Mercury, the closest planet to the Sun, is a true underdog of the Solar System. Most people are so unfamiliar with it that they sometimes mistake it for the Moon. An elusive planet of extremes, Mercury is a planet worth getting to know with its speedy, oddball orbit, countless pockmarked craters, and wildly fluctuating temperatures. Read on to discover some interesting Mercury facts.
Facts about Mercury
1. Mercury is the smallest planet in the Solar System.
Mercury has been the smallest planet in our Solar System since 2006 when Pluto was reclassified as a dwarf planet. With a mean diameter of 4,879 km (3,032 miles), Mercury is only slightly wider than the contiguous United States. Earth is 2.6 times larger than Mercury.
Mercury is even smaller than two natural satellites in the Solar System – Jupiter’s moon Ganymede and Saturn’s moon Titan. However, Mercury is more massive than both.
Furthermore, Mercury is so small that its volume is almost 18 times less than Earth’s and around 23.2 million times less than the volume of the Sun.
2. Mercury has the most eccentric orbit of all the planets in the Solar System.
Mercury has the most elliptical, or least circular orbit of any planet in the Solar System. Orbital eccentricity is the amount a planet’s orbit deviates from a perfect circle. If an orbit is a perfect circle, the planet’s eccentricity is zero and the more elliptical the orbit is, its eccentricity increases.
Mercury has an eccentricity of 0.206, which is significantly higher than that of Earth’s (0.017) or Venus’s (0.007). Mercury’s bizarre, elliptical orbit means that on average it lies at a distance of about 57 million km (35 million miles) from the Sun.
At its closest (perihelion), Mercury is 46 million km (29 million miles) from the Sun. At its furthest (aphelion), Mercury is 70 million km (43 million miles) from the Sun.
3. It takes 3.2 minutes (193 seconds) for the Sun’s light to reach Mercury.
In comparison, it takes 8.3 minutes (499 seconds) for the Sun’s light to reach Earth.
4. Mercury is named after the Roman messenger god.
Mercury gets its name from the fleet-footed Roman messenger god of the same name because it revolves around the Sun faster than any other planet in the Solar System.
5. A year on Mercury is as long as 88 days on Earth.
The closer an object is to the Sun the faster it needs to move to maintain its orbit. Since Mercury is the closest planet to the Sun, it has the shortest orbit and fastest orbital velocity (47.36 km/s) of all the planets.
At this speed, it takes Mercury 87.969 days, or the equivalent of 0.24 Earth years, to complete one full orbit around the Sun. Thus, it can be said that a year on Mercury lasts almost as long as 3 months here on Earth.
6. A day on Mercury lasts almost 59 Earth days.
The length of a day on a planet is the time it takes for the planet to rotate or spin once on its axis. Mercury rotates at a speed of 10.83 km/h on its axis as compared to the speed at which the Earth rotates (1,674 km/h).
Its slow speed makes Mercury the second-slowest spinning planet in the Solar System, after Venus. Thus, due to its tardy rotational speed, Mercury takes 58.646 Earth days (1,407.6 hrs) to spin once on its axis.
7. Mercury is locked into a 3:2 spin-orbit resonance where it rotates three times on its axis for every two orbits around the sun.
One of the most interesting Mercury facts is that its rotation is a unique case in the Solar System since the planet is locked into a 3:2 spin-orbit resonance, with its rotational period being exactly two-thirds of the orbital one.
Mercury rotates once every 58.646 days, which is exactly 2/3 of its orbital period of 87.969 days. This gives Mercury a spin-orbit resonance of 3:2, which means that the planet makes three complete rotations on its axis for every two orbits it makes around the Sun.
As a result of its bizarre 3:2 spin-orbit resonance, there is a significant difference between the time it takes for Mercury to rotate once on its axis (a sidereal day) and the time it takes for the Sun to reappear in the same place in the sky (a solar day). On Mercury, it takes 176 days so that the Sun appears in the same position in the sky.
8. Nobody knows who first discovered Mercury.
If you’re wondering who discovered Mercury, the problem is nobody really knows. No one can take credit for “discovering” Mercury because the planet is easily visible to the naked eye and has been observed since antiquity by the people of many different cultures.
9. Mercury has almost no atmosphere.
Although Mercury has little to no atmosphere, it does possess what is called an “exosphere”, a tenuous collection of atoms from the planet’s own surface.
The powerful solar wind and striking asteroids cause the atoms on Mercury’s surface to blast off and escape and form this exosphere. Mercury’s exosphere is largely composed of oxygen, hydrogen, helium, sodium, and potassium.
10. Mercury has no moons or rings.
Along with Venus, Mercury is one of two planets in the Solar System that has no moons. It is theorized that the planet’s proximity to the Sun makes having moons impossible, as the Sun’s strong gravitational pull would likely pull them out of the planet’s orbit.
