Craters On The Moon

With the Artemis mission around the moon this week, let's review my view of it's craters.

An interesting question is why meteors traveling through space often fall into orbit around the earth but don't immediately impact it's surface. If it will be under the influence of the earth's gravitational field then why wouldn't it fall to the surface? It is because that would be creating energy out of nothing and that is something that can never be done. This simple fact actually explains a lot.

A natural object, made of rock or metal, passing through space is called a "meteor". After it impacts a planet or moon it is called a "meteorite". 

Meteors usually come from the Asteroid Belt. It is generally believed that the asteroids would have coalesced by gravity into a planet but the powerful gravity of Jupiter, the most massive of the planets, prevented them from coalescing. 

A higher position or a higher orbit is a higher energy position. An object ten meters above the ground can be seen to have more potential energy than one five meters above the ground because, if both objects are dropped, the one that was higher will impact the ground with greater force. Likewise a higher orbit has more energy than a lower one. Gravity operates by the Inverse Square Law so that if we give an object in orbit three times the orbital energy it will orbit at nine times the original altitude, but with only one-third the speed. 

What happens is that the orbit of Jupiter is further from the sun, and thus a higher-energy orbit, than the orbits of the asteroids. The mutual gravity between Jupiter and an asteroid gradually slows the asteroid, every time it passes Jupiter in it's orbit. This transfers orbital energy from the asteroid to Jupiter, causing the asteroid to fall into a lower orbit closer to the sun. Eventually the asteroid encounters the inner planets, and may collide with one of them or the moon. 

Here is the mystery. We can see that there has been countless meteorite Impacts on the moon and on Mercury, as evidenced by the number of craters, but much fewer on the earth and on Mars. Part of the explanation is erosion that can erase evidence of meteorite impacts over time. There is weather on earth and wind and dust on now-dry Mars. But that cannot fully explain why there are so many more meteorite impacts on the moon and Mercury than there is on earth and Mars. There is another explanation that I have noticed.

Venus is also an inner planet but I am leaving it out of this explanation because it is shrouded in dense cloud and we cannot see it's surface.

Now here is what we must remember, and it is simple. It is that energy can never be created out of nothing. 

When an object falls toward a planet it undergoes acceleration due to the planet's gravity. On earth the acceleration due to gravity of a falling object is 32 feet, or 9.8 meters, per second squared. For an object to fall into orbit around the earth it will have come from further away from the sun than the earth. When it is in orbit around the earth it is still also in orbit around the sun, since the earth itself is in orbit around the sun, the additional orbital energy from it's original higher orbit will go into it's orbit around the earth. The total orbital energy of the object will be that of it's orbit around the sun plus that of it's orbit around the earth.

But the fact that energy can never be created out of nothing determines whether the object will fall to earth as a meteorite or remain in orbit. If the object falls to earth it will be subject to the acceleration due to gravity of a falling object and will impact the ground with force. But there is no energy in gravity itself, it is just a force, and if the energy with which it would impact the earth exceeds it's original orbital energy around the sun, minus the orbital energy of the earth around the sun, then it would be creating energy out of nothing for it to impact the earth. Since this can never be done the object has to stay in orbit around the earth, rather than impacting the earth. 

This means that, the more massive a planet is, the more likely that a passing object will fall into orbit around it, rather than impacting it, to avoid the impossibility of creating energy out of nothing. Also the closer a planet is to the sun the less it's orbital energy and thus the more likely that a passing object will have sufficiently more orbital energy so that impacting the planet, after undergoing it's acceleration due to gravity, would not be creating energy out of nothing. So the less massive a planet is, or the closer to the sun it is, the more likely it is to be impacted by meteors. 

This explains why there are so many more meteorite impacts on the moon and Mercury than there are on earth and Mars. 

Mars is not really a massive planet. But it is the furthest of the inner planets from the sun, thus having the highest orbital energy per mass. It is also the closest inner planet to the asteroid belt. This means that an asteroid that was descending to a lower orbit around the sun, due to lost orbital energy, would have the least orbital energy difference with Mars, compared to the other inner planets. Thus the asteroid would have less chance to not have to create energy out of nothing, which is impossible, if it impacted Mars after undergoing it's acceleration due to gravity. 

This is why Mars does not have that many meteorite impacts and it's two small moons, Phobos and Deimos, are widely believed to have originated as asteroids that fell into orbit around the planet but have never impacted it, because to impact Mars would have required creating energy out of nothing. 

Since the moon is in orbit around the earth it is the same average distance as the earth from the sun. But the moon is much less massive than the earth, it's surface gravity is only 1 / 6 that of the earth. This means that approaching meteors require much less additional energy than the orbital energy of the moon, per mass, to be able to impact the moon without creating energy out of nothing. This is why the moon gets struck by meteorites much more than the earth, at least per area. 

Mercury is the planet that is closest to the sun, meaning that it has the least orbital energy of the planets. This is why it really gets hammered by meteorite impacts. It is also the smallest planet, if Pluto isn't included, although Mercury does have a high density because so much of it's interior is made of iron. Mercury was impacted with such force by one meteorite, the Caloris Impact, that the resulting shock wave affected the terrain on the diametrically opposite side of the planet.