The Universe Within (9 page)

A
trip in a time machine to
Earth of 4.5 billion years ago would not only be eerie; it would be perilous. With an atmosphere lacking free
oxygen and raining acid, you’d need a space suit far beyond the technology of modern science to survive. Impact after impact of
rock and ice from space made the surface sometimes roil at thousands of degrees Fahrenheit. With this heat, there were no oceans: liquid water may have formed a few different times, only to evaporate away. As a break from this desolation, you might hope for beautiful moonlit nights. Forget about it. There was no moon.

Artifacts of the trans
formation of this primordial world into our modern one are strewn across different bodies of the
solar system. Six missions landed on the
moon and returned samples to Earth. Carrying mini geological kits, astronauts collected rocks from craters, highlands, and lowlands of the lunar surface. The specimens are today stored in liquid
nitrogen in repositories in Houston and San Antonio. A number of small moon fragments have been given as gifts to foreign dignitaries, while others grace public exhibits. The bulk of the rocks, about 850 pounds in all, remain to be studied. The few samples that have made it to labs tell important stories of the origin of our world.

One of the biggest lessons from
moon rocks is how normal many of them are. In terms of mineral content and structure, moon rocks are more similar to those on Earth than others in the solar system. One similarity is particularly telling.
Oxygen atoms can exist in different forms, defined by the number of
neutrons in the nucleus. By measuring the neutron-heavy and neutron-light
versions of
oxygen in any rock, a very informative ratio can be calculated. Each body in the solar system carries a unique chemical signature written in the proportion of different versions of oxygen in their rocks. The reason is that the oxygen content inside a planet’s rocks is sensitive to its
distance from the
sun when it formed. The oxygen composition of moon rocks, though, is virtually identical to those of Earth. This means that the moon and Earth formed at the same distance from the sun—perhaps in the same orbit.

With all of these similarities, there remains one very significant difference between moon rocks and those of Earth. Moon rocks almost entirely lack one class of elements, the so-called
volatiles. These elements—nitrogen,
sulfur, and
hydrogen—share one important geological fact: they tend to vaporize when things get hot (hence the name volatiles). Some great event in the distant past must have baked the moon rocks, releasing their volatiles.

The lessons of the moon rocks are clear—the minerals on the moon formed at the same orbital distance from the sun as Earth and then suffered some kind of blast. What do these facts tell us of the origin of the moon?

The current theory for the formation of the moon envisions something like a cosmic
demolition derby. In these automotive mosh pits, common at fairgrounds in the 1970s, cars intentionally smashed into one another, with the last car running being the winner. Along the way, cars would slam into each other with wild abandon. The most violent of these collisions would eject the light outer layers of the cars, hubcaps and bumpers, leaving the inner ones hopelessly entangled.

This type of collision offers an insight into how the
Earth-moon system came about. Over 4.5 billion years ago, a large, perhaps
Mars-sized,
asteroid is thought to have collided with the forming Earth. Much like the twisted mélange of car parts in a demolition derby crash, the collision ejected lighter parts of each body while the heavier pieces fused. The lighter debris, consisting of dust and smaller particles, now depleted of
volatile elements, began to orbit Earth as a disk. Over time, this debris disk coalesced as the moon. The cores of the two bodies did not propel into space but liquefied under the great heat of the impact, only later to cool and solidify as the new core of Earth. In addition, the impact so whacked Earth that it left a 23.5-degree tilt in its axis of
rotation.

Initially, there were two large bodies in the same orbit of the sun. Then they collided, forming what we know as Earth and moon today. Ever since that impact, the two bodies have been locked in an orbital dance—Earth and moon exert gravitational pull on each other, while the laws of physics and momentum tie the speed of the spinning of Earth to the rotation of the moon. The impact on our lives is as straightforward as it is profound: the length of
days and of months, like the workings of the
seasons, derive from the Earth-moon system. Every clock and
calendar, like the cells of our bodies, holds artifacts of a cataclysm that took place over 4.5 billion years ago.

The Romans had an effective way of controlling troublesome officials in the far-flung regions of their empire. Instead of gerrymandering districts to stay in power—to help friends and get rid of foes—Caesar and his cronies found the ultimate way to retain control. They gerrymandered the calendar. Have a political friend in one region? Add a few extra days to his term. Want to get rid of a foe in another place? Lop days of his rule off the year. This was wonderfully effective; however, over time, not only did the
decentralized calendar make ruling difficult, but the year became a patchwork of political kludges, fixes, and compromises.

The nature of Earth’s
rotations in space makes it ripe for these
kinds of abuses. We all learn this material in school, but most of us forget the meaning of the planet’s
rotations by the time we are in college. A recent survey of
Harvard undergraduates asked the simple question: What causes the
seasons? Over 90 percent of them got the answer totally wrong. The answer has nothing to do with the amount of light that hits Earth during winter and summer, nor with Earth rocking back and forth, nor with the planet getting closer to the sun over the course of the year.