And along with Venus, Earth, and Mars, it is also one of the four planets in the Solar System that doesn’t have a set of rings around it.
11. Mercury is one of the four terrestrial planets in the Solar System.
Mercury is one of the four terrestrial planets (along with Venus, Earth, and Mars) meaning that it has a rocky, solid surface made up primarily of metals and silicate materials, and also has a dense metallic core.
12. Mercury is the second densest planet in the Solar System.
Although it is less than half the size of the Earth and not much bigger than our Moon, Mercury is only second to Earth in terms of density among planets. In comparison to Earth’s density of 5,514 kg/m3, Mercury has a density of 5,429 kg/m3.
13. Mercury has the biggest core in proportion to the planet’s size of any planet in the Solar System.
One of the lesser-known facts about Mercury is that it has the largest core of all the planets relative to its size. The iron core of Mercury (which may be partly molten) takes up nearly 3/4 of the planet’s diameter and makes up about three-quarters of its mass.
In comparison, the cores of Earth and Venus are only about one-third of their mass.
14. If you weigh 100 kilograms (220 lbs) on Earth, you would weigh 38 kilograms (84 lbs) on Mercury.
Looking to shed weight instantly? Then head to Mercury! The planet has the lowest surface gravity (3.70 m/s2) of all the planets in the Solar System, only behind that of Mercury.
The surface gravity of Mercury is 0.38 times the gravity on Earth (9.8 m/s2). Hence, you would be 62% lighter on Mercury than on Earth. Weirdly, Mars also has 38% of the gravity of Earth.
15. Despite being the closest planet to the Sun, Mercury is not the hottest planet in the Solar System
Logic would suggest that Mercury is the hottest planet, being in such close proximity to the Sun. But, with no thick atmosphere to trap energy, Mercury cannot retain the significant amount of heat it receives from the Sun.
Mercury has an average surface temperature of 167°C (333°F). While that is pretty blazing hot, it’s no comparison to the average surface temperature of Venus (464°C/867°F), making it the hottest planet in our solar system.
16. Mercury experiences the most extreme temperature changes of any planet in the Solar System.
Given its proximity to the sun, it’s hardly a surprise that temperatures on parts of the surface of Mercury during daytime can reach up to 464°C (801°F). However, the planet’s slow rotation makes for stark differences in temperature between its dark and light sides. Thus, the side that faces away from the Sun can be awfully cold – as frigid as -173°C (-279°F).
Mercury’s lack of an atmosphere means that there is nothing to shield its surface from sun rays, and there is nothing to retain the warmth and transport it to the dark side of the planet.
Thus, you can expect a temperature swing of around 600°C (1080°F), undoubtedly the most extreme temperature change of all the planets.
17. Mercury is the closest neighbor, on average, to each of the other seven planets in the Solar System.
One of the most unbelievable Mercury facts is that it is the nearest planet to all other planets in the Solar System, including Earth and even Neptune—the planet farthest from the Sun. Well, on average at least. So, how is it possible that Mercury is the closest planet, on average, to every other planet in the Solar System?
This might seem impossible, but it makes sense when you think that every planet spends about half its time on the opposite side of the Sun. Mercury isn’t the closest planet to the other planets all the time but is the one that is closest to every planet more of the time than any other due to its tight orbit around the Sun.
To further elucidate, while Mercury is never really close to any of these planets, its tiny (in space distances) orbit never takes Mercury far away from any of them. In the case of the Jovian planets (Jupiter, Saturn, Uranus, and Neptune), their orbits are humongous, it makes the separation between the planets when they are at a maximum distance from one another, i.e. on the opposite sides from the Sun, absolutely colossal.
18. Mercury is the planet with the smallest axial tilt in the Solar System, so it doesn’t experience any seasonal variations.
Planets experience seasonal variations based on the degree of their axial tilt (the measure of the angle between the planet’s rotational axis and a line perpendicular to its orbital plane). The greater a planet’s axial tilt, the more pronounced the seasonal changes as it allows the sun to heat the planet’s surface at different times during the year.
While Earth’s axis is tilted 23.5 degrees, Mercury’s axial tilt is only 0.03°. This virtual lack of tilt means that Mercury’s northern and southern hemispheres receive a uniform amount of the Sun’s energy.
However, Mercury as a whole does have seasons of unequal size because Mercury’s highly elliptical orbit brings the entire planet closer to the Sun for short periods and farther away for longer periods. This causes some parts of the year on Mercury that are hotter and parts that are colder.
19. Mercury is the second-most spherical planet in the Solar System.
Due to a combination of gravity, rotation, size, and density, no planet in the Solar System is a perfect sphere. Rather, all the planets are described as “oblate spheroids”, because they all bulge slightly around the middle.
