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Subsequent Evolution in Earth Origin | Article 4

This is Fourth Article of Origin of Earth.

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Subsequent evolution question during the last 20 years earth origin.

The planets were originally thought to have formed in or near their current orbits. However, this has been questioned during the last 20 years. Currently, many planetary scientists think that the solar system light might have looked very different after its initial formation: several objects at least as messive as Mercury were present in the inner solar system, the outer solar system was much more compact it is now, and that was kuiper belt was much closer to the sun.

Earth Origin
Earth Origin

Terrestrial planets

At the end of the planetary formation epoch the inner solar system was populated by 50—100 moon to mars-sized  planetary embryos. Further growth was possible only because these bodies collided and merged, which took less than hundred million years.These objects would have
gravitationally interacted with one another, tugging at each others orbits until they collided, growing larger until the four terrestrial planet we know today took shape. One such giant collision is thought to have formed the moon, while another removed the outer envelope of The Young Mercury.

One unresolved issue with this model is that it cannot explain how the initial orbits of the proto terrestrial planets, which would have need to be highly eccentric to collide,  produced the remarkably stable and nearly circular orbits they have today. One hypothesis for this eccentricity dumping is that the terrestrial formed in a disk of gas still not expelled by the sun. The gravitational drag of this residual gas would have eventually lowered the planets energy, smoothing out their orbits. However, Such gas, if it existed, would have prevented the terrestrial planets orbits from becoming so eccentric in the first place. Another hypothesis is that gravitational drug occurd not between the planets and residual gas but between the planets and the remaining small bodies. As the large bodies moved through the crowd of smaller objects, the smaller objects, attracted by the largest planets gravity, formed region of higher density, a "gravitational wave", in the larger objects path. As they did so, the increased gravity of the wake slowed the larger objects down into more regular orbits.

Asteroid Belt

The outer edge of the terrestrial region, between 2 and 4 AU from the sun, is called the asteroid belt. The asteroid belt initially contained more than enough matter tu to form 2-3 Earth like planets, and, indeed, a large number of planetesimals formed there. As with the terrestrials, planetesimals in this region later coalsced and formed 20-30 moon to Mars sized planetary embryos however, the proximity of Jupiter meant that after this planet formed, 3 million years after the sun, the regions history changed dramatically. Orbital resonances with Jupiter and Saturn at particular is strong in the asteroid belt, and gravitational interaction with more massive embryos scattered many planetesimal into those resonances.Jupiter's gravity increased the velocity of object within these resonances, causing them to shatter upon collision with other bodies, rather than accrete.

As Jupiter migrated inward following its formation, resonances would have swept across the straight belt, dynamically exciting the regions population and increasing their velocities relative to each other. The cumulative action of the resonances and the embryos either scatter the planetesimal away from the asteroid belt for excited the orbital inclinations and eccentricities. Some of those massive embryos too were ejected by Jupiter, while others may have migrated to the inner solar system and played a role in the final accretion of the terrestrial planets. During this primary depression period, the effects of the giant planets and planetary embryos left the asteroid belt with the total mass equivalent to less than 1% that of the earth, composed mainly of small planetesimals. This is still 10-20 times more than the current mass in the main belt, which is now about 0.0005 M. Secondary depletion period that brought the asteroid belt down close to its present mass is thought to have followed when Jupiter and Saturn and entered temporary 2:1 orbital resonance.

Earth Origin
Earth Origin

The inner solar system speed of giant impact probably played a role in the earth acquiring its current water content from the early asteroid belt. Water is too volatile to have been present at Earth's formation and must have been subsequently deliver from outer, colder parts of the solar system. The water was probably delivered by planetary embryos and small planetesimals thrown out of the asteroid belt by Jupiter. UP population of men belt comets discovered in 2006 has been also suggested as a possible source of Earth's water. In contrast, comets from the kuiper belt for further regions delivered not more than about 6% of Earth's water. The planetesimals hypothesis holds that life itself we have been deposited on earth in this way, Although this idea is not widely accepted.

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