Mercury is the smallest and the innermost planet in our Solar System. Being the closest to the Sun, it is a dry and desolate world. Yet, this planet has intrigued astronomers for centuries and in the recent years, missions sent to explore it have revealed that the planet is far from boring. Orbiting at a distance of 50 million kilometres from the Sun, exploration of the planet has been minimal since the beginning of the space age. This makes Mercury one of the least known planets in the Solar System. However, scientists believe that the mysteries locked away on this tiny world could lead to interesting conclusions about the planet and the early Solar System.

This post covers most of what we know so far about Mercury, what future missions could reveal and what it would imply for astronomy as a whole. Let us begin!!

Size, Mass and Orbit

Having a radius of 2440 km, Mercury is not only the smallest planet in the Solar System, it is even smaller than Jupiter’s moon Ganymede and Saturn’s moon Titan. In fact if Mercury, orbited another planet, it would have been classified as a satellite instead of a planet. Although it is very small, it is quite massive for its size with a mass of 3.3022×1023 kg. This gives the planet a density of 5.427 g/cm3 making it the second densest planet in the Solar  System, after our very own planet Earth (5.515 g/cm3).

Mercury rotates very slowly on its axis, making one day on the planet as long as 58.646 days on Earth. A year on Mercury (or the time taken by it to orbit the Sun) is however very short, extending to a mere 88 days on Earth only. This is due to Mercury’s close proximity to the Sun. This planet shows a peculiar behaviour in which it completes three rotations on its axis for every two rotations around the Sun. For the scientifically inclined, Mercury has a spin-orbit ratio of 3:2 as it is tidally locked to the Sun.

When it comes to oddities and mysteries, Mercury is one of the main players. The planet has the most eccentric orbit in the Solar System with an eccentricity of 0.205. In layman terms, the orbit of Mercury is the most elliptic or the least circular.  To get a fair idea of this, consider the fact that at it’s just 46 million km  from the Sun at its closest approach (perihelion) but 70 million km at its farthest (aphelion).

Another fun fact about Mercury’s orbit is that is shifts its perihelion closer to the Sun by a factor of 0.16°. This may seem very insignificant but the interesting part was, it could not be explained by using Newton’s Law of Gravitation. This was a big deal as Newton’s theories has been used by astronomers to calculate the trajectories of almost all the bodies in the Universe but this small, pesky planet broke the rules.

Artist’s exaggerated impression of Mercury’s shifting of orbit

It was only after the discovery of Einstein’s Theory of Relativity in 1916, that the mystery was finally solved. Einstein provided a better theory of gravity based on the bending of space itself which could easily explain Mercury’s orbit without much error. We won’t get into the details of it as it requires us to understand Relativity which is not the topic of this post. Another record held by Mercury is the lowest axial tilt of any planet in the Solar System (0.027°). This means the planet’s axis of rotation is almost perpendicular to the plane of the Solar System, unlike most of the other planets. Earth for example has an axial tilt of 23.5°.

Planetary Composition

Mercury is one of the four terrestrial planets of the Solar System, along with Venus, Earth and Mars. Measurements suggest that the planet is composed of 70% metallic and 30% silicate material. The oddities of Mercury are not only refrained to its orbit, but to its structure as well.

But before that, let us examine the general, not so weird facts about structure and composition of the planet. Mercury’s crust, is about 100 – 300 km thick, whereas the interior is composed of a molten iron core which is surrounded by a 500 – 700 km mantle of silicate material as shown in the image below.

Screenshot 2019-02-11 at 12.55.10 PM
Artist’s depiction of Mercury’s interior (notice the size of the core)!!

And that’s all that is normal about the planet. In fact the oddities outnumber the regularities!! Staring off from the interior, Mercury’s core is very large compared to the planet itself which is not common in other planets. You can’t help but notice this fact in the above image as well. Also it contains a higher iron content than the cores of any other major planet in the Solar System, and no one is sure why.

The most widely accepted theory is that what we see as Mercury today was once itself the core of a larger planet!! This was struck by another body measuring several thousand kilometres in diameter, thus stripping off the planet’s outer layers leaving only the core and a few inner layers. And hence, today we see a planet with a major core component.

This theory can also be modified to state that Mercury may have formed from the solar nebula before the Sun had stabilised as a full-fledged star. In this scenario, the original planet of which Mercury was the core would have been subjected to temperatures of 25,000 to 35,000 K as the early Sun contracted. This process would have vaporised much of the planet’s surface rock, reducing it to its current size and composition.

