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There is no logical reason, other than relative size, to consider comets, asteroids, or meteoroids as completely different classes of objects. They appear to be different only because each is at a different stage of its life cycle, starting as high-velocity inbound comets from other galaxies that gradually shed fragments and dust particles that become building materials for bodies already in our solar system. Pure chance, size, and relative gravitational power dictate where each fragment or dust particle eventually ends up as part of some larger body. They form a continuum from comets to asteroids to meteoroids to dust and back again into planets, to larger planets, and then into suns and stars that eventually explode into supernovae to repeat the process--Nature’s ultimate recycling plan.
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Accreation of a planet begins with the Sun's gravitational capture of a stray hyper-velocity comet that brings it into a large parabolic solar orbit. The origin of comets is unknowable, but their extreme velocities suggest they are fragments of some planetary body flung into space as the result of a supernova explosion of a solar body in this, or some other, galaxy. This also provides an explanation for the wide range of sizes and chemical compositions seen in meteorites.
The orbit of any newly-captured comet, whether prograde or retrograde, is completely random and entirely dependent upon the comet's initial angle of entry into the Sun's gravitational sphere or that of any solar body that might capture it or its fragments. A retrograde or highly inclined orbit suggests the body is a relative newcomer to its host system and has not had time to be drawn into its host’s ecliptic plane.
Each solar orbit gradually reduces the comet's velocity and size by fragmentation and ablation until it reaches a stable size, either as an asteroid or meteoroid, orbiting the Sun in a more regular orbit--or collides with an existing solar body to create a larger body or disintegrate both into smaller pieces that may or may not re-amalgamate into a larger body. A major factor in such amalgamations are the relative sizes and paths each body is traveling at the time of impact. If both are traveling in the same general direction at nearly identical velocities it is possible both could come together as gently as two feathers.
Over time, each orbiting solar body, whether planet, asteroid or meteoroid, attracts smaller particles and dust, thereby slowly but continuously increasing in size and gravitational power. Its increased size and gravitational power then enables it to attract ever-increasing volumes of meteor dust, micrometeorites and meteorites--accelerating the accreation process over time.
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This passive accretion of extraterrestrial material causes each solar body to slowly increase in size and mass until it reaches spherical shape at a diameter of ~400-600 km. Sphericity is the critical point at which gravitational force can be focussed omnidirectionally on the exact center of the proto-planet to produce gravitationally-induced compressive heating that initiates formation of an expanding molten core and commences distillation of minerals, water and other volatile gases known to exist in all rocks. Water makes up 8-10% of most rocks.
Upon attaining critical sphericity, the passive accreation process becomes dynamic. Expansion of the heated molten core creates an irresistible tectonic force that eventually ruptures its confining outer shell and releases magma and gases to the surface through a volcanic vent or surface graben (such as volcanos or Valles Marineris on Mars), thereby commencing formation of an atmosphere and hydrosphere, while any extruded magma creates or modifies surrounding surface features of the protoplanet. Continued thickening of the atmosphere also increases the ablative effect of friction on inbound meteoroids.
With increasing size and formation of an atmosphere, solar insolation and molecular H2O can combine to activate dormant biotic cells (if they exist) in the soil to begin the cycle of life on the planet. (Some will argue, correctly, that there is no proof of dormant biotic cells in extraterrestrial matter that have survived a journey of billions of years through space, but recent discoveries of living organisms in rocks deep underground may change this view. The only alternative is to assume that life forms have arisen spontaneously on Earth as combinations of chemical elements. Scientists are still struggling with this profound conundrum, but western religions find their own explanations in Genesis.)
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Gradually, "creation by accretion" moulds the surface configuration and chemical composition of each planet, dependent on the unique chemical composition (stony or metallic) of the accreting dust and meteorites. Earth's known meteorites come in all shapes and sizes, but only 10% are composed of nickel-iron and immediately recognizable as extraterrestrial. The other 90% of Earth's meteorites (chondrites) are indistinguishable from ordinary field stones. The chondrites ablate easily during atmospheric entry to produce very fine dust and micrometeorites, but larger meteors often break up on transit through the atmosphere and produce sizable stones that lie unnoticed. One of the most recent discoveries was a 2.2 pound (1 kg) meteorite containing halite crystals that fell to earth 22 March 1998 near Monahans, Texas, and is now being studied by NASA.
For analog examples of the accreation process, closely examine photos of the surfaces of Moon and Mars, each of which is now the same size as the archaean Earth had reached at a similar stage of development. The Moon's surface is covered with very fine dust, and older impact craters are now filled in by dust particles, as shown by later impact craters within larger craters. The Moon has no atmosphere to ablate impacting meteorites, so accreted dust covering the near side of the Moon consists of small particles of cosmic dust and micrometeorites that have partially buried the larger impact craters.
In contrast, the far side of the Moon is more heavily cratered, but still shows evidence of accreted dust. Because of its proximity to Earth and the fact the Moon does not rotate about its own axis, heavier cratering on the far side may result from the Moon "shortstopping" meteors that might otherwise strike Earth when both are aligned with meteor stream trajectories. When their positions are reversed, the Earth's larger size protects the near side of the Moon even more effectively. (Explanation provided by Emily Compson, one of my eight lovely granddaughters.)
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