Fascinating details about the smallest planet in our solar system

Aptly named after the speedy messenger god of the Roman pantheon (Hermes in the ancient Greek one), the planet Mercury is the “speed freak” of our Solar System.

Not only that, but Mercury, like all of our other planet neighbors, has a unique set of characteristics and circumstances that have dictated its history till the modern-day. Join us then as we take a tour to the nearest planet to our own Sun. 

When was Mercury first discovered?

Like the “discovery” of most natural phenomena, it is challenging to pinpoint a single person who can get the credit for discovering Mercury. The main reason for this is that Mercury is one of the five inner rocky planets that can be seen without using a telescope or binoculars.

However, Mercury is a little “trickier” to find than other planets as it tends to not travel too far from the Sun (from our point of view on Earth). This makes it harder to spot in the sky and probably meant it was one of the later planets to be found when compared to the other five, like Mars, Jupiter, Saturn, or Venus.

facts about mercury planet
An artist’s impression of Mercury. Source themotioncloud/iStock

That being said, we can be very confident that anyone, at any time, could have seen it simply by looking up into the night’s sky. For this reason, a better question might be to ask who were the first to record their observations of Mercury? 

From what we can ascertain today, the Sumerians might have been the first. They had records showing the planet’s existence around 3000 BC. The ancient Babylonians called the planet Nabu the god of writing and learning. The ancient Greeks originally thought that Mercury was two planets, but in the 4th century BCE, they realized it was just one object. Fast forward a few millennia, the great Copernicus developed his Sun-centered solar system model, published in 1543, which clearly showed that Mercury was a planet.

The first person known to observe the planet using a telescope directly was the late, great Gallileo. In the 17th century, he provided the first “proof,” so to speak, that Mercury was, indeed, another planet. He also confirmed that Venus must be orbiting the Sun, as it appeared and disappeared at regular occurrences on either side of the Sun. 

What’s so special about Mercury?

Mercury is the smallest planet in our solar system and is the closest to the Sun. It is only slightly larger than the Earth’s moon and has a diameter of about 3,031 miles (4,879.4 km), slightly larger than the width of the United States. About 4 billion years ago, an enormous asteroid is also thought to have slammed into Mercury. Rather than destroying the planet, it actually managed to survive the impact.

The asteroid that hit Mercury is estimated to have been 60 miles (99.5 km) wide and created a vast crater now named the Caloris Basin which is 960 miles (1,545 km) across. That is enormous. 

To put that into perspective, the asteroid that is thought to have killed off the dinosaurs was about 6.2 miles (10km) wide.

If this isn’t enough to make the planet special, then some other aspects of Mercury also make it strangely unique. For example, while it is nearest to the Sun, it is not the hottest planet in the solar system. 

facts about mercury photo
Source: NASA/Wikimedia Commons

That accolade goes to its neighbor Venus. Mercury is freezing at night, and instead of an atmosphere, the planet has a thin exosphere of atoms blasted off the surface by the solar wind.

Another special feature of Mercury is its magnetic field. Mercury’s magnetic field is offset relative to its equator. Even though the strength of the planet’s magnetic field is only 1% that of Earth, it interacts with the solar wind to cause magnetic tornadoes, channeling the fast, hot solar wind plasma down to the surface.

Mercury has an eccentric, egg-shaped orbit that ranges from 29 million miles (47 million kilometers) to 43 million miles (70 million kilometers) from the Sun. The planet tilts on its axis by just 2 degrees with respect to the plane of its orbit around the Sun. This means that it spins while almost completely upright, so does not have any seasons.

Because Mercury is the closest planet to the Sun, it has the shortest orbit, speeding around the Sun every 88 days. It travels through space at speeds of around 29 miles (47 kilometers) per second, faster than any other planet. However, the planet spins very slowly on its axis, completing one rotation every 59 Earth days.

Is Mercury hot or cold?

The short answer is that it depends on when and where you look. 

Thanks to Mercury’s fragile and non-protective atmosphere, you cannot find any weather phenomenon except for some, frankly, crazy fluctuations in temperature.

For example, Mercury experiences extreme temperatures, either very hot at up to 840 degrees Fahrenheit (450 degrees Celsius) or extremely icy at minus 290 degrees Fahrenheit (-180 degrees Celsius). The average temperature on Mercury is, however, around 354 degrees Fahrenheit (179 degrees Celsius).

However, some places remain cold enough for water ice. Complex radar studies have shown water ice from comet residue appears to be present on the shady side of craters on Mercury’s North pole, for example.

facts about mercury image from space
Closeup image of Mercury from the recent European-Japanese BepiColombo mission. Source: ESA

These extremes in temperature on Mercury probably rule out any organisms surviving there, but there may be “goldilocks” parts of the planet that possibly could support life. But, it is widely accepted that this is extremely unlikely. 

What are some cool, while also sizzling, facts about Mercury?

Not had your fill of facts about Mercury? Then read on to find out some other fascinating details about our enigmatic and strange neighbor. 

