Comparison of the Planets in Our Solar System

To this day, Earth is the only planet in our solar system that has life that we know of. This simple fact in and of itself makes Earth a particularly unique planet in our solar system. There are many other things that make our planet and the other planets unique as well as surprisingly similar. Throughout this paper I will discuss some of these similarities and differences and how these characteristics came to be on each planet. To begin the comparison, I will begin with the planet we know the most about, our home, Earth.

The Earth is an average distance of 1.00 AU from the Sun. Its average temperature is 290 K, and its density 5.52 g/cm³ . It is a terrestrial planet and therefore, made up of rocks and metals. (Bennett et al., 2007, p. 207). The interior of Earth, like all of the other terrestrial planets, has a core, mantle, and crust. The layering of these three parts of the interior happens though differentiation, the more dense the material sinks while the less dense material rises. The core of a planet is "the dense central region of a planet [. . .]", consisting mostly of nickel and iron. (Bennett et al., 2007, p. G-3 & p. 249). Earth's core is solid in the inner region and has a liquid outer region. The Mantle is made up of rocky material that surrounds the core, this material is less dense then the material that makes up the core. Lastly (unless you count the atmosphere as a layer as well), the crust is the outer most layer of the planet. It is made from low density rock like granite and basalt. (Bennett et al., 2007, pgs. 249-250).

"Smaller planets generally have smaller cores." (Bennett et al., 2007, p. 252). However, this statement brings us to Mercury. One of Mercury's unique characteristics is its unusually large core. Scientists believe that even though Mercury was formed where planetesimals were believed to be metal-rich, it wasn't a contributing factor. They attribute its large core to a possible large impact that destroyed much of its mantle and crust. This left it primarily with highly dense materials. Another unique characteristic of Mercury is its amazing cliffs; they are over 3km high and run for hundreds of kilometers long. When Mercury's core cooled, it shrunk in size. This caused the mantle and lithosphere to shrink as well, causing the tectonic stresses that the cliffs were made from. (Bennett et al., 2007, p. 266). Tectonics; "the disruption of a planet's surface by internal stresses",(Bennett et al., 2007, p. 256), is one of four different processes that can shape a planet. Most of the geological features of Earth were shaped by tectonics. Every time there is an earthquake the shape of the Earth changes, sometimes it changes more drastically than other times. Earthquakes are caused when the lithosphere is fractured and the individual plates move around. (Bennett et al., 2007, p. 260). This process is called plate tectonics; and to our knowledge, is unique to Earth.

Another process that shapes the surface of the Earth is erosion, or the "wearing down and building up of geological features by wind, water ice and other phenomena of planetary weather". (Bennett et al., 2007, p. 256). Erosion has carved canyons, glaciers, builds sand dunes, and makes river deltas. (Bennett et al., 2007, p. 260). These features of erosions would be thought evident on Earth's "sister planet", Venus. However, Venus's surface is to hot for any water and it has a slow rotation causing it not to have any weather at all. (Bennett et al., 2007, p. 272). Another key characteristic of Venus is its volcanic activity. No eruptions have been observed, however, due to the sulfuric acid found in its clouds, it is assumed that the volcanoes are still active. There are three very basic volcanic features; volcanic plains, shield volcanoes and stratovolcanoes (steep-sloped mountains). Venus has evidence of lava plains, and stratovolcanoes. It also has evidence of the third feature as well, shield volcanoes. Shield volcanoes are formed when lava solidifies before it can spread out. These volcanoes are not steep but they can be tall. The Hawaiian volcanoes are an example of this formation on earth. In fact, the tallest known shield volcano is Olympus Mons, on Mars. Its base is large enough to cover the state of Arizona and the peak is about three times higher above sea level than Mount Everest. Mars also has tectonic features of long, deep valleys that extend over one fifth of the way, around the planet, along its equator. Scientists aren't quite sure how Valles Marineris formed, however because the canyon is enclosed by high cliffs on all sides, they can rule out flowing water or lava. (Bennett et al., 2007, p. 269).

Along with tectonics, volcanic activity, and abundant evidence of features caused by erosion; Mars, like Earth, also has impact cratering, or craters caused by an asteroid or comet hitting the planets surface at extreme speeds. Mars's southern hemisphere has numerous large impact craters, the most notable is Hellas Basin. (Bennett et al., 2007, p. 268). The effect of impact cratering on Mars is small in comparison to the effect of impact cratering on the Moon.

The entire surface of the Moon should be riddled with craters; however, regions known as the lunar maria, look smooth and dark. When photographers photographed the far side of the moon they found it to also be heavily cratered and at a higher altitude. The mystery of the lunar maria can be explained by volcanic activity. The difference in altitude allowed lava to fill the craters only on the near side (the side visible to Earth), which is why the far side looks as expected. The lava filled the craters making the maria look smooth. The dark color comes from the hardened lava, or basalt. (Bennett et al., 2007, p. 264). The Moon is unique to the other planets, wherein it orbits the Earth and the other planets orbit the Sun. All of the planets mentioned, including Earth, are terrestrial planets and therefore made up of rock and metals. The next planets however, are the Jovian planets. They are "radically different from the terrestrial worlds in size, density, and composition". (Bennett et al., 2007, p. 328). These planets are also different among themselves. Saturn and Jupiter are both as visible as other planets. They both are made up of layers, like the Earth, but with material made from pure hydrogen and helium gases, instead of rock or metal. Their core is a small percent of their mass; Saturn's is slightly larger than Jupiter's, it is made up of hydrogen compounds and small amounts of rock and metal. The next layer is the metallic hydrogen layer. This layer is much larger in Jupiter than Saturn. The next layers are liquid hydrogen and gaseous hydrogen. Jupiter, like Earth, experiences some forms of weather. In fact Jupiter is most famous for its storm the Great red Spot. Saturn is unique because of its oblong shape; when gravity pulls materials in, the rapid rotation flings the material toward the equator, causing its shape to be less then spherical. (Bennett et al., 2007, p. 332-333). Uranus and Neptune have cores made of rock and metal; a layer of water, methane, and ammonia; and then a layer of gaseous hydrogen. Like Saturn, they both have visible rings, though they encircle the planet at a vertical angle. Like the Earth, the Jovian planets all have at least one moon, in fact Neptune has the least with 13.

As you can see there are many differences and similarities between Earth and the other planets. Some differences as extreme as the Jovian planets, and some similarities as surprising as the commonalities we share with Mars or Venus. We also explored the interesting facts and features that was to unique to that planet. After all is said and done, "there's no place like home." (Dorothy, Wizard of Oz).
References

Bennett, J., Donahue, M., Schneider, N., Voit, M., (2007). The cosmic perspective (4^th ed.). San Francisco: Pearson Education, Inc..