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FAQ - Impact Cratering

1. How can you tell a volcanic crater from an impact crater?

Volcanic craters may have a cone or flanks associated with the crater. There may also be flows present. Impact craters may have central peaks, ejecta, raised rims and floors that are lower in elevation than the surrounding terrain that can distinguish them from volcanic craters. During an impact event, the rocks that are impacted are shocked.

2. Is the ejecta always relative to the size of the crater?

The geometric relationships between an impact crater and the ejecta surrounding it is remarkably similar over an enormous range of sizes: from sand craters (made in one of the PSI workshops) to the giant craters found on the Moon and the planets. As can be seen in the image below of Kuiper crater on Mercury, the "continuous" ejecta – the bright annulus around the crater – extends about 4 times the crater radius. The discontinuous ejecta – the streaks - extend much further. This pattern is roughly the same for all planetary-scale impact craters.

3. How could we relate the height at which we drop an object to relative velocities?

The simple (and correct) answer: if you double the height, an object will hit with twice as much energy ("force"). For the sake of time, its not worth getting detailed into the math-- but velocity goes as the square root of height. So, to double the velocity, you need four times the height. However, energy goes as v2, so, if you are at four times the height, you have four times as much (potential) energy in the thing you are dropping.

4. What is the difference when an object impacts a large body of water versus land?

The main difference is in the type of material that is ejected, and the final crater formed. The basic cratering event is the same, that is, a huge explosion to start with (think about hitting the surface of a pool at high speed: the higher the speed the more it hurts, so at the velocities of impact of asteroids and comets, the surface of the ocean is not very different from a solid surface). Obviously, an impact in the ocean would not leave a lasting crater in the water; the ocean floor may or may not be altered, depending on the size of the impactor. If the impactor is large enough it will not be stopped by the water and reach the bottom of the ocean producing a crater. It is estimated that for an ocean about 4 km deep this requires a rocky asteroid at least 2 km in diameter (this impact event is large enough to have worldwide effects). Therefore, oceanic impacts leave behind a much smaller crater, if any, than land impacts. The material ejected in an oceanic impact is mostly water droplets and water vapor, with small amounts of dust, while in a land impact most of the material ejected is solid and melted rock.

5. How does the NASA Ames gun work? Does it make good examples of how craters are made through an atmosphere?

The NASA Ames Vertical Gun works pretty much the same as a real gun. A projectile is shot through a barrel (as it goes through there are appropriate traps that measure the speed of the projectile) and it eventually reaches the chamber, where there is an appropriate target of a given material. The chamber and barrel can hold different pressures, from atmospheric surface pressure down to very low pressures. The main difference between real cratering events and laboratory ones is the energy involved: the projectiles are much smaller as are the impact velocities. As a result, while crater formation in the laboratory is representative of what happens in reality on planetary surfaces, at least up to the formation of the final transient crater (before the onset of crater collapse), the magnitude of the shock generated by the impact is generally lower. It is then very hard to melt target material (and even harder to vaporize it!) in laboratory impacts.

6. Do the different kinds of rocks such as granite or mica come from meteorite sites because of their characteristics?

All types of rocks are modified by impact cratering. Generally impact rocks are local rocks that have been modified by the impact event. However, their chemical composition is that of the local rock, where the impact occurred.

7. How is a shatter cone formed?

The actual formation of shatter cones is still debated. There are different theories on how shatter cones form. They all associated with the interaction of the shock wave generated in the impact event with the rocks. One theory suggests that shatter cones are formed as a result of the compression of the rock as the shock wave goes through. Another suggests that the shatter cones are the result of the tension in the rock that occurs after the shock wave as gone through and the pressure in the rock goes back to normal. One thing is clear: shatter cones are formed only when the shock wave has weakened below the point of melting or completely fracturing rocks. For example, you cannot find shatter cone at the bottom of a simple crater (unless there is massive slumping of the crater rim), but you may find shatter cones in the central peak of a complex crater, because the central peak is formed by uplifting of material located below the bottom of the original transient crater, where the shock wave was low enough.

8. How does the crater erosion on Earth compare to the Moon, Mars, and Venus?

Crater erosion on Earth is much larger than on Venus, Mars, or the Moon. Earth, Venus, and Mars have atmospheres, thus some sort of climate. This causes erosion of the craters (think sand blasting). All objects experienced some volcanism. The main difference is that on the Moon volcanism ended pretty soon after its formation, it ended a bit later on for Mars, and much later on Venus. On Earth volcanism is still going on today; lava flows over craters covering them. Earth has also plate tectonics that cause the continuous destruction of crust, which obviously include the destruction of any crater on that crust. We do not believe Mars and Venus have plate tectonics, thus one less crater erosion (or destruction!) factor. Earth also has a vast amount of its surface covered by liquid water. The ocean floor is continuously changed by the deposition of sediments that would quickly cover any crater formed on the bottom of the ocean. One other thing that clearly puts the Earth on a different level as far as crater erosion is life. Vegetation is a powerful eroding agent, and humans are certainly not that inferior!

So, in summary from the highest to lowest amount of crater erosion we have: Earth, Venus, Mars, and the Moon. Indeed, in terms of numbers of craters known on the surface of these objects, this is the same listing in terms of smallest to largest number of craters.

