Facts About Mercury

Mercury is the closest planet to the Sun and the smallest since Pluto was downgraded to a dwarf planet. It is also one of five planets visible by the naked eye, along with Saturn, Jupiter, Mars, and Venus. Despite its proximity to Earth, Mercury is among the least explored planets due to its proximity to the Sun. Nevertheless, researchers do not expect life to exist on the planet due to its unconducive environment.

Mercury has water ice and organics.

This may sound surprising given that the planet is so close to the Sun, but the ice is in permanently shadowed craters that don’t receive any sunlight. Organics, a building block for life, were also found on the planet’s surface. While Mercury doesn’t have enough atmosphere and is too hot for life as we know it, finding organics there demonstrates how those compounds were distributed throughout the solar system. There’s also quite a bit of sulfur on the surface, something that scientists are still trying to understand since no other planet in the Solar System has it in such high concentrations.

The water ice appears younger than we would expect.

Close examination of the ice shows sharp boundaries, which implies that it wasn’t deposited that long ago; if it was, the ice would be somewhat eroded and mixed in with Mercury’s regolith surface. So somehow, the ice perhaps came there recently — but how? What’s more, it appears the ice deposits on the Moon and the ice deposits on Mercury are different ages, which could imply different conditions for both of the bodies.

A forced perspective view of Mercury's north pole (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
A forced perspective view of Mercury’s north pole (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Mercury has an atmosphere that changes with its distance to the Sun.

The planet has a very thin atmosphere that is known as an “exosphere” (something that is also present on the Moon, for example.) Scientists have detected calcium, sodium and magnesium in it — all elements that appear to change in concentration as the planet gets closer and further from the Sun in its orbit. The changes appear to be linked to how much solar radiation pressure falls on the planet.

Mercury’s magnetic field is different at its poles.

Mercury is somehow generating a magnetic field in its interior, but it’s quite weak (just 1% that of Earth’s). That said, scientists have observed differences in the north and the south pole magnetic strength. Specifically, at the south pole, the magnetic field lines have a bigger “hole” for charged particles from the Sun to strike the planet. Those charged particles are believed to erode Mercury’s surface and also to contribute to its composition.

Illustration of MESSENGER in orbit around Mercury (NASA/JPL/APL)
Illustration of MESSENGER in orbit around Mercury (NASA/JPL/APL)

Despite Mercury’s weak magnetic field, it behaves similarly to Earth’s.

Specifically, the magnetic field does deflect charged particles similarly to how Earth does, creating a “hot flow anomaly” that has been observed on other planets. Because particles flowing from the Sun don’t come uniformly, they can get turbulent when they encounter a planet’s magnetic field. When plasma from the turbulence gets trapped, the superheated gas also generates magnetic fields and creates the HFA.

Mercury’s eccentric orbit helped prove Einstein’s theory of relativity.

Mercury’s eccentric orbit relative to the other planets, and its close distance to the Sun, helped scientists confirm Einstein’s general theory of relativity. Simply put, the theory deals with how the light of a star changes when another planet or star orbits nearby. According to Encyclopedia Britannica, scientists confirmed the theory in part by reflecting radar signals off of Mercury. The theory says that the path of the signals will change slightly if the Sun was there, compared to if it was not. The path matched what general relativity predicted.

A hot flow anomaly, or HFA, has been identified around Mercury (Credit: NASA/Duberstein)
A hot flow anomaly, or HFA, has been identified around Mercury (Credit: NASA/Duberstein)

Mercury is hard to spot in the sky, but has been known for millennia.

Mercury tends to play peekaboo with the Sun, which makes it somewhat of an observing challenge. The planet rises or sets very close to when the Sun does, which means amateur astronomers are often fighting against twilight to observe the tiny planet. That being said, the ancients had darker skies than we did (no light pollution) and were able to see Mercury pretty well. So the planet has been known for thousands of years, and was linked to some of the gods in ancient cultures.

