For 4.25 billion years, the asteroid impact caused the Moon’s axis of rotation to rotate back and forth by about 10 degrees. Such a small change means that the ice hidden in the craters at the moon’s poles is less affected, and it also means that future lunar exploration can continue.
The discovery mission of NASA’s lunar exploration program: Gravity Recovery and Interior Laboratory (GRAIL) provides a very detailed map of the moon’s gravity field. David Smith thought: “The moon is full of craters, which represents the distribution of the gravitational field data. Why not flatten one of the craters and take a look?” Smith’s team started from this idea and picked out craters with a diameter of more than 20 kilometers one by one. And corresponding to the GRAIL gravity data, about 5,200 craters and basins, the time is reversed back to the unimpacted appearance by year.
Looking at the impact of each crater alone is minimal, of course, when there are enough craters, minus one crater, it is found that the moon’s poles are slowly returning to their positions billions of years ago. Combined, these large and small craters are almost equal to the gravitational effect caused by the South Pole Aitken Basin (about 2,500 kilometers in diameter) on the far side of the moon, and almost a quarter of the lunar surface.
If the impact is large enough, craters at the moon’s poles can be pushed into places where the sun shines. If it does, volatiles in the shadows at the bottom of the previously icy crater will sublimate, leaving less or no ice behind. The new understanding of the moon also affects future lunar exploration, such as NASA’s upcoming manned Artemis mission, as scientists investigate the ice that persists at the poles.
The first phase of the team’s work focused on craters over 200 kilometers in diameter. The results of the analysis were very compelling, and it was a good start, but not the whole story. There is more work to be done. The moon has many craters that are beyond the parameters of the team’s current study; albeit small, they will have an impact. In addition, the lunar geology is not as calm as it is now, and previous volcanic activity may also have altered the distribution of gravity. It seems that every aspect is important.
“There are things we haven’t thought about yet, but we want to remind you: Little craters that people ignore are small, influential and important things,” said Sander Goossens, a planetary scientist at NASA’s Goddard Space Flight Center.
The research was published in Planetary Science Journal.
▲ The gravity distribution map of the GRAIL moon, showing dense craters. (Source:Vishnu Viswanathan）
▲ A topographic map of the lunar poles (North Pole on the left, South Pole on the right). The pole displacement route is shown as the black line, the white circle marks the time information, and the red circle marks the position of the pole at 4.25 billion years ago: 80.4°N, 0°E and 80.4°S, 180°E. (Source: Planetary Science Journal)
(This article is reproduced with permission from the Taipei Planetarium; the source of the first image: NASA)
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