June 5, 2023
Learning from impact sites on the Moon - Sky observation

Learning from impact sites on the Moon – Sky observation

If you look at the Moon with binoculars or a small telescope, two types of terrain stand out.

There are lighter mountainous areas that have large craters. They are such intense craters that it is difficult for an incoming object to create 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 (Sea of ​​​​Tranquillity), Mare Procellarum (Sea of ​​​​Storms), Mare Nubium (Sea of ​​​​Clouds) Mare Crisium (Sea of ​​​​Crises) and so on. The maria are not water, but ancient lava flows.

At various points in its history the Moon has been struck by large objects, which blasted huge craters. Lava rose to fill the craters and then overflowed onto the surrounding land. We can see craters that are partially buried by this lava. The poetic name “Sinus Iridum” (gulf of rainbows), is defined by a crescent of mountains that is the rim of a large crater buried by the lava that forms the Mare Imbrium (Sea of ​​Rain). However, one thing about these lava plains stands out, they are much less cratered than the highlands, which suggests that when the maria formed, the frequency of impacts was greatly reduced.

The Apollo astronauts brought back many rock samples from the Moon. Some of them were from the mountainous regions and others from the lava plains. Using radioisotope dating it was possible to determine when the rocks solidified – in other words, when the rock was formed.

As expected, the rocks from Maria were younger than the highland rocks, but not much younger. Like Earth, the Moon formed about 4.5 billion years ago. The mountainous terrain dates back to about four billion years ago. The impacts that formed the other Maria are estimated to have occurred between three and 3.5 billion years ago.

The maria’s relatively light cratering indicates that by the time they formed, most of the bombardment that shaped the highly cratered mountainous terrain occurred within half a billion to a billion years after the Moon’s formation. The bombing has not stopped and the effects are still happening, but at a much lower rate.

This fits our ideas about how the Solar System formed. A cloud of collapsing dust formed clumps. These pieces grew by colliding and sticking together. The largest piece formed the Sun and other pieces formed the planets.

As the newborn planets orbited the Sun, they swept through the material sharing their orbits. When the trajectories were “cleared”, the impacts became much less frequent, but did not stop completely. Even today there are objects crossing the Earth and the Moon around the Sun, many of which pose a risk of collision. These orbiting bodies are a constant threat because objects in safe orbits continue to be moved into dangerous orbits by Jupiter’s gravitational pull.

So, about 3.5 billion years ago, the great bombardment was over. This was lucky for us on Earth, because along with the other planets, we were hit at least as often as the Moon. The geologic record here on Earth suggests that life emerged after the worst of the bombardment ended and conditions were stable enough for living things to survive, reproduce, and thrive. We don’t know if there were any false starts.

In an age of huge telescopes, looking further out into the world and back to the beginning of the universe, it is interesting that important information about how our world began is written on the face of a familiar object that lights up our night sky.

•••

• 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 was written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

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