2.2.1 Planetary heating
Early modeling of planetary formation assumed that planets formed cold and heated up through later decay of radioactive elements (Toksoz and Hsui, 1978; Solomon, 1979). However, the realization that accretionary processes dominate planetary formation led astronomers to realize that planets are formed hot and gradually cool over time (Grossman, 1972; Benz and Cameron, 1990; Boss, 1990). The heat comes from two main sources: the kinetic energy of impacting bodies is transformed in part into heat during accretion (accretionary heating), and radioactive elements, particularly short-lived radioisotopes, generate heat during decay. Complete melting of a Mars-sized object requires approximately 2X1030 joules (J) of energy. For Mars, accretionary heating likely produced ~4X1030 J (e.g., Wetherill, 1990). Among the short-lived radionuclides, 26Al likely contributed the most heat, with estimates around 2X1030 J (Elkins-Tanton et al., 2005). Thus, accretional heating and short-lived radionuclide decay can easily produce enough energy to melt Mars.
This semi-fluid state of the planet allows materials to segregate into layers, a process called differentiation. Differentiation occurs because of density variations and differing chemical affinities of the elements. The variation in density among different elements causes the denser material to sink towards the planet's center while less dense materials rise to form the surface. In particular, iron tends to sink towards the center to form a core while oxygen compounds rise towards the surface to create the crust. Certain elements, called siderophile elements, have a chemical affinity for iron and will follow iron (Fe) to the core. Examples of siderophile elements include nickel (Ni), cobalt (Co), iridium (Ir), and platinum (Pt). Elements that follow oxygen (O) are called lithophile elements; examples include potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), and aluminum (Al). Elements that tend to form compounds with sulfur (S) are called chalcophile elements and include zinc (Zn), copper (Cu), and lead (Pb). The combination of density and chemical affinities explains why iron and sulfur compounds are typically found deep in planetary interiors while planetary crusts are largely composed of oxides.
Information about Mars' early evolution comes from analysis of radioactive elements and their decay products in martian rocks. None of the spacecraft missions to Mars has yet returned soil or rock samples which can be analyzed in terrestrial laboratories. The lander/rover missions are beginning to supply mineralogic information which can provide some insights into the planet's bulk composition, but instrument capabilities limit the amount of information obtained by these missions. However, nature has provided scientists with samples of the martian surface in the form of martian meteorites. Geochemical analysis of the 37 martian meteorites currently in our collections provides important insights into the early history of the martian interior. The thermal evolution of Mars is constrained through both geochemical analysis and geophysical modeling.
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