As for the extra abundance of iron, another hypothesis suggests that the solar nebula from which our entire Solar System formed, dragged on the young planet Mercury, stripping away lighter particles and leaving dense elements like iron behind. However, all of these models are just theories and further analysis is needed before any of them can be confirmed or ruled out.

Surface Features

Moving on to its surface, we see a terrain marked with numerous narrow ridges extending up to hundreds of kilometres in length. The ridges are theorised to have formed as Mercury’s core and mantle cooled and contracted while the surface solidified.

Mercury looks very similar to the Earth’s moon in the sense that it has a dry landscape pockmarked by numerous craters and vast plains. The presence of these plains, suggest that the planet has been geologically inactive for billions of years. Here lies another odd feature of the planet. Unlike our Moon, which contains significant stretches of similar geology in a certain area, Mercury’s surface appears much more jumbled with irregular, mixed and mashed up terrain suggesting a chaotic past. All these features support a commonly held theory known as the period of the Late Heavy Bombardment.

It states that about 4 billion years ago, the Solar System had barely formed and there was chaos everywhere. Planets migrated from place to place flinging asteroids and comets in all directions. The Solar System was one major shooting gallery. A young Mercury during that time was heavily bombarded by comets and asteroids. Due to this, the planet received a lot of craters on its entire surface, mostly because of the lack of any atmosphere to slow the impactors down. The planet was volcanically active, releasing magma which later cooled and spread to form smooth plains.

Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins hundreds of kilometres across. The largest known crater on the planet is called the Caloris Basin, which measures 1,550 km in diameter, almost quarter the size of the planet!! The impact that created it was so powerful that it sent shock waves throughout Mercury. These waves travelled throughout the planet and converged on the opposite side of the impact causing lava eruptions which created mountains altogether. Today we see a ring over 2 km tall surrounding the impact crater. Overall, about 15 impact basins have been identified on those parts of Mercury that have been surveyed.

Screenshot 2019-02-11 at 1.00.51 PM
A false coloured image of the Caloris Basin, taken by the MESSENGER probe

Other common features on the planet’s surface include dorsa (aka. “wrinkle-ridges”), highlands, montes (mountains), planitiae (plains), rupes (escarpments) and valles (valleys).

These features follow a similar naming system as with features on any planet. Commonly, valleys are named after radio telescopes; craters are named after artists, musicians, painters, and authors; depressions are named after works of architecture; ridges are named after famous scientists; planes are named after what ‘Mercury’ is called in various languages; mountains are named after what the word ‘hot’ is called in different languages and escarpments are named after scientific expeditions.

Mercury also has a significant, magnetic field that is about 1.1% the strength of Earth’s. It is theorised that just like for the Earth, this magnetic field is generated due to the churning of iron in the core of the planet. Also like the Earth, Mercury’s magnetic field is strong enough to deflect the solar wind emanating from the Sun, thus creating a magnetosphere. The planet’s magnetosphere, is strong enough to trap solar wind, which would otherwise wear down the planet’s surface.

Recent missions have confirmed the existence of water ice and even organic molecules in deep craters on Mercury’s surface even though the planet is subject to extremely hot and harsh environments. This is due to the fact that the floors of deep craters are never exposed to direct sunlight, and temperatures remain at a level that supports the formation of such compounds.

Craters like these on Mercury are in permanent darkness, and have suitable temperatures for the formation of water ice and organic compounds.

These icy regions are believed to contain as much as 1015 kg of frozen water. The reason they don’t evaporate is because a layer of regolith covers them and prevents escape. However the origins of such ice on Mercury is not yet known, but the most widely accepted theories suggest that they were deposited there during the Late Heavy Bombardment when water rich comets crashed onto Mercury’s surface.

We don’t know what the presence of water ice on Mercury could mean for life on the planet although it seems very unlikely due to other factors like intense radiation and lack of an atmosphere which are harmful for any life as we know it.


Due to its close proximity to the Sun, Mercury is constantly bombarded with solar radiation which makes it too hot to retain a considerable atmosphere. However, the planet does consist of a mild and variable atmosphere made up of a variety of ages such as hydrogen, helium, oxygen, sodium, calcium, potassium and water vapour. The atmosphere is so weak that the pressure on the planet’s surface is one-quadrillionth of the atmospheric pressure on Earth.

