Why do Mercury and Venus have no moon?
Colossal planetary collisions were common in the first chaotic solar system, but they do not guarantee a moon at all.
Scientists think Mercury may also have experienced a high-speed impact that removed its entire outer layer and caused the material to come out into space. But unlike Earth, Mercury is too close to the Sun and not as massive as having kept this material in orbit against the Sun’s attraction, and therefore has no moon. Interestingly, this scenario would also explain why Mercury’s core occupies more of the planet’s volume than Venus, Earth, and Mars.
Venus without a moon is more disconcerting. Being about twice as far from the Sun as Mercury and almost as massive as Earth, any ancient planetary collision would probably have produced a Venusian satellite. Scientists think that Venus experienced not one, but two giant impacts, the first created a moon, but the second fell Venus in a way that caused the moon to move inward and eventually collide. with the planet. This “double-impact” scenario would also explain Venus ’extremely slow rotation speed of 243 Earth days and the fact that it rotates in the opposite direction to most planets.
How giant planets like Jupiter reach their many moons
When giant planets such as Jupiter are formed, their greater gravity can attract large amounts of material, which orbit the planets as dense circumplanetary disks. Just as the disks around new stars forge planets, the material of these circumplanetary disks joins over time to form moons, the purest examples of which are the great Galilean moons of Jupiter: Io, Europe, Ganymede and Callisto.
Saturn’s moon Titan, which is larger than the planet Mercury, also formed from a circumplanetary disk. Uranus also has five major satellites formed from a disk, but its order remains mysterious because the outer satellites are counterintuitively much more massive than the inner ones. We have even detected a circumplanetary disk around the young Jupiter-like planet PDS 70c outside our Solar System, from which large moons could form.
The higher gravity of giant planets sometimes makes them simply steal a moon. At some point in the past of the Solar System, Triton passed close enough to Neptune to be captured in orbit around the ice giant. It is believed that Phoebe, Saturn’s moon, has also been captured because its orbit is very elliptical and is inclined with respect to Saturn’s plane of rotation.