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A. CAUSES OF IMPACTS

Impact craters are features created on the surface of a planetary body when a meteoroid strikes the surface creating a bowl-shaped hole. Meteoroids can be particles of cosmic dust, parts of asteroids, planets, moons, leftover material from the formation of our Solar System, or even comets – objects made of rock and ice that travel through our Solar System. Small meteoroids burn up in our atmosphere. Those that are large enough to make it through the atmosphere, strike the surface creating an impact crater. The most common meteorites found on Earth are fragments of asteroids and other planets. The number and sizes of impact craters found on the surface of a planetary world vary.

Believe it or not, you have probably seen hundreds of impact craters! Every time you look up at the Moon you are looking at evidence of ancient collisions in our Solar System. Impact craters are found on almost all of the rocky (terrestrial) planetary worlds (including planets, moons, and asteroids) in our Solar System. The impact process is the most common geologic process seen across our Solar System.


B. FORMATION OF CRATERS

CraterFormation.pngThere are three main stages involved in the formation of craters:
  • Contact/compression stage: A meteor (a meteoroid that has successfully made it through our atmosphere) traveling 10-15 kilometers per second strikes a planetary surface. As it strikes, shock waves compress the surface and cause rocky material to almost liquefy or melt.

  • Excavation stage: Material gets ejected or thrown out of the newly formed hole in the ground.

  • Modification stage: The ejecta settles out onto the surface and material in the walls of the newly formed crater slump. (Slump is when material moves a short distance down a slope.)

The entire crater formation process occurs in seconds. Usually much of the meteoroid is vaporized. Fragments that remain are called meteorites. The final crater will continue to be modified by gravity, erosion and/or other geologic processes shaping the surface.


C. CRATER CHARACTERISTICS

There are 5 main parts or physical characteristics of a crater. These include:
  • Rim: The raised area around the edge of the crater
  • Wall: The sides of the crater
  • Floor: The bottom of the crater
  • Central peak: An uplifted mound in the floor of the crater. This peak is created when melted rock rebounds or gets uplifted during the impact event and then solidifies in that uplifted position.
  • Ejecta: Material from inside the crater that is thrown out during the impact event. Ejecta can appear as rays or as a blanket of material surrounding the crater.

There are two general types of craters:
1. Simple Crater: (Crater A shown below)
  • Simple bowl shape
  • Generally smaller and younger than complex craters.
2. Complex Crater: (Craters B, C, and D shown below)
  • Much larger and older than simple craters.
    • Characteristics frequently include one or more of the following:
    • Central peak (visible in crater B)
    • Ring of peaks (visible in crater C)
    • Multi-ring structure (visible in crater D)
    • Material that has slumped along the walls giving them a terraced, step-like, or inner ring appearance.
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D. USING CRATERS TO REVEAL THE GEOLOGIC HISTORY OF A SURFACE

Impact craters and their physical characteristics open up a window into a planetary world’s history and geology. Once a crater is formed, making observations of how it is modified or eroded tells us a lot about that planetary world. Craters can be modified by wind, water, tectonics, volcanic processes, or even other impacts. As part of these processes and others, craters can be covered or filled in by water, lava, sediments (sand, dust, etc), landslides, vegetation, or even ejecta from other craters.

When scientists examine the surfaces of planetary worlds, it is common for them to use relative terms for age dating. They will often refer to a surface as being older or younger relative to another. Additionally, they often refer to a process as having occurred recently or long ago. In geologic terms, recent may be 50,000 years ago, especially if you consider the Solar System as being 4.6 billion years old.

When looking at planetary surfaces, a younger surface is considered one that has been recently resurfaced by some process (wind, water, volcanics, or some other process). An older surface is one that has NOTbeen resurfaced by any process in a long time. Think of it this way…consider the street in front of your house. Over time, the street will become worn and will likely crack and even get pot holes. With every passing day, the street “ages”. One day, the street gets repaved. That resurfaced street, if we think in relative terms, is now younger. The older street is still there – it is now under the newer surface. This works in a similar way with planets. A planet may be 4.6 billion years old, but if a volcano erupts lava onto the surface, that lava can fill in craters or even cover them up completely. That 4.6 billion year old surface basically just got “repaved” and this new surface is younger, with the older terrain underneath.

Geologic Principles
Scientists use geologic principles or rules to help determine relative ages. By applying these rules you can gain insight into the sequence of geologic events that took place in a region. Three of these principles include:
1. Principle of Superposition:
  • The order of layers or geologic features found on the surface provides information about which features are older or younger.
  • Features found on top are the youngest.
  • Relating to craters, if a crater is found on the floor of or overlapping the rim of another crater it must be younger. The crater on the bottom must have been there first, making it older.
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Answers: Layer A is younger; Crater B is younger.

2. Crater Density: When comparing areas of equal sizes, the more craters on the surface, the older that surface is.
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Region A is older.

3.Crater Classification: The more modified a crater, the older it is. Craters generally flatten and lose shape with age. The relative ages of craters can be determined by classifying them into one of the following categories:
PRESERVED CRATERS:Youngest, best preserved craters
  • Circular craters
  • Raised rim
  • Look fresh
  • Can sometimes see ejecta blanket or rays of ejecta

MODIFIED CRATERS:Middle aged craters with evidence of modification
  • Rim may appear uneven or somewhat irregular in shape
  • Floor may be partially filled in with sediment
  • Appear to be modified or eroded by wind, water, lava, or other process(es)
  • Can range from being slightly to severely modified

DESTROYED CRATERS: Oldest craters that have been severely altered
  • Broken rims
  • Almost completely filled in by sediment, lava, or other material
  • Appear very flat and very worn away (severely eroded)
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CONTINUE READING BACKGROUND INFORMATION (PART 2)


*Keep track of your sources in your bibliography.