Zircon crystal analysis suggests Earth may have been habitable as little as how long after its formation?

• A. 100 million years
• B. 1 billion years
• C. 10 million years
• D. 500 million years

Answer: (A)

Zircon crystals are incredibly durable record-keepers of early Earth, preserving clues about crust formation and interactions with water. Scientists use isotopic systems in zircons, especially oxygen and hafnium, to infer when liquid water was present and when the continental crust started to form. In some of the oldest zircons, around 4.4 billion years old, evidence of water-rock interaction suggests that oceans and a surface environment capable of supporting liquid water existed not long after the planet formed. Since Earth formed about 4.56 billion years ago, this points to habitable conditions arising roughly on the order of 100 million years after formation. The other timeframes are too short or too long to match the earliest water signals we glimpse in these zircons, making about the 100-million-year mark the best fit.