Can the Moon have its own moon? ⚡ экспресс
Submoons — satellites of satellites — could be habitable. Scientists simulated their tidal evolution. Earth would only retain asteroid-mass submoons (~10^15 kg), while a massive one (~4.6·10^17 kg) would push the Moon away by 30%. For a warm Jupiter Kepler1625b, chances are better: a submoon at 10% lunar mass is stable if its moon is 68% farther away. Giant submoons (1.8 M⊕) are stable, but for life a mass of ~0.5 M⊕ is needed (tectonics, atmosphere), where stability drops. At around ~0.1 M⊕ — a minimum due to the balance of tides and the Roche limit. This "bottleneck" could make habitable submoons rare.
The Moon can have its own tiny moon — like a nesting doll, but in space. The outer doll is a planet, the middle one is a moon, the inner one is a small moon-satellite, or submoon. Ньютон showed that all bodies attract each other, and gravity holds them together. But gravity is not uniform: on the near side of the satellite it's stronger than on the far side. This difference creates tidal forces that squeeze and stretch the object, like clay. If a submoon gets too close to the main moon, it gets torn apart — possibly just like the dusty ring around Saturn's moon Rhea was torn apart. искривление пространства-времени explains this as a deformation of space itself.
Calculations show: Earth could host an asteroid-sized submoon. If it were bigger, the Moon would move 30% farther away from us. For экзопланеты Kepler-1625b — a warm gas giant — submoons could be as large as Earth. For life to arise on such a moon, жидкая вода, an atmosphere, and plate tectonics are needed, meaning the mass should be about half of Earth's. But precisely at that 'half' mass the orbit becomes unstable — a paradox that might make habitable submoons extremely rare.
🎯 Saturn's moon Rhea may have once had a dusty ring — almost submoons.