Target Earth: evidence for large-scale impact events

Area of Science:

• Planetary Science
• Geology
• Impact Cratering

Background:

• Earth retains few impact structures due to geological activity and surface renewal.
• Over 150 impact structures and 15 stratigraphic impact events are known on Earth.
• Terrestrial impact structures share morphologies with those on other planets.

Purpose of the Study:

• To analyze the terrestrial impact record and its implications.
• To determine the terrestrial impact rate and composition of impactors.
• To assess the threat of impacts to civilization.

Main Methods:

• Analysis of known terrestrial impact structures and stratigraphic evidence.
• Geochemical analysis of trace elements in impactites.
• Comparison of terrestrial impact rates with astronomical observations.

Main Results:

• A terrestrial cratering rate of 5.6 +/- 2.8 x 10(-15) km(-1) a(-1) for structures >= 20 km.
• Approximately 50% of larger impactors (>1 km) are chondritic.
• Impacts represent a significant threat to human civilization on timescales of 10^5-10^6 years.

Conclusions:

• The terrestrial impact record, despite biases, is crucial for understanding impact flux.
• Impact events are a stochastic, long-term threat to civilization.
• Further evidence of impacts is expected with ongoing research.