary of the solar system.
(Continues...)
Excerpted from From Dust to Life by JOHN CHAMBERS. Copyright © 2014 John Chambers and Jacqueline Mitton. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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