Mercury is the smallest planet in our solar system and the closest one to the Sun. Aptly named after the swift Roman messenger god, it has a short year of only 88 days meaning you would get 4 times the number of birthdays if you lived on Mercury. A 5 year old on Earth would be 20 years old on Mercury, but you would only be 2 1/2 days old, as Mercury has a very slow rotational speed of around 7mph with a solar day (what we call a day, which is sun rise to sun rise) of 176 days.

Let’s hope you were born on a Friday. Most of your 2.5 days were at the weekend.

Mercury also follows an elliptical orbit meaning its distance from the sun changes thought out its year. This high eccentricity means that Mercury's distance from the Sun varies considerably over the course of its orbit. At its closest approach (perihelion), Mercury is about 46 million kilometers from the Sun, while at its farthest distance (aphelion), it is about 70 million kilometers from the Sun.

Mercury’s orbit also precesses, or rotates, over time due to gravitational interactions with other planets and the Sun’s shape. This means the long axis of the oval orbit will rotate over time. To visualise precession, think of a hula hoop spinning around your waist. As you move, the hula hoop might shift and turn in different directions around you. The hula hoop still goes around you, but its path changes over time.

Speedy Mercury

Key Stats

By rights, mercury should be the hottest planet as it is closest to the Sun. But it has a very thin atmosphere, so thin it's essentially a vacuum. You cannot breathe it; more importantly, it can't retain heat, leading to extreme temperature changes between day and night. During the day, temperatures can reach 427°C, about eight times hotter than the hottest place on Earth. At night, temperatures can drop to -173°C, colder than any place on Earth. These are the greatest temperature variations of any planet in the solar system. So, it's not a great place to live.

Because Mercury lacks an atmosphere, it’s surface is heavily cratered and resembles our Moon. On Earth, our atmosphere protects us from a lot of metorite impacts, and those that get through are covered up eventually through weathering. But Mercury has no such protection nor weather, so it preserves the craters from the impacts asteroids and comets over billions of years. Some are vast in size, at over 1500km in width. As big as France!

Hard to breath

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution Of Washington, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Mercury’s geology

Mercury does not have the active geology of our planet, but it does have some interesting surface features.

For instance, Mercury’s surface has regions known as "hollows," which are shallow, irregular depressions. These hollows are bright and are found in or near impact craters. Their formation is still a mystery, but they might result from the outgassing of volatile materials from the planet’s interior, suggesting there are ongoing geological processes.

More extreme and ancient features of Mercury are its "lobate scarps" – huge cliffs caused by the planet shrinking as it cooled over billions of years. These scarps can be hundreds of kilometers long and up to 3 kilometers high, providing evidence that Mercury is still geologically active. Interestingly planetary shrinkage was an early idea to explain earths geological features such as mountains.

Understanding Mercury’s magnetic field, geological activity, and exosphere can help scientists develop better models of planetary magnetic fields and geodynamics. This knowledge is crucial not only for understanding Mercury but also for applying these models to exoplanets in other star systems.

Mercury does have a magnetic field, far stronger than is expected for a small planet. This is due to its large iron core, which makes up an incredible about 75% of the planet's diameter (on earth the inner and outer cores make up around 50%). It creates a magnetic field that is only 1% of the earth’s but still strong for a small planet.

Mercury has been visited by two NASA missions so far: Mariner 10 and MESSENGER, with another, BepiColombo en route to arrive in 2025. Mercury is quite close to earth but few missions have travelled there as its hard to reach. First you need to deal with the extreme heat and radiation close to the sun. Next, due to quirks of orbital mechanics, a probe going towards the sun has to slow down, requiring a burn to decelerate and fall in towards the sun. But it has to do this in a controlled way as the influence of the Sun’s gravity increases as you get closer. So a delicate dance of manoeuvres is needed along with clever engineering and careful planning.

The BepiColombo mission, for example, uses a series of flybys of Earth, Venus, and Mercury itself to gradually adjust its trajectory and speed, allowing it to enter orbit around Mercury without being pulled into the Sun. Hence why it will take 7 years to reach its target. You have to be patient to study the planets.

Exploration of Mercury

How to see Mercury

Mercury is one of the five planets visible to the naked eye from Earth. However, it can be challenging to observe because it is so close to the Sun. And care must always be taken when using a telescope of binoculars near the sun. Here’s how you can see Mercury with the naked eye, binoculars, and a small telescope:

• Naked Eye: Mercury can be seen just after sunset or just before sunrise. It appears as a bright "star" close to the horizon. The best times to observe Mercury are during its greatest elongations, which are the points in its orbit where it is farthest from the Sun in the sky. During these times, Mercury is higher above the horizon and easier to see.

• Binoculars: Using binoculars, you can get a better view of Mercury’s brightness and its position relative to the horizon. Binoculars can help you spot Mercury more easily during twilight when it is not yet dark.

• Small Telescope: A small telescope will allow you to see Mercury as a small disc. During its phases, similar to the Moon, you can observe the changing shape of Mercury as it orbits the Sun. However, even with a small telescope, surface details are challenging to see due to Mercury’s small size and proximity to the Sun.