Final answer:
Early in the solar system's history, the cratering rate was higher due to the era of giant impacts and a large number of debris present after the solar system's formation. Over time, this rate decreased due to clearing of debris and geological processes that have erased craters on Earth and other planetary surfaces.
Step-by-step explanation:
Early in solar system history, the cratering rate was much higher than at present. This high rate of cratering was due to the era of giant impacts, which is believed to have taken place during the first 100 million years of the solar system's existence. During this time, after the dissipation of the dust disk, planets and other bodies were heavily bombarded by the remnants of the solar system's formation, which included numerous asteroids, comets, and other debris. As time progressed, the rate of impacts decreased, particularly after the first few hundred million years, when orbital changes of Jupiter and Saturn may have scattered many asteroids out of the inner solar system, reducing the number of potential impactors.
Another factor contributing to the higher crater rates in the past was the relative freshness of planetary surfaces. When we look at regions such as the lunar highlands, we find more craters compared to regions like the maria. This indicates that surfaces with more craters are generally older because they have been exposed for longer periods of time without being resurfaced by geological processes such as lava flows or new impact events that could modify or cover existing craters. Over time, as these surfaces experienced geological activity, the number of visible craters diminished, leading to the lower cratering rates we observe today.
Additionally, on Earth, the presence of a thick atmosphere and active geology, including erosion and tectonics, has also erased evidence of early impacts. Comparisions between Earth's relatively crater-free surface and the heavily cratered surfaces of airless bodies in the solar system can tell us much about Earth's active geological history and atmospheric protection.