## Out With the Tide

We have a word for things that move farther from Earth: up. The Moon is steadily moving up and up. As anyone who climbs long flights of stairs knows, it takes energy to move up. If the Moon is moving up, where does the energy come from? There is only one place it could come from: the spin of Earth. This is a zero-sum game that physicists call the conservation of angular momentum: The total angular momentum in the Earth-Moon system, which is related to the energy stored in both rotation and revolution, must remain the same. Since the Moon is gaining angular momentum as it spirals away, Earth must lose the same amount of angular momentum.

Earth loses angular momentum because the high tide closest to the Moon is trying to get back directly underneath the Moon, while the high tide farthest from the Moon is trying to get as far away from the Moon as possible. Consequently, the high tides flow westward, and in so doing, they encounter continents and islands. The water pushes against these land masses, which, because of rotation, are moving eastward. The net result is that the eastward rotation is retarded by the westward motion of the tides -- slowing down the rotation. The day is getting longer by about 0.002 seconds per century. It doesn't sound like much, but over billions of years it adds up.

If Earth is slowing down, it must have been rotating more rapidly in the past. By counting the growth rings in 400-million-year-old coral fossils and in 3-billion-year-old stromatolites, geologists calculate that Earth was rotating four times faster when it formed than it is today. The tidal effects of the Moon and, to a much lesser degree, the Sun have lengthened the day from six hours to 24 hours.

We can also work backwards in time for the Moon. Since the Moon is moving away, it must once have been closer. The closest the Moon could have been was about 7,300 miles above Earth's surface, 1/20th its present distance -- any closer, and the tides created on it by Earth would have ripped the Moon apart, turning it into a ring. This limit on the Moon's distance is consistent with the theory of how our satellite formed.

The Moon did not form with Earth. The chemistry of the Moon's rocks and other evidence indicate that the Moon was once part of Earth. When a huge asteroid hit Earth early in our planet's history, a huge volume of rock literally splashed into orbit. The young Earth had been a parched, steaming world of volcanoes and oozing rivers of molten rock, with an unbreathable atmosphere of carbon dioxide and virtually no surface water -- in short, an inhospitable, lifeless world. The impact shattered Earth's tenuous crust, sending superheated gas and water vapor out into interplanetary space. At the same time, large quantities of the Earth's mantle and crust (its outer layers) went into orbit around our planet. This material quickly coalesced into the Moon. This impact has been reproduced successfully in computer simulations.

## Tidal Waves

Let's assume that the Moon formed 10 times closer to us than it is today. In this case, the tides on the young Earth were 1,000 times higher than they are today, since tidal forces vary inversely with the cube of the distance. These humongous tides plunged miles inland and withdrew every three hours (remember, the day was only six hours long). As they moved over the land, the awesome volumes of water scraped and pounded the primeval rock, removing and pulverizing a considerable amount of it. Every time the tide retreated, it dragged this material back into the ocean. Continually churned up in the water, these chemicals formed the broth in which life probably formed.

With this information as background, we are ready to consider what Solon, a moonless Earth, would be like:

The length of the day

On Solon, the only tides would be from the Sun. The Sun accounts for one-third of the tides on Earth today. Therefore, Solon would still experience some tides and its rotation would still slow down, but not nearly by as much as Earth's. Solon's day would be around eight hours long at this time in its life, 4.6 billion years after it formed.

Winds

The faster a planet rotates, the faster its winds blow. We see the effects of extreme rotation by looking at Jupiter, which rotates every 10 hours. There, the winds are pulled into east-west flowing patterns, with much less north-south motion than occurs on today's Earth (see figure 3). Furthermore, the wind speeds on Jupiter are typically between 100 and 200 miles per hour. This indicates that the winds on Solon would flow more east-west than they do on Earth and that their speeds would be much higher. Winds of 100 miles per hour would occur daily, and hurricanes would have even higher wind speeds.
 Figure 3 Jupiter, as seen by the Hubble Space Telescope. The dark circle on the upper left of the jovian disc is the shadow of the innermost jovian moon, Io, seen to the right of the circle. As Jupiter spins once every 10 hours, it drags its outer atmosphere with it -- creating high winds that blow east-west around the planet. These winds are highlighted by the dark belts and light zones that gird the gas giant. The winds have only a limited north-south motion. Photo courtesy of Harold A. Weaver and T.E. Smith, Space Telescope Science Institute; John T. Trauger and R.W. Evans, Jet Propulsion Laboratory; and NASA.

Origins of life

The high lunar tides filled Earth's early oceans with the chemicals necessary for life to evolve under the influence of the Sun's radiation. While Solon would experience the same radiation, its oceans would fill at a snail's pace with the chemical building blocks of life. The paltry tides on young Solon would contribute little to enriching the oceans. The primary way that chemicals would enter the ocean would be through river flow. We see the same thing happening today at the mouths of rivers, but the rate is very, very slow compared to the effects of monster tides. Therefore, it would have taken longer for a critical mass of chemicals to fill the oceans. As a result, it would likely have taken much longer for life to evolve.

Biological evolution

Both the higher winds and the shorter days on Solon would have major effects on evolution. The winds would mediate against tall life forms that are not stabilized by their weight, broad bodies, or deep roots. Palm trees are a good example of a life form unlikely on Solon: These trees have shallow root systems and are easily knocked down by strong winds.

Tree-dwelling life would have a more difficult time on Solon, since tall trees there would sway more than they do on Earth. This does not necessarily mean there would be no ape-like creatures. Rather, it means that such creatures would have to be even more responsive to their environment than arboreal creatures on Earth. This could actually lead to even more complex brains in Solon's tree dwellers and, perhaps, different mental capacities.

Try to imagine what day-to-day life would be like with only three or four hours of sunlight each day. For one thing, life forms would evolve biological clocks with different cycles than those on Earth. Many activities of terrestrial life forms are regulated by internal biological clocks: Waking, sleeping, hunger, and mating depend on the circadian rhythms. Studies have shown that biological clocks in most creatures do not cycle in exactly 24 hours. For example, the dominant human circadian rhythm has a 25-hour cycle. Fortunately, sunrise resets, or entrains, clocks that are not precisely 24 hours long. But entraining can only occur if a biological clock is within three hours of the day-night cycle. On Solon, with its eight-hour day, animals possessing Earth-like biological clocks would quickly get out of sync. They would be sleeping when they should be awake, hunting when they should be mating, and so on. They would become vulnerable to attack by better-adjusted predators.

<< previous page | 1 | 2 | 3 | next page >>

back to Teachers' Newsletter Main Page