Terrestrial Planets, Page 1: [Previous Page] [Next Page] [Class Home Page] Textbook pages 156-160

Comparative Planetology

Comparative planetology compares the different natures of the nine planets in our solar system. This helps in our understanding of each planet, as we contrast possibly similar phenomenon occurring under differing conditions. A major aim of studies of the terrestrial planets, Mercury, Venus & Mars is in helping us to understand the origin, history and future of the most important planet, the planet Earth. The next four giant planets , also called the gas giants, Jupiter, Saturn, Uranus, & Neptune, are all much larger and rich in ices and gaseous hydrgen compounds. Pluto falls in NEITHER category, and its standing as a planet is currently debated.

Our Early Earth Earth recycles it's surface. Evidence from it's early history is mostly erased (Earth's Impacts). The Moon's surface (left) tells us much about the conditions of the early Solar System, which has been erased on Earth. Curiously, Venus shows few craters (right), like Earth. What does this say about Venus's geology?

Volcanism around the Solar System Venus (left) and Earth (right) both show geological evidence for volcanism, as demonstrated in the pictures here. However, the lava flows formed on Venus do not extend very far from their source. Such comparison's help us to understand volcanism on Earth and Venus.

Water flows on Mars There is no water on Mars now, but satellite images of Mars (left) shows features nearly identical to those seen on Earth (right) which are currently caused by water. Is this sufficient evidence to support past water on Mars?

When is an object a Planet and when is it a Moon or a Star or something else?
Stars fuse elements in their cores via nuclear reactions. This requires very high temperatures and densities at their cores.
Planets are too low in mass to obtain the temperature and density at their cores to ever fuse elements. They have (so far) only been found to orbit Stars.
Satellites orbit Planets. Sometimes Satellites have very small satellites.

What about Pluto? The debate about whether Pluto is a planet has been fueled by the recent discoveries of >30 new objects in the outer part of the solar system - (part of a large collection of objects, Pluto-sized and smaller, that form a disk-shaped cloud beyond Pluto's orbit - called the Kuiper Disk). But these new Kuiper Disk objects are about 10 times smaller than Pluto. Many planetary scientists consider Pluto to be an outer asteroid, since it shares all its most important characteristics (orbit, density, chemistry) with these Kuiper Disk objects. Currently (Fall 2004) the International Astronomical Union still has Pluto designated as a planet. So, for this class, we will study the NINE planets of our Solar System.