The Insider’s Guide to Planet Mercury and Its Characteristics [2019]

Mercury planet

Planet Mercury is the smallest planet in our solar system & nearest to the Sun. When Mercury is closest to the Sun, it is only 47 million km away, and when it is furthest away from the Sun the distance is 70 million km.

Even though it is small it is one tough planet, surviving brutal attacks from comets, asteroids and the mighty Sun.

Mercury has always been a difficult planet to study. Even the Hubble telescope is forbidden from viewing Mercury because it is too close to the Sun and it may damage the telescope’s optics.

A Little History

Timo Charis was the first person to make observations on Mercury. He first thought that it was a star.

Galileo made the first telescope observations of the planets with a 20x magnification telescope. But his telescope was too small to see Mercury’s phases but he did see Venus phases.

Pierre Gassendi was the first to observe the transit of Mercury around the Sun. More can be found in this book about Gassendi’s work available on Amazon.

What does Planet Mercury mean?

The Romans named the planet after the Roman messenger god, MERCURY. Hindu Mythology used the name Buddha for Mercury.

The Roman god Mercury had wings on his helmet by which he could travel very quickly from one place to another.

The planet Mercury moves very quickly around the Sun. This is how it got its name.

How far is Planet Mercury from the Sun?

Not far but very close. Mercury makes its closest approach to the Sun at 47 million km and furthest at 70 million km.

From Mercury’s surface, the Sun will appear more than three times larger than it does from Earth’s surface.

What is the color of Mercury?

Mercury has a dark grey rocky surface covered with dust. Mercury’s dark grey color comes from molten lava which cooled many million years ago.

What is the size of Mercury?

Planet Mercury size compared to other planets
Image source:-

Mercury is the smallest planet in the solar system. It is smaller than Saturn’s moon Titan and only slightly larger than Earth’s Moon.

Mercury’s diameter is only 4880 km which is very small compared to Earth’s 12,756 km.

How hot is Mercury?

Mercury is both very hot and very cold. The daytime temperatures on Mercury can reach up to 430 degrees Celsius and at night the temperature can drop to a chilling -180 degrees Celsius. Mercury is still not hotter than Venus though. (know why)

How long is the day on Mercury?

Mercury rotates very slowly around its axis taking it 59 Earth days to rotate around itself which means that one day on Mercury is 59 Earth days long.

However, Mercury rotates very fast around the Sun taking it just 88 days to make one full orbit around the Sun making it the planet with the smallest year of 88 Earth days.

Mercury has only one sunrise every 175 days!! Weird as it gets.

Atmosphere and Surface

Mercury’s thin atmosphere or exosphere is composed mostly of Oxygen(O2), Sodium (Na), Hydrogen(H2), Helium (He) and Potassium(P).

Mercury is a rocky planet, also known as a terrestrial planet. Mercury has a solid, cratered surface much like Earth’s moon.

No evidence of life has been found on Mercury yet.

Mercury is a very strange planet when it comes to temperature variations.  The daytime temperature on Mercury can reach 430 degrees Celsius and drop to -180 degrees Celsius at night.

These temperature variations are mainly because of two reasons-

1. Mercury has a very slow spin. It has one sunrise every 180 Earth days. So, it has a lot of time to heat up and a lot of time to cool as well.

2. Mercury doesn’t have a strong atmosphere to trap the heat. So, it cools very fast after the sunset.

Mercury was heavily bombarded with comets and asteroids following its formation. These impacts created a lot of craters on its surface similar to the moon. Mercury’s thin atmosphere also didn’t help to slow down the impacts.

Gravity on Mercury is 38% of the gravity on Earth. A man weighing 100 kg would lose 62 kg just by landing on the planet’s surface. In the past, Mercury was an active volcanic planet but the eruptions ended about 3.5 billion years ago.

volcanos on planet mercury
volcanos on Mercury by MESSENGER

Mercury is the second densest planet after Earth. It has a large metallic core having a radius of about 2000km, that’s about 85% of planets diameter.

Mercury’s core is partly molten. Because of its enormous core, Mercury is shrinking. Over the years the core cools pulling the planet inward. As if Mercury isn’t small enough.

Space explorations

Only two spacecraft have visited Mercury till now, Mariner 10 and MESSENGER

Mariner 10

South pole of mercury
The south pole of Mercury by Mariner 10

Mariner 10 was the first spacecraft to visit Mercury. It was launched in 1973. It also studied Venus on its way to Mercury.

Mariner 10 used the gravity of Venus to reach Mercury. Mariner flew past Mercury 3 times managing to detect its magnetic field.

Mariner was only able to image 40-45% of Mercury’s surface because the same side of Mercury was sunlit each time.

Mariner confirmed that Mercury has a metallic core comprising about 80% of planets’ mass and temperature ranging from 187 degrees Celsius at dayside and -183 degrees Celsius at nightside.

You can read the full details about the mission on NASA’s Mariner 10 mission page.


North pole of
North pole by MESSENGER

Messenger (Mercury Surface, Space Environment, Geochemistry, and Ranging) was the second mission to Mercury and was the first to orbit the planet.

Messenger used a complex series of flybys to reach Mercury. It made a flyby of Earth in 2005 and two flybys of Venus in 2006 and 2007.

It made the first flyby of Mercury in January 2008, second in October 2008 and third in 2009. It entered Mercury’s orbit in 2011.

The messenger was able to map 100% of the planet which Mariner 10 didn’t.

Messenger discovered all sorts of weird things about Mercury, not the least of which is polar ice on a planet with ungodly surface temperature. The ice was hiding in shadows from the Sun. This ice is up to 164 feet (50 meters) thick in some areas.

It confirmed that Mercury’s core is about 85% of its radius which is huge compared to Earth’s 50%. It also detected past volcanic eruptions on the planet.

Messengers primary mission ended in 2012 having taken 100,000 images of the planet. Mission was later extended.

Spacecraft also photographed two comets: Comet 2P/Encke and Comet C/2012 SI (ISON).

On April 16, 2015, NASA announced that spacecraft would impact the surface of Mercury by April 30, 2015. It fell on Mercury’s surface on April 30 creating a new crater approximately 16m wide.

You can read more about the mission in the book written by Cambridge Planetary Science.


BepiColombo is a joint space mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) to the planet Mercury.

It was launched on 20 October 2018 and is planned to reach Mercury in December 2025. The spacecraft will perform one fly-by of Earth, two flybys of Venus and six flybys of Mercury.


To conclude, Mercury is small but a tough guy who is in a constant battle to escape from the Sun’s gravity.

Mercury is a planet with short years and long days.

This unique planet despite being so close to the Sun has water on it which makes it even more unique.

Despite being so small we still have so much to learn from this tiny little world.

How do you think the water originated on Mercury?

Prajyot Kumbharjuvekar

Co-founder of Astronomiac

Prajyot is a writer and co-founder of Astronomiac. Writer at day and reader at night, he writes everything about astronomy.

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