Mercury: curiosities of a planet where solar days last longer than years

Of the eight known planets in the Solar System, Mercury is the smallest and closest to our star, the Sun.

The images of the surface of Mercury captured by the BepiColombo mission
Mercury Transit: beautiful images that we will not see from Earth until 2032

Mercury has been known since the time of ancient Babylon, in the 14th century BC. Its name comes from the messenger of the gods in Roman mythology, a god known for his great speed. Three space missions have been sent to this planet so far. Two of them were organized by NASA: Mariner 10 (1973) and Messenger (2004). Currently, there is another mission underway, BepiColombo (2018), launched by the European Space Agency (ESA) and the Japanese Space Agency (JAXA).

The scarcity of missions to Mercury is due to its extreme difficulty, caused by the gravitational well created by the Sun. To avoid being trapped by our star's gravity, spacecraft require a vast amount of energy, more than would be needed for a mission to Pluto. This is compounded by the need for substantial heat shields to prevent the spacecraft from being destroyed by the extreme heat of Mercury's orbital zone.

Mercury has a diameter of 4,880 km, meaning that its size is one-third that of Earth. In size and appearance, it closely resembles the Moon, as its surface is rocky and riddled with craters. It has an almost negligible atmosphere, and life as we know it would be impossible there due to the extreme temperatures: 430ºC during the day (remember that paper burns at a temperature between 232 and 233ºC) and -180ºC at night. There are areas of the planet never exposed to sunlight that contain large quantities of water ice.

Unlike on Earth, on Mercury, days are longer than years. This is because the planet's orbital period (the time it takes to complete one revolution around the Sun) is 88 Earth days. Mercury orbits at an impressive speed (it is, in fact, the fastest planet in the Solar System, reaching about 172,000 km/h). However, its rotation is slower. Although its sidereal days (the time it takes to rotate 360° on its axis) are 1408 hours (58.6 Earth days), a Mercury solar day (the time between sunrises) is 176 Earth days.

Mercury's days have another curious feature: a double sunset and a double sunrise. This occurs during the eight days of maximum orbital speed and at certain points on the planet. Due to its slow rotation and eccentric orbit, when sunset on this planet, the Sun appears to move backward for a moment and then descend again, a phenomenon that is repeated at both sunrise and sunset.

Furthermore, Mercury is a planet composed of 70% metallic materials. It is the planet in the Solar System with the highest iron content. In addition, it has a considerably large core, which occupies 57% of its volume (on Earth, the core accounts for 15% of the planet). This characteristic suggests that in the early Solar System, Mercury may have been struck by a large object that tore away part of its crust, a process similar to the one that may have led to the formation of the Moon, but much more violent.

Finally, Mercury is a solitary planet. Like Venus, it has no natural satellites. The small planet revolves around the Sun, whose proximity is a curse for Mercury. Our messenger of the gods will be the first planet in the Solar System to disappear, in about 5 billion years, when the Sun begins to transform into a red giant. It will be a process that humans will no longer witness, at least not from our home planet, which by then will be an uninhabitable world, scorched by the heat of our star and with its atmosphere evaporated. In the meantime, we inhabitants of Earth will still be able to see the transit of Mercury between us and the Sun every few years. Here you can see the images of the 2019 transit. The next one will occur in 2032.

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Main image: NASA.