EARTH

The Magnetic Shield Switches On

~3.9 billion years ago (claims reach 4.2 Ga)

Archean · Eoarchean

Currents of liquid iron, churning in the Earth's core, generate a magnetic field that reaches far beyond the atmosphere. It deflects the stream of charged particles the Sun throws out, a wind that would otherwise strip away the upper air, molecule by molecule.

Why it matters

Mars has no such field; Mars also lost most of its atmosphere and nearly all its water. Earth kept both. Much of the difference is a core that never stopped churning.

Dating & uncertaintyDisputed

Magnetic grains trapped inside ancient crystals have been read as a record of the field 4.2 billion years ago. An electron microscopy study argues that every grain examined formed long after its host crystal, which would cancel that reading; the team that made it has answered with fresh microscope and age measurements, arguing that some grains are original after all. Crystals from Barberton, in South Africa, give a steadier record from about 3.9 billion years.

Sources

1. Tarduno et al. (2015), Science · A Hadean to Paleoarchean geodynamo recorded by single zircon crystals
2. Tang et al. (2019), PNAS · Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo
3. Tarduno et al. (2020), PNAS · Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo

Earth history as one day

03:22:5920 h 37 min before midnight

if Earth’s 4.54 billion years were compressed into a single day