Theories suggest that this atmosphere, also called an exosphere is constantly renewed as the Sun blows away the older gases while volcanic outgassing and debris from micrometeorite impacts produce new gases. Thus, nothing sure can be said about its exact composition. However, scientists at NASA have discovered that Mercury is actually followed by a sodium trail. This owes to the fact that the solar wind constantly blows sodium atoms away from the planet, hence leaving a trail of sodium atoms.

An infrared image of Mercury’s sodium tail, taken by MESSENGER probe.

Because Mercury lacks a viable atmosphere, it does not retain the heat from the Sun. Most of this heat is reflected back into space. This results in a peculiar effect on the planet. During day time, the surface temperatures reach up to 427° C (700 K), while during night, there is a drastic dip to only -173° C (100 K). We don’t see this on Earth as our atmosphere traps solar energy which prevents the planet from cooling during night.


Humanity’s curious nature has always driven us to explore the unknown and Mercury surely fits into our list. Our very first attempt to explore this planet was NASA’s Mariner 10, which conducted 3 flybys of the planet between 1974 and 1975. During the course of the mission, it was able to capture the first close-up images of Mercury’s surface, revealing the above mentioned heavily cratered, lave flow filled, Moon-like terrain.

Unfortunately, Mariner 10‘s orbit was such, that the same face of the planet faced the probe at each of its close approaches. This made observation of the whole planet impossible, and resulted in the mapping of less than 45% of Mercury’s surface.

Many of the mysterious properties of Mercury we talked about were discovered by Mariner 10. During its first flyby, its instruments detected a magnetic field, which was a surprise to planetary geologists as their models suggested that such a small planet must not have a magnetic field. The second flyby was mostly dedicated for imaging. However, the third flyby was again used for research of the magnetic field, The data revealed that the planet’s magnetic field is like Earth’s, which deflects the solar wind around the planet.

Mariner 10

On March 24th, 1975, just 8 days after its third flyby, Mariner 10 ran out of fuel, resulting in the closure of the mission. The probe is still thought to be orbiting the Sun, passing close to Mercury every few months.

The second mission to Mercury was the MESSENGER probe also known as MErcury Surface, Space ENvironment, GEochemistry, and Ranging probe. The mission launched from Cape Canaveral on August 3rd, 2004 and made 3 flybys of Mercury on January 14th, 2008, October 6th, 2008, and September 29th, 2009 over the course of a year. Most of the hemisphere that was not imaged by Mariner 10 was mapped by MESSENGER. On March 18th, 2011, the probe successfully entered an elliptical orbit around the planet and began taking images by March 29th.

The probe carried imaging devices that captured images of a higher resolution images of the planet than Mariner 10. Along with this it also had a variety of other instruments on board. A few of them were spectrometers which helped to determine the elemental composition of the crust, and magnetometers which were used to measure velocities of electrically charged particles and study the magnetic field.

The main purpose of this mission was to solve 6 major mysteries related to Mercury, namely – its geological past and activity, its high density, the existence, nature and cause of its magnetic field, how it gets its weak atmosphere, the large structure of its core with high iron content and whether or not it has ice at its poles.

After mapping the planet for over a year, the mission was extended for another year  until 2013. MESSENGER’s final manoeuvre took place on April 24th, 2015, after which it was depleted of fuel. The probe slowly descended until it crashed into Mercury’s surface on April 30th, 2015.


On 20th October 2018, the European Space Agency (ESA) and the Japan Aerospace and Exploration Agency (JAXA) launched a joint mission called BepiColombo. This robotic space probe is third mission ever sent to the planet. BepiColombo is predicted to reach Mercury in 2025 after a flyby of Earth, 2 flybys of Venus and 6 flybys of Mercury before finally settling into orbit.

The mission consists of two probes: a mapper probe and a magnetosphere probe. As the name suggests, the magnetosphere probe is tasked to study the ever so elusive magnetic field of the planet.  The mapper probe will go on to map and image the planet in many different wavelengths including infrared, ultraviolet, X-ray and gamma ray using an array of spectrometers similar to those on MESSENGER.


With only 2 complete mission conducted so far since humanity’s entry into the space age, Mercury remains one of the least known planets in the Solar System and hence holds a lot of mysteries. From high density, iron rich core to a molten surface containing water ice and organic molecules, with a small discernible atmosphere, a surprisingly strong magnetic field and an orbit that shifts every century or so, Mercury holds many secrets just waiting to be uncovered in the ages to come.





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