Mercury
Overview
Mercury is the closest planet to the Sun, and it is also the smallest planet in our solar system. It is only slightly larger than our Earth’s moon.
Mercury is mostly made of metallic substances such as iron and nickel. Seventy percent of its mass comes from metallic substances, and the remaining thirty percent comes from silicate materials. You can read more on its unique internal structure in the next section.
Though it is the closest planet to the Sun, it is not the hottest planet in the solar system; that honor goes to Venus.
Unique Interior
Like our Earth, the interior of Mercury consists of four layers:
- A thin crust (solid rock) at the top.
- A mantle (molten rock) layer underneath the crust.
- A liquid Metallic layer beneath the mantle.
- A solid metallic layer at the bottom.
What is unique about Mercury is its high proportion of metallic layers in its interior, as the graph below illustrates.
The graph above is a comparison of the internal structures of Earth and Mercury. In the case of the Earth, the bottom metallic layers (shown in blue and dark blue) make up 55% of its radius. However, in the case of Mercury the metallic layers go up to 85% of its radius.
There are several theories, describing different possible incidents or scenarios, to explain the high proportion of metallic interior. However, none of them is certain.
Interesting Facts
Moon: Mercury does not have any moon, nor does it have any ring system.
Atmosphere: Mercury does not have atmosphere.
Size: The radius of Mercury is about 2439 kilometers, and it is only slightly bigger than our Earth’s moon which has the radius of about 1738 kilometers. The picture below compares the sizes of Mercury and our moon.
Orbit: Comparing with the orbits of other planets, Mercury’s orbit around the Sun is the most eccentric (elliptical). Its distance from the Sun varies from 46 million kilometers (at perihelion – the nearest point) to 70 million kilometers (at aphelion – the farthest point). The picture below shows how Mercury’s orbit looks like.
Day and Year: It is interesting to note that Mercurian day is longer than Mercurian year. One Mercurian day (from one sunrise to the next sunrise) is 176 earth days while one Mercurian year is 88 earth days.
Surface temperature: During the daytime (the sunlight-lit side), when it is closest to the Sun at perihelion, Mercury’s ground temperature reaches 870° Fahrenheit; hot enough to melt lead. However, during the night it plunges to minus 295° Fahrenheit. What could be the reason for this extreme swing? Lack of atmosphere is the cause for the extreme surface temperature variation (between the night-side and daytime-side) as there is no blanket on the surface to trap and retain the heat.
Mercury's Precession &
Einstein's General Theory of Relativity
You may be wondering, what does Mercury’s orbit have to do with Einstein’s relativity theory?
First, let us discuss an issue with Newton’s law of gravity. Though Newton’s law of gravity could explain (and predict accurately) the orbit of every planet in the solar system, it could not fully explain the anomalous precession of Mercury. But Einstein’s theory of gravity was able to. Before getting into the details of Einstein’s theory of gravity, let us understand what ‘precession of perihelion’ is.
When we think of an orbit of a planet, we may picture a planet starting at a point and coming back to the same point after completing one orbit. Right? But that is not what is happening. You may be surprised to know that planets do not come back to the same point after completing one revolution. Planets orbit the Sun pivoted to the perihelion point, and that point keeps moving ahead from one orbit to the next. This is known as precession of perihelion. Though this precession of perihelion is tiny for other planets, it is quite significant for Mercury.
In the 19th century, astronomers discovered that the actual precession of Mercury is more than what they had computed using Newton’s law of gravity. It differed by about 43 arc seconds per century. One arc second is one-3600th of one degree angle.
This difference remained a mystery until Einstein discovered the new theory of gravity.
Albert Einstein, in the year 1915, published his new theory of gravity which is also known as the ‘general theory of relativity’. Using his newly discovered gravitational theory, he was able to explain and predict Mercury’s precession accurately. The precession predicted by the general theory of relativity matched up with the actual precession.
That was one of the great validations for Einstein’s general theory of relativity.
Sources:
- The Cambridge Guide to the Solar System by Kenneth R.bLang, Cambridge University Press.
- https://solarsystem.nasa.gov/solar-system/our-solar-system
- https://solarsystem.nasa.gov/planets/mercury/in-depth/