The Formation and Evolution of the Solar System

The Origin of Earth's Water and the Role of the Asteroid Belt

Earth's abundant water, with an estimated mass of 6 x 10^21 kilograms, is widely believed to have been delivered from the early asteroid belt during a period of intense bombardment in the inner Solar System. Initially, Earth was too hot for water to condense, and the inner Solar System too warm for water to be incorporated into the forming planet. Thus, water was likely brought to Earth by icy planetesimals and planetary embryos from the colder, outer regions of the Solar System, influenced by the gravitational perturbations of Jupiter. The discovery of main-belt comets in 2006 provided additional support for the asteroid belt as a significant source of Earth's water. In contrast, isotopic studies suggest that comets from the Kuiper belt or more distant Oort cloud contributed a relatively small fraction of Earth's water. The panspermia hypothesis, which posits that life could have been transported to Earth via cometary or asteroidal material, is an intriguing concept but remains speculative and is not the prevailing explanation for the origin of life on Earth.

The Migration of Gas Giants and the Architectural Evolution of the Outer Solar System

The current positions of the ice giants, Uranus and Neptune, challenge the predictions of the nebular hypothesis, which suggests that planets form in situ from a protoplanetary disk. Given the sparse material and long orbital periods at their present distances from the Sun, it is more likely that these planets formed closer to the gas giants, Jupiter and Saturn, and subsequently migrated outwards. This migration, occurring over hundreds of millions of years, is a key factor in explaining the structure of the Solar System's periphery, including the Kuiper belt, the scattered disc, and the Oort cloud. These regions consist of numerous icy bodies, which are the progenitors of many comets. The Nice model provides a framework for understanding the dynamical evolution of the giant planets' orbits post-formation, including a pivotal 2:1 resonance between Jupiter and Saturn that precipitated the dispersal of Kuiper belt objects and the outward journey of Uranus and Neptune.

The Late Heavy Bombardment and the Continuation of Accretion in the Solar System

The outward migration of the gas giants is hypothesized to have instigated the Late Heavy Bombardment, an intense phase of asteroid and comet impacts on the terrestrial planets approximately 4 billion years ago. This cataclysmic event, which may have spanned several hundred million years, is recorded in the numerous impact craters on the Moon and Mercury. The bombardment period aligns with the earliest evidence of life on Earth, suggesting a possible connection between these impacts and the origin of life. Accretion, the process of growth by the accumulation of additional material, is ongoing, as evidenced by events such as the collision of Comet Shoemaker–Levy 9 with Jupiter. The Oort cloud, a distant reservoir of cometary nuclei, was formed from objects scattered by the giant planets and later sculpted by the gravitational forces of galactic tides, passing stars, and interstellar clouds.

The Dynamic History of the Asteroid Belt and Moon Formation

The asteroid belt underwent significant changes after the Late Heavy Bombardment, primarily due to collisional evolution. These collisions often led to the breakup of larger bodies and the creation of new asteroids. Some asteroids have moons, which are thought to have coalesced from the debris ejected during these collisions. The origins of moons in the Solar System are diverse, including formation from a circumplanetary disk, from the remnants of a massive impact, or by the capture of a passing body. For instance, the prevailing theory for the Moon's origin involves a colossal impact that sent debris into Earth's orbit, which then aggregated to form our Moon.