9.3 Impact Craters
Impact Craters
Learning Objectives
By the end of this section, you will be able to:
Compare and contrast volcanic and impact origins of craters.
Explain the process of impact crater formation.
Discuss the use of crater counts to determine relative ages of lunar landforms.
Introduction to Lunar Craters
Historical Context:
The Moon serves as a benchmark for understanding our planetary system's history.
Impacts greatly affect solid worlds; however, Earth's geological activity has erased much of this history.
The Moon has preserved its impact history, providing insights that can be applicable to other worlds.
The Moon is unique as it has shared Earth's history for over 4 billion years.
Volcanic Versus Impact Origin of Craters
Initial Misconception:
Until the mid-20th century, lunar craters were largely thought to be volcanic due to the rarity of impact craters on Earth.
Craters on Earth, usually volcanic, led scientists to erroneously conclude the same for lunar craters.
Grove K. Gilbert's Contribution (1890s):
Gilbert, with the US Geological Survey, proposed that large lunar craters had an impact origin based on the unique characteristics of lunar craters.
Lunar craters differ from terrestrial volcanic craters:
Lunar craters are larger and have mountain-rimmed, circular features with floors below the surrounding plains.
Terrestrial craters are smaller, deeper, and typically sit atop volcanic mountains.
Key Observations:
Gilbert noted the different shapes and sizes, concluding that the impact origin was the most plausible explanation for lunar craters.
His hypothesis was initially unaccepted but laid the foundation for modern lunar geology.
Understanding Escape Velocity
Definition of Escape Velocity:
The minimum speed needed for an object to break away from the gravitational pull of a celestial body.
Escape velocities:
For Earth: V_e = 11 ext{ km/s}
For Moon: V_e = 2.4 ext{ km/s} (approximately 5400 miles per hour).
Impact Mechanics:
Projectiles typically arrive with speeds greater than the escape velocity due to gravitational attraction.
The kinetic energy produced during impact results in a violent explosion:
The impact creates a shock wave that fractures the target rocks and generates heat that vaporizes the projectile.
Resultant explosion leads to a crater formation that is generally symmetrical.
Cratering Process
Impact Dynamics:
Projectiles penetrate two to three times their diameter before stopping upon impact, transferring energy into shock waves and heat.
The crater size is primarily a function of impact speed, typically measuring 10 to 15 times the projectile's diameter.
Stages of Impact Crater Formation:
(a) Impact occurs.
(b) Projectile vaporizes and shock waves spread through the rock.
(c) Ejecta is expelled from the crater.
(d) Ejected material falls back, filling the crater and possibly forming a central peak.
Figures:
Figure 9.14 illustrates the stages of crater formation.
Figure 9.15 shows King Crater on the Moon, displaying typical features of impact craters.
Features of Impact Craters
Central Cavity:
Initially bowl-shaped; stress from the impact can lead to the formation of a flat floor and occasionally a central peak.
Ejecta Blanket:
Surrounding the crater rim, the ejecta blanket consists of debris from the explosion, creating a rough terrain, extending outward to form bright crater rays.
Size and Distance:
Higher-speed ejecta: fall farther from the crater, digging small secondary craters elsewhere.
Bright crater rays can extend hundreds of kilometers, notably in young craters like Kepler and Tycho.
Observing the Moon
Amateur Astronomy:
The Moon is observable with small telescopes and binoculars, revealing topographical features.
Viewing Craters:
Full moon offers minimal detail due to direct sunlight causing no shadows.
Best views occur during first and third quarters when sunlight creates contrast through shadows, revealing craters effectively.
Visual Impact of Phases:
Under different lighting phases, lunar craters and features change drastically in visibility.
At full phase, contrast highlights features like the maria and rays from craters.
The Effect of Earthlight
Earthlight Illumination:
The term “the new Moon in the old Moon’s arms” describes how the entire lunar disk can be faintly seen during the crescent phase, illuminated by sunlight reflected from Earth.
Earthlight is approximately 50 times brighter than sunlight reflected from the Moon.
Using Crater Counts to Determine Ages
Crater Counting Methodology:
For planets and moons that lack significant erosion, crater counts can estimate surface age.
These ages refer to the time since significant surface disturbances occurred, such as volcanic eruptions on the Moon.
Challenges:
Direct measurements of crater creation rates are hindered by the infrequency of large impacts.
Example of Meteor Crater in Arizona indicates crater is about 50,000 years old.
Crater Production Rates:
Estimates indicate that:
A 1 km diameter crater forms approximately every 200,000 years.
A 10 km crater forms every few million years.
A 100 km crater forms once or twice every billion years.
Implications of Findings
Surface Age Estimation:
The Moon has been formed over billions of years based on current cratering rates correlating with radioactive dating, suggesting the maria are approximately 3.3 to 3.8 billion years old.
Crater Density Discrepancies:
Highland regions show about ten times more craters than maria but are only slightly older (around 4.2 billion years).
This indicates that impact rates were not constant, with higher activity early in the Moon's history, leading to the “heavy bombardment” theory.
Conclusion
The study of impact craters on the Moon provides critical insights not only into lunar history but also into planetary processes across the solar system. The variations in cratering rates and the characteristics of craters themselves reveal much about the historical impact events that shaped both the Moon and other celestial bodies.