As we’ve known since the
days of
Copernicus and his contemporaries, the moon rotates around Earth, while Earth retains its constant 23.5-degree tilt as it rotates around the sun. The angle that sunlight hits the planet changes at different parts of the
orbit. Direct light generates the long days and heat of summer; tilted and less direct light gives us shorter and colder winter days. The seasons aren’t generated by Earth rocking back and forth; they derive from the planet having a constant tilt as it rotates around the sun.

Because of the different orbits that affect our lives—ours around the sun and the moon around us—there are choices to make when constructing a
calendar. Of course, the length of a year is based on the rotation of Earth around the sun. If we know the longest and shortest days, we can carve up the year into months based on the seasons. Another way to do this is to base the calendar on the position of the moon as it goes from full to partial to new every twenty-nine days. The problem is that you can’t synchronize a
lunar calendar with a
seasonal, or solar, one. The number of lunar cycles does not correspond easily to the number of seasonal ones.

So what do we do? We add fudge factors.
Julius Caesar’s calendar had a leap year every three
years to keep the months in line with the seasons. The problem with this calendar for the Catholic Church was the extent to which the date of Easter wandered. To rectify this situation,
Pope Gregory XIII initiated a new calendar in 1582. Italy, Spain, and a few other countries launched it
immediately following the papal bull, resetting October 4, 1582, to October 15, 1582, losing eleven
days. Other countries followed to different degrees. Britain and the colonies, for example, only accepted it in 1752. One of the most important issues to iron out, naturally, was when to collect taxes.

Years, months, and days can, at least in theory, be based on celestial realities, but minutes and seconds are mostly conventions. Our calendar has seven days because of the biblical story of a six-day creation, followed by a day of rest. Minutes and seconds are in units of 60 due to a matter of convenience. The
ancient Babylonians had a number system based on 60. It turns out that 60 is a wonderful number because it is divisible by 1, 2, 3, 4, 5, and 6.

Humans are a timekeeping species, and much of our history can be traced to the ways we parse the moments of our lives. These intervals are based as much on astronomical cycles as on our needs, desires, and the ways we interact with one another. When the necessities of shelter, hunting, and survival were highly dependent on days and seasons, humans used
timepieces derived from the sun, moon, and stars. Other early timepieces relied on gravity, with
hourglasses that used sand or
water
clocks such as those first seen in Egypt in 4000
B.C.
Our need to keep time has itself evolved; an ever-increasing necessity to fragment time corresponds to the demands of our society, commerce, and travel. The concept of moments parsed into seconds would have been as alien to our cave-dwelling ancestors as seeing a jet plane.

There are clocks in our world that do not rely on convention, political choice, or economic necessity. The DNA in our bodies can serve as a kind of timepiece. Averaged over long periods of time, changes to some parts of the DNA sequence happen at a relatively constant rate. This means that if you compare the DNA structure of two species, you can estimate how long ago they shared an ancestor, because the more different the strands of DNA from two species are, the more time they have changed
separately. As we’ve seen with
zircons, atoms in
rocks also tell time. Knowing the ratio of different versions of the elements
uranium,
argon, and
lead can tell us how long ago the minerals in the rock crystallized.

The different clocks in bodies and in rocks don’t tick independently; they are part of the same planetary and solar metronome. Comparisons of the DNA inside humans, animals, and
bacteria speak of a common ancestor of all three that lived over 3 billion years ago. This is roughly the age of the
earliest fossil-containing rocks. The broad match of dates from rocks and DNA is all the more remarkable given how the rocks have been heated and heaved over the same billions of years that DNA has mutated, evolved, and been swapped among species. Agreement between these different kinds of natural clocks leads to confidence in our hypotheses. On the other hand, discordance between the clocks in DNA and those in rocks can also be the source of new predictions. Whale origins are a case in point. With some of the largest species on the planet, blowholes in the middle of their heads, ears specialized for a form of sonar, and odd limbs, backs, and tails,
whales are among the most extreme animals on Earth. Yet, as observers have known for centuries, their closest relatives are mammals: they have hair, mammary glands, and innumerable other mammalian affinities. But which mammals are their closest relatives, and when did whales enter the seas? Comparison of the DNA of whales with that of other mammals revealed that whales likely diverged from odd-toed ungulates such as hippos and deer. The differences in the genes and proteins implied that the split happened nearly 55 million years ago. But this created a whole new puzzle for
paleontologists. Not only were there no
fossils that showed transitional organs in the shift; there was nothing that ancient with whalelike features in the fossil record. The gap served as a challenge. Vigorous paleontological exploration brought confirmation: the discovery of whale skeletons with ankle bones similar to those of hippos and their relatives inside
rocks over 50 million
years old. And it all happened by relating the different clocks
in rocks and DNA.