Characteristics and Origins of the Solar System
Lecture
12
October
4, 2000
The Earth is in the same part of space as the Moon, and it formed at the same time as the Moon, 4.5 – 4.6 billion years ago. Accordingly, it must have undergone the same bombardment that the Moon did in the Age of Bombardment. The craters are less obvious because the active geology of Earth, and its hydrology, remove the obvious signs with the passage of time.
Less obvious, however, does not mean nonexistent. Given clues from our studies of the Moon, we can begin scrutinizing the Earth.
· The Earth continues to get pelted by objects from outer space. Every time you see a meteor in the atmosphere, a solid object from outer space has collided with the Earth.
· The Barringer Meteor Crater in Arizona is a lunar-style crater a mile across. It was formed 50,000 years ago by an iron-nickel meteor that was 50 meters in diameter and released as much energy as a 15 Megaton nuclear weapon. There is a picture of it in Figure 7.17 of your book.
· From sophisticated geological studies, we can find paleocraters, the craters left by impacts from early in the history of the Earth. >>>>> transparency of known paleocraters on Earth. (examples are impact craters in southern Germany).>>>>>> Big Impact web page.
Impacts like Barringer crater would not have had global effects. But impacts of larger objects could drastically change the climate of the Earth. In last few decades there has been a remarkable discovery in this respect.
65 million years ago was a huge kill-off of life on Earth. Estimates are that 60% of plant and animal species became extinct in a short period of time. Included were all of the species of dinosaurs. For long time was a considerable mystery as to what had caused this.
In late 1970’s a physicist at the University of California suggested that a large asteroid impact on Earth at this time had done this. Some key aspects of this theory are as follows.
· There is a world-wide layer of sediment from this period, enhanced in the element Iridium.
· For a sufficiently large explosion, there could be a planetary-wide cloud of ash and dust thrown up which would take years to settle out. Sunlight would have been extinguished at ground level.
· In the last ten years, the apparent crater has been found. Chicxulub on the coast of Yucutan. It is 200 kilometers in diameter (comparable to large lunar craters). It dates from this period.
· To produce such a crater would have required an impactor 20 kilometers in diameter. An artist’s conception of it colliding with the Earth is shown in Figure 7.18 of the textbook.
Paleontologists have identified in the fossil record 5 cases of mass extinctions, including the Cretaceous-Tertiary. The most severe was the End-Permian, 250 million years ago, when apparently 90% of all species became extinct “instantly” on a geological scale.
>>>>>> Anecdotes about properties of End-Permian extinction.
It is at the present unknown if meteoric impact is responsible for others of these extinctions (perhaps all of them?). The mean time between the extinctions is about the expected time between impacts of “planet killers”, asteroids 10 to 20 kilometers in diameter.
· Steven J. Gould and the evolutionary significance of Big Impacts.