Background

When two objects collide it is called an impact event

The basic idea is simple-- objects in space sometimes run into one another. If they are the same size (and perhaps moving very fast with respect to one another), like two asteroids, the collision might result in both of the bodies getting smashed to pieces. To the left is artist William Hartmann's conception of two asteroids colliding.

But if one body is much bigger than the other, the larger body will probably survive the collision. Scientists may call the larger of the two the 'target' and the smaller, the 'impactor' or ' projectile'. This can be seen in the image by artist Don Dixon. In this case the 'target' will survive, but... there will be a big scar left over to tell the tale of the event!

This scar is the impact crater

In the image below, artist Don Davis has imagined what a very large asteroid impacting Earth might look like. The target, Earth in this case, will survive the impact intact. But the surface will be forever changed by the energy of the event, and the massive impact crater formed as a result.

What happens when a large object impacts a gaseous planet? Do we see the 'scar' of the impact preserved? Well, in 1994 we did witness a series of very large impact events (caused by the breakup of the Shoemaker-Levy 9 comet) on Jupiter, and we saw huge scars on the visible (gaseous) surface of the planet. However, much like when we throw a rock into a pond, the effect disappears after awhile, and today those scars are not visible on Jupiter's surface.

Impact craters are the scars left behind when objects in space hit one another and they are the best record scientists have of the collisional history of the solar system.

(B) Why do scientists want to study impact craters? Why do we care?

There is a great deal of evidence that supports the idea of a very large impact event that caused the extinction of dinosaurs about 65 million years ago. The dinosaurs may have already been on the edge of survival, and an impact event would have been more than enough to change the Earth's climate, making it unfriendly and inhospitable for dinosaurs to survive. This artwork by Joe Tucciarone shows what the final minutes for these dinosaurs may have been like!

In the 1990's, scientists found the place where this 'killer' asteroid struck, in what is now the Yucatan Peninsula of Mexico. In the painting above, artist Don Davis imagined what the impact event may have looked like. The devastation created by such an event was probably the death knell for the dinosaurs and 50% of all species on Earth!

These destructive effects are one of the main reasons scientists study impact craters. For more information, check out the Terrestrial Impact Craters and their Environmental Effects website. Also check out the Earth Impact Effects Program website if you want to explore the possible effects at a given distance from an impact event of some size.

Impact craters have been seen on planets and satellites all over the solar system. Some surfaces have so many impact scars that they pile up on top of one another, wiping out the ones that were formed earlier. There must have been a time when impact events happened regularly. And yet, there are other surfaces that don't have many craters at all. Is that because there are fewer impacts nowadays or because something has quickly erased the craters before new ones could form?

The image collection above shows surfaces of various planets and satellites of the solar system such as Mercury, Mars, the Moon (satellite of Earth), Europa and Ganymede (satellites of Jupiter), Phoebe and Tethys (satellites of Saturn), and the asteroid Eros.

What can we learn from the number and distribution of craters on planetary surfaces? What is responsible for all this variation among planetary surfaces, especially the number, size, shape, and density of craters? To get an idea, see PSI's Introduction to Cratering Studies website. Scientists have to study the impact craters on these planets, including Earth, before these questions can be answered.

And many other questions remain-- How do impact craters relate to other planetary events, like earthquakes or volcanisms? What about other extinctions on Earth? Are they linked to impact events? How about today, should we be expecting an impact event to happen sometime soon? How big of an event, and how big of a crater will be created? Can we figure out how or where?

Scientists have a lot of motivation to study impact craters, and we have lots of reasons to care about the answers they find!

(C) What are the stages of impact crater formation? What are the processes that shape impact craters?

You might think that impact craters are all the same, but they are not ! In fact, the differences between craters provide a lot of information about how the craters formed, when they formed, and why.

Simple Craters

The image to the left is a very simple, bowl shaped crater on the Moon and is typical of small craters that have formed relatively recently. It has a raised rim around the edge, and nice, sharp features. These sorts of craters are usually only a handful of miles across, at the most, on planets like the Earth, Mars, and the Moon.

A beautiful example of a simple crater right here on Earth is Barringer, a.k.a. Meteor Crater. As the images above and below show, it clearly has a raised rim-- can you see it above the road? It kind of looks like a flat mountain range when driving up to it and its interior has the shape of a bowl.

From the air, as shown above, it looks very much like the simple crater on the Moon.

It does not look like it would take something too dramatic to make this hole, but don't be fooled. Try to imagine yourself at the rim of Meteor Crater, like at the observation deck, which is the small platform in the lower right of the image below. See the folks standing on it? You are pretty small compared to the crater, which is about a mile wide.

Important thing to know: an impactor does not create a hole by pushing aside material to form an impact crater. What really happens is an explosion !

Even small craters are created by very energetic events-- impactors that plow into a planet like the Earth are moving very fast, anywhere from 15 to 70 kilometers per second, that is around 36,000 to 160,000 miles per hour!! Compare that to a jet fighter, which may travel 1800 miles per hour, or a rifle bullet that only reaches speeds of 2200 miles per hour.

All that speed means a lot of momentum, and a lot of energy. That energy gets transferred right into the ground, making dramatic changes to the rocks, the most noticeable of which is the huge explosion that creates the impact crater itself. Because of the energies involved, it doesn't take a very big impactor to create a big crater. Take Meteor Crater for example-- we know that the impactor was an iron meteorite around 40 meters in diameter (about 1309 feet, a little bigger than a football field), but the crater is 1200 meters (about 4000 feet) across!

