Two primary geological forces affect terrestrial planetary bodies: basaltic volcanism (internal) and impact (external). Of the two, impact is more apparent because of the craters and basins that it creates. The public has become more aware of the hazards due to impact recently, with the multiple impacts of Comet Shoemaker-Levy 9 into Jupiter in the summer of 1994, movies such as “Deep Impact” and “Armageddon” and the widespread acceptance of the Impact Hypothesis for the extinction of the dinosaurs.
Impact has played a major role in the evolution of terrestrial bodies in the solar system. The very formation of the solar system was an impact process, known as collisional accretion. The early history of the solar system was an era of heavy bombardment, the scars of which are easily visible on the Moon, Mercury, and the moons of the outer planets, and less visible on the surface of Mars.
Energy Considerations
The energy released by an impact is enormous. The maximum amount of energy that can be released in an impact is given by:
E = mv2 (1)
E is the energy in Joules
m is the mass of the impactor in kg
v is the impact velocity in m/s
The mass of the impacting body can be estimated by assuming a spherical impactor of constant density, leading to:
m = (4/3) p r R3(2)
r is the density of the impactor in kg/m3
R is the radius of the impactor in meters
If you wish, you may use the following equation to determine the mass of an ellipsoidal impactor with axes a, b, and c:
m = (4/3) p r abc(2a)
In most cases the density of the impacting object will have to be an estimate. Table 2 may prove useful for such estimates. These two equations can be combined into:
E = (2/3) p r R3 v2 (3)
This equation tells us how much energy is released for a given impact, if we can make an estimate for the velocity of impact, the mass of the impactor, and the density of the impacting object.
The energy increases with the square of the velocity, but with the cube of the radius. In other words, increasing the velocity by a factor of 10 increases the energy by 100, but increasing the radius of the impactor by a factor of 10 increases the energy by 1000. This explains why the impact of a large comet or asteroid has globally-devastating consequences, and the impact of even a moderately-sized body can produce widespread devastation.
Below is a table of impact energies. The first object on the list is the asteroid 4581 Asclepius (1989 FC), which passed by the Earth on March 22, 1989 at a distance of some 700,000 km. I include this one because there was some media attention at the time. The second object is a potential future threat to the Earth-Moon system: Comet Swift-Tuttle, which passed Earth in 1992 and is scheduled to return in 2126. It has been estimated that there is a 75 percent chance that Swift-Tuttle will impact either the Earth or the Moon within the next million years. The third object is the Chicxulub impactor, the trigger for the extinction of the dinosaurs. The fourth is Comet Shoemaker-Levy 9 Fragment Q, and the last is the asteroid responsible for Meteor Crater in Arizona.
Table 1: Impact Energies
| Object | Radius (m) | Density (kg/m3) | Impact Velocity (m/sec) |
| Asclepius | 100 | 3000 | 30,000 |
| Comet Swift-Tuttle | 5000 | 1000 | 60,000 |
| Chicxulub impactor | 5000 | 3000 | 32,000 |
| SL9 Fragment Q | 2150 | 1000 | 60,000 |
| Meteor Crater | 12 | 7800 | 6,200 |
| Object Name | Energy (Joules) | Energy (TNT Equivalent) |
| Asclepius | 5.665 x 1018 | 1.35 gigatons |
| Comet Swift-Tuttle | 9.42 x 1023 | 225,000 gigatons |
| Chicxulub impactor | 8.03 x 1023 | 191,793 gigatons |
| SL9 Fragment Q | 7.5 x 1022 | 18,000 gigatons |
| Meteor Crater | 1.09 x 1015 | 260 kilotons |
| Material | Density (x 103 kg/m3) |
| Water (ice) | 0.94 |
| Water (liquid) | 1.00 |
| Carbonaceous chondrite meteorite | 2.5 |
| Plagioclase | 2.7 |
| Ordinary rock (average) | 3.0 |
| Pyroxene | 3.3 |
| Olivine | 3.3 |
| Ordinary chondrite meteorite | 3.5 |
| Iron sulfide | 4.8 |
| Iron | 7.9 |
How Big is Big?
A very common question is “How big was the object that made that crater?” This is not an easy question to answer, because of a number of different factors that come into play during crater formation. I have included an equation to estimate the size of crater that will be made, but it is not terribly accurate (it is usually within a factor of two). It also looks rather messy, but it can be used to get a feel for the general size of a crater that will be made from an impactor of a given size into a given target material of a given density. (For Earth I use rt = 2600 kg/m3, for the Moon, 2900 kg/m3).
D = 1.8 rp0.11 rt -1/3 g-0.22 L0.13 E0.22 (sin q)1/3 (4)
rp is the density of the impacting object (r in the above equations)
rt is the density of the target material in kg/m3
g is the acceleration due to gravity at the surface in m/sec (Earth = 9.80)
L is the diameter of the impacting object in meters (NB: diameter, not radius)
E is the energy in Joules, from equation 3, above
q is the angle of impact in degrees
An interesting measure of the power of an impact is to calculate the Richter scale equivalent of the energy release. See Table 3, below.
