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Top Ten Facts About Mercury

Ashley is a UND space studies grad student. She currently works in astrophysics communication at the NASA Goddard Space Flight Center.


10. Mercury is named after the Roman messenger of the gods.

Mercury is the closest planet to the Sun, orbiting about three times closer in than we are. It travels very quickly as it orbits, too—faster than any of the other planets in our solar system. In fact, if you compare it to our outermost planet, Neptune, it is moving at a speed about 9 times greater!

This swiftness determined Mercury's name. In Roman mythology, Mercury was a god who was known for being very, very fast. He was often depicted as having wings on his shoes and helmet, and he served as a messenger for the other gods. The Greek equivalent of this god is Hermes.

9. It takes Mercury 88 Earth days to orbit the Sun one time.


Mercury is very close to the Sun—only about 36 million miles away. That's about 39% of Earth's distance from the Sun, yet its year is 24% as long as ours. This means it must be moving more quickly than we are—but why?

Its orbit is comparatively very small (due to its proximity to the Sun), so of course it takes less time for the planet to travel all the way around the Sun. Its distance from the Sun affects the duration of its year in another way as well, however. The two factors determining gravitational pull are mass and distance. Mercury isn't very massive, but since it's so close to the Sun it's much more strongly affected by the Sun's gravitational pull. Therefore, it travels more quickly as it orbits.

8. A Mercurian day lasts 59 Earth days.

We were first able to measure the length of Mercury's day in 1965 via radar. For some planets we can just watch them spin until the same surface features come back around again, but Mercury's features tend to get washed out since it's so close to the Sun--it's too bright to easily observe, since it never gets very far from the Sun from our vantage point on Earth.

Although it is moving so quickly as it travels around the Sun, it actually spins quite slowly. It takes it 59 Earth days to make just one complete rotation. This is one of the factors that causes a wide temperature range on the planet. Due to the timing of its rotation and orbit, a "day" (taken to mean the time to go from one sunrise to the next) actually lasts two of Mercury's years.

The day side has an awfully long time to get hotter and hotter, while the night side is losing all its heat. People have actually talked about building a mobile base on Mercury, which would move to always stay in the shadow side of the planet. There are of course some obstacles they would have to conquer, such as the tremendous amounts of dust Mercury features damaging the equipment—but it's still an interesting idea to consider!

This image compares the average temperatures of each of the planets in our solar system. Note that even at Mercury's hottest (800 degrees Fahrenheit) it still isn't as hot as Venus!

This image compares the average temperatures of each of the planets in our solar system. Note that even at Mercury's hottest (800 degrees Fahrenheit) it still isn't as hot as Venus!

7. Mercury experiences the widest range in temperature of any of our planets.

Since Mercury is closest to the Sun, most people would naturally believe it to be the hottest planet in our solar system. That would be the case if it weren't for a couple of other factors. First, Mercury has practically no atmosphere to lock in the heat it receives from the Sun. Because of this, it actually gets super cold during the long Mercurian nights—around -300 degrees Fahrenheit!

But even during its day it doesn't get nearly as hot as our hottest planet: Venus. The surface of Venus is almost 900 degrees Fahrenheit, whereas Mercury only averages about 350 degrees (and peaks at 800 degrees). Unlike Mercury, Venus has an incredibly thick atmosphere, which serves to trap in heat. Venus experiences global warming on a grand scale due to the greenhouse effect this causes (learn more about Venus here!).

(So basically Venus is like a person who is bundled up in 20 blankets, while Mercury's walking around pretty much in the buff.)

6. Mercury has as exosphere that is over 40% oxygen.

Not long ago, scientists debated the presence of an atmosphere on Mercury. As time went on, they grew to believe it must not have a significant atmosphere. In 1974, data sent back to us from the Mariner 10 spacecraft confirmed that this was indeed the case. We received even better measurements from the MESSENGER spacecraft in 2008.

Mercury's tenuous (thin) atmosphere is so weak it's referred to rather as an exosphere. This means the molecules are held in by the planet's gravity, but they aren't thick enough to behave the way we would typically expect of gases in an atmosphere. Surprisingly, Mercury's exosphere's primary element is oxygen, accounting for over 40% of its composition. This is expected to come from the planetary body's makeup, whereas the hydrogen and helium in its exosphere are believed to come from the Sun via solar wind.

5. The surface of Mercury is COVERED in craters.


In addition to causing an extreme variation in temperature, Mercury's weak atmosphere has also led to its pockmarked appearance. The many craters we see on Mercury's surface were caused by impacts of meteoroids and other objects. Hopefully you've been able to witness a meteor shower, where you see streaks of light (this visible phenomenon is called a meteor) race across the night sky. What's happening is that an extreme level of friction is produced when the meteoroids encounter our atmosphere. This generates so much heat that the they nearly always completely burn up. Even when they don't they're still typically very small when they reach the Earth's surface—much too small to create a crater. However, Mercury doesn't have much of an atmosphere to shield it from incoming objects. The objects don't really even decrease in size before they impact the planet, causing all of the craters we see on its surface today.

