Why Don’t Mercury and Venus Have Moons?
The answer is possible because they are too close to the Sun. Any Moon with too great a distance from Venus and Mercury would be in an unstable orbit and be captured by the Sun. And if they were too close to these planets, tidal gravitational forces would destroy them.
So, it all comes down to orbital dynamics and a lot of gravitational interactions. Both of these objects have a lot less gravity, which is why they’re not that good at holding other objects, such as Moons, in their orbits.
Both Venus and Mercury probably had Moons, but they just disappeared for two simple reasons. One, the proximity to the Sun and two, the rotation of the two planets. And this is important when it comes to having moons orbiting planets.
Earth and Mars Have a Similar Rotation Speed
For example, both Earth and Mars have a relatively similar rotation speed earth does a single spin in about 24 hours. And mars does it in approximately 25 hours.
On the other hand, neither venus nor mercury spin fast. They spin quite slow. They are slow rotators. So this plays a significant role when it comes to the stability of moons.
Because the Moon and Earth have a specific gravitational relation toward each other, the pool from the Moon causes the Earth to have these two so-called tidal bulges. One of them is formed by the attraction from the Moon, and the other one is the so-called inertia.
The Moon is Tidally Locked to Earth
And because of this and because the Moon has a lot less mass eventually, the Moon became tidally locked to Earth which is why we always get to see the same side of the Moon no matter where we are on Earth.
So, because of all of this causes a little bit of pull on the Moon’s front side, this pull causes the Moon to accelerate slowly. And also, because of this, the Moon is gradually receding away from Earth roughly about 3 centimeters or about 1 inch per year.
But what if the Earth was not spinning as fast and was spinning similar to Mercury and Venus. Well, in this case, the bulge would be behind the Moon at all times simply because the Moon would be orbiting around faster than the spin of the planet.
This would obviously slow down the object, which is why we believe all of the moons that existed around Mercury and Venus eventually fell back to their surfaces.
Hill Sphere
The other feature we need to address is the so-called hill sphere. The Hill sphere approximates the gravitational sphere of influence of a smaller body in the face of perturbations from a more large body. It was established by the American astronomer George William Hill. It was based on the work of the famous French astronomer Édouard Roche.
So, the gravitational parameters formed by the Sun and Earth form this unique sphere around our planet known as hill sphere which is the only region around our planet where we can easily hold on to the moons and other objects.
Beyond this hill sphere, nothing really can stay in orbit and actually becomes part of the Sun’s orbit. This hill sphere around planet earth is roughly around 1.5 kilometers in radius, which is about one-fifth of the radius of where the Moon is currently located.
For Planets Closer to The Sun Their Heliospheres Decreases in Size.
But for planets closer to the Sun the heliosphere decreases in size. For Venus it’s about one million kilometers for Mercury it’s significantly lower which, of course, means that Mercury cannot really hold on to objects as easily as Earth simply because the Sun just kind of steals them.
And due to this, only close moons are possible around Mercury. But if the Moon is too close to Mercury, within only a few hundred million years and possibly even sooner than that due to the tidal interaction it basically just falls back to the surface of the planet.
So as it stands right now, there’s really no way for Mercury to have a stable moon for longer than a few million years. It will disappear and become part of the Sun, or it becomes the property of the surface of Mercury. And this is pretty much the same happens to Venus.
So, in a nutshell, this is why neither Venus nor Mercury has any moons.
That’s it. I hope you enjoyed this article. If you want to know more about our Moon, then head over to this interesting article named; How Does The Earth’s Rotational Gravitation Affect The Moon?
Watch the video below on why Why Doesn’t Mercury have a Moon? Credit: Lunartic.