The theory that Mercury was once a moon of Venus that was knocked out of orbit by an asteroid impact is just one hypothesis proposed to explain the origin of Mercury's unique characteristics. However, it is important to note that this hypothesis is not widely accepted by the scientific community, and alternative theories exist.
That said, there are a few lines of evidence that have been suggested to support this hypothesis:
Resonance: Some proponents of the hypothesis argue that the current resonance between Mercury's orbital and rotational periods, known as a 3:2 spin-orbit resonance, could be a result of its previous orbit around Venus. This resonance suggests that Mercury rotates three times on its axis for every two orbits around the Sun. It has been proposed that this resonance could have been established during the moon's time around Venus and later persisted after its capture by the Sun's gravity.
Orbital Eccentricity: Mercury has a highly eccentric orbit, meaning its distance from the Sun varies significantly throughout its orbit. Proponents of the hypothesis suggest that the gravitational perturbations caused by an asteroid impact could have disrupted its original orbit around Venus, leading to its current eccentric orbit around the Sun.
Chemical Composition: Another argument made in support of this hypothesis relates to the similarities between the surface compositions of Mercury and Venus. Both planets have a relatively high abundance of volatile elements like sulfur. Proponents argue that this similarity in composition could be explained if Mercury originated as a moon of Venus, as the two bodies could have shared a common origin.
It's important to note that while these pieces of evidence are intriguing, they are not conclusive proof of the moon capture hypothesis. The origin and evolution of Mercury remain topics of ongoing scientific investigation, and alternative theories, such as the giant impact hypothesis or early solar system dynamics, are also considered as possible explanations for Mercury's characteristics.