7 - Ages of planetary surfaces

Abstract

On Earth, geologists have many techniques available to determine the absolute ages of minerals, rocks, and unconsolidated deposits; but obtaining absolute ages requires actual samples of materials collected in controlled ways from known locations. To date, such samples exist only for a handful of locations on the lunar surface. For all other extraterrestrial surfaces, ages are constrained through geologic mapping and the Principles of Superposition, and combining models of impactor flux with observed impact crater size-frequency distributions (CSFDs). Unfortunately, many processes can affect observed CSFDs: both erosion and deposition can alter observed crater diameters; surface properties (such as composition) affect both the initial crater dimensions, and how those dimensions change with time. Thus, there are few absolute age constraints on volcanic products throughout the Solar System. Mercury’s volcanoes were mostly active ~3.9–3.5 Ga. On Venus, the small number of impact craters gives an average surface age of ~700–800 Ma; there is evidence suggesting that Venus may still be volcanically active. Lunar volcanism is volumetrically dominated by the maria, and most of the maria were emplaced between ~4.0 and 3.6 Ga. Mars appears to have experienced near global volcanic activity until ~3.7 Ga; by 1.6 Ga, volcanism was focused in the Tharsis and Elysium regions. In the outer Solar System, Jupiter’s innermost moon Io is volcanically active and uniquely displays no impact craters on its surface. In contrast, Ganymede and Callisto display some of the most heavily cratered surfaces in the Solar System.