The first atmosphere was mostly water vapor, but by 3.8 million years ago the atmosphere had changed to mainly CO2 (carbon dioxide) and methane. What happened to the water vapor? A. Chemical reactions from the heat and solar radiation changed it into the CO2 and hydrogen gas.
B. The early earth's gravitational field wasn't strong enough to trap it, and it escaped into space.
C. Condensation over time converted it to rain and then eventually oceans.
D. The water vapor bound with silicon, sodium, and other light metals forming sediment that settled on the surface and fused into the earth's crust.