GEOLOGIC HISTORY PRACTICE SCENARIOS
Scientists apply these geologic principles (and others) to study impact craters which can help them learn about the history of our Solar System. As the impact process is one of the most dominant processes that has affected terrestrial worlds, craters help provide clues about how our Solar System has changed over time. In addition to applying the geologic rules you just learned, other important aspects to pay attention to include:
  • How the frequency of impacts has changed over time.
  • How the size of material that has struck the surfaces of planets has changed over time.
  • Evidence of geologic processes that may have modified craters such as wind, water, or volcanic processes. This helps scientists determine not only the types of geologic processes that have shaped (or continue to shape) the surface of a planet but also provides clues as to when those processes may have occurred.

BarringerVredefortsm.pngIt is important to note that the classification of craters can be useful in determining when processes may have occurred on a planetary surface. For example, preserved craters are considered relatively young craters. In addition to being relatively young, they can also help you make inferences about whether or not there are active geologic processes shaping the surface. The surface of the Earth, for example, is constantly being modified by Earth’s active processes. Do you think there are many preserved craters on Earth? Probably not. Between weathering and erosion, even the youngest of craters on Earth would likely be classified as modified. The crater on the right is Barringer Crater in Arizona. This is one of the youngest craters on Earth (~50,000 years young). As the rim appears somewhat irregular in shape and there is evidence of wind erosion, it makes sense to classify this young crater as modified.

Can we really state the actual age of a crater, as is mentioned with Barringer Crater above? For craters on Earth, we can. Scientists are able to use specialized dating techniques that allow them to examine rock samples collected from areas impacted to determine actual ages of craters on Earth. Based on these analyzed samples, Barringer Crater (1.2 km in diameter) is ~50,000 years old. One of the oldest craters on Earth is the Vredefort crater in South Africa. This complex crater is over 2 billion years old and is ~160 km in diameter. For most planetary worlds we do not have samples we can match as being from specific impact craters. Therefore scientists must rely on geologic principles to make inferences about relative ages.

Let’s see if you can apply what you have learned about craters and geologic principles to help make inferences about the geologic history of a planetary world. Two practice scenarios are given below. For each scenario you will be asked to think about two questions designed to help you gain practice in thinking about 1) the relative age of a planetary surface and 2) what you can infer about the geologic processes affecting the planet. As you answer each question, use the hints provided below to help guide your answers.
Question 1: Is the planetary surface relatively young or old? Explain.
Answer Hints: Think about the following as you answer this question:
  • Does the planetary surface have many or few impact craters on the surface?
  • Does the planetary surface have many large impact craters?

Question 2: What can you infer about the geologic processes affecting this planet? Explain.
Answer Hints: Think about the following as you answer this question:
  • Are the impact craters modified? This may mean there are (or were) active geologic processes shaping the surface.
  • Are some impact craters preserved? This may mean there are not (or were not) any active geologic processes changing the surface when those craters formed.

SCENARIO #1:
This planet has many impact craters. Craters range in size from relatively small (~1 km) to very large craters (~100+ km). Most of the larger craters are complex and some have visible central peaks, central rings of peaks, or look to be multi-ring basins. Most of the larger craters are modified. A few very large craters are destroyed. Smaller craters have raised rims and look preserved.

Question 1: Is the planetary surface relatively young or old? Explain.

Question 2: What can you infer about the geologic processes affecting this planet? Explain.

SCENARIO #2
This planet has few impact craters. Craters range in size from relatively small (~1 km) to very large craters (~100+ km). All of the craters are modified or destroyed. Surface appears rugged in some areas; some areas appear to have evidence of water or ice; sand dunes are visible in other areas.

Question 1: Is the planetary surface relatively young or old? Explain.

Question 2: What can you infer about the geologic processes affecting this planet? Explain.

Were you able to successfully determine details about the geologic history of the planetary worlds included in the two scenarios? It certainly requires you to critically think about many aspects and is not always completely straight forward. For example, while we can state that larger craters are generally more common early in the history of our Solar System, we must also realize that early in history there were craters of all sizes – both large and small. As you think about the geologic history of different planetary worlds, being prepared to justify your thinking is extremely important.


As you investigate planetary worlds within our Solar System, you may end up asking some of the same questions scientists have asked for years. Why are the planetary worlds in our Solar System different from one another? Aside from Earth, are there any other planetary worlds that may have once been able to support life? How has the Solar System changed over time? How have the sizes of impacts changed over time? In the future, will Earth be struck by a large object? Is there data to support the idea that material impacting planetary surfaces today is relatively small compared to material that has impacted surfaces in the past? Although you may not be able to answer all of these questions, these ideas are part of thinking about the bigger picture.

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