A magnetic shield may have saved Earth's air from the fate of Mars
About 3.5 billion years ago Earth switched on its magnetic field. The young Sun was blasting out a solar wind roughly 100 times stronger than today's, and without that shield our atmosphere might have been stripped away, as probably happened on Mars. Even so, the early magnetosphere reached only about half its present size.
Earth assembled about 4.54 billion years ago from the disc of dust and gas left over after the Sun formed. Volcanism and constant collisions kept it molten at first, until the outer layer cooled into a crust. Soon afterwards the Moon appeared, probably after a giant impact; recent potassium isotope work favours a smaller, very energetic blow that sliced off a large piece of Earth, some of which merged back and some of which gathered in orbit. Volcanic gases built a first atmosphere, and condensing vapour made oceans, although a 2020 study suggested enough water may have been present from the start.
The earliest eon, the Hadean, is really defined by a lack of surviving rock; the oldest dated zircon crystals are about 4.4 billion years old. A proposed Late Heavy Bombardment, some 4.1 to 3.8 billion years ago, would have cratered the inner planets, but sceptics note it rests on samples from only a few lunar hotspots. Whether plate tectonics ran from the beginning or replaced some vertical style of crustal churning is also argued over. Together, the three eons before the Cambrian cover roughly 90 percent of geologic time.
Continents have gathered and scattered repeatedly. Rodinia, the earliest known supercontinent, began splitting around 750 million years ago; the pieces regrouped as Pannotia between 600 and 540 million years ago and later as Pangaea, which broke up 200 million years ago. The first glaciations came in the Proterozoic, peaking in the Snowball Earth of the Varangian glaciation.
Ice has shaped life too. At the end of the Ordovician, with Gondwana, the great southern landmass, sitting over the pole while carbon dioxide fell from about 7,000 to 4,400 parts per million, a short ice age triggered the second-largest mass extinction, between 447 and 444 million years ago. Only the Permian-Triassic die-off was bigger. Our own cycle of glaciations started roughly 40 million years ago, and its most recent cold spell ended around 10,000 years ago.
Source: Geological history of Earth
