Named by Preston Cloud for the Greek underworld, the Hadean Eon refers to that time period when the earth began as a planet circling the sun (not just a thick dust cloud, but a real planet) to the time of formation of the oldest rocks that still exist in the geologic record. Lacking hard-rock data, much of the characteristics of the Hadean are speculation: what is known to have been in existence by the time of the beginning of the next eon, the Archean, by necessity, must have been created in the Hadean.
So, let’s go back in time starting with the earth’s birth at ~4.54 billion years ago (bya) and “speculate” what the earth was like until the end of the Hadean around 3.8 bya.
In the beginning there was…
--No moon. The moon was created shortly after the earth; possibly it was born from a collision of the young earth with a large asteroid, or was pulled out of the earth by the passing of a large celestial body several hundred years after the earth’s creation. However formed, the young lava-rich moon was much closer to the earth than it is today. The tides in speculative incipient seas of the young earth would have been tremendous. But more horrendous would have been the planetary tides caused by the pull of gravity between the earth and moon, tearing apart the interior of the planet on a regular basis. This sort of tide causes a great rise in the internal frictional heating within the planet and could induce overwhelming volcanism.
--A planet under continual bombardment of meteors, asteroids, debris remaining from the solar system’s formation. By comparing the relative abundance of impact craters on the moon, this continuous shelling waned throughout the Hadean until nearly its end. At about 3.95 billion years ago, there was a resumed period of meteorite strikes so heavy as to earn the title of the “Late Heavy Bombardment.” Some speculate that this bombardment was connected to the destruction of a possible 5th planet of the solar system, today represented by the asteroid belt past the orbit of Mars. There is also speculation that any early attempts at life might have been annihilated by such a bombardment.
--There was no distinct core or mantle within the earth. It is thought (borderline speculation) that the earth’s interior was primarily heated by the decay of radioactive minerals, something that goes on today but at much lower activity. Consider as well: with no molten core, there would not have been a magnetic field; without a magnetic field, there would be no protection from incoming solar radiation. Perhaps the formation of a magnetic field and differentiation of the planetary interior are the Hadean’s great achievements.
--A faint young sun. This sun of the Hadean was not the powerhouse we know today, but gave the young earth ~ 70% of the energy it does now. Think what affect this lack of solar energy had on temperatures within the early atmosphere. Surface temperatures would have been far too low for liquid water. So was the surface cold? But what about the heating affects from the prevalence of lavas and hot blasts of meteorite strike, steam from volcanic eruptions, not to mention the possible “greenhouse” effects of the early atmosphere?
--An early atmosphere totally alien to what we know today. Many speculate the early atmosphere to be rich in hydrogen and helium because these gasses would have been left over from the formation of the solar system. The entrance of gasses into the atmosphere from the coalescing of the interior of the planet is given a name: outgassing, but this doesn’t really explain how this happened. Active volcanoes could have provided water vapor, ammonia, methane, carbon dioxide, and sulfur-rich vapors. Even a faint sun could slowly break that ammonia down to release nitrogen (the most abundant element in today’s atmosphere). But for there to be a significant greenhouse warming of the atmosphere during this epoch of the faint young sun, the atmospheric pressure would have to be much greater than that present today.
--There definitely wasn’t any oxygen in the atmosphere. Bacteria that makes up the earth’s oldest fossils (~3.4 bya) was anaerobic; conditions of bacterial life at the start of the Archean were less than 1-2% of the present atmospheric levels of oxygen.
--There was no crust of the earth as we know it. There are many speculations on what the initial surface of the world was like. Possibly the world view was one of vast surface expanses of lava or equally possible it was to a poorly patched together surface of meteoritic-rich material frozen beneath the faint young sun. Zircons dated at 4.3 billion years suggest the existence of some incipient crust. Though the oldest rocks are 3.8 million years in age, these are gneiss, meaning they were metamorphosed from rocks even older, which means some early form of the rock cycle was already in operation.
--No oceans. The earth probably cooled enough to support liquid water on its surface within several hundred million years of its formation. Yet it is questionable if there were indeed oceans or, if there were early oceans, they would have been able to survive the Late Heavy Bombardment period. The oldest sedimentary rock yet identified (dated at 3.8 billion years) attests to the activity of water’s ability to aid in deposition of rocks by the latest Hadean. There may not have been an ocean, but that water was going somewhere.
--There was no life… or was there? Speculations suggest that there were life-conducive conditions in the earliest Hadean, conditions at least as favorable as those of deep oceanic hot springs and within cracks inside shallow warm rocks. Possibly early life would have been blasted into extinction by the Late Heavy Bombardment, making that the first great extinction on earth.
Speculate: All these processes mentioned above were occurring simultaneously. What do you think the early hell on earth was like?
And by the way, this time distinction is not universally accepted by stubborn stratigraphers in charge of assigning geologic eras to earth history, possibly because the Hadean lacks strata.
Photo: a collage of my graphics: Upper left: the early moon. Lower left: zircons, some of the few remnants left from this period. Center right: the “faint young sun.” Center: some early gneiss. Background:http://www.dipity.com/