Final Project

Kim S.

Legislator:  Jane Nelson, Senator

In man’s conquest of space, one should be aware of the geological landscapes of those planets that have been chosen for man’s arrival. Two of these extraterrestrial worlds, the moon and mars, have different atmospheres, rotational periods and may appear to be astronomically different from the earth, but geologically they are not. On Mars and more closely related, the Moon, the primary geological features are that of impact craters. In fact, these features that result from collisions between two orbiting bodies provide a commonality between all solid masses in the universe. Whether one is looking to Jupiter, the asteroid belt, up at the moon, or simply to earth, one will always find impact craters.

Impact craters may be defined as geological structures that are formed when a meteorid is not completely burned up in a planet’s atmosphere and reaches the ground. As stated earlier, impact craters are found not only on Earth, but also on all of the terrestrial planets, asteroids and many of the moons that orbit the outer planets. They are created when large bodies such as asteroids or comets strike a planet, and thus produce an impact crater or impact basin. Impact craters are bowl-shaped depressions that measure up to approximately miles (twenty-five kilometers) in diameter. They have shallow, flat floors and uplifted centers. Impact basins are larger than craters, and inside their rims, there are one or more rings on the planet's surface.

Throughout the solar system, if an impactor reaches the surface of a planet, it forms a crater with a morphology that is dependent on the target surface and on the dimension of the impactor. Pieces of the impactor will sometimes be found within the crater after the incident, but in massive events, the pressure and temperature will reach very high levels and can completely vaporize the meteorite, which will leave behind only the created crater structure. In addition, during these events, the impactor and target rocks will generally become melted and mixed together. This will create and lead to a new chemical composition, the impact melt, which is characteristic of impact events. Other than the intense heat, the shock wave generated by the impact will also be responsible for the production of irreversible chemical and physical changes in target rocks. This phenomenon is known as shock metamorphism.

Not all craters are made equal; there happen to be several physical characteristics of craters that form a signature of the impact origin of the structure. These characteristics are a good instrument for scientists to recognize the impact nature of a crater. The floor of the crater can be either bowl shaped or with a central uplift, while around it the walls will form a raised rim. An ejecta blanket generally surrounds the crater, while the floor is covered with breccia, a course-grained rock, which is composed of broken rock fragments. As previously mentioned, during the impact both the target rock and the impactor are melted together, and then are dispersed into various impact deposits and ejecta, including some small particles called tektites. By looking at the size, composition and features of an impact site, scientists may make several deductions about the estimated mass of the meteoroid and where it originated.

The Earth is home to over 150 known craters, among them Meteor Crater in Arizona, and the largest on the planet, the Chicxulub Basin that is centered in Mexico's Yucatan Peninsula. The diameter of the basin is about 190 miles. Rock samples obtained by drilling into the basin indicate that an asteroid struck the earth there about sixty-five million years ago, which was about the time the last dinosaurs became extinct. However, even the largest and most devastating impact crater on earth is nothing in comparison to that which the moon possesses.

Craters are the most numerous feature of the moon's surface. The entire landscape is littered with craters. In addition, there are so many craters that the moon has craters within craters and even connected craters. Scientists estimate that the moon has half a million craters that are more than one mile wide, and a total of approximately thirty thousand billion craters that are at least one foot wide. They are so abundant on the moon due to the lack of atmosphere, which entails that even the smallest meteoroids form craters. In addition, erosion on the moon progresses so slowly that craters only one foot in diameter remain for millions of years.  Most of the large craters on the moon were probably formed when comets or asteroids hit the moon. However, some scientists believe that the moon's largest and oldest craters may have been created by the impact of planetesimals, solid objects that perhaps crashed together and formed the moon itself.

Generally, all of the small craters on the moon are simple bowl-shaped pits with low rims. Craters that range from five to ten in width have high walls and level floors. All craters that are wider than fifteen miles have hilly floors or central peaks and are rimmed by mountains and have steep, terraced walls. The moon’s largest crater, the Imbrium Basin, is about 700 miles wide. The floor of it is covered by dark lava, which forms one eye of the familiar "man in the moon."

Another form of crater on the moon is that of ray craters. These craters are surrounded by light gray streaks known as rays. The rays appear as splashes of bright material and extend out in many directions. Around Tycho, a crater fifty-four miles wide, a few rays are between ten and fifteen miles wide and can be traced for nearly 1,000 miles. In the rays are vast amounts of small secondary craters that were most likely formed by rocks thrown out of the ray craters. The rays are probably mixtures of broken rocks thrown from the ray craters and rock fragments that splashed out of the secondary craters. Scientists know that the ray craters were formed late in the moon's history because their rays cross over maria, mountains, and other craters.

By looking at the battered landscape of the moon, humans are reminded how fragile the earth actually is in comparison to the universe. The only barrier preventing a massive extinction on the earth is the thin atmosphere that prevents extraterrestrial objects from reaching the surface. This small and seemingly insignificant barrier is the only reason that the earth does not look like the moon. We must keep it that way, by increasing the amount of funding to help spot meteors that may strike earth. It is not a lost cause, because the only way we can save ourselves is with knowledge. I believe that we must also travel to the moon and learn first hand about the craters, how they were made, and how we can stop them. Because an impact crater could cause serious destruction on earth, it is imperative that this specialty receive top priority, because when one looks at the moon one can see that seemingly miniscule particles of a few grams can be destructive when traveling thousands of miles an hour. Luckily, the small rocks do not penetrate the surface of the earth; but, if an asteroid is large enough to break through the atmosphere, size will not matter, for it is the velocity that will make all the difference between life and death.

“Earth Impact Craters.” March 1, 2001.

http://exobio.ucsd.edu/Space_Sciences/earth_impact_craters.htm (May 25, 2001).

“Educator's Guide to Impact Craters Courtesy of the Jet Propulsion Laboratory.” http://marple.as.utexas.edu/~rocks/site/craters.html (May 22, 2001).

Short, Nicholas M. “Impact Crater.” 2001.

http://www.britannica.com/eb/article?eu=120707 (May 21, 2001).

Last Updated: 09/07/01