Mars is the fourth planet from the sun and one of the four terrestrial worlds in our inner solar system, including Earth's moon. While each of the terrestrial worlds holds secrets about planetary evolution, Mars has the ideal combination of features for a study of rocky planet evolution. Why?
- Mars is big enough to have undergone most of the early processes that shaped the other terrestrial planets: accretion (or the melding of space debris), melting, core separation and differentiation, and subsequent cooling by conduction, convection and volcanism. So an understanding of Mars' evolution can essentially be translated into an understanding of Mercury, Venus, Earth and the Moon's evolution, too.
- Mars is small enough to have retained much of its historical record. In contrast, the Earth, with its vigorous internal convection and a plate tectonic cycle that recycles its crust through subduction and sea floor spreading, retains very little record of its initial building blocks. Similarly, Venus appears to have undergone complete resurfacing of its crust. Mercury, though smaller than Mars, may have evidence for early processes, but its surface is so inhospitable that a detailed investigation of its interior may be impossible. And due to its small size, pressures inside the moon are simply too low to subject rocks to the same processes that occurred inside the other rocky planets in the solar system. Mars, on the other hand, is like an history book that simply requires a little "digging into" to yield its secrets.