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 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 kilometers miles in diameter. The largest impact basin on the Moon is kilometers 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:. 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.
The Moon has almost no erosion because it has no atmosphere. That means it has no wind, it has no weather, and it certainly has no plants. Almost nothing can remove marks on its surface once they are made. The second thing is something called tectonics. Because of tectonics, the surface of Earth is recycled many times throughout its long history.
As a result, very few rocks on Earth are as old as the rocks on the Moon. The Moon has not had tectonics for billions of years. The third thing is volcanism. Volcanic flows can cover up impact craters. This is a major way impact craters get covered up elsewhere in our solar system, but it is less important than the recycling of crust here on Earth. It starts to glow, and if it's heated enough, it may break apart and send showers of debris to the surface. When impactors strike the surface of a world, that sends a shockwave out from the impact site.
That shock wave breaks up the surface, cracks rock, melts ice, and digs out a huge bowl-shaped cavity. The impact sends material spraying out from the site, while the walls of the newly created crater may fall back in on themselves. In very strong impacts, a central peak forms in the bowl of the crater.
The surrounding region may get buckled and wrinkled into ring-shaped formations. The floor, walls, central peak, rim, and ejecta the material scattered out from an impact site all tell the tale of the event and how powerful it was. If the incoming rock breaks up, as it usually does, then pieces of the original impactor can be found among the debris. The Moon isn't the only world with craters dug out by incoming rock and ice. Earth itself was pummeled during the same early bombardment that scarred the Moon.
On Earth, most craters have been eroded away or buried by shifting landforms or sea encroachment. Only a few, such as Meteor Crater in Arizona, remain.
On other planets, such as Mercury and the surface of Mars , craters are quite obvious, and they haven't been eroded away. Although Mars may have had a watery past, the craters we see there today are relatively old and still look in fairly good shape. Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads.
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