29 Magnificent Facts about Mercury

By James Israelsen, Associate Writer

Published August 9, 2018

Mercury is the closest planet to the sun as well as the smallest planet in our solar system, in terms of both mass and volume._[1]
Mercury cannot be seen by the naked eye when the sky is fully dark. It only appears at dusk and dawn._[1]
Though it is the most difficult of the visible planets to see, Mercury's existence has been known since ancient Sumerian times, roughly 5,000 years ago._[1]
The ancient Greeks called Mercury Apollo when it was visible at dawn and Hermes at night._[1]
Mercury held religious importance for many ancient civilizations including the Egyptians, Assyrians, Chaldeans, and the Phoenicians._[5]
The Norse people called Mercury Odin, the father and ruler of the gods._[5]

The Greek god Hermes and his Roman counterpart Mercury were light-hearted and fleet of foot

Mercury is named for the Roman god of commerce, trickery, and communication. His Greek equivalent is the god Hermes._[1]
Early ancient peoples believed that Mercury was two distinct stars, one appearing in the east at dawn and the other in the west in the evening. By the time of Plato, however, around 350 BC, Greek astronomers realized it was in fact a single planet._[5]
Of all the planets, Mercury has the most eccentric, or elliptical, orbit._[5]
Mercury and Venus are the only two planets to show "phases" like Earth's moon._[5]
It only takes 88 days for Mercury to travel around the sun, but it takes 58 days for it to revolve around its axis. This means that a day on Mercury is roughly 2/3 the length of a Mercury year._[5]
From the surface of Mercury, the sun appears two and a half times larger than on Earth._[4]
Mercury's gravity is only 38% that of Earth's._[4]
Mercury is the least researched of the inner planets. To date, the only in-depth exploration missions carried out have been done by the Mariner 10 space probe in 1973 and the MESSENGER in 2008._[3][4]

The Mariner 10 was the first spacecraft to use a gravitational slingshot maneuver in its approach to Mercury

Mercury is so close to the sun that daytime temperatures reach 400 °C (752° F ). That's hot enough to melt lead._[4]
Since it has almost no atmosphere to trap heat, Mercury's surface cools to a freezing -170 °C (-274° F) at night._[4]
One of Mercury's most prominent features is the Caloris Basin, a giant crater likely formed when an asteroid collided with the planet._[4]
Mercury's surface reveals that it was once flooded by lava, but all geological activity ceased some 3,000 million years ago._[4]

Earth's moon is the most similar object to Mercury in the night sky

Mercury is remarkably similar to Earth's moon, with nearly identical surface features and crater formations._[4]
Mercury has a very thin atmosphere containing traces of hydrogen, helium, and oxygen._[3]
Mercury is considered a "terrestrial planet," along with Venus, Earth, and Mars. These planets are composed mainly of silicate rocks and metals, unlike the gas giants such as Jupiter and Saturn._[5]
Though Mercury is the smallest planet in the solar system, it is the second densest planet, after Earth._[3]
Because of its proximity to the sun, Mercury is constantly bombarded by solar wind and radiation._[3][5]
Like Earth's moon, Mercury has "mascons," massive gravitational concentrations emanating from impact craters._[3]

Mercury's core contains more solid iron than any other planet in the solar system

Mercury has a weak magnetic field, which indicates a partially molten core. This came as a surprise to scientists, many of whom expected Mercury to be completely solidified._[5]
Mercury's core is massive, making up about 85% of its radius._[3]
The MESSENGER probe discovered water-ice on a shaded portion of Mercury. It appeared to be coated with a dark organic material, which puzzled the scientists who observed it._[3]
Mercury is one of only two planets in our solar system without moons or rings. The other is Venus._[4]
Mercury's thin atmosphere, or "exosphere," changes slightly depending on how close the planet is to the sun._[3]