Mercury has no moons or rings.

Scientists are still trying to understand how the Solar System formed, and one of the ways they do so is by comparing the planets. Interesting to note about Mercury: it has no rings or moons, which makes it different from just about every other planet in our Solar System. The exception is Venus, which also has no moons or rings.

Longer Day Than Year

It takes Mercury 176 Earth days to complete a rotation around its axis and 88 days to complete a revolution around the Sun, meaning that a day is longer than its year. Mercury and the Sun are nearly in a tidal lock, which has slowed the planet’s rotation. Additionally, Mercury has the greatest orbital eccentricity, ranging from 29 million mi to 43 million mi.

Mercury’s Unique Orbit

Mercury revolves around the Sun at 112,000 mph, which is the fastest of all planets. Its orbit is immensely elliptical, as its distance from the Sun varies between 29 million mi and 43 million mi. The elliptical orbit also influences its visibility from Earth, as it can move between clear visibility or not be visible at all.

Difficult To Spot In The Sky

Mercury plays peekaboo with the Sun, as the planet rises and sets close to when the Sun does, which can make it challenging to see it in the sky. Ironically, the planet’s existence was documented by ancient astronomers before the discovery of Venus and Mars. Current astronomers believe that the darker skies of the past made it possible for ancient astronomers to see Mercury.

Mercury Has No Rings Or Moons

Mercury and Venus are the only two planets in the Solar System without either a moon or ring. Astronomers believe that in the past, both planets had moons, but that the Sun’s immense gravity pulled them away. Moons offer a better understanding of the formation of planets, and the lack of a moon leaves certain questions unanswered.

Second Hottest Planet

Mercury may be the closest planet to the Sun, but it is not as hot as Venus. Mercury has a light atmosphere that allows heat to escape, unlike Venus, which has a dense atmosphere that traps the heat. The side of Mercury that faces the Sun experiences temperatures of approximately 427 °C, while the alternate side can be as cold as -173 °C. Venus has an average temperature of 427 °C, as its dense atmosphere circulates heat evenly throughout the planet.

Molten Core

Recent studies show that Mercury indeed has a molten core, which is contrary to the solid iron core theory proposed throughout the 20th and early 21st centuries. The cores of smaller planets cool and solidify rapidly, but extensive research concludes that this is not the case for Mercury. Researchers believe that the core is made up of lighter materials that have lower melting points, such as sulfur.

Mercury Has Organic Material And Water Ice

Surprisingly, given its proximity to the hot Sun, Mercury contains water ice. However, this ice is located within permanently shadowed craters that do not receive sunlight throughout the year. Organic materials, which are the building blocks of life, have also been discovered, but the harsh conditions on Mercury cannot support life as it exists on Earth. The concentration of sulfur on Mercury is unlike any other planet in the Solar System.

The Most Cratered Planet

While other planets “self-heal” after collisions with comets and asteroids, Mercury does not, which has led to numerous craters on the planet’s surface. Most are small craters, while a few are more than 50 mi wide. The largest crater is the Caloris Basin, which has a diameter of 963 mi.

Two Probes Have Visited Mercury

It is challenging to travel between Earth and Mercury not only because the closest distance between the two planets is over 48 million mi, but also due to the immense temperature from the Sun. Only two probes have reached Mercury: NASA’s Mariner 10in 1974-75 and NASA’s MESSENGER in 2004. In 2018, the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) launched the joint BepiColombomission that is expected to reach the orbit of Mercury in December 2025.

Changing Atmosphere

Mercury has a fragile atmosphere known as an exosphere. It is unable to retain heat, which is why the dark side of the planet is frigid. Research shows that the concentration of magnesium, sodium, and calcium in Mercury’s atmosphere fluctuates depending on its distance from the Sun. Although there is no conclusive evidence regarding why this occurs, astronomers link it to solar radiation pressure.

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