Mercury is the first planet from the sun in the solar system. Aside from being the first planet, Mercury is also the smallest of the planets with a mean diameter of about 4,880 km. Since it the innermost planet in the solar system, it also means that it revolves around the sun in the shortest time (89.97 days) compared to the other planets. Most of the planets and other bodies (such as some asteroids) in the solar system have moons or similar satellites. However, Mercury, just like Venus, does not have a moon.
Scientists back in the 1970s briefly thought that they had evidence of a moon orbiting mercury. The scientists realized later on that what they saw was a star and not a moon. The confusion they had was understandable because of the proximity of Mercury to the sun. In fact, until 1995, the planet itself was not seen in the infrared spectrum. All confusions were cleared with finality after the MESSENGER spacecraft from NASA orbited Mercury between 2011 and 2015. The spacecraft did not detect any moons.
How Natural Satellites Occur
For an understanding of why Mercury does not have a moon, it is important to understand how moons occur in other planets. A moon may occur from a disk of materials orbiting a planet. The disk eventually becomes a big enough body that it may end up having a spherical shape. Experts believe that a similar process is responsible for the formation of most of the moons in the case of Jupiter, Neptune, Uranus, and Saturn.
In some cases, a planet can acquire a moon from a smaller body that has a moon. In these cases, the larger body acquires the smaller one due to the larger body’s superior gravitational pull. Experts believe that the two moons of Mars (Phobos and Deimos) were acquired through a similar process. The process also applies to some of the small moons in the case of Neptune, Saturn, Uranus, and Jupiter.
The third method is that a planet gets a moon after a huge collision between a planet and another astronomical body that causes the ejection of material. This material eventually coalesces to form a moon. This process is believed to be responsible for the formation of the earth's moon, which is a hypothesis known as the giant-impact hypothesis, the Theia Impact, or the Big Splash.
Why Mercury Has No Moon
Since Mercury is extremely close to the sun, the gravitational pull from the planet is extremely weak. Another thing that makes this gravity weak is the small size of the planet. For a space object to hold another body using gravity, the other body has to enter the holding body’s region known as a hill sphere. Due to the weak gravitational pull of Mercury, it is likely that the other body would be pulled in by the stronger gravitational pull from the nearby sun.
Aside from the weak gravity, the orbit of mercury does not have that many materials that could potentially form a moon. This scarcity could be because of things like solar winds and the inability of some substances like methane and hydrogen to condense.
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