9. What is the process of testing craters? How can you test craters on other planets?

It is assumed this question is related to the characterization of craters, including determining that they are actually of impact origin. The determination of the impact origin of a crater on Earth is based on very specific indicators of shock. This means that on the Earth, to determine if a crater is of impact origin, one must go to the crater itself, pick up rock samples from the crater or its vicinity and identify some of the diagnostics of shock in rocks, which are: shocked quartz, multiple planar deformation features in various minerals, shatter cones, evidence of melt with the composition of the surface rocks at the crater site, presence of lithic (only solid fragments) or melt (solid and melt fragments) breccias.

On other planets we do not have the luxury of getting to the crater and collecting samples. In these cases, we can only rely on the images we collect. We must understand what are the characteristics of the region where the crater is, specifically, is it a volcanically active region, is it a tectonically active region, what is the level of erosion, etc., before we can infer if the crater observed is of impact origin. In general, there are basic differences between volcanic and impact craters, and it is usually pretty straightforward to distinguish a volcanic crater from an impact one. Also, keep in mind that there are different types of impact craters, depending on the size of the crater (where the progression from simple to complex craters to multi-ring basins goes with increasing size), which do not have a volcanic equivalent. For example, we do not know of any process other than impact that can generate a peak ring or multi-ring crater.

Impact craters and volcanic craters often display similar features: a volcanic caldera like Crater Lake, OR looks a lot like complex craters with raised rims and small subsequent eruptions that produce small mountains inside the crater like central peaks. The similarities gave rise to the arguments in the 1800s and 1900s over impact vs. volcanic origin of the craters on the Moon. Sometimes it is hard to tell the difference from appearance (morphology) alone. The definitive test—that of finding impact-shocked rocks—can only be done for craters on the Earth or the Moon from which we have samples.

However, having said that, with experience, one can usually tell the difference between an impact and a volcano, even on other planets and moons: impacts are more regular in outline (shape and height of the rim) than volcanoes, have distinct texture and distribution of ejecta and morphology of features on the walls and floors. Volcanoes do not show multiple rings like two and three ring craters, and volcanic “central peaks” are usually off-center. Impacts are always holes in the ground with a raised rim; volcanoes are usually holes in a mountaintop, and if on flat ground, often lack a raised rim. Finally there are issues of geologic association: impact craters can occur anywhere, singly or in groups; volcanoes usually occur in characteristic geologic settings and groupings. It may seem like a dodge, but with some practice we could teach you to spot impacts vs. volcanoes on appearance alone anywhere in the Solar System with 90% accuracy. They really do look different!

10. Does water absorb the impact better than land does?

This is a complex question, mainly because of the implications of “absorb.” Water is a fluid and as such it deforms easily and later goes back to its original position just as easily. So when an object hits an ocean, it first creates a big crater which is later wiped out by the returning water, so no final crater will be visible at the surface. Does this mean that an impact in water is harmless? Of course not! An impact is equivalent to a huge explosion, no matter if it happens in water or on land. In oceanic impacts, the water can mostly stop the impactor if the object is small compared to the depth of the ocean. Therefore, a shallow sea does not do much as far as “absorbing” the energy of an impact. Objects comparable with the ocean depth will crater the ocean floor on top of creating a crater in the water. The impact will still eject a large amount of material, but in an oceanic impact most of the ejected material is going to be water. Much of the ejected ocean water will reach the upper atmosphere where it can stay for quite some time and affect the chemistry of the atmosphere. In particular, water and the salts it contains will increase ozone destruction; ozone in the upper atmosphere is the main shield we have against the harmful UV radiation from the Sun. Once removed in the entire atmosphere or even in a limited region (ozone hole anyone?), all living organisms will be affected by much higher levels of UV radiation. Once the water and salts are removed from the atmosphere, the ozone will be replenished, but that may take years. Another problem with water impacts is that the impact will create powerful tsunami waves that may crash into nearby shores. Any city on those shores will be devastated by the tsunami.

11. What are the reasons why you get multiple central peak rings—why not just a peak or one ring?

The complexity of the craters depends on the size of the event, essentially on the energy involved. As you increase the energy involved you will see an increase in the complexity of the resulting crater. Think of the material affected by the cratering event as if it was fluid (a thick oil, mud, or something like that); the more energy is involved in the cratering, the higher the rebound at the bottom of the crater (the effect of gently throwing a rock in a pond is very different from the effect you get if you throw the rock as hard as you can); the resulting peak will tend to collapse upon itself into a ring (too big to hold itself up against gravity), that expands out while a new rebound occurs, generating a new peak. In water this process goes on until the water goes back to being flat; rocks behave in such a fluid way only for a short time, and eventually they return to behaving as a solid (or brittle). This happens at different stages for impact events with different energies. The higher the energy involved the more complexity the crater has. Thus, the progression from simple to complex, complex to central peak, central peak to peak ring, etc.

12. What is the value of counting craters?

There are a number of reasons for counting craters: 1) for estimating how often things get hit and how big the impactors are, we can estimate how often something will hit the Earth and how big. 2) It helps us understand the history of planetary surfaces—how old, what processes are going on that can affect the surface, etc.

13. How can we evaluate the ocean floor for craters?

One can map the ocean floor and look for circular features (done all the time). Also, where any drilling is done, one can look at the core samples and see if there is any indication of shocked material, a sign of an impact.

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