The Solar System - Mercury. A Tiny Planet Full Of Surprises.

Image source: nasa.gov

Have you heard your mom complain that when it comes to doing chores around the home you do it so slowly but when it comes to going out to play you run like the wind?

If planet Mercury had a mom, we would hear a similar complaint.
It would go somewhat like this: “he is so lethargic that I have to prod him to turn even a little bit, but when it comes to travel around the Sun, he is so swift, making me dizzy”

Mercury, the smallest of the eight planets, is also the fastest to go around the Sun, completing one orbit every 88 earth days. It is this speed that got the planet its name, honoring the Roman Messenger god whose helmet and shoes had wings.

Mercury is the second densest planet after Earth, with a huge metallic core roughly 3,600 to 3,800 km wide. This means that 75 percent of the planet comprises of its core. Mercury's outer shell is only 300 to 400 miles thick. The planet has an abundance of volatile elements and many scientists are puzzled by these and the size of its core.


Image courtesy: sciencenews.org

A few facts of Mercury:

• It has no moon.
• It is one of the rocky planets. The other three are Venus, Earth, and Mars.
• Mercury is the smallest of all the eight planets, a little bigger than Earth's moon in size. However, it is heavier than the moon, because it is made up of heavy materials like iron.
• It is about 18 times smaller than Earth.
• Mercury has a very thin atmosphere. The strong Solar winds keeps blowing the atmospheric particles away; this means that the atmosphere has to be replenished constantly, making it unstable. Due to this constant blowing and replenishment, it would be apt to call Mercury’s atmosphere as exosphere.
• Mercury’s surface is covered with craters that were caused by space rocks. Due to the thin atmosphere, the rocks do not burn up and therefore cause impacts. The surface of Mercury looks like Earth's moon, full of craters, as a result of these impacts.
• Due to the planet’s proximity to the Sun compounded by the thin atmosphere, Mercury experiences a humongous difference between its daytime and night-time temperatures. Daytime temperatures rise up to 700K (427 °C); the moment that area faces away from the sun, due to the thin atmosphere, the heat immediately dissipates into space, and the temperatures dips to −173 °C. The Polar Regions are constantly at -93°C.

Until 1962 it was believed that the planet was tidally locked to Sun. Tidal locking is when the orbital period is the same as the rotation period. (Our moon is tidally-locked to Earth - 28 days rotation and 28 days orbit – the reason why we see the same side of the moon.)

In 1965, Mercury was observed to rotate three times every two years. It takes 58 Earth days for Mercury to rotate once on its axis, known as sidereal rotation period.

However, due to its proximity to the Sun and rapid speed with which it circles it, it takes the equivalent of 176 Earth days for the Sun to reappear in the same place in the sky. Hence, while the planet rotates once every 58 Earth days, it is roughly 176 days from one sunrise to the next on Mercury, which is every 2 Mercury years.


Mercury also has a highly eccentric orbit. At its nearest point on the elliptical orbit (perihelion), Mercury is only 46 million km from the Sun and at its farthest point (aphelion) it is 70 million km.

There takes place a phenomenon on Mercury that does not happen elsewhere in the Solar system.

If we were to be on Mercury at perihelion during sunrise we will see the sun rise, then reverse, set in the horizon, and then rise again and continue on its path. If we were at perihelion during sunset, we would see the sun set, rise again towards east, and then eventually set.

While it is a little difficult to visualize, this is what happens.

• It is a well known fact that due to our Earth’s rotation its axis we experience Sunrise in the East and Sunset in the West.
• If our Earth did not rotate, the Sun will rise in the West and set in the East.

Now let’s go to Mercury.

• At perihelion (closest point to the Sun) Mercury’s speed increases.
• The sun rises as usual, but because Mercury speeds up near perihelion point there is not enough time for the planet to rotate and appears to be almost standstill.
• We know when a planet does not rotate the sun moves in the opposite direction.
• The sun now moves in the opposite direction, and when the rotation continues it rises again as Mercury leaves perihelion point.

Below is a more scientific explanation:
At certain points on Mercury's surface, an observer would be able to see the Sun rise about halfway, then reverse and set before rising again, all within the same Mercurian day. This is because approximately four Earth days before perihelion, Mercury's angular orbital velocity equals its angular rotational velocity so that the Sun's apparent motion ceases; closer to perihelion, Mercury's angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a retrograde direction. Four Earth days after perihelion, the Sun's normal apparent motion resumes



Mercury continues to shrink in size! This is because the tiny planet is made up of a single continental plate over a cooling iron core. As the core cools, it solidifies, reducing the planet's volume and causing it to shrink.

Visitors
Only two spaceship have visited Mercury.

Mariner 10 was the first to visit Mercury. It flew by in 1974 and 1975. NASA's MESSENGER spacecraft flew by Mercury in 2008 and 2009.

MESSENGER spacecraft continues to send in crucial data about the planet.

Mariner 10 made an unexpected discovery that Mercury possessed a magnetic field.

Theoretically the planet should not possess a magnetic field – it is too small, and its iron core should have cooled off due to its slow rotation. On further investigation scientists think that the iron core is still molten at the center of the planet and only the outer boundary cools.

Mercury sighting.

Mercury is at the lower right. Venus is top left.


Image courtesy: science.nasa.gov.

Mercury is very hard to see as it is very close to the sun. We can still see the planet sometimes as seen in above picture.
This planet also occasionally passes between Earth and Sun – known as Mercury transit. Mercury transit occurs 13 or 14 times per century.

The last Mercury transit was on May 9, 2016.


The next transit will take place November 11, 2019.
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Information sources: NASA, Space.com, Universe today.

 

Thank you @iyerviji . Vijikka.
It is a great that you should say so. I feel blessed.
Thanks for your first feedback.

 

Thank you @justanothergirl .
It took me also many days to understand that . It took one full Mercury year to visualize, and write about the double sunrise and double sunset. Had to read multiple sources and piece them all together. I literally did experiments rotating and orbiting a small ball around a "sun" to grasp the phenomenon.

 

@justanothergirl
Here is an illustration that explains the rotational day and the real daytime on Mercury.




Image courtesy: www.enchantedlearning.com

 

Thank you @Gauri03 .
The picture is one my favorites - a beautiful blue, our only home, alone in the vast world.

 

Thank you @Novalis for this interesting information.

Yes, long long ago, when the solar system was just forming, Jupiter was much closer to the Sun than where it is now. So was Saturn. Slowly the giant gas planets moved outwards and the terrestrial planets moved inside. This movement of the gas giants is a reason on why the asteroids could not coalesce into another planet.

All planets have the same elements, give or take a few, because all formed from the same materials leftover after the sun formed. Later, as they settled into what is the current solar system, some elements ended up being in abundance and some in lesser amounts. Many factors contribute to these differences.
What the shy student informed is also one of them.

One other theory on why the volatile elements did not just vaporize on Mercury is that the planet has less oxygen and more iron. First, without oxygen, the volatile elements could be behaving differently than they do on Earth. Second, the dense iron could have kept the volatile elements closer to the surface of the core than the crust.
Besides, Mercury is still an active planet.
Its volcanoes conitunue to erupt and spew materials, which include the volatile elements, onto the surface.

There is so much to write about Mercury alone. Too many demands on my time are my nemesis.