Craters on the Moon
Learning Targets
- Simulate an impact between an object and Moon's surface.
Success Criteria
- I can model what happens when different objects strike the Moon's surface.
- I can use Lunar collisions to describe the origin and age of a moon or planet.
- I can use Lunar collisions to infer the origin and age of the solar system.
Questions to Ponder
- Now that you know a little bit more about how craters are formed, what are some observations you can make about moon craters?
- Do you think that craters are different in different places of the solar system?
Reading
- Read the following article about how impacts can help shape a planet.
- As you read think about the following.
- What factors affect the shape of craters?
- What can the existence of craters tell us about the origin and age of a planet?
- What can the existence of craters tell us about the origin of the Solar System?
Shaping the Planets: Impact Cratering
Impact cratering is the excavation of a planet's surface when it is struck by a meteoroid. Impacts are instantaneous events. They leave very characteristic features.
What are craters?
Craters are roughly circular, excavated holes made by impact events. The circular shape is due to material flying out in all directions as a result of the explosion upon impact, not a result of the impactor having a circular shape (almost no impactors are spherical). Craters are the most common surface features on many solid planets and moons—Mercury and our Moon are covered with craters.
This portion of the Moon is covered by numerous circular holes. These are impact craters, each of which was formed when an asteroid or comet collided with the Moon's surface. The large number of craters in this region indicates that this part of the Moon is quite ancient. Geologic processes have not erased the craters with time.
What happens when an impactor hits?
When an impactor strikes the solid surface of a planet, a shock wave spreads out from the site of the impact. The shock wave fractures the rock and excavates a large cavity (much larger than the impactor). The impact sprays material — ejecta — out in all directions. The impactor is shattered into small pieces and may melt or vaporize. Sometimes the force of the impact is great enough to melt some of the local rock. If an impactor is large enough, some of the material pushed toward the edges of the crater will slump back toward the center and the rock beneath the crater will rebound, or push back up, creating a central peak in the crater. The edges of these larger craters also may slump, creating terraces that step down into the crater.
What are the major parts of a crater?
- Floor – The bottom of a crater, either bowl-shaped or flat, usually below the level of the surrounding ground.
- Central peaks – Peaks formed in the central area of the floor of a large crater. For larger craters (typically a few tens of kilometers in diameter) the excavated crater becomes so great that it collapses on itself. Collapse of the material back into the crater pushes up the mound that forms the central peak. At the same time, the rock beneath the crater rebounds, or bounces back up to add to the peak.
- Walls – The interior sides of a crater, usually steep. They may have giant stair-like terraces that are created by slumping of the walls due to gravity.
- Rim – The edge of the crater. It is elevated above the surrounding terrain because it is composed of material pushed up at the edge during excavation.
- Ejecta – Rock material thrown out of the crater area during an impact event. It is distributed outward from the crater's rim onto the planet's surface as debris. It can be loose materials or a blanket of debris surrounding the crater, thinning at the outermost regions.
- Rays – Bright streaks extending away from the crater sometimes for great distances, composed of ejecta material.
What are the different kinds of craters?
Simple craters are small bowl-shaped, smooth-walled craters (the maximum size limit depends on the planet).
- This image shows a simple crater on Mars that has no central peak or terraces around its edges. The crater is 2 kilometers (about 1 mile) wide. An extensive blanket of ejecta covers the area around the rim.
Complex craters are large craters with complicated features. Larger craters can have terraces, central peaks, and multiple rings.
- Copernicus is a large crater (93 kilometers or 60 miles wide) on the Moon. The inner walls of the crater have collapsed to form a series of step-like terraces, and a central peak is visible in the center of the image.
- A complex crater in the northern region of Mars. This crater is about 20 kilometers (12 miles) across and has a large central peak and terraces around its rim. The ejecta blanket has lobes, which may indicate wet material was ejected, suggesting that subsurface water or melted ice was mixed into the debris.
Impact basins are very large impact structures that are more than 300 kilometers (185 miles) in diameter. The largest impact basin on the Moon is 2500 kilometers (1550 miles) in diameter and more than 12 kilometers (7 miles) deep. Large impact basins are also found on other planets, including Mars and Mercury.
- The large circular dark areas in the image are impact basins, created as huge impactors struck the Moon. Lava later flowed across the low floors of the basins, giving them a darker, smoother appearance than the surrounding, brighter highlands. The dark basins can be seen by the naked eye.
Scientists describe other types of craters as well:
- Multi-ring basins – A very large impact basin surrounded by as many as five or six circular rings of mountain chains in addition to the main basin rim.
- Irregular craters – Craters with irregular shapes or multiple impact craters formed at the same time. Oblong craters can be created by impacts striking the surface at a very low angle.
- Degraded craters – Craters that have become eroded due to weathering, lava flows, impacting, or downslope movement of material.
How are large craters different than small ones?
Small craters often are simple bowl-shaped depressions. The structure of large craters is more complex because they collapse, forming terraces, central peaks, central pits, or multiple rings. Very large impact craters greater than 300 kilometers (185 miles) across are called impact basins.
What influences the size and shape of a crater?
The size and shape of the crater and the amount of material excavated depends on factors such as the velocity and mass of the impacting body and the geology of the surface. The faster the incoming impactor, the larger the crater. Typically, materials from space hit Earth at about 20 kilometers (slightly more than 12 miles) per second. Such a high-speed impact produces a crater that is approximately 20 times larger in diameter than the impacting object. Smaller planets have less gravitational "pull" than large planets; impactors will strike at lower speeds. The greater the mass of the impactor, the greater the size of crater.
Craters most often are circular. More elongate craters can be produced if an impactor strikes the surface at a very low angle — less than 20 degrees.
How can craters be used to determine the age of a planet or moon?
Scientists record the size and number of impact craters — and how eroded they are — to determine the ages and histories of different planetary surfaces. Early in the formation of our solar system (before 3.9 billion years ago) there was lots of large debris striking the surfaces of the young planets and moons; these older impact basins are larger than the more recent craters. As a rule of thumb, older surfaces have been exposed to impacting bodies (meteoroids, asteroids, and comets) for a longer period of time than younger surfaces. Therefore, older surfaces have more impact craters. Mercury and the Moon are covered with impact craters; their surfaces are very old. Venus has fewer craters; its surface has been covered recently (in the last 500 million years!) by lava flows that obscured the older craters.
After Reading
Use the Learning Targets and Success Criteria to answer the questions from the beginning of the reading...
- What factors affect the shape of craters?
- What can the existence of craters tell us about the origin and age of a planet?
- What can the existence of craters tell us about the origin of the Solar System?
Group Presentation
- Your team will be assigned to answer (in detail) one of the questions from above.
- As you prepare your answer, go back to the reading and the experiments that we did to model impacts.
