Tom Yulsman

Earth was formed through cataclysmic collisions and mergers of baby planets called planetesimals. Simply by chance, the rocky body that would grow through these mergers into our planet developed into one of the bigger guys in its neighborhood. Because it was more massive, it swept up everything within its gravitational feeding zone. The resulting mega-collisions are believed to have knocked our planet's spin axis off kilter. By causing variations in the amount of solar energy reaching different parts of the globe at different times of the year. Earth's tilt endowed us with seasons, and it may also contribute to the comings and goings of ice-age glaciers. Both aspects of Earth's climate system—seasonal variation and long-term climate cycles like ice-ages—have encouraged biodiversity by creating a wide variety of habitats and putting pressure on species to adapt to new conditions.

But if Earth's tilt, or obliquity, were to get too far out of whack, life-threatening climate swings could result. Luckily, the Moon's gravity appears to prevent that from happening. According to Lynn Rothschild, an astrobiologist at the NASA Ames Research Center, without the Moon's stabilizing grip. Earth's obliquity could vary enough to cause large variations in the amount of solar radiation reaching us. To survive, 'you'd better be very small, very versatile, and live in the water,' she says. And what about humans? 'Earth would not be a good place for us.' Where did the Moon come from? It appears to be another legacy of the Earth's violent upbringing. Scientists believe it formed when an object the size of Mars crashed into the Earth about 4.51 billion years ago. According to this theory, the impact splashed a huge volume of melted debris into orbit-some from the Earth and some from the impactor itself—which eventually cooled and coalesced to form the Moon.

The pummeling from planetesimals, comets, asteroids and other bodies while Earth was accreting had other important effects as well. The impacts delivered cargoes of water and gases from the solar nebula, leaving the Earth a steaming and fuming mess. Once the rate of impacts trailed off enough, and the Earth cooled sufficiently, the planet's gravity clutched gases such as nitrogen and carbon dioxide close to the surface, forming the atmosphere. Later, water vapor began to condense and rain out, forming a hydrosphere—a salient event in the pre-history of life. Liquid water is considered a prerequisite for life because nutrients and waste products can dissolve in it, essential chemicals can be moved within it, and biochemical reactions occur with it.

The construction of planet Earth and the Moon was essentially complete by about 4.47 billion years ago. Unfortunately for geologists, detailed evidence of what happened on Earth over the next 500 million years has been obliterated by erosion. Not so on the Moon, however, where wind and water have never abraded and scraped the surface. As a look through a pair of modest binoculars will reveal, the Moon's face bears the scars of continued bombardment by solar system debris. Analysis of those scars and rocks brought back by Apollo astronauts have allowed scientists to calculate the rate of impacts on both the Moon and Earth.

There is some question as to when the bulk of the big, early impacts occurred. According to the conventional view, a continuous but slowly declining rain of solar system debris fell down on the Moon and Earth after the lunar-formation event. These were not planet-sized bodies like the impactor responsible for the formation of the Moon. The bodies included comets and very large asteroids, as well as smaller chunks of solar system flotsam and jetsam. The very largest of these impacts occurred earlier and trailed off in frequency as time passed. But based on another interpretation of the lunar impact record, some scientists believe the rate of impacts dropped steeply right after the Moon formed, and then spiked catastrophically about 3.9 billion years ago in an event known as the late-heavy bombardment.

Whether they were concentrated in a spike or spread out over some 600 million years, some of the impacts that occurred even after the Moon and Earth were complete were huge. The largest, telling impact scars on the Moon are dark basins on the front side that vaguely suggest a human visage. The biggest seems to be the South-Pole/Aitken Basin, which is about as wide as the distance between Chicago and the southern tip of Florida, or London and Athens. To gouge out that much rock, the colliding object (which hit about 4.1 billion years ago) had to have been about 200 kilometers across, or roughly as wide as the state of Massachusetts. Just a little smaller is the Imbrium Basin, 1500 kilometers across and gouged out by an object at least 100 kilometers in size. According to Kevin Zahnle, a scientist at the NASA Ames Research Center, the Moon suffered about a dozen such impacts in its early history.

The Moon clearly suffered terribly as it took one hit after another on the chin. In the region of the big impact basins, its rocky crust thinned so much that molten rock welled up to the surface, filling the basins with oceans of magma that subsequently hardened into darker, lowland rocks. Meanwhile, the Earth, being a heavyweight in comparison to the Moon, had a more powerful gravitational grasp. 'Earth, being bigger, picks out the largest available impacts,' Zahnle says. As a result, our planet suffered hundreds of impacts of the kind that dug out the big basins on the Moon.

Zahnle uses an unconventional method for categorizing the various sizes of impacts that afflicted Earth: the U.S. Department of Agriculture's ratings for olives. So a Super Colossal impact is big enough to splash out enough material to form the Moon. A Colossal melts the entire crust of the Earth. A Jumbo vaporizes the oceans. An Extra Large vaporizes the photic zone of the oceans. And a Large cauterizes the continents.

Just a single jumbo impact would have created a hell on Earth. As Zahnle describes such an event, the energy of the impact evaporates large amounts of rock. The result? A noxious, rock vapor atmosphere 'a hundred times thicker than the atmosphere we know and love,' he says. Toward the top, cooling causes clouds to form—not the ordinary kind made of water but rock clouds made of silica. Lower down, temperatures exceed 2000 Kelvin, causing the sea surface to boil off. 'It takes a few months at most to evaporate the oceans,' Zahnle says. Then the rains come. But not water. As the rock vapor condenses, rock drops pelt Earth's surface. Once the rock rains out, a steam atmosphere is left behind. With this insulating blanket thrown over the planet, heat escapes slowly. According to Zahnle, it takes about 3000 years for conditions to cool enough for the steam to condense. Then, in perhaps just a few years, torrential downpours 'give you your oceans back,' he says.

But pacific conditions would not have lasted for long. Based on the lunar impact record, scientists have estimated that during its early years, about five ocean evaporators hit Earth. The last of these cataclysms could have occurred as late as 3.8 billion years ago. So woe to any early life forms that may have managed to get it together in a fetid tide pool during this period.

  The World was all before them, where to choose
Their place of rest, and Providence their guide:
They, hand in hand, with wand'ring steps and slow,

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Through Eden took their solitary way.