The origin of the planets and their mottled moons is a challenging, as-yet unsolved problem. Most of our knowledge of the Solar System’s formative stages comes from studies of galactic clouds, fallen meteorites, and the Sun and Moon, as well as from various planets and moons examined from a distance with peering telescopes and sometimes closer with robotic probes. Added insight now also comes from observations of planets circling nearby stars, a new and exciting area of research that has led to an avalanche of discoveries in recent years. Ironically, studies of Earth itself don’t help much since our planet’s youngest stages eroded away long ago.
All signs point to an Earth that formed nearly 5 billion years ago. Initially cool or moderately warm, our planet gradually heated enough to melt completely, partly from without because of violent, macroscopic asteroid bombardment but mostly from within owing to serene, microscopic radioactive heating. During its first 0.5-billion years of existence, Earth’s interior evolved, its crust solidified, and much of its atmosphere escaped. Change was initially rampant, energy flows surging. And although that change slowed thereafter, it produced nonetheless mountain ranges, oceanic trenches, and atmospheric rejuvenation. As environmental change continued, the stage was progressively set for the origin and evolution of life—including human life, an especially interesting, if seemingly anthropic, chapter in the cosmic-evolutionary story.
The learning goals for this epoch are: