Craters and Surface Ages

Directions: Three buttons in the lower right allow you to control the interplay between two geologic processes on a planetary surface. The processes of impact cratering and erosion are the focus of this simulation.

The counters in the upper right display the passage of time. The upper readout displays the time since the simulation began or since the RESET button was pressed. The lower readout displays the age of the surface indicated by the arrow pointer. This pointer is controlled by the mouse.

The GO/STOP button starts or stops the passage of time. Small bodies strike the surface of the planet at random and produce impact craters of various sizes. As time passes, the surface becomes more heavily cratered just as we see on the planets and moons of the Solar System.

The ERODE button stops the passage of time and erases the impact craters from a small portion of the planetary surface. For the purposes of the simulation, the cause of the erosion is unimportant. It may be due to wind, water, or volcanic activity. To begin the passage of time again, press the GO button.

The RESET button removes all craters and erosion events from the planetary surface. The "Time since Start" clock is also reset to 0 million years. You can use this control to rejuvenate the planetary surface if you would like to start over.

Questions to Consider:

1. How does the number of craters on a planetary surface change as time passes?
(Click for Answer)

With no other geologic activity (like wind, water, volcanism, or tectonic), impact craters represent the only way a planetary surface can change. As time passes, the number of crater will increase. Younger terrain, like a fresh lava flow, will have relatively few craters. Ancient terrain, perhaps billions of years old, will be heavily cratered.

In this simulation, an erosion event completely obliterates the craters underneath. In reality, craters wear away slowly. For example, the rims of ancient craters may still be visible as circular features in the landscape. Lava flows may partially fill a crater or erase only a portion of the crater rim.

2. Earth has less than 1000 recognized impact craters on its surface. Earth's Moon has many millions of impact craters on its surface. What factors are responsible for the very different appearance of Earth and the Moon?
(Click for Answer)

Earth's surface is only 25% dry land. No crater is formed when an object strikes the oceans (unless the object is very large and penetrates to the ocean floor). Any object of any size striking the Moon produces a crater.

Wind, water, volcanic activity, and tectonics erase craters on Earth's surface relatively quickly (geologically speaking). There is no active geology on the Moon today. Even the Moon's volcanic activity ended billions of years ago. The Moon's surface is dominated by impact cratering.

3. Test your lab partner's ability at crater counting. Have your partner turn away while you create a planetary surface for them to investigate. Run the simulation for about 2 billion years, placing several (2 to 4) erosion events on the surface at different times.

With the planetary surface ready, your lab partner should try to determine which erosion event occured farthest in the past and which occured most recently. What features should you look for to determine the relative age of a surface?
(Click for Answer)

Recall that impact cratering occurs over time. The longer a surface is exposed to impacts, the more heavily cratered it will be. A young surface will appear relatively free of craters, while an old surface will contain many craters.

4. Run the simulation long enough to span at least 2 billion years. Hit the ERODE button at an early time (say, 500 million years) and again at a late time (say, 1.5 billion years). How does the number and sizes of craters at early times compare to the number and sizes of craters at late times?
(Click for Answer)

Early in the history of the Solar System, the leftovers from the formation of the planets struck the surfaces of the planets and moons often. Many larger bodies still orbited the Sun but had not yet accreted onto a planet or moon. These large bodies produced large craters like the impact basins seen on the Moon, Mercury, or Mars. But there were many small bodies as well. Early in the history of the Solar System, many more small craters formed.

After billions of years, most of the debris from that early era of planet formation is gone. Today, impacts are much less common. While many of the larger bodies struck a planet or moon long ago, smaller bodies remain. Large impacts like those that formed giant craters are rare today, but smaller impacts are still common.

Created by Kevin Healy, 2008