Age of Mars

Constraints on the accretion and differentiation of Mars come from the application of four isotopic systems to Martian meteorites  

Pu-136Xe. Accretion is not directly datable by any of these clocks, so we must rely on dates from early differentiation events to put constraints on the accretion history of Mars. 

Chronology of the solar system from radioactive isotopes 

Plot of the cumulative frequency of craters versus crater diameter, for three geologic units on the Moon. Crater density measurements provide a means of ordering units in relative time. Modified from Neukum etal. (2001). 

Absolute time measurements using radioactive isotopes are presently limited to samples that can be carefully processed and analyzed in the laboratory. Estimates of the relative times of formation of various geologic units on other planets can be gained from spacecraft imagery, by crater counting. The older a surface is, the longer it has been exposed to meteorite impacts and thus the higher the density of craters on that surface. However, very old surfaces can become saturated with craters, when every new crater effectively obliterates an older one. 

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