Complex Craters

As mentioned before, not all craters are the same. While the smallest craters on a planet will be nice, simple bowl shapes, the medium to large-sized craters will have a more complex form. Scientists refer to these as 'complex' craters. Like the crater shown here to the left, they can have ridges or 'terraces' inside of their rims with flatter floors, and a central peak, or ring of peaks.

The same goes for the lunar crater on the right. The floor is flatter, the crater rim is broken by many terraces, and the central group of peaks is in the form of a ring.

Impact Basins

The largest craters found on a planetary body are called 'basins' and they hardly look like craters at all. Basins can be huge, many hundreds of kilometers across, and have multiple rings of rims. They have flat floors between the rings, and sometimes it can be hard to identify the largest ring! Orientale Basin on the Moon (left) is one of the best examples of such an impact basin.

Simple versus Complex

Another way of thinking about simple and complex craters is to see them from their sides. Imagine if we were to slice a crater and remove one half, so we can see the inside. The diagram to the left is a slice through or 'cross section' of the two types of craters you've been reading about.

The first, (a) is a simple crater. 'D' is the diameter of the crater from rim to rim, and the red areas are crater fill materials, such as rocks that were melted, cooled and re-hardened from the impact event. The blue areas are materials that were thrown out, or ejected from the impact event, and cascaded down around the crater. This material is called 'ejecta' and it can sometimes be found many tens of crater diameters away from the site of the impact.

The second, (b) is a complex crater. The floor is much flatter, and is also covered by hardened melt rocks. Ejecta is also cast out over the rim. Notice the many terraces inside the final crater rim.

So, you've seen images and diagrams of craters that were formed in the past. But what does the impact process look like step-by-step? The diagram to the right shows the stages of crater formation. When an impactor plows into a target (makes contact), it brings a lot of energy with it. That energy is what drives the creation of the impact crater.

For simplicity, we can split the formation of a crater into 3 stages: contact and compression, excavation, and modification. During the first stage, the energy forces the target rocks down and compresses them. A transient crater starts to form-- we call it 'transient' as this early crater will change. Material is then melted, even vaporized, and starts to be thrown out of the rapidly expanding crater during the excavation stage. For relatively small impact events (craters < 2-4 kilometers across on Earth) the transient crater is relatively stable and we end up with a simple crater, such as Meteor Crater.

For larger impact events, however, this transient crater is unstable-- its basically too deep and wide. Rocks at the bottom of these craters resist being compressed and deformed, and eventually 'snaps back' during the modification stage. This is the process that pushes up the central peak in complex craters. Finally, the ejecta falls to the ground, and the rim and center of the crater slump a bit and settle into their final shapes.

All of this happens within a few minutes, although for larger craters the melted rocks can take a very long time to cool and harden again, and the rim and peaks may fall and slump a bit more. And then of course...there is time.

Image: Adler Planetarium and Astronomy Museum

Just like people, craters age too. They change with time, given what is going on around them in their environment. On the Moon, where there is no wind, rain, or atmosphere to speak of, craters can remain fresh-looking for quite a while. But even with no Earth-like atmosphere, craters can slowly erode, and eventually more craters form on top of the older, beat up craters.

(D) Are impact craters rare? Are they hard to find?

The Earth is a tough place to be a crater. Once formed, impact craters are immediately subjected to wind, rain, earthquakes, landslides, volcanism, and even plate tectonics. All of these processes act slowly, and not so slowly, to change their original appearance, making them hard to identify. In addition to these natural geologic processes at work, there are biologic ones as well-- craters can be covered by plant life and trampled by animals. Humans have even built cities over them, never realizing they were there!

But how can that be? Something that big ought to be pretty easy to spot, even if its worn down, right? Well, things can look very different on the ground than they do from the air. A nice, fresh, small crater might be easy to identify, even looking at it from an angle, like this one to the left.

And at the rim of Meteor Crater, this is something you wouldn't miss, as you saw from the earlier air photo.

But what about much larger craters? They too can be very difficult to identify. You might be standing right next to one and not realize it. Note this lunar crater. This picture was taken from an orbiter, so we are still above the surface. Even so, the crater is difficult to identify. What if we were standing on or near the rim?

The image above was taken standing on the surface along the edge of a crater. This is a panoramic view of the Haughton impact structure in Canada. Can't quite see it? Does anything stand out? Does it look circular at all? Check out a short movie from stop 1 of the Haughton Virtual Tour. Still not sure? Well, as we said, they can be difficult to identify and even scientists have to work hard to see them.

And how about the Moon's Smithii Basin to the right? At more than 740 kilometers across, this is one large impact basin! But that very fact makes it incredibly difficult to recognize from the ground. From inside the crater, you would never be able to see the outer rim-- it would be beyond the horizon. You would never know you were standing in a crater... unless you knew what to look for

But scientists have identified lots of impact craters on Earth. The map to the left shows a total of 174 that have been discovered around the world (see the Earth Impact Database of the University of New Brunswick). So, if scientists have found this many, they must looking for very specific things to really know that they are impact craters.

And what is it that scientists look for?