The Late Heavy Bombardment (LHB) was a period of intense bombardment of the inner Solar System that is thought to have occurred about 3.85 billion years ago, when the Solar System was only about 700 million years old. Most of the craters on the Moon, including the giant lava-filled basins (also called Mare), are thought to have formed during this brief but intense period of bombardment. Although the Earth would have suffered a similar bombardment, geologic processes have since erased any such craters that formed then.
To study this possible bombardment of the Earth, Jorgenson and colleagues studied ancient sedimentary samples they collected in Greenland that are 3.9 billion years old (see the abstract of his paper). These samples were probably deposited around the time of the LHB. They find that the iridium abundance in those samples are elevated by a factor of 7, which indicates that comets (and not asteroids) are the principal source for the LHB impactors. Evidently, the entire inner Solar System was bombarded by icy comets that likely originated in the outer Solar System. This cometary bombardment might also have been triggered by a sudden rearrangement of the outer planets orbits, Jupiter through Neptune.