M = 0.67 log10 E - 5.87(5)
M is the equivalent magnitude on the Richter scale
E is the energy, calculated from 3, above
Table 3: Richter scale equivalents
| Object | M |
| Asclepius | 6.69 |
| Comet Swift-Tuttle | 10.19 |
| Chicxulub | 10.14 |
| SL9 Fragment Q | 9.46 |
| Meteor Crater | 4.20 |
Impact Erosion
One outcome of a very large impact is known as impact erosion, which results in the loss of atmosphere due to hypervelocity expansion of a vapor plume from the impact; in other words, the impact literally blows off some of the atmosphere to space. There are two basic requirements for impact erosion to occur. The first is that the impactor must strike at a velocity high enough to form a vapor plume that will expand at a speed greater than the planet’s escape velocity. The second is that the mass of the vapor plume exceeds the mass of atmosphere above the plane tangent to the impact. (i.e., everything from the horizon upward). A rough estimate of the mass of vapor created during an impact is given by
Mv = (0.4 vi2 Mp) / ev(6)
Mv is the mass of vapor
vi is the impact velocity
Mp is the impactor mass
ev is the specific energy of vaporization of silicate rocks (5.7 x 107 J/kg)
The estimated minimum impactor mass for impact erosion is 4 x 1015 kg, and the estimated minimum velocity is 25,000 m/s.
Impact erosion probably played a significant role in the early history of planetary atmospheres. Repeated bombardment by large impactors would have certainly resulted in the loss of some atmosphere.
Impact Frustration
Another outcome of a large-scale impact is to sterilize a planet. Any sufficiently large impactor will cause so much damage that conditions will be incompatible with life. It is possible that, at the very beginning of life on Earth, impacts sterilized the planet and life had to start over. Impact frustration is defined to be that time in Earth’s history when the interval between devastating impacts was less than the minimum timescale needed to establish life. This establishing timescale is estimated to be on the order of 100 million years.
General Impact Processes
We will examine the process of impact by reconstructing the events which took place during the Chicxulub impact 65 million years ago.
The impactor struck the Earth at a velocity of some 32,000 m/s. The impact vaporized the impactor and a lot of the target material as well. Hundreds of cubic kilometers of dust and water vapor were thrown out of the forming crater. As the crater formed, ejecta was thrown for hundreds of kilometers, thickest near the crater and thinning out further away.
Some of Earth’s atmosphere was probably blown off during the impact, and some of the impact debris was probably blown into space (this may be the source of the microtektites which have been found). The expanding shock wave created tremendous tsunami which carried away loads of rock. These tsunami reached the nearest land masses, dumped their load of rock and continued inland for many kilometers before dying out. Winds created by the impact blew away from the impact point, reaching speeds of hundreds of kilometers per hour.
The energy released by the impact may was probably sufficient to cause gases in the atmosphere to chemically react with one another, forming nitric and nitrous oxides. Combining with water, they formed nitric and nitrous acids, which rained out. The heat released by the impact triggered world-wide forest fires. The smoke and ash generated by such fires remained suspended in the atmosphere for months, possibly years. Much sooty material has been found at the K/T boundary around the world. As many as half the world’s forests may have burned.
The fireball rising from the impact point carried tremendous amounts of dust high into the stratosphere, where it could have remained for years before settling out. It is estimated that over 1400 cubic km (400 cubic miles) of dust was injected into the atmosphere during the impact. Much of this dust came from the target rock at the impact point, but some of it was from the impactor. The dust from the impactor is the source of the world-wide iridium anomaly at the K/T boundary.
The crater excavated by this impact was 160 km (100 miles) wide and 8 km (5 miles) deep.
The Death of the Dinosaurs
The dust from Chicxulub was the culprit in the K/T mass extinction. The dust injected into the stratosphere was sufficient to prevent light from reaching the surface, for months or even years. Temperatures world-wide probably dropped by tens of degrees. Plants around the world died. The phytoplankton in the oceans probably disappeared within 100 days or so. With the phytoplankton gone, along with all of the other plants, the ecosystem completely collapsed.
Over 75 percent of families of organisms disappeared as a result of this impact—a major blow to terrestrial biodiversity. Other mass extinction events in Earth history may be impact related. The Permian extinction was more destructive—some 90 percent of families disappeared.
Sources
Consolmagno, Guy, and Martha Schaeffer, Worlds Apart: A Textbook in Planetary Sciences, Prentice-Hall: Englewood Cliffs, NJ, 1994.
Cowles, Dennis J., “The Energy of Impact.” Paper presented at the annual conference of the Southeastern Planetarium Association, 1995.
Takata, T. and Ahrens, T.J., “Fragment and Progenitor Energy of Comet Shoemaker-Levy 9 and the Frequency of Such Impact Events.” Lunar and Planetary Science Conference XXVI Abstracts. (3 vol.) Lunar and Planetary Institute: Houston, TX, 1995.
Norton, O. Richard, Rocks from Space, Mountain Press: Missoula, MT, 1994.
Shirley, James H., and Rhodes W. Fairbridge, Encyclopedia of Planetary Sciences, Chapman and Hall: New York, NY, 1997.
Shoemaker, Eugene M., “Asteroid and Comet Bombardment of the Earth.” Annual Review of Earth and Planetary Sciences, 1983. 11: 461 – 494
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