SOLAR SYSTEM FAMILY-FACTS ABOUT PLANET MERCURY

 Compared to modern times, the people of ancient civilizations had enough leisure to gaze upon the night sky. The awe inspiring sky became the abode of gods, goddesses, and mythological beings, as thousands of twinkling blips of bright and faint light adorned the heavenly realm. Although the twinkling stars remained unchanged for generations, among them, there were some who always ran away. These wandering dots came to be known by the Greeks as ''ASTERES PLANETAI''(wandering stars) or commonly, planets. In spite of our solar system being a family of eight planets- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, the ancients saw only the first five(excluding Earth). 

A Bit of Mythology- Mercury, being the son of a nymph, Maia and the supreme Roman God Jupiter, was the God of commerce, communications, travelers, financial gain, boundaries, luck, trickery, and thieves. Mercury was the cleverest among the Olympian Gods and also their messenger. In Hindu mythology, Mercury was worshipped as God Budha, the son of the moon god Chandra and goddess Tara(wife of God Brihaspati/Jupiter).

 Planet Mercury/Credits: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington / Public domain

Basic Facts: Mercury is the innermost and the smallest terrestrial planet in our solar system. This little planet hasn't got any moons, and is itself smaller in size compared to some of Jupiter/Saturn's natural satellites. Because of being close to its parent star, Mercury is visible just before sunrise or after sunset; and is seen like a star like object near the eastern horizon at dawn and the western horizon at dusk. 

Planetary Characteristics:   

• Rotational & Orbital Characteristics: Mercury is the fastest revolving planet in our solar system, completing its orbit around the Sun once in every 88(87.969) Earth days. Mercury's orbit also has the highest eccentricity of 0.2, where eccentricity is defined as the amount by which a planetary orbit deviates from being a perfect circle. Mercury is closest to the Sun at perihelion(separation of 46 million km) and is farthest at aphelion(separation of 70 million km). Because of this high eccentricity and fast space, an observer standing on the planet would witness a weird phenomenon. The Sun would appear to rise and set and rise again on the same day. This is called retrograde motion. The same thing would happen during sunset; when the sun will set, and rise again before finally going below the horizon. Mercury has a 3:2 spin-orbit resonance, meaning the planet rotates about its axis three times for every two revolutions around the Sun. Again, an observer on  mercury would experience only one day every two Mercurian years.

• Internal Structure: Mercury being a terrestrial planet has got a solid inner core, a molten outer core, a rocky mantle, and a thin crust. The planet is composed of 70% metallic and 30% silicate materials. It is the smallest planet of our solar system with an equatorial radius of 2440 km. Mercury is also smaller than some of our solar system's largest moons namely Titan(radius of 2600 km), and Ganymede(radius of 2550 km). Mercury is also a dense planet, having a density of 5.42 g/cc compared to 5.51 g/cc of Earth, with its core occupying 55% of the total volume. 

• Surface geology: Mercury has a heavily cratered surface, having sustained the bombardment of countless asteroids, meteors and micrometeorites for billions of years. In absence of a significant atmosphere, the space debris hits the planet with high velocity and leaves a mark for billions of years. The craters on mercury range in size from a few meters to hundreds of kilometers across the diameter. These craters form impact basins, with the largest one being the Caloris Basin having a diameter of 1500 kilometers. The Caloris Basin is the most elegant among all Mercurian craters, and is also bounded by some mountains. Other impact basins are Tolstoj Basin(400 km), Beethoven Basin(625 km), Dostoevskij Basin(430 km), Rachmaninoff Basin(305 km), Goethe Basin(317 km), Michelangelo Basin(230 km), and Apollodorus Basin(41 km, nicknamed as ''spider''). The surface of mercury reveals an interesting feature about the planet's internal structure. The presence of some lobate scarps and wrinkles made geologists conclude that the planet had been shrinking in size; the solid iron core cools, and as a result, the thin crust contracts. It is estimated that the planet has lost 7 km of diameter since its fiery birth, billions of years ago.

        Hokusai Crater/Credits: NASA Goddard Space Flight Center from Greenbelt, MD, USA / CC BY  

                      

• Surface Temperature and Possibility of Liquid Water: In spite of being closest to the Sun, Mercury is not the hottest planet in our solar system. The surface temperature on Mercury ranges from 100K at night to 700K at day(-173°C to 427°C) at the most extreme places on 0°N, 0°W or 180°W. However, the temperature does not exceed more than 180K(-93°C) at the poles. Astronomers think that Mercury might contain water-ice in deep craters near the polar regions because these regions are not exposed to direct sunlight and temperature remains below 102K(-171°C). However, this speculation is only based on radar reflection from the poles as water ice strongly reflect radar signals. 

