The most famous impact event that affected Earth is perhaps the Chicxulub impact event, which has been linked to the extinction of the dinosaurs 65 million years ago. It is, however, only one of many impact events that have affected our planet and not even one of the largest. The record of these processes is very poor on Earth, because our planet has a lot of geologic processes that destroy or mask evidence of impact events. Mountain building processes crumple impact craters, plate subduction consumes impact craters, erosion dissects impact craters, and lava flows and sediments often bury any remaining impact craters. While scientists continue to discover a handful of impact craters each year, the number of known craters is quite small (currently ~160; see Map for a current listing of impact craters).
A much better record of the impact cratering history of the Earth is preserved on our neighboring Moon. Because water does not flow across its surface, lava no longer erupts from its volcanoes, and plate tectonics has never occured there, the record of bombardment in the Earth-Moon system is far better preserved. Over 300,000 impact craters the size of Barringer Crater (or Meteor Crater; ~1km in diameter) in Northern Arizona and larger exist on the Moon. They have been created as asteroids and comets pelted its surface, at the same time similar object pelted the Earth. However, because of the Earth's larger size, it would have suffered about 10 times more impact events, or over 3 million impact craters greater than 1 km in diameter!
These impact craters have been produced on the Earth and Moon over their 4.5 to 4.6 billion year history. Studies of the impact cratering histories of the Earth-Moon system and observations of asteroids and comets in our solar sytem indicate that impact events continue to occur on both the Earth and Moon and that they occured more frequently earlier in Earth's history. But an additional twist of this story began to emerge when scientists began studying rocks collected on the Moon by Apollo astronauts.
Analyses of lunar samples collected by Apollo astronauts revealed a surprising feature: the crust of the Moon seems to have been severely heated ~3.9 billion years ago, metamorphosing the rocks in it. Scientists (Tera et al., 1974) suggested this metamorphic event may have been created by a large number of asteroid and/or cometary collisions in a brief pulse of time, <200 million years, in what was called the lunar cataclysm.
If a lunar cataclysm really occurred, then lots of impact melted rocks with that same age should also exist. And, indeed, additional analyses of impact melts collected by Apollo astronauts revealed a range of impact ages, but, significantly, none older than 3.85Ga (Dalrymple and Ryder, 1993; 1996). This also seemed to imply a lunar cataclysm ~3.9 Ga, which completely destroyed or metamorphosed impact melts produced by older impact events.
Maps showing regions of the Moon that were resurfaced
during Nectarian Period (brown) and
Imbrian Period (blue), and outlines of larger basins. Also shown are pre-Nectarian units (darkest brown)
and areas of unclear stratigraphic relationships (gray) (Wilhelms, 1987).
However, the interpretation of these data were controversial, because all of the Apollo landing sights were restricted to the nearside equatorial region of the Moon. This restriction was a mission limitation because it was necessary to maintain communication with Earth. The result, unfortunately, was a potentially biased set of samples. Critics of the luanr cataclysm hypothesis suggested the Apollo samples are dominated by the effects of the large impact basins (Nectaris, Cresium, Serenitatis, and Imbrium) in the nearside equatorial region of the Moon and do not reflect a global impact record.
More recently, a new set of samples was discovered that can help test the hypothesis of a lunar cataclysm. Begining in 1981, scientists began finding meteorites that looked very similar to the rocks collected by Apollo astronauts. It was soon realized that these meteorites came from the Moon and were delivered to the Earth by recent impact events on the lunar surface. Since impact events occur randomly on the surface of the Moon, the collection of lunar meteorites provided a set of samples from a much larger region of the lunar surface than the Apollo collection. They were a perfect set of samples to test the lunar cataclysm hypothesis.
Among the lunar meteorite collection was a set of rocks called regolith breccias. These are rocks composed of the broken fragments of lots of other, older rocks. Some of these fragments are samples of impact melts that occured farther back in the moon's history. To test the lunar cataclysm hypothesis, one could extract these fragments and determine when they had formed. If they also failed to record impact events prior to 3.9 Ga, like the Apollo samples, then this would seem to confirm the lunar cataclysm hypothesis.
Examples of the Largest Craters
by the Cataclysm on the Moon
||Diameter (km)||Age (Billion Years)|
|Imbrium||1200||3.85 +/- .01|
|Hertzprung||570||3.89 +/- .009|
|Serenitatis||740||3.895 +/- .017|
|Nectaris||860||3.89 - 3.91|
In a study by Cohen, Swindle, and Kring released in 2000, the ages of impact melts in 4 lunar meteorites were reported. The collection of impact melt samples had a range of ages, meaning they had been produced in impact events that occurred from 3.9 to 2.7 billion years ago on the Moon. Significantly, none of the ages were older than ~3.9 billion years ago. In the case of the Moon, this event appears to have nearly resurfaced the entire planet.
Observations of impact craters on the Moon indicate that >1,700 impact craters with diameters >20 km were produced during the cataclysm. This implies that >17,000 impact craters with diameters >20km were produced on the Earth during this same period of time, which lasted from 20 to 200 million years. Each of these impact events is large enough to have produced global effects and some of the largest would have produced impact craters with diameters that exceeded 1,000 km. That is, impact craters the size of continents were being produced on the Earth (e.g. Kring, 2000; Kring and Cohen, 2002).
Scientists are now wondering if this cataclysmic bombardment may have affected life on Earth or been involved in life's origins. The earliest isotopic evidence of life we have is from rocks ~3.8 billion years old, immediately after the cataclysm.
More about these investigations can be found on the Origin of Life page.
- The Moon
- Impact Cratering
- Origin of Life
This web site is based on information originally created for the NASA/UA Space Imagery Centerís Impact Cratering Series.
Concept and content by David A. Kring.
Design, graphics, and images by Jake Bailey and David A. Kring.
Any use of the information and images requires permission of the Space Imagery Center and/or David A. Kring (now at LPI).