Hold on tight, some of these facts will blow your mind.

1. Mercury’s surface is a land of contradictions

facts about mercury burning
Source: ESO

Since Mercury only has something like 38 percent of Earth’s gravity it becomes challenging for the planet to hold on to its atmosphere. This becomes even more challenging because solar winds from the “nearby” Sun constantly buffet and erode it. However, the very same solar winds, micrometeorites dust, and radioactive decay also partially replenish the gases escaping into space.

Mercury’s atmosphere is a fragile one and has challenges retaining heat, culminating in the freezing temperatures on the side of the planet in shade. There is also some research that shows that the distance of Mercury from the Sun determines the concentrations of sodium, magnesium, and calcium in the atmosphere over time.

Though there is no conclusive evidence supporting why this happens, astronomers link the fluctuations to solar radiation pressure.

Though we don’t really know.

2. Mercury inferior, Jupiter superior

mercury inferior, juptier superior
An artist’s conception of our solar system. Source: NASA/JPL-Caltech/T. Pyle (SSC)

As we’ve previously highlighted, Mercury (being so much closer to the Sun than Earth) loves to play a game of peekaboo with us here on Earth. While the planet is in front of the Sun (from our vantage point) it is relatively easy to sport, but it completely disappears for some parts of its orbit only to reappear later. 

Mercury is also known as an “inferior planet,” which might sound like a veiled insult, but is actually related to its relative position in the Solar System.

This term simply means, in this context, that it has an orbit nearer to the Sun than the other planets, especially the Earth. All other planets further out than Earth are, unsurprisingly, referred to as “superior”. 

This is ironic because astronomers noted Mercury’s existence before discovering Mars or Venus.

Interestingly, it is currently believed that the darker skies that existed before our megacities of today were built probably made it easier to spot Mercury.

3. Mercury wears its scars like badges

If you like craters, Mercury is the planet for you. Mercury has no atmosphere or weather conditions, so erosion is basically nonexistent. There is also a distinct lack of volcanic activity and weather in the form of wind, clouds, storms, etc.

For craters, this is good news, as they won’t be eroded away over time. 

Mercury’s iron core takes up about 75 percent of the planet’s radius, and scientists think its rapid formation may have formed a thin crust over the large iron core. 

In early research, astronomers believed that Mercury had a smooth surface. However, in November 1973, the Mariner 10 spacecraft flew past Mercury and soon revealed the planet’s pitted and smashed surface. This large amount of craters has given Mercury the title of the “Most Cratered Planet” in the Solar system.

Unlike other planets, these numerous craters form because Mercury does not “heal itself,” for want of a better term, after a collision. While most craters on Mercury are small, a few may be more than 50 miles (80 km) wide. The largest crater on Mercury is the Caloris Basin, with a diameter of 960 miles (1,545km). 

However, the planet does have other features. As the planet’s large core cools, the planet “shrinks,” creating cliffs hundreds of miles long and up to a mile high, as well as great valleys up to 620 miles long (1,000 km) and two miles deep (3.2 km). 

4. Mercury might be covered in diamonds

mercury diamonds
Mercury might have loads of diamonds! Source: alex-mit/iStock

Another interesting discovery of late is the possibility that Mercury is covered with diamonds. Since its crust has a high concentration of carbon in the form of graphite, it is plausible that it also has a large number of diamonds. 

Unlike here on Earth, where diamonds form from high pressure squeezing in the Earth’s crust, diamonds on Mercury, if present, probably form wherever the planet gets pummelled by an asteroid. 

“The pressure wave from asteroids or comets striking the surface at tens of kilometers per second could transform that graphite into diamonds,” says Kevin Cannon, a geologist at the Colorado School of Mines and a key member of the research team. 

The research in question goes as far as even attempting to quantify how many diamonds could be lying around on the surface of Mercury – 16 quadrillion tons!

5. 176 earth days is equal to one solar day on Mercury

mariner image of mercury
Enhanced Mariner 10 image of Mercury. Source: Wikimedia Commons

Mercury takes 176 Earth days to complete a rotation about its axis. The planet also takes 88 days to finish a complete revolution around the Sun.

Because of this, amazingly, solar days are longer than years in Mercury. The reason for this is that Mercury and the Sun have a near tidal lock, slowing Mercury’s rotation.

Another Mercury fact is that it travels 180,00 km/h along its elliptical orbit. Unlike most other planets, Mercury also has a near-perfect circular spherical shape, the most nearly circular of all the planets besides Venus.

6. Water Ice and organic material are actually present on Mercury

close up of mercury
Source: NASA/Flickr

Surprisingly, close examination of Mercury has revealed the existence of water ice, despite its proximity to the Sun. However, you can only find this ice in the permanently shadowed craters that don’t receive sunlight throughout the year.

Amazingly, organic molecules, some of which are thought to be the building blocks of life, also exist in Mercury. However, as we’ve previously explained, it is highly unlikely that the planet itself can support life.

