Hadean

The Hadean is the earliest geologic eon in Earth's history, spanning roughly from 4.6 to 4.0 billion years ago. The name, drawn from Hades, reflects the intense heat and volatile conditions believed to have characterized the planet's formation and early evolution.

During the Hadean, Earth accreted from the solar nebula and experienced a colossal, Moon-forming impact that shaped its early structure. The planet was likely covered by magma oceans and subjected to heavy bombardment by planetesimals. The early atmosphere is thought to have been outgassed from the interior and comprised predominantly water vapor, carbon dioxide, nitrogen, and sulfur-bearing species, with light gases gradually escaping to space. Surface conditions were extremely hot, and the crust, if present at all, was continually remelted and recycled.

Evidence for Hadean processes is primarily geochemical and isotopic rather than intact rock records. The oldest minerals, notably zircon crystals found in younger rocks, date to about 4.4 billion years ago, indicating that crustal differentiation and some presence of liquid water may have occurred by late Hadean times. However, no continuous rock record from the full eon survives, so much of what is known comes from indirect evidence and theoretical modeling of early Earth conditions. The emergence of more stable crust and a growing hydrosphere is commonly placed toward the end of the Hadean, marking the transition to the Archean around 4.0 billion years ago.

Understanding the Hadean informs models of planetary formation, early atmospheric evolution, and the conditions that preceded the origin of life on Earth. The eon is defined by time, not by a preserved stratigraphic sequence, making its study dependent on radiometric dating and extraterrestrial insights.