Mercury. Astronomy with the Deaf World #9

Welcome to another episode of Astronomy with the Deaf World! Have you watched the episode about the Sun yet? If not, I strongly encourage you to do so. [For a full understanding of the text below, we encourage you to watch the video].

Mercury – the planet… boring

And today we will deal with the first planet from the Sun – Mercury. Compared to the Sun, Mercury is… boring. There are no spectacular explosions or temperatures reaching millions of degrees. There is not even an atmosphere on Mercury. That is, there is a minimal one, but it is so small that we can ignore it and say that it is a planet without an atmosphere.

The importance of the atmosphere for a planet

When a planet has an atmosphere, the atmosphere protects the planet from impacts from asteroids – that is, space debris of various sizes, stones, which is why I will continue to use the “stone” sign. These stones just burn up in the atmosphere, and even if they don’t burn up completely, they will burn up in part and slow down, so they will hit the planet at a much lower speed, causing much less damage. And since Mercury has no atmosphere, they hit the surface directly. Therefore, on the surface of Mercury you can see many traces of impacts with other objects (rocks). These are so-called impact craters – the same as we see on the surface of the Moon. And these two factors – craters and the lack of an atmosphere – make Mercury very similar in appearance to our Moon.

When a planet has no atmosphere

The lack of an atmosphere has its own interesting consequences. Mercury is closest to the Sun, but it is not the hottest planet at all. The hottest planet is Venus, but we will make a separate video about Venus.

The temperature on Mercury is variable – on the part of the planet that faces the Sun, the temperature reaches more than 400°C, but on the side turned away from the Sun, where it is night, it drops to -173°C!!! A huge difference, right? This is precisely due to the lack of atmosphere.

The atmosphere on the planet acts similar to a warm blanket that wraps it and keeps it warm. The same is true on Earth – the atmosphere distributes heat throughout the planet, so we don’t have such extremely different temperatures as on Mercury. We can easily see this on a cloudless night – it is usually a little colder then than when the sky is full of clouds, because it is the clouds that keep the heat close to the ground. Meanwhile, the lack of an atmosphere on Mercury means that all the planet’s heat on the day side, is lost when the sun sets.

Water on Mercury?

Interestingly, despite such extremely high temperatures, there are places on the planet where sunlight never reaches. This means that these are places where there are constantly extremely low temperatures. Such a place could be the bottoms of craters, of which Mercury has a lot. Astronomers believe there may be water ice in such places.

Mercury’s (and Earth’s – for comparison) rotation around its axis and around the Sun

Mercury circles the Sun at a different time than the Earth does. It also has a different rotation around its axis than the Earth does. This causes an interesting situation. But to understand it well, let’s recall how it looks on Earth, and compare with Mercury.

And so: The Earth rotates on its axis once every 24 hours. To be precise, it’s once every 23 hours, 56 minutes, 4 seconds. But for the sake of simplicity, we say once every 24 hours. Let’s look at the graphic (in the video). The blue circle is our Earth, as if we were looking at it “from above.” And the red arrow? It only serves to make us clearly see the Earth’s rotation, which is just 24 hours. Well, and what does it look like on Mercury?

Mercury also rotates, but very, very slowly. This rotation takes as long as 58.6 Earth days! To be precise, it is 58 days, 15 hours and 26 minutes. So in that time, while Mercury rotates just once, the Earth rotates more than 58 times!

Of course, every planet in the Solar System revolves around its axis and around the Sun. It takes the Earth roughly 365 days to do this, or more precisely, 365 days, 6 hours, 9 minutes and 9.54 seconds. In our graphic it looks like this (see in the video). Here we see the Earth’s rotation around the Sun, taking 365 days. The yellow section is the part illuminated by the Sun. Of course, we don’t have the size scale preserved here!

Mercury orbits the Sun in 88 days, which is much faster than the Earth. At the same time, as we just learned, it takes it 58 days to rotate around its axis. This causes an interesting situation. See the graphic. Again, the red arrow helps us understand the planet’s rotation around its axis, and the yellow box shows the part illuminated by the Sun. See how long one part of the planet is illuminated!

Solar day on Mercury

In astronomy, there is a concept called the “solar day” – it is the period between two consecutive sunrises. The solar day on Earth lasts about 24 hours. But the solar day on Mercury lasts 176 of our earthly days. Since a year on Mercury lasts 88 Earth days, this means that a solar day on Mercury is twice as long as a year on Mercury!

Mercury’s orbit

Often, when we think of planets orbiting a star, we imagine planets that move in a perfect circle, that is, at the same distance from the star. However, this is most often not the case, and the orbit more or less resembles an egg than an equal circle, and once it moves closer to the star and then further away. And precisely among all the planets of the solar system, Mercury has the greatest fluctuations in distance from the Sun. It is closest to the Sun at a distance of 46 million km, and farthest at a distance of 69.8 million km.

Mercury’s mass and size

Mercury has a large iron nucleus. This causes that although Mercury is small, it is very heavy. Mercury’s diameter is 4879 km. So it is significantly smaller than the Earth and not much larger than the moon. To visualize this more easily, let’s see animations – first a comparison with the Earth, then with the Moon and finally with the Sun.

Mercury in the sky

We can see Mercury with the naked eye in the sky. However, since Mercury is much closer to the Sun than the Earth, from our perspective, when we view it from Earth in the sky, it is always close to the Sun. This makes it very difficult to observe Mercury from Earth – during the day it is quite invisible, because it is lost in the strong glare from the Sun. So we can see Mercury only at dawn and at sunset.

What probes have flown to Mercury?

And although we can see Mercury with the naked eye, we have not studied the planet very well. Only two probes have so far flown to Mercury: the first in 1974-1975 by the Mariner 10 probe, and the second in 2008, 2009 and 2011-2015 by the Messenger probe. In 2025, the BepiColombo probe is expected to reach the planet, which will certainly provide us with a lot of new information about the planet.

Astronomy with the Deaf World

So much for Mercury – the first planet from the Sun. In the next episodes we will learn more about more planets and other astronomical curiosities. And finally, I leave you with a beautiful video showing the so-called transit of Mercury against the Sun. What is a transit? It is when one object is seen against the background of another, as here, where Mercury is seen against the Sun. Mercury is that little black dot. Tiny, right? See you in the next episode of “Astronomy with the Deaf World.”