A few weeks ago, I posted a TNRTB describing the special circumstances in Earth's early history that ensured we had an adequate supply of osmium and iridium with which to develop a technological civilization.
Further developments indicate that the carefully orchestrated events on early Earth played an even more critical role in establishing this planet's habitability. Without those events, Earth would not enjoy an abundance of liquid water or the life-essential plate tectonics it facilitates.
Consider this brief description of how the solar system formed. A gas cloud began collapsing, resulting in a disk of material around the protosun at the center of the cloud. As the particles in the cloud interacted, they grew in size. At some point, these particles began to gravitationally attract other material from the disk until they became planet-sized. The last step of planet formation occurred when these planet-sized objects merged to form Mercury, Venus, Earth, and Mars. The gas giants formed in a similar fashion except they grew rapidly enough to gravitationally attract a sizable fraction of hydrogen and helium before the solar wind blew these gases out of the solar system.
Until recently, scientists believed that the oceans that currently cover Earth (similar to the water that once covered Mars and Venus) arose when water in the formation material escaped to the surface. However, a growing body of evidence now indicates that these Earth-forming materials did not contain enough water to form the oceans. This is because Earth resides inside the snowline, locations closer to the Sun than the asteroid belt. Inside the snowline (indicated by the red region in the image-link below) high temperatures kept any ice from forming within the materials that made up Earth. Consequently, the solar wind would have driven all the water from this region before it could be incorporated into the planets.
This new evidence meant astronomers needed to modify their model to explain why Earth has abundant liquid oceans. The simplest modification added a period of asteroid bombardment at least a hundred million years after Earth formed. A suitably large number of asteroids that originated outside the snowline could deliver enough water to account for the oceans and the water inside Earth. As with any good model, additional data must support the model's predictions. A review article in Nature provides data to support this updated model.
Here are some of the results the paper presents:
- The amount of zinc and potassium (compared to uranium) in terrestrial material and Martian meteorites falls well below the carbonaceous chondrites that represent the primordial material in the solar system. These elements condense at lower temperatures than uranium but at higher temperatures than water. Thus, if the material that formed Earth is depleted in zinc and potassium, it was certainly depleted of water also.
- The depletion of heavier isotopes of zinc matches the depletion of the lighter isotopes. If the volatile materials were lost during the accretion stage, the heavier isotopes would show less depletion compared to lighter isotopes because the Earth's gravity would bind them a little more tightly.
- Analysis of the radioisotopes hafnium and tungsten indicate that the impactor that formed the Moon occurred around 30 million years after Earth started forming. Additionally, rocks from the Moon exhibit less water than Earth's mantle. If Earth had a significant amount of water at the time the Moon formed, the moon would have ended with much more water.
- Analysis of xenon and lead isotopes indicate that most of these elements arrived at least 100 million years after the solar system started condensing. This is consistent with a model predicting a bombardment of asteroids that brings Earth its water.
- Without water, plate tectonics does not operate on a planet Earth's size. Without the addition of substantial water after the moon impact event, all of Earth's water would have been buried deep in Earth's interior (like the fate of Venus' initial water supply). However, a late verneer of water arriving from asteroid impacts would ensure that Earth's surface remains covered in water even after 4.5 billion years of plate tectonics. During that period, roughly half the water would be subducted into Earth's interior, matching measurements that indicate roughly an oceans' worth of water resides inside Earth.
So what is the bottom line?
Without an event that brought an abundance of asteroids from the outer regions of the solar system crashing to Earth, our planet would not have maintained a stable water cycle. Had this bombardment occurred too early, the water would have ended up buried deep inside the Earth instead of forming a life-essential liquid ocean. Only by the proper timing of this asteroid bombardment did Earth become habitable. Advances in our understanding of how our home developed continue to support the idea that a super Intellect worked to provide a place for humanity to reside.