A Walk Through Time

Settling In

4400 Million Years Ago

Radioactive elements inside Earth decay and planetesimals bombard Earth and her sister planets, causing tremendous heating. Heavy metals sink to form cores, while volcanoes spew lighter elements into the atmospheres. This "out-gassing" transforms the early atmospheres.

The sister planets of the inner solar system — Mercury, Venus, Earth, Mars — reach their current configurations during this phase.

Mars-scape. We journey into deep time when we journey into space. As we study planetary evolution, we better understand the environmental evolution of Earth and the central role life plays in shaping our home planet. (Photograph courtesy NASA)

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