On the Earth, geological activity helps to smooth and erase craters. Wind, water, volcanoes, etc wear away at craters on the surface of our planet. Mercury, however, doesn't have much geological activity to elicit this effect—at least not anymore. There is evidence that there was once volcanic activity there sometime earlier in its formation, and as a result some of the older craters are somewhat smoothed. Today, the appearance of its surface remains more constant (though new craters still pop up due to new impacts).

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4. If you weigh 150 pounds on Earth, you would weigh less than 60 pounds on Mercury.

Weight is a measure of gravitational pull, so it can change based on location. As previously stated, the factors affecting the amount of gravitational pull exerted on an object are mass and distance. Mercury's gravity is only about 38% of ours.

Its low mass accounts for its lower gravitational pull, but the gravity experienced there is almost exactly the same as the gravity that would be experienced on the surface of Mars. Mars is nearly twice the mass of Mercury, so how can that be? It's actually Mercury's small size that accounts for it, since that means objects on its surface would be closer to its center of gravity—so they would be more affected by its gravitational pull. Mars may be more massive, but the matter it's made up of is more spread out. Its lack of density means that objects on its surface would be farther from its center of gravity.

We have actually been able to learn quite a bit from the gravitational affects of the Sun on Mercury and its elongated orbit. According to 30 Second Astronomy, "[Mercury's] orbit does not quite fit Isaac Newton's theory, but Albert Einstein's theory of gravity, known as General Relativity, solved the anomaly." Continuing, it also notes that, "This was the first proof that General Relativity was better than Newton's theory" (François Fressin, 2014).

3. Mercury's core is larger (proportionally) than any of the other inner planets' cores.


Mercury is only slightly larger than our moon, but its core makes up a much larger percentage of the planet by volume than the cores of any of the other inner planets. To put this into perspective, the Earth is about 17% core, whereas Mercury is over 40% core. Scientists expect that this is a result of some unique phenomenon which occurred during Mercury's formation, but they're not sure what it was yet (perhaps a major impact that blasted away much of Mercury's exterior?).

Its crust is believed to be even thinner than ours, and its mantle average sized. The planet is made up of metal and rock, and the core is so hot that the metal it's composed of is liquefied. Mercury's core contains the most iron of any planet in our solar system.

2. Mercury features a weak magnetic field.


The iron in Mercury's core creates the planet's magnetic field, which was discovered in 1974 by data sent back from the Mariner 10 spacecraft. Though it's about 99% weaker than ours, Mercury's magnetic field is actually rather active. This is due largely to its proximity to the Sun. The solar wind carries hydrogen and helium, which is pulled to Mercury's magnetic poles by its magnetic field. This is where Mercury derives much of its thin atmosphere from.

1. Thanks to the Messenger space probe, we have discovered evidence of water ice on Mercury.

Before 2008, the best information we had of Mercury came from three flybys of Mariner 10 between 1974 and 1975. It was able to photograph nearly half of Mercury's surface, and it discovered the magnetic field. In 2008, the MESSENGER space probe reached Mercury. It performed a few flybys and then successfully entered orbit in 2011. The images we now have of the planet's surface are of a much higher quality, due to the many technological advances that have been made since the '70s. Now, we have discovered something truly incredible on Mercury's surface: evidence of water ice. The two sources which could cause water ice to exist on Mercury are outgassing (material being released from the interior of the planet) or bombardment by water-bearing meteors.

Still, technological advancements are not great enough yet to allow confirmation of this potential discovery. We will need to send an advanced probe or orbiter to know for sure.


For each question, choose the best answer. The answer key is below.

  1. Mercury's atmosphere has the highest concentration of which of the following elements?
    • Hydrogen
    • Helium
    • Oxygen
  2. True or false: Mercury is the hottest planet in our solar system.
    • false
    • true
  3. How long is a Mercurian day?
    • 59 hours
    • 59 Earth days
    • 59 Earth weeks
  4. How long is a Mercurian year?
    • 88 Earth days
    • 88 Mercurian days
    • 88 Earth weeks
  5. Why is Mercury covered in craters?
    • Its thin atmosphere doesn't burn up objects that enter it, so they impact the surface.
    • Mercury has little if any geological activity to smooth the surface.
    • Both answers are correct.
    • Neither answer is correct.
  6. If you weigh 150 pounds on Earth, about how much would you weigh on Mercury?
    • 33 pounds
    • 60 pounds
    • 100 pounds
  7. Mercury is made up predominantly of which of the following?
    • Crust
    • Mantle
    • Core
  8. What is believed to be the source of Mercury's magnetic field?
    • The Sun
    • Iron in its core
    • Geological activity
  9. How many moons does Mercury have?
    • 0
    • 1
    • 3
  10. Which of the following spacecrafts have NOT studied Mercury?
    • Voyager 2
    • Mariner 10

Answer Key

  1. Oxygen
  2. false
  3. 59 Earth days
  4. 88 Earth days
  5. Both answers are correct.
  6. 60 pounds
  7. Core
  8. Iron in its core
  9. 0
  10. Voyager 2

© 2015 Ashley Balzer


SteelCup on March 25, 2015:

The thing that caught me off guard the most about Mercury is that dust is an obstacle that has to be considered when trying to explore it.

Miran Shuleta on March 14, 2015:

Great Hub, very interesting indeed.

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