Podcaster: Avivah Yamani

Title: Why is it that Mercury and Venus didn’t have a moon?

Organization: Astrosphere new Media

Link :

Description:  Why is it that Mercury and Venus didn’t have a moon?

Bio: Avivah is a project manager of 365 days of Astronomy


Welcome to the 365 Days of Astronomy podcast Q&A the question and answer series for we answer your cosmic question . If you have a question which you’d like us to answer, visit our G+ or facebook pages and fire away!

Today’s question is Why is it that Mercury and Venus didn’t have a moon?

The Solar System has eight planets, and most of them have moons as a companion. Moons and satellites have many shapes, sizes and types. Only one has a significant atmosphere and most of them formed from the discs of gas and dust circulating around planets in the early solar system.

Astronomers have found 173 moons orbiting planets in the Solar System, as well as at least eight orbiting dwarf planets.  Of the terrestrial (rocky) planets of the inner solar system, neither Mercury nor Venus have any moons at all, Earth has one and Mars has its two small moons. In the outer solar system, the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune have numerous moons. Asteroids also have moons. There are 76 object in the asteroid belt with satellites. Event planets around other stars have satellites.  As the planets grew in the early solar system, they were able to capture objects with their large gravitational fields.

So how do planets get their moons?

There are three theories on the acquisition of moons based on origin. The first type are those moons that formed by accretion; that is, during the formation of the Solar System, material orbiting a young planet joined together gravitationally to form a moon.

The second type of moon is an object that formed elsewhere in the Solar System, not orbiting a young planet, and was later captured by a planet’s gravitational force. An example of this type of moon is Mars’ moon Phobos, which is believed to be a captured asteroid.

The third scenario is where a planet can get its satellites through an impact between the planet and another object. The debris from this collision is trapped in the planet’s orbit and starts to orbit it. Eventually the debris accretes gravitationally and forms a moon.

There are many theories for what might have caused a planet as small as Earth to have such a large moon as our Moon. The most popular theory assumes an impact between Earth and a Mars-size object, where the debris of the collision — a mix of the material from Earth and the other body — was trapped by Earth’s gravity and gave birth to the moon. This body then stayed in orbit about the Earth, forever bound to its new home.

But what about Mercury and Venus ?

According to the existing scenarios, Mercury and Venus could have satellites.  But both of them are very close to the Sun making it hard for any satellite to have a stable orbit around them.

The most probable case for Mercury and Venus is that both planets had a moon or moons in the early stages of the planets’ formation. Both planets could have gotten moons from an impact with other objects, captured asteroids or accretion.

Venus did have moons in the ancient past. That’s because Venus is rotating “retrograde” or backwards from the rest of the planets. This strange rotation is evidence that Venus was probably whacked hard in the past by a planetesimal; a similar event to what is believed to have happened to the Earth billions of years ago, forming the Moon.  It’s possible that this collision with Venus threw up material that coalesced into a moon, or even moons.

Once that moon formed at Venus, it needed to be the right distance from the planet to remain stable. If its too close it will crash into the planet. And if it’s too far it will be captured by the Sun. So what’s happened to the moons of Venus? Well, Instead of orbiting the planet for billions of years, it appears to have crashed back into the planet. It is thought that the tidal forces from the Sun made the orbit unstable.

So too with Mercury. It has the same problem as Venus. The zone for a satellite to remain stable around them is really thin, making it hard to keep their satellites for very long.

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