Mercury also has the highest levels of sulfur of all the planets – about ten times the amount of sulfur than is found on Earth.

From pictures and photographs taken of Mercury’s surface, the planet appears wrinkled because its iron core has cooled and contracted. Scientists call these wrinkles “lobate scarps” and these can reach up to hundreds of miles long and a mile deep.

Additionally, the planet’s north and south poles are cold and shadowy, which could allow the craters in these regions to hold frozen water. The craters and features on Mercury are named after famous deceased artists, musicians, authors, etc.

7. Mercury’s core shouldn’t be molten, but it is

mercury core
Source: Ariel/Wikimedia Commons

The inner core of Mercury has fascinated astronomers for centuries. Today, scientists are fairly certain that Mercury’s core (roughly 1,119 miles/1,800km in radius) makes up most of the planet itself. Following recent discoveries, scientists at NASA believe that the solid iron core on planet Mercury is probably molten.

This is unusual, as the cores of smaller planets can be expected to cool rapidly due to their large ratio of surface area to volume. But, after extensive research, this doesn’t appear to be the case for Mercury.

These results contradict the “solid core theory” proposed throughout the 20th and the early 21st centuries. In effect, Mercury’s core has a higher concentration of molten iron in its core than any other planet in the galaxy.

Scientists also believe that Mercury’s core contains sulfur, which lowers the melting temperature of the planet’s core material. Where the Earth’s core makes up around 17% of the planet’s volume, Mercury’s inner core could make up as much as 85% of the planet’s volume.

8. Poor old Mercury orbits alone

mercury surface
Color image of Mercury’s surface. Source: NASA/Flickr

Unlike most of the planets in our Solar System, Mercury doesn’t appear to have any natural satellites or moons. It also lacks rings. Venus, as it happens, also lacks rings or moons. 

We are not entirely sure why, but astronomers believe that these planets actually did have them once upon a time. 

However, the Sun’s intense gravity probably pulled them away because Mercury has a weak gravitational pull due to its size and distance from the Sun. 

9. Mercury actually has geomagnetic storms like on Earth

mercury auiroras
Source: KeongDaGreat/iStock

While much smaller than Earth, Mercury actually has its own geomagnetic storms like the Earth’s various auroras. A recent study found that the planet has a ring current consisting of charged particles that flows laterally around the planet and excludes the poles. This ring is then able to generate geomagnetic storms at the poles too. 

According to Hui Zhang, a space physics professor at the University of Alaska Fairbanks Geophysical Institute, “The processes are quite similar to here on Earth. The main differences are the size of the planet and Mercury has a weak magnetic field and virtually no atmosphere.”

However, unlike on Earth, these auroras are only visible in the X-ray and gamma-ray spectrums, not as visible light. 

10. Mercury is the second densest and second hottest planet in our Solar System

Mercury is the second densest and second hottest planet in the solar system. With regards to temperature, it is second only to the planet Venus (despite it being closer to the Sun). The reason for this is that Venus has a dense atmosphere that helps it trap heat.

In this regard, it is sort of the opposite of Mercury, which has little to no atmosphere that lets heat escape from its surface. Heat is also better distributed around Venus when compared to Mercury.

This is because Venus has highly volatile “weather” that is able to move heat around the planet’s surface. Mercury, on the other hand, has areas of incredibly hot and incredibly cold. 

With regards to density, even though it is relatively small, Mercury is disproportionately dense compared to most other planets. This is mainly due to the composition of Mercury, which is a mix of heavy rocks and metals. In fact, the only planet denser is the Earth.

11. Mercury may have once had volcanoes

mercury lava flows
Presumed ancient lava flows on Mercury’s surface. Source: The Smithsonian

While the crust of Mercury is effectively dead today, it may have looked very different billions of years ago. A close study of the surface of the planet seems to show evidence of long-dead volcanoes. 

Until we can actually get some probes down there to take a closer look, it is currently believed that Mercury may have been very volcanically active about 3.5 billion years ago. For some as yet unknown reason, at around this time, its volcanic activity completely stopped, which is much, much earlier than other planets like Mars or, of course, Earth.

However, we do have some indirect evidence from orbital surveys of Mercury using NASA’s Messenger probe. This was able to determine that many of the craters on the planet’s surface have tilted over time, suggesting more recent volcanic activity. Images also showed deep lava flows. The current best guess for the early end to this activity is due to Mercury’s relatively small size. 

This probably means Mercury lost much of its internal heat early on in its life, resulting in the planet contracting and the crust effectively sealing off any magma conduits, choking off volcanic activity. 

And that brings our short tour of Mercury to an end for today.

Mercury might be the smallest of the planets in our Solar System, but it certainly punches above its weight for intrigue. A strange planet compared to what we’re used to here on Earth, it is certainly a place full of other wonders yet to be discovered.

And, of course, potential resources yet to be explored. 

Who knows, there may be a time when couples get engaged using Mercurian Diamonds in the not too distant future? 

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