The Heavy Bombardment and Life on Earth

It is estimated that Earth formed around 4.5 billion years ago, yet life-even the simplest sort of microscopic life that must have started things off_did not begin right away. Until about 3.9 billion years ago, Earth was subject to an intense barrage of large objects from space, a time called the “heavy bombardment.” Radiometric dating of Moon rocks shows that most of the Moon’s visible impact craters, or holes formed on its surface from impacts, must have formed during this early period of the solar system’s history. This is not surprising: According to our modern theory of solar system formation, the planets were built as larger and larger chunks of rock (sometimes mixed with metal or ice). or planetesimals, collided with one another. When a colliding planetesimal stuck to a growing planet, the planet got larger, increasing its gravity and allowing it to draw in even more planetesimals. Even after the planets had reached essentially their current sizes, there must still have been many planetesimals floating around; some of them still remain today, as the objects we call asteroids and comets. Those planetesimals that had orbits intersecting the orbits of the planets were doomed to eventual collisions. and most of those collisions must have occurred early in the solar system’s history, when the number of planetesimals was still large. In other words, the heavy bombardment was the period of time during which impacts were most common, and the evidence from the Moon tells us that this period ended by about 3.9 billion years ago.

Some of the planetesimals were quite big. We have good reason to think that the Moon itself was created when a planetesimal the size of the planet Mars struck the young Earth within just 20 to 30 million years after Earth’s formation. This “giant impact” is thought to have blasted rock from Earth’s outer layers into space, where some of it settled into Earth’s orbit and then was collected together by gravity to make the Moon.

Once the Moon formed, it became a record of the continuing impacts, not only telling us when the heavy bombardment occurred but also telling us about the sizes of the impacting objects from the sizes of the craters they left. Because human spaceflight missions visited and brought back rocks from only six sites on the Moon, we have only incomplete data about lunar cratering. Nevertheless, these data point to two key ideas: First, while there were no more Mars-size impacts (fortunately!), the Moon continued to be pelted by objects tens of miles to a couple hundred miles across. Second, some of the largest impacts occurred as the heavy bombardment was ending, marking what many scientists now call the “l(fā)ate heavy bombardment.” These large impacts created the smooth lunar maria (large, flat surface areas), that you can see easily with a pair of binoculars.

Because the heavy bombardment was a phenomenon of the solar system, it cannot have been unique to the Moon. This explains why we see craters on so many other planets arid moons. Earth, too, must have been frequently scarred by large impacts during the heavy bombardment. In fact, Earth should have been hit even more than the Moon, because our planet presents a bigger target and Earth’s stronger gravity would have drawn in more objects and accelerated them to higher speeds by the time they hit the ground. The only reason we do not see the craters from these impacts on Earth is that they were erased long ago by volcanic eruptions, erosion, and other geological processes that occur here but not on the Moon.

What does all this have to do with the origin of life? Calculations suggest that some of the larger impacts would have had a devastating effect on life. For example, the impact of an object larger than about 225 miles across would have released enough energy to completely vaporize the oceans and raise the global temperature to more than 3,000F. Such an impact probably would nave sterilized our planet, wiping out any life that existed when it occurred. Somewhat smaller impacts would have vaporized all but the deepest ocean water, killing off any life that was not either living near the ocean bottom or in rock deep underground. This means that life on Earth probably could not have begun until after this stage.

1.It is?estimated?that Earth formed around 4.5 billion years ago, yet life-even the simplest sort of microscopic life that must have started things off_did not begin right away. Until about 3.9 billion years ago, Earth was subject to an intense barrage of large objects from space, a time called the “heavy bombardment.” Radiometric dating of Moon rocks shows that most of the Moon’s visible impact craters, or holes formed on its surface from impacts, must have formed during this early period of the solar system’s history. This is not surprising: According to our modern theory of solar system formation, the planets were built as larger and larger chunks of rock (sometimes mixed with metal or ice). or planetesimals, collided with one another. When a colliding planetesimal stuck to a growing planet, the planet got larger, increasing its gravity and allowing it to draw in even more planetesimals. Even after the planets had reached essentially their current sizes, there must still have been many planetesimals floating around; some of them still remain today, as the objects we call asteroids and comets. Those planetesimals that had orbits intersecting the orbits of the planets were doomed to eventual collisions. and most of those collisions must have occurred early in the solar system’s history, when the number of planetesimals was still large. In other words, the heavy bombardment was the period of time during which impacts were most common, and the evidence from the Moon tells us that this period ended by about 3.9 billion years ago.