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# 6.25 Comparative Planetology

In the 1970s, space probes radically increased our data on other planets in our solar system from space probes. Researchers were able to use this information to compare and contrast the different planets, looking for processes that work universally across planets with similar conditions. According to this technique, called comparative planetology, we can learn more by looking for the unifying principles different planets share, rather than by studying each planet as an unrelated system.

The planets in our Solar System all formed from essentially the same "stuff," yet they are now strikingly different in appearance and surface composition. To understand how this differentiation occurred we must look at both external processes (such as the delivery of water by comets) and internal processes (such as volcanism). The comparison of greenhouse warming on Venus, Earth, and possibly Mars, is one example of a process that occurs in different levels on different worlds. The presence of life and liquid water is another example. They are present on Earth, but our two closest neighbors, Venus and Mars, currently lack either. Nature has performed various experiments for us, placing planets of different sizes at different distances from the Sun. We can compare the results of the experiments to learn more about how planets work in general, and especially about how the Earth itself works. Life, just like volcanism, effects our planet's atmosphere, chemistry, and surface. Studying other, simpler, systems can help us understand the Earth more completely.

There are some simple, general rules we can derive from this method that helps us understand planets. Here are some examples:

• Older Surfaces Have More Impact Craters.

• Bigger Planets Have More Internal Heat and More Geological Activity.

• Bigger Planets have Stronger Gravity and Retain More Atmosphere.

• CO2 is the "Normal" Composition for the Atmosphere of a Terrestrial Planet.

• Free Oxygen Means Something Strange is Going On.

The tenets of comparative planetology are all based on simple physical principles related to gravity, chemistry, and the kinetic theory of matter. Since we have evidence that these physical properties are universal, it's a good bet that these rules apply to planets beyond the Solar System, too. With thousands of extra solar planets now known ,we are on the cusp of being able to measure their detailed properties. The exciting idea that we can predict the geology and atmospheric composition of yet-undiscovered planets is an example of the long reach of the scientific method.