So – you’re still interested in astronomy? Mercury5 min read

If you remember, last time we took a look at 10 things you wanted to know about Venus. This time, I thought it would be a great idea to consider 10 facts you might find interesting about one of the smallest places in our Solar SystemMercury.

1. Is Mercury really one of the smallest places?

For a planet, she certainly is. Mercury is only 4,879 KM (3,032 miles) in diameter. Just look at this comparison picture of relative sizes.


Relative_satellite_sizes[1] (2)


Do you see? Mercury – (not a dwarf planet) But an actual fully fledged planet – is smaller than two of the moons of our solar system, Ganymede and Titan

2. So where is Mercury?

Mercury is the first planet out from the sun, and orbits our star at a distance of 46 million km (28.5 million miles), once every 88 Earth days. She is classed as an inferior planet, because like Venus, she is closer to the Sun than Earth.

3. Why does Mercury have that name?

Simply because she appears to move through the sky so quickly when viewed from Earth. That rapid motion is thought to have led to her being named after the Roman deity, Mercury, the fast flying messenger of the gods. She’s such a speedy planet, that her year is four times shorter than ours.

4. Mercury must be really hot then?

Well, yes and no.

Remember, last month’s blog revealed that Venus is the hottest of all the planets of our Solar System because of the special conditions prevalent in her atmosphere.
However, Mercury isn’t too far behind. Because she’s so close to the sun, her atmosphere has been stripped away. Due to this, the heat built up during the day cannot be retained. As such, she experiences the greatest temperature variation of all the planets.

At night, conditions can fall as low as 100K (-173C/-280F). But, in the day, that rises to 700K (427C/800F)

5. Is it like that all over?

Well, here’s an interesting point. Mercury’s axis has the smallest tilt of any planet in the Solar System. When you combine that factor to the lack of an atmosphere, you find a very interesting phenomenon occurs. Despite being so close to the sun, Mercury’s polar areas experience extremely low temperatures. Did you realize, the conditions experienced at those two locations are constantly below 180K (-93C/-136F)? Wow!

It’s strange to think that it’s possible to freeze to death on the closest planet to the sun, eh?

6. So, because she has no atmosphere, Mercury can’t have any weather?

Aha! You might think that, but you’d be mistaken. Mercury’s magnetic field has been seen to leak from time to time. During a fly-by in on October 6, 2008, satellite Messenger discovered huge magnetic tornadoes wreaking havoc on the surface.

Basically, they form when the magnetic charge carried by the solar winds, connect to Mercury’s magnetic field. As the solar wind blows past, the joint fields react, and the planetary charge is sucked up into the air and twisted into a vortex. This actually creates an open window through which the solar wind can reach the surface and create 800 km wide storms. Impressive!

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7. So, Mercury does actually have an electromagnetic field then?

Oh yes! Despite her size and slow rotation, Mercury has a significant global magnetic field. It’s strong enough to deflect the solar wind around the planet, to create a magnetosphere. Though it can fit within Earth’s volume, Mercury’s magnetosphere is strong enough to trap passing solar winds and this is thought to be a major contributing factor to the weathering effect we see on the surface.

8. Are there any other surprises?

There certainly are!

Although small, Mercury consists of approximately 70% metallic and 30% silicate material. Her density is the second highest in the solar system, only slightly less than Earth. – (look at picture 1 to see how much smaller she is).

Remember, Earth’s density results appreciably from gravitational compression, particularly at the core. Mercury is much smaller and its inner regions are not compressed. Therefore, that density must be due to the fact the core is large – in relation to the planet itself – and rich in iron




Geologists estimate that Mercury’s core (3) has a radius of about 1,800 km and occupies about 42% of her volume. (Earth’s core occupies only 17%). The mantle (2) is thought to be between 500 – 700 km thick. Based on information from the Mariner 10 expedition, Mercury’s crust (1) is only 100 – 300 km thick.

9. Anything else?

Well, although the surface of Mercury is said to look a lot like the Moon, there is one distinctive feature that sets her apart. Look at this…



Despite numerous craters, Mercury is covered in a multitude of narrow ridges up to several hundred kilometers long. They scar the entire surface. It is believed this feature formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.
Of course, being so close to the Sun also plays a factor. Her surface is flexed by significant tidal bulges from the Sun itself.

(The effect being 17 times stronger than the tides generated by our Moon).

10. Is there anything about Mercury we still don’t know?

There certainly is.

One of them relates to something of a mystery. Observations by the 70m Goldstone telescope and the Very Large Array in the 1990’s revealed patches of “very high radar reflections” near to Mercury’s poles. Astronomers are sure this is due to the presence of ice. (yes – ice on Mercury). Now, the origins of that ice in not yet known. However, two of the most likely causes are thought to be, outgassing – (from within the planetary crust) or that the ice was deposited there by comet impacts.
Whatever the cause, the presence of ice/water on any planet is a cause for further investigation…because life as we know it cannot exist without water.

So, there you go. I hoped you enjoyed this brief look at Mercury. Next time, we’ll look at 10 interesting things about someone who plays a major role in the life of the Solar System. Jupiter.

See you the.



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Andrew Weston

Andrew P. Weston is Royal Marine and Police veteran from the UK who now lives on the beautiful Greek island of Kos with his wife, Annette, and their growing family of rescue cats. An astronomy and law graduate, he is the creator of the international number one bestseller, The IX, and also has the privilege of being a member of the Science Fiction and Fantasy Writers of America and the British Fantasy Society. When not writing, Andrew devotes some of his spare time to assisting NASA with one of their remote research projects, and writes educational articles for and Amazing Stories. He also enjoys Greek dancing and language lessons, being told what to do by his wife, and drinking Earl Grey Tea. If you would like to find out more, visit his blog or website at:

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