Fig. 3. Schematic timeline for Mars and Earth.

Comparison of the key events in the environmental histories of Earth and Mars. For example, Mars has lost the bulk of its early atmosphere, followed by loss of its hydrosphere, whereas Earth has always retained both. Therefore, the formation of hydrated minerals continues on Earth but ceased early in Mars’ history. On the other hand, Mars can offer evidence for geochemical conditions similar to Hadean and Archaean Earth during the epoch of Earth’s prebiotic chemistry. This evidence has been mostly erased from Earth’s rock record: This is illustrated by the panels marked “Surface age,” referring to crater retention age for Mars, and preserved rock record for Earth.