Zircon evidence implies Earth's habitability could have existed as early as how long after formation?

• A. 50 million years
• B. 200 million years
• C. 900 million years
• D. 100 million years

Answer: (D)

Zircon crystals act like tiny time capsules, preserving clues about early Earth conditions that survive geological recycling. The key idea here is that certain isotopic signatures in ancient zircons show the presence of liquid water and the formation of continental crust very early in Earth's history.

Ancient zircons found in the Jack Hills record ages around 4.4 billion years. Their oxygen isotope ratios indicate interaction with surface water, while hafnium isotopes reveal early crust formation. Putting those clues together means Earth could have had a surface environment with water and a stable crust not long after it formed—roughly 100 million years after formation. That makes this the earliest strongly supported window for habitable conditions based on zircon evidence.

Options implying 50 million or 200 million or 900 million years after formation don’t align with the zircon-derived timeline and the earliest crust-water signatures, which point to around 100 million years.