• A Thin Atmosphere: This little planet is also devoid of a significant atmosphere. Because of its small size, close proximity to the Sun and a lesser gravity(38% of Earth), Mercury failed to hold whatever atmosphere it had in the past. Nevertheless, the planet is still blessed with a very thin exosphere, consisting of gaseous hydrogen, helium, oxygen, sodium, potassium, and water vapor. Again, this thin atmosphere is not stable, as it gets depleted and replenished from a variety of processes every now and then.

• Magnetic Field: Mercury has got a fairly good magnetic field, with the magnetic poles being nearly aligned with the planet's spin axis. The field is strong enough to deflect solar wind around the planet and form a magnetopause. Though the magnetopause is small, it can trap solar plasma and aid in space weathering of the planet's surface.

• Natural Satellite(s): This planet has got no natural satellite(s).
  
  
  

        Processed Image of Mercury/Credits: Pablo Carlos Budassi / CC BY-SA

Biological Considerations: Till now we haven't found some Mr. E.T on Mercury and probably there aren't any life forms to look for. But recently there had been a scientific support about extremely primitive life forms having existed on the planet.

Observations & Explorations: 

• Observations: Ancient cultures from all over the world observed the planet and knew Him by different names. To the Babylonians, He was god Nabu; to the Greeks, Hermes; to the Romans, Mercurius; and to the Indians, Budha.  

                                                          The first telescopic observations of mercury were made by the Italian astronomer Galileo Galilei during the early 17th century. But his telescope was not powerful enough to discern its phases; which was later discovered by another astronomer Giovanni Zupi in 1639. 

                    

  Mercury Transit/Credits: Elijah Mathews / CC BY-SA

• Explorations: Because of being extremely close to the Sun, it is extremely difficult for astronomers to observe this planet. Mercury is visible just a few moments after sunset and before sunrise. So, telescopic observations had been difficult and with the advent of radio astronomy scientists were able to observe Mercury in much detail. But, sending a probe to this planet is still technologically challenging as it is hard to maneuver in the gravitational potential well of the Sun. And that's why, until today only two space probes, MARINER 10 and MESSENGER have visited the planet.

       1.   MARINER 10- The first probe to visit our tiny member was the MARINER 10, launched on November 3, 1973, by NASA. It also became the first spacecraft to reach a planet with the aid of gravitational sling-shot from Venus! The probe did three flybys around mercury and collected data about the planet's magnetic field, surface features, temperature, atmosphere, and many more. But because of technological limitations at that time, MARINER could only map 45% of the planet's surface. The first flyby provided data about the magnetic field, which surprised scientists to a great extent. The second flyby had to map and image the planet's surface. It revealed a heavily cratered surface and gathered proof about the planet's shrinking core. The third flyby obtained extensive magnetic data and revealed a magnetic field very similar to earth. The probe was finally shut down on March 24, 1975, when it ran out of fuel. 

        2.   MESSENGER- NASA made a second voyage called the MESSENGER(short for MErcury Surface, Space ENvironment, GEochemistry & Ranging) on August 3rd, 2004. MESSENGER was created to answer some of the most important questions regarding the planet's structure and its history of formation. The MESSENGER probe collected immense data to help scientists throw light upon the planet's high density, it's geological history, magnetic field, the structure of the core, polar ice, and its thin atmosphere. The probe was finally shut down and crashed with a small crater on the planet's surface, on April 30, 2015.

       3.    BepiColombo- The ESA and JAXA launched a joint mission called BepiColombo, designed to orbit Mercury with two probes. It has been launched in October 2018 and is scheduled to reach Mercury in 2025.

 Mercury as compared to Earth/Credits: NASAMercury image: NASA / APL (from MESSENGER) / Public domain

                                                   

Very Distant Future: In the future, the human species might become capable of building an interstellar launch station for solar sails on this planet. Mercury receives the greatest amount of solar radiation, and this could accelerate solar sails to pretty high velocities and help in interstellar travel.

                                          

                                          But the very very far future is a little bleak for this tiny member. Our Sun is essentially a star, and like all stars it is destined to expand hundreds of times of its initial size and shed its outer layers. In about 5-7 billion years from now, the Sun will devour Mercury, as the former turns into a Red Giant. And unfortunately, Mercury would become one with its parent star.

Some Quick Facts!