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Picture of Mercury. Color enhanced beyond visible light to show variations we can’t see

Table of Contents

  1. Basic Facts
  2. Layers of Mercury
  3. Surface Composition
  4. Atmosphere?
  5. For Educators

Basic Facts of Mercury

Mercury orbits at 38 thousand kilometers from the Sun and weighs 3.285 × 10^23 kg, which is about a tenth of the weight of Earth. It orbits the sun once every 88 days(We orbit the Sun once every 365 days, which constitutes a year). Mercury has the most elliptical orbit out of all the planets. Planets do not orbit the Sun in a circle, but rather as an ellipse(an oval) which is explained by Kepler’s Laws. The year to day ratio of mercury is 2/1. This means that one day on Mercury is 2 years on Mercury! That sounds awfully confusing but to understand, the year to day ratio of Earth is 1/365. This means each year, 365 days occur. A day of a planet is how long it takes for the planet to turn one full rotation(360 degree). A year of a planet is how long it takes for the planet to make one revolution around the Sun. Thus, this means that Mercury orbits the Sun twice before it finishes one rotation around itself. This is because Mercury is really close to the Sun, so it can orbit around the Sun very fast. It also rotates around itself very slowly, which helps account for the odd ratio I provided. Finally, in terms of Earth days, Mercury orbits around the Sun every 88 days and rotates around itself every 176 days(thus the 2 to 1 ratio).

Layers of Mercury

Mercury, being a terrestrial planet, is dense. This is accounted for by the inner most layer of Mercury, the inner core, which is solid iron. The pressure from the rest of the planet(basically the weight of all this iron and the stuff on top of it) accounts for the solid behavior. After this comes a liquid outer core of similar composition. Then comes the mantle, which is separated from the outer core by a layer of solid iron. Explaining this is difficult, and I am not entirely sure why such a layer exists. Generally with less pressure comes a liquid form, and I have not come across explanations for this. The mantle of mercury is made of silicates(compounds made of silicon and oxygen) and is very thin, only 500 kilometers. Finally is the crust, which is considered the surface of mercury. This is discussed next.

Comparison of Earth’s interior to Mercury. Mercury is very small compared to Earth(about a tenth in mass) and has a significantly different interior).

Surface of Mercury

Mercury has a unique surface, but a great way to think of it is similar to the moon. There are many craters caused by impacts from different space objects like debris, asteroids, and planetesimals. The largest crater on mercury is Coloris Basin. The cracks of the smooth regions of mercury, rupes, are long cracks on the older surface of mercury caused by faults on Mercury(like fault lines on Earth where earthquakes happen). Final thing to know is the temperature changes of Mercury. As I will talk about next, Mercury does not have an atmosphere. Thus, temperature changes are extreme. It ranges between -300 to 800 degree Fahrenheit based on if it is day or night! Note that by extreme I mean the range of the temperatures, the change in temperature is not constantly fluctuating as the planet spins very slowly. It gradually decreases as night approaches.

The line through the middle is one of the rupes we discussed above. It is formed by a thrust fault(a fault like that of where an earthquake occurs).

Mercury’s Questionable Atmosphere

It is easiest to say that Mercury has no atmosphere. An atmosphere is the gases held by gravitation of a planet. Because of Mercury being so light, it does not have the mass to hold an atmosphere. Another issue is that Mercury is too close to the Sun. Remember the phenomena we talked about with the Sun? Those phenomena and just the flow of charged particles out of the sun produces what is called a solar wind. This wind blows away most of the “atmosphere” that the planet could hold onto. This means there is essentially nothing to consider as an atmosphere. However, there is still a little bit of gas held by the planet. This is mainly oxygen, sodium, and hydrogen. For reference, it is 10^34 times sparse than Earth’s atmosphere. That is Earth’s atmosphere divided by 1 with 34 zeroes in front of it.

For Educators

Every post I do was intended to have a for educators section, but when it comes to an individual planet, I do not think it is necessary. Each planet should only be discussed briefly, when teaching astronomy, its more important to give the big picture and get people attracted to astronomy. As such, I am going to skip this section for most of the planets. I think a short talk about each planet is sufficient. If you have anything you’d like in this section, please let me know.

Thank you

Thank you for reading! Any questions feel free to contact me at!

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