Wherever the water came from, Earth was awash in it from the start. As the sterilizing steam rained down for the last time, the curtain rose on a new drama. What was it like here when our planet first became infused with something that could be called life?
The young Earth was heavily cratered and largely covered in oceans. The circular rims of the largest craters formed rugged mountain arcs jutting sharply from the sea. Still reeling from the mighty collision that had made the Moon, Earth spun rapidly, completing a day and night in about five hours.* A dim Sun and a nearby, looming Moon bolted across the sky. The atmosphere that remained after the steam condensed was largely composed of carbon dioxide.
Baby Earth needed all the help she could get to keep warm, as several factors conspired to threaten a deep freeze. In its early days, the Sun was much dimmer than it is now. Throughout its life, our star has gradually been brightening as the hydrogen in its core burns slowly to helium, increasing the Sun's density, and requiring a hotter nuclear flame to fight off gravity. If the modern Sun is a hundred-watt bulb, at the time of Earth's formation it was only about a seventy. This wimpy Sun could not have kept Earth very warm without help. If the greenhouse effect had been as feeble then as it is now, the oceans would have completely frozen over. Earth would have been the solar system's largest skating rink, and it is questionable whether life of the kind we know would have formed.
The earliest known rocks, some 3.8 billion years old, are constructed from deposits of water-borne sediments, betraying the presence of a liquid-water cycle well before that date. Clearly, something was keeping the young Earth warm against the weak Sun and the frigid vacuum of space.
Given that our neighboring planets Venus and Mars have atmospheres of nearly pure carbon dioxide, the consensus these days is that the early Earth also had a thick atmosphere consisting mostly of CO2. As you well know from presidential debates and cereal boxes, CO2 is a greenhouse gas that warms a planet. Back then, the dreaded greenhouse effect was a good thing1 that efficiently trapped the weak sunlight, keeping Earth cozy and warm. Global warming to the rescue.
*The moon itself, dragging on Earth through the tides, has gradually slowed us to our current twenty-four-hour day.
fIt still is, actually. Without it we wouldn't be here. But anthropogenic global warming could quickly cause too much of a good thing.
A weak young Sun was not the only problem for a newborn Earth struggling against the elements to stay warm. Making matters worse were the thick clouds of obscuring dust constantly lofted high into the air by the continuing bombardment. Though the early, fearsome ocean-vaporizers, the two-hundred-mile-wide monsters, were finally gone from the inner solar system, a steady rain of lesser impacts continued to pepper the planets. Many of these were big enough to cause severe environmental changes.
An object only a few miles across falling from space will raise enough dust to darken the skies globally for a couple of years. When this happens, most incoming sunlight is absorbed by dust in the upper atmosphere and the surface grows dark and cold. Even now, such events still happen every 10 to 100 million years. The last big one was the "K/T impact" 65 million years ago, which did in the dinosaurs.* Long ago, when the solar system was still sweeping up the mess from planet formation, impactors came much more frequently. Between 4.3 and 4.1 billion years ago, several objects this large were hitting Earth every century, causing enormous temperature oscillations at Earth's surface. A thick cloud of light-absorbing dust intermittently shrouded our planet.
How can we be so sure about all this? After all, we're talking about a time that is, as I've already admitted, older than any preserved surface on the planet, and older than the oldest Earth rock ever found. Aren't we just guessing? Nope. Fortunately a well-preserved record extends back to this time. You can see it with your own eyes on any moonlit night.
Our nearest neighbor has not had nearly as interesting or eventful a life as has Earth. While Earth's surface has continually been remade by mountain building, weather, and life, destroying all traces of the earliest rocks and landscapes, the Moon is dead. Geologically, meteorologically, and biologically, it's dead. It just sits there, passively taking whatever space tosses its way, never washing its face with rain or regenerating its skin with plate tectonics. The moon's entire surface is much, much older than any place on Earth, and it has been getting shot up with craters for billions of years. Consequently, its pockmarked face preserves a record of the intense bombardment that hammered both Earth and Moon when remnants of the preplanetary swarm still men
*The "Cretaceous/Tertiary impact event" which ended the Cretaceous geological age and ushered in the Tertiary.
aced the inner solar system. The Moon serves as a cosmic rain gauge, recording the environment of near-Earth space back to circa 4.1 billion years ago. (Before that, the cratering itself was so vigorous that it continually obliterated all traces of earlier surfaces, so we have no direct record of the bombardment rate for the first half billion years of our planet's existence.)
The pockmarked face of the Moon tells us, without a doubt, that conditions on Earth's surface were dominated by the effects of explosive impacts right up to the time when life here first got started. The entire globe oscillated between periods of freezing dark gloom and hotter spells when the skies cleared and the surface was bathed in intense, deadly solar ultraviolet irradiation (the protective ozone layer would not be invented for billions of years).
These surface conditions were not healthy for children or other living things. For this reason, we think that life may have originated deep underground, or at the bottom of the oceans, places that provided natural fallout shelter from the cosmic bombs still wreaking havoc at the surface. A currently popular location for life's origins is at hydrothermal submarine vents on the ocean floor. Plenty of chemical energy was supplied by the hot, mineral-rich waters pouring out of these vents, and the deep ocean was relatively immune to the extreme environmental hazards plaguing the surface at the time when life seems to have gotten its start. As the impact storm raged above, the first glimmerings of life on Earth may have been safe and warm below the storm in an octopus's garden beneath the waves.*
*If life can get started at the bottom of a planet's ocean, caring little about hazards plaguing the surface environment, this could have interesting implications for life beyond the Earth. Keep this in mind when, in a few chapters, we return to the question of life on Europa, Jupiter's oceanic moon.
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