Evolution of the Surface Features of the Terrestrial Planets



LUNAR CHRONOLOGY

The chronology for the Moon is very good because we have been there, seen it. This has allowed the gathering of rock samples from various terrains on the Moon. The importance of this is that the rocks could be age-dated using the technique of radioactive age dating which allowed us to set a firm chronology for the evolution of the lunar surface features. This is important not only for the Moon but also for the inner Solar System in general as, presumably, the chronology of the Moon roughly follows the timeline as the rest of the inner Solar System (that is, it tells what the cratering rate was for all of the inner Solar System bodies.)


Radiometric Age Dating

Radioactive elements provide accurate clocks. If one can measure the relative abundances of the radioactive element (parent) and the decay product (daugher), one can infer the age of the rock through the half-life of the material. The half-life of an element is the time it takes for one-half of the parent nuclei to decay (transmute) to the daughter nuclei. See the plot to the right.


Some further useful elements (for the Moon and Earth) are:
  • Potassium-40 -----> Argon-40 with half-life of 1.3 b. years

  • Rubidium-87 -----> Strontium-87 with half-life of 47 b. years

In practice, a number of parent-daughter pairs are used to date a rock. The result is usually accurate within about 1 to 5%. This age is usually thought of as solidification age after which rock has stayed undisturbed.


Using radioactive age dating, Moon rocks were found to be:

Here, we present the chronology of the Moon's evolution and the cratering rate for the inner Solar System over the last 4 billion or so years. Not shown is that largest impact suffered in the Moon-Earth history, however, that is the origin of the Moon itself;


ORIGIN OF THE MOON


TECTONIC ACTIVITY

The most massive Terrestrial planets, Earth and Venus, are likely to show active geologies. Mars may show geological activity. Mercury and the Moon are probably dead from a geological standpoint. The sorts of geology we expect are volcanism and tectonic activity, both manifestations of the fact that the interiors of the planets are hot.

A consequnce of geological activity (and wind and water erosion on the Earth) is to produce a relatively young surface. (By young, we mean that the surface features were formed recently.) The surface of Venus is around 300 - 800 million years old, while the majority of the Earth's surface features are much younger. For example,

These ages are to be compared to the ages of the planets and Solar System of 4.6 billion years.

  • Tectonic Activity on the Terrestrial Planets?