Impact Cratering
I. Reasons to study impact caters
A. Without samples craters are the only indicator of the age of a planetary surface.
B. Impact cratering is the most common geologic process in the solar system.
C. Can be used to give various other types of special information on the planets:
1. Impact craters excavate materials from depth.
2. How the crust of a planet responds during and after crater formation yields information about the mechanical properties of the crust.
3. On Earth at lease one, and perhaps several, extinction events are impact-related.
II. A brief history of impact studies
A. First interest in impact craters occurred when they were discovered on the moon.
~1609 - Galileo coins terms "crater" Greek for mixing bowl.
B. Until this century craters were generally accepted to be of volcanic origin. Why?
1. Craters bear some resemblance to volcanic calderas.
2. A lack of source materials; the existence of meteorites as extraterrestrial rocks was not confirmed until the 1800's.
3. No one could explain why craters are all round; why no oblong craters caused by oblique in past?
C. Some breakthroughs in cratering studies:
1. 1893 - Gilbert (the head of the USGS) matched several properties at lunar craters with low velocity impact experiments, but could not figure out why almost all craters are round.
2. 1916 -Opik suggested that high velocity impacts are similar to explosion craters, and thus should always be round.
3. 1949 - Baldwin compared the morphometric properties of explosion craters, lunar craters, and volcanic craters, and found lunar craters to be much more similar to explosion craters.
III. Stages in formation of an impact crater:
A. Contact and compression stage:
1. Impactor strikes rock at several times the speed of sound; a typical impact velocity is 20 km s-1.
2. The impactor initially pushes material out of the way, accelerating it to a high percentage of impactor velocity. A small amount of melted material may be "jetted" out of the crater at high velocities.
3. A shock wave propagates outward from the point of impact. Shock wave - a wave traveling faster than the speed of sound in the propagating material.
4. Shock wave pressure = 100's GPa; lots of melt and vapor produced.
5. This stage ends when projectile releases pressure (i.e., it's been stopped). Total time is less than 1s.
B. Excavation stage:
1. Shock wave propagates outward, closely followed by a rarefaction caused by reflection off the free surface of the planet.
2. The decline in pressure outward of the shock wave/rarefaction combined with the increase in pressure with depth sets up a large pressure gradients after passage of the shock wave. Pressure gradients define stream tubes for excavation. Material travels along a stream tube on its way out of the crater.
3. Material travels out of crater in ever-widening curtain at ~45o angle.
4. Properties of resulting "transient cavity":
a. depth of cavity is 1/3 diameter.
b. depth of excavation is 1/10 diameter.
c. cavity has parabolic shape.
d. however, region of melt/vapor, pre-excavation, is hemispherical.
5. Modification stage - smaller, "simple" craters retain transient cavity shape. Larger craters collapse inward and produce a variety of "complex" features.
IV. Morphology (appearance) of craters
A. Lunar craters - typical of craters on an airless rocky body
1. Simple craters - essentially a "transient cavity" with a little breccia and melt on the floor.
a. largely parabolic shape
b. rim-floor depth ~1/5 diameter
c. rim height above surrounding terrain ~1/20 diameter
d. simple craters are <15 km diameter
2. Complex craters - more complicated features form at larger diameters.
Onset diameter on moon:
a. lower depth/diameter ratio 11 km
b. central peaks 20 km
c. terraces 20 km
d. flat floors 12 km
e. peak rings 140 km
f. multiple rings 350 km
3. Ejecta blankets - on the moon ejecta is emplaced ballistically. Larger craters have a "hummocky" zone where ejecta has plowed into the surface.Some fresh craters have "rays" that extend around the globe.
B. Venusian craters - emplaced in the presence of a dense atmosphere.
1. There are very few simple craters because the atmosphere prevents small meteoroids from reaching the surface.
2. Smaller craters (<15 km diameter) are often irregular in shape because the incoming meteoroid breaks up and disperses in the atmosphere. Sometimes dispersal is enough that "multiple-floored" craters and "crater fields" occur.
3. Except for 1&2 above, the interiors of Venusian craters are similar to lunar craters. However, the onset diameters for complex crater features (e.g., central peaks, peak rings) are less because the higher gravity enhances the collapse of the transient cavity.
4. Ejecta - ejecta is not emplaced ballistically but instead is entrained in a cloud around the crater.
a. the ejecta blankets for craters >20 km in diameter are lobate and perhaps emplaced as a debris flow.
b. some craters have very fluid "outflows" that sometimes extend hundredsof km from the crater.
c. craters larger than ~50 km in diameter have largest clasts emplaced ballistically forming "secondary" craters.
C. Martian craters - generally similar to lunar craters with the exception of rampart craters - ejecta appears to have been emplaced as a mud flow. Size range of rampart craters is 5-50 km diameter.
D. Icy satellites - although formed in ice or ice-rock mixture, generally similar to lunar craters except for:
1. central pit craters - craters that have a pit in the center instead of a peak
2. multi-ring basins - very large craters on Callisto appear to "crack" the surface like a windshield on a car.
V. The influence of planetary gravity and crustal strength.
A. Higher gravity and lower crustal strength has the following effects:
1. Lower onset diameter for central peaks.
2. Lower onset for peak rings.
3. Lower onset for lower depth-diameter ratio.
4. Lower depth-diameter ratio.
5. No effect on central peak diameter.
6. No effect on peak ring diameter.
B. Implication of A: Horizontal dimensions of a crater controlled by the transient cavity diameter, but the collapse and vertical dimensions of a crater are controlled by gravity and crustal strength.
VI. Using craters to date a planetary surface and help determine resurfacing mechanisms. Basically, use the number of size of craters on a planet's surface to determine what has happened to the surface and how long it has been since geologic activity occurred.
A. Calibration of the data
1. The moon - take a surface of known formation age and count the craters and measure their size. Example: mare serenitatis formed ~3.2 Ga. For a 2 diameter range, the number of craters is N = 9.55 x 10-2 D- 1.91/(106 km2). This function defines the "incremental" size-frequency curve. If one counted the total number of craters above a particular diameter, this would be the "cumulative" size-frequency curve. For airless bodies these functions are usually fairly constant over a large size range.
2. The meteoroid flux - by surveying the Earth-crossing asteroids, their probability of colliding with Earth, and estimating the resulting crater size from collision with Earth, one can estimate the formation rate of craters on Earth.
3. To apply 1&2 above to other planets one must account for differences in meteoroid flux, impact velocities, and cratering efficiency (the size of crater formed by a given meteoroid).
B. Interpretation of the incremental size-frequency distribution. Some examples:
1. A single resurfacing event buried all the existing craters at the time - counting the number of craters per unit area indicates the time since the resurfacing event.
2. A continuously occurring process that fills X m per year of craters - affects small craters more than large ones, causing change of slope of size- frequency curve.
3. A single resurfacing event that buries small craters but not large ones - causes a break in the incremental size-frequency; counting small craters per unit area gives time since resurfacing event.