Why don't mercury & Venus have moon?

Earth has the Moon as its companion. Mars has Phobos and Deimos. Jupiter and Saturn both have more than 60 moons. Ok, 62 for Saturn and 67 for Jupiter. Uranus has 27 natural satellites while Neptune has 14 moons. Even Pluto and Eris, the largest dwarf planets have moons. Pluto, the planet that has been downgraded to a dwarf planet, has 5 moons!

But… the nearest planets to the Sun have no moons at all. How come? Isn’t it weird?

Ok, let's start with how the satellites or the moons formed. There are a few theories on how the moons formed. Let’s explore them.

In the beginning, when the Solar System formed, an object will accrete material (gas and dust) to have enough mass so it can become a planet. In this theory, a natural satellite is a leftover material which was not accreted by the planet. The material then forms a satellite and orbits the planet under its gravitational influence.

But, a planet can also have a moon by capturing another object such as an asteroid or comet and trap them in its orbit under its gravitational influence. In this case, Mars’s moons Phobos and Deimos are good examples of asteroids captured by Mars, becoming its satellites.

A planet can get its satellite through a collision between it and another object. The leftover material is trapped in the planet’s gravitational influence and ends up coalescing to form a moon orbiting the planet. This is a likely scenario for our Moon. So a few billion years ago when the Earth was still young… our beautiful Earth collided with a Mars-sized object. During the collision, some material was ejected and was trapped in Earth’s gravity. This ejected material then merged and formed the Moon.

So how about Mercury and Venus?

From these possible scenarios, both planets should have at least one small satellite orbiting each of them. But why they are they moonless?

The biggest problem for moons for Mercury and Venus is (as real estate agents say) location, location, location! Both planets are too close to the Sun. But yes, according to one scenario of satellite formation, Mercury and Venus have the possibility to host a satellite.

And it’s probably right.  From those 3 theories, only 2 are possible for the case of Mercury and Venus. It is possible for Mercury and Venus to have satellites through the collision theory similar to the Earth/Moon scenario, or accretion of the material in the early Solar System. Satellites in both planets formed when the Solar System was still very young.

But it is impossible for both planets to capture a comet or asteroid. The problem is their close distance to the Sun. At this distance, the gravitational influence of the Sun is dominating the system and if there is a comet or asteroid passing by then they will be captured by the Sun.

Base on this possibility, there was a time when Mercury and Venus could have had their own companion.

Satellite modeling for Venus shows us that there was a time when Venus gained a satellite from a collision with another large object. It is believed, there were 2 collisions that changed the fate of the satellite as well as Venus’s spin. If the first collision we gain a moon for Venus, then in 10 million years Venus had another impact in an opposite direction which not only made the moon spiral inward and collide with Venus but which also reversed the planet’s rotation.

Another model shows that Venus only had one large impact which gained it a moon and reversed its spin at the same time. But it is also possible for Venus to form a Moon through accretion.  Similar theories also apply for Mercury except for the part of spin reversing.

Once both planets have moons, there is another problem that comes up. Both planets are too close to the Sun. Which means any moon at too great a distance from the planets would be in an unstable orbit and would be captured by the Sun. If they were too close to Mercury or Venus they would be destroyed by tidal gravitational forces. The zones where moons around these planets could be stable over billions of years is probably so narrow that nobody was ever captured into that exact orbit, or created in situ when the planets were first being accreted.

In the end, Mercury and Venus will never be able to have a moon.