One problem is understanding the capture mechanism.[19] A close encounter with Earth typically results in either collision or altered trajectories. For this hypothesis to function, there might have been a large atmosphere around the primitive Earth, which would slow the movement of the Moon by natural aerobraking before it could escape. That hypothesis may also explain the irregular satellite orbits of Jupiter and Saturn.[20] This hypothesis also has difficulty explaining the essentially identical oxygen isotope ratios of the two bodies.[4]