How much a planet bulges out is called oblateness, or ellipticity. Mercury has an oblateness value of 0.0009, which is very near zero (Earth’s is 0.003). Thus, Mercury is an almost perfect sphere and is the second-most spherical planet in the Solar System, after Venus.
In case you’re wondering, Saturn is the most oblate, or flattest planet in the Solar System.
20. Mercury is covered in hundreds of craters, making it the most cratered planet in the Solar System.
Mercury’s thin atmosphere does little to prevent or slow down impacts from extraterrestrial objects and space debris, leaving the surface of Mercury littered with craters.
The planet is home to craters of all sizes, the largest being Caloris Planitia, which is 1,550 kilometers (960 miles) in diameter and is surrounded by a ring of mountains 2 km (1.2 miles) high.
21. Craters on Mercury are named after famous musicians, artists, and writers.
Interestingly, all craters on Mercury are named after celebrated musicians, artists, and writers. Some of the craters include Bach, Chekhov, Dostoevsky, Goya, Goethe, Keats, Lennon, Twain, Mozart, Picasso, Shakespeare, Tolstoy, Yeats, Hemingway, Botticelli, Tolkien, Beethoven, Dali, and Dickens.
22. There is water ice on Mercury.
Although Mercury is the closest planet to the Sun and can be extremely hot over most of its surface, there are deposits of water ice at the poles of Mercury.
Mercury spins almost vertically on its axis, so its poles are never fully sunlit. Thus, water ice on Mercury is in shadowed craters of the planet that don’t receive any sunlight.
23. Mercury has a weak magnetic field whose strength is about 1% of the magnetic field on Earth.
Despite its small size and tardy rotation around its axis, Mercury has a global magnetic field, the strength of which is just 1% of that of Earth. However, it is interesting to note that like Earth’s the magnetic field of Mercury is dipolar and pretty active.
24. Mercury’s surface looks like it has wrinkles known as ‘Lobate Scarps’.
Lobate scarps are long, curvilinear structures found on some planetary bodies that form as a result of compression. They were formed as the metallic core of Mercury cooled and contracted over millions of years, and its crust compressed. The lobate scarps on Mercury can be up to 3.2 kilometers (2 miles) high and hundreds of kilometers long.
25. Mercury passes directly between Earth and the Sun in a rare event called a “transit.”
In astronomical parlance, a “transit” takes place when a smaller body passes in front of a larger one.
The transit of Mercury takes place when it passes directly between the Sun and Earth, becoming a silhouette against the bright face of the Sun. During a transit, Mercury can be seen from Earth as a small black dot moving across the face of the Sun. A typical transit lasts a few hours.
Transits of Mercury take place on average 13 or 14 times per century either in May or November. They also usually come in pairs separated by 3 years, with each pair separated by about a decade. The previous Mercury transits occurred in 2016, 2006, and 2003. The next one will happen in 2032.
Since Mercury is so small relative to the Sun, observing its transit is a rather challenging affair. It is not possible to observe and photograph the transit of Mercury without the use of a telephoto lens or a telescope.
26. Mercury’s eccentric orbit helped prove Einstein’s theory of relativity.
Mercury was the only planet in the Solar System whose peculiar orbit around the Sun couldn’t be fully explained by Isaac Newton’s universal law of gravitation.
As astronomers charted the progress of the planets, they realized that Newton’s law worked pretty well for predicting the motion of planets as well as objects on Earth but not Mercury. Mercury’s orbit made its round faster than predicted. What this means is that each successive perihelion of Mercury occurs in a slightly different direction as seen from the Sun
Einstein’s theory of relativity showed that mass warps space. The theory of relativity correctly predicted that due to the curvature of spacetime created by the huge dent imposed by the mass of the Sun, the perihelion of Mercury advances slightly more than is predicted by Newtonian gravity.
27. Only two spacecraft have ever visited Mercury.
Mercury isn’t very far away (on a cosmic scale), but it’s incredibly hard to get there. Due to its proximity to the Sun, putting a probe in Mercury’s orbit is a really difficult task. A real challenge is slowing the spacecraft down relative to Mercury’s velocity to be captured by its gravity.
The enormous gravity of the Sun means that as you launch a spacecraft towards the Sun it starts to speed up. Thus, more energy is required to enter a stable orbit around Mercury than to send a mission to Pluto!
Space agencies have been hesitant to send even unmanned spacecraft to Mercury as the threat of coronal mass eruptions damaging the sensitive technology is simply too high.
Only two spacecraft, both robotic and launched by NASA, have visited Mercury to date, making it the least explored terrestrial planet. The first was called ‘Mariner 10’ which flew by Mercury 3 times during 1974 and 1975.
In 2011 the special Mercury-exploration spacecraft ‘MESSENGER’ entered an orbit around the planet and explored it until 2015. Mercury will soon be visited a third time. The BepiColombo joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) is scheduled to arrive in 2025.