Mercury: the Ultimate Guide

An image of Mercury enhanced to show surface and chemical features. 
Credit: NASA

The closet planet to the Sun, Mercury, has been observed by NASA in two different ways. The first was the Mariner 10 spacecraft which flew by Mercury three times from 1974 to 1975. This spacecraft took photos of the small planet but captured only half of its surface. This probe’s mission was to fly by Mercury and Venus and was the first time any probe had done flybys of more than one planet. 

30 years passed until we visited the planet again, but this time we stuck around a little longer. The Mercury Surface, Space Environment, Geochemistry, and Ranging spacecraft (commonly known as MESSENGER) began its 4-year orbit of Mercury in 2011 with the mission of observing the planet in depth. MESSENGER has provided us with most of what we know about the planet, including mapping the planet completely. When the spacecraft finally ran out of fuel in 2015, lasting 3 years longer than originally intended, it crashed into the surface of Mercury and created a new crater. 

Mythology

Mercury is named after the Roman messenger god Mercury. In Greek mythology, this god is called Hermes. A prominent trait of this god was his speed, which suits the fastest planet in the Solar System perfectly. Mercury was visible from Earth in the time of the ancient Greeks twice a day, so it was originally thought that Mercury was two different planets. They called the morning planet Apollo and the afternoon planet Hermes. When it was later discovered that the two planets were the same, the name Hermes (or Mercury) was chosen over Apollo. 

The Roman god Mercury
Source: Guido Bonatti, De Astronomia Libri X (Basel, Nicolaus Pruknerus, 1550)

Mercury also has a role in astrology. Similar to the messenger god in Greek and Roman mythology, astrology views Mercury as the planet of communication. The style of communication, or “your Mercury”, is determined by where Mercury was at the time of your birth. 

Mercury has had quite an influence on pop culture as well. The planet has acted as a muse for composers like Gustav Holst who composed a movement in his orchestral suite The Planets entitled “Mercury, The Winged Messenger.” It has also appeared as a setting or subject for countless science fiction books, poetry, films, tv shows, and other forms of media.

How big is Mercury and its orbit?

Mercury is the smallest planet in our solar system at a diameter of just 3,032 miles (4,880 kilometers), a little less than the distance from New York to Ireland. Its mass is 1/18 of Earth’s mass, at about 3.3×1023 kilograms, and has a surface area of 28.88 million square miles. 

While the Earth takes 365 days to complete one full orbit around the Sun, Mercury only takes 88. Mercury is the fastest planet in the Solar System, orbiting the Sun at 29 miles (47 kilometers) per second. Mercury is, on average, 1/3 the distance of Earth from the Sun. Its path of orbit around the Sun is the most elliptical of all the planets which means that its distance from the Sun ranges from 29 to 43 million miles (46 to 70 million kilometers). 

Mercury’s elliptical orbit around the Sun.

Geology of Mercury

The geology of Mercury is our window into the history of the planet. This desolate planet was not always so bland. In the early years of our solar system Mercury was a geologically active planet. As its core cooled, the surface of the planet contracted to form daunting cliffs and long narrow ridges. The young planet also had volcanic activity which helped form its solid crust. Although all geological activity ceased about 3.5 billion years ago, the core of the planet, which is about the size of our moon, is said to still be partly molten. 

There are theories about why Mercury’s core is so large in comparison to the other inner planets. Some think that the crust of the planet was originally much thicker but was destroyed by impacts from very large celestial bodies early in its life. Others think that the young Sun may have stripped it of its outer layer with violent solar wind. 

The largest crater in the center of the image is the crater Prokofiev which has a diameter of 68 miles.
Credit: NASA/JPL

Now, the most abundant surface feature on the planet are thousands of impact craters. These craters are up to 68 miles (110 kilometers) in diameter. These craters can be much larger than those found on other terrestrial planets because Mercury does not have a thick, protective atmosphere. 

What would it be like to live on Mercury?

Mercury’s absence of a developed atmosphere causes a few problems when it comes to habitability. First, and most important, you cannot breathe on Mercury as there is only a thin layer of hydrogen, helium, and oxygen. Second, without a thick atmosphere, the planet cannot regulate its heat. At night, the temperature gets as low as -300 degrees Fahrenheit (-184 degrees Celsius) and during the day it reaches 800 degrees Fahrenheit (427 degrees Celsius). These intense temperature changes and lack of breathable air make living as we know it on this planet impossible. 

Other aspects of life on Earth, like seasons and length of a day, are also different on Mercury. Seasons, like summer and winter, are caused by the tilt of a planet’s axis of rotation. Mercury’s axis only tilts by 2 degrees, compared to Earth’s 23.5 degrees, so technically it shouldn’t experience seasons. However, since Mercury’s orbit is so elliptical, there is a form of summer and winter. Unlike the predictability of seasons on Earth, Mercury moves so fast in its orbit that it is hard to tell when one season ends and the other begins. There is also some form of weather beyond just hot and cold days. Usually the magnetic field of a planet will protect it from solar radiation, but Mercury does not have a very strong one. When its magnetic field fails, tornadoes of solar wind plasma, a gas-like substance, spiral down to the surface of the planet. Unfortunately, these tornadoes don’t lead to Oz. 

An explanation of sidereal vs solar days. 
Credit: Swinburn University of Technology 

Have you ever felt like the day drags on forever? On Mercury, it certainly does. One day-night cycle, a solar day, on Mercury lasts for 176 Earth days. The planet’s solar day lasts so long because the planet is moving on its orbit as well as spinning on its axis. It only takes the planet 59 days to spin on its axis, known as a sidereal day. Once it has completed a sidereal day, the planet must further rotate to face the Sun again for a solar day to be completed. This would mean that the Sun would rise and set twice during one Mercury day. Maybe two sunrises a day doesn’t sound too bad but, on this planet, the Sun is 3 times larger and 7 times brighter than it appears on Earth.  

What is it like to stand on Mercury? Mercury is gray and dusty, like our moon. During its long nights, it would be incredibly dark due to the absence of any moons. Mercury has a little over a third of Earth’s surface gravity. This means if you weighed 100 lbs on Earth, you would way a little over 33 lbs on Mercury. It also means that if you jumped off a chair on Mercury, you would fall two thirds slower making for a much gentler landing. 

There is some evidence for water on Mercury. Evidence found by the MESSENGER spacecraft suggests that there is some scarce water in the form of ice. The adjacent image shows highlighted spots on the north pole of Mercury where water ice is speculated to be found. The quantity and form that the water is said to be in is not sufficient enough to survive on. That being said, if there was any place to live on Mercury, this pole would be it. 

The north pole of Mercury taken by the Mercury Dual Imaging System (MDIS)
Credit: NASA

The Future of Exploring Mercury

As of now, there are no plans of visiting Mercury within the next 5 years. Since Mercury is uninhabitable and geologically inactive, scientists might be aiming up and out to learn more about other mysterious aspects of our solar system and universe. 


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