Photo: contributed
The moon.
If you look at the Moon with binoculars or a small telescope, two types of terrain stand out.
There are lighter colored and heavily cratered mountainous regions. They are so intensely cratered that it is difficult for an incoming object to make a new crater without hitting an existing one.
Surfaces like this are called “saturated”. The plains are called “maria” because early astronomers thought the Moon had seas. They have romantic names like Mare Tranquillitatis, Mare Procellarum, Mare Nubium, Mare Crisium, etc. The maria are not water, but old lava flows.
At various times in its history, the Moon has been hit by large objects, which exploded huge craters. The lava flowed until it filled the craters and then overflowed onto the surrounding land. We can see craters partially buried by this lava. The poetic name “Sinus Iridum” (Bay of the Rainbow), is defined by a crescent of mountains that are the edge of a large crater that was buried by the lava forming the Mare Imbrium (Sea of Rain ). However, one thing that stands out about these lava plains is that they are much less cratered than the mountainous areas, suggesting that when the maria formed, the frequency of impacts had greatly decreased.
The Apollo astronauts brought back many rock samples from the Moon. Some of them were from the mountainous areas and others from the lava plains. Using radioisotope dating it was possible to determine when the rocks solidified, that is, when the rock was formed.
As expected, the rocks in the maria were younger than those in the mountain areas, but not much younger. Like the Earth, the Moon formed about 4.5 billion years ago. The mountainous terrain dates back some four billion years. The impacts that formed the other maria are estimated to have taken place between three and 3.5 billion years ago.
The maria’s relatively light cratering indicates that at the time they formed, most of the bombardments that shaped the heavily cratered mountainous terrain occurred between half a million and a billion years after the Moon formed. The bombing has not stopped and impacts are still occurring, but at a much lower rate.
This fits with our ideas about how the Solar System formed. A cloud of collapsing dust formed clumps. These clumps grew by bumping and sticking together. The largest clump formed the Sun, and other clumps formed the planets.
As the newly born planets orbited the Sun, they swept away the material sharing their orbits. When the orbits had been “cleaned”, the impacts became much less frequent, but did not stop entirely. Even today there are objects that cross the Earth and the Moon around the Sun, many of which pose a risk of collision. These orbiting bodies are an ongoing threat, because objects in safe orbits continue to be moved into dangerous orbits by the gravitational pull of the planet Jupiter.
So, about 3.5 billion years ago, the big bombardment was over. It was lucky for us on Earth, because along with the other planets, we were hit at least as often as the Moon. The geological record here on Earth suggests that life appeared after the worst of the bombardment was over and conditions were stable enough for living creatures to survive, proliferate and develop. We don’t know if there were any false starts.
In an age of huge telescopes, peering further into the cosmos and the beginning of the universe, it is intriguing that important information about how our world began is written on the face of a familiar object that lights up our night skies.
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• After sunset, Jupiter is in the southeast and Saturn is in the south. Mars rises later.
• The Moon will reach its last quarter on November 16.
This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.