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...