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 its atmosphere and nearly all its water. Earth kept both. 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. 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