In our solar system, Mercury is the second smallest planet. In size, it is slightly larger than the Earth’s moon. Being at a distance of around 58 million kilometers (36 million miles), Mercury is the closest planet to the Sun. The complete orbit around the Sun by the planet is in just 88 Earth days. Mercury comprises a solid, cratered surface; which is similar to the Earth’s Moon. The planet neither comprises rings around it nor does it have any natural satellites. Mercury’s exosphere or thin atmosphere is mainly composed of oxygen, sodium, hydrogen, helium and potassium. Mariner 10 in the year 1974–1975 and Messenger are the only two space aircrafts to have visited this rocky planet. Until today, no evidence for life has been found on this planet. The Sun would appear more than three times larger than it does on Earth, if standing on the surface of the planet, at its most distant point to the Sun. Being a rocky planet, Mercury is also sometimes referred as a terrestrial planet.
Mercury is popularly known as the Morning star or the Evening Star as it can be seen during dawn and twilight. In mythology, Mercury was the Roman version of the Greek god Hermes. The god was the swift messenger of other gods. The reason being Mercury is often depicted in pictures along with winged sandals. In addition to delivering messages, the god was also believed to be the saviour of travellers and merchants.
The tidal waves of the Sun have an enormous impact on Mercury, which has slowed down its rotations over the years to match its revolution around the Sun. So each day on Mercury lasts for about 58–1/2 days on Earth. In contrast, its revolution around the Sun is comparatively fast. The planet completes one orbit around the sun in only 88 Earth days, which is only about three months on Earth. That is why the planet was named Mercury, as in mythology it was a swift messenger similar to the planet’s fast revolution.
Mercury being comparatively small has a very stunted gravity to hold on to an atmosphere. Any gases released from the planet would in a flash escape into space. Due to the planets proximity to the Sun, any atmosphere is swiftly blown away by the Sun’s solar winds. Mercury’s thin atmosphere contains gases like helium, hydrogen, oxygen, sodium, potassium, magnesium and calcium. It is the only inner planet besides Earth to have a magnetic field. At surface, Mercury’s magnetic field is about hundred times weaker than the Earth.
Mercury has sparse or no atmosphere to regulate its temperatures. So the surface of Mercury which faces the Sun can reach about 800°F (426°C), while in the night the temperatures can plummet to -300°F (-184°C) as the planet does not have an atmosphere to retain that heat.
Mercury is so close to the sun that it is comparatively very difficult to observe it from Earth; except during dawn or twilight. Though the planet does make an appearance for about thirteen times in each century. The observers on Earth can view the rare phenomenon of Mercury passing across the face of the sun, through an event known as a ‘Transit’. These ‘Transits’, fall within several days of the months from May to November. The first two ‘Transits’ of the planet in the 21st Century occurred on the 7 May 2003, and on 8 November 2006. The rare phenomenon can be next observed on 9 May 2016 and on 11 November 2019.
The surface of Mercury resembles to that of the Earth’s moon, which is basically very scarred by many impact craters which resulted due to collisions from comets and meteoroids. The large impact basins on the planet including Caloris (1,550 km), and Rachmaninoff (306 km) were created due to asteroid impacts on the planets’ surface early in the solar system’s history. There are also large areas comprising smooth terrains. There are also lobe-shaped scars, which are around hundred miles long and rise up to a mile high; which were initially formed as the interior of the planet cooled and contracted over billions of years since its formation.
Being the second densest planet after Earth, Mercury comprises a large metallic core having a radius of around 2,000 km and about 80% of the planet’s radius. Recent research found evidence that the core is partly molten (liquid). The outer shell of the planet in comparison with Earth is only about 400 km thick.
Reaching the planet from Earth poses significant technical challenges, as the planets orbit is closer to the sun than Earth. Mercury also presents difficulties to study by space probe.
Mariner 10 – In the year 1974, Mariner 10 was sent in orbit around the Sun and the space probe passed the planet on three occasions. Mariner 10’s encounters with Mercury included two close night-side encounters and one distant dayside pass. Mercury was discovered to have evidence of rotating clouds, and a very weak magnetic field. It revealed more or less moon-like surface. Mariner 10 also discovered that Mercury also comprises a tenuous atmosphere and a large iron-rich core. The day and nighttime temperatures of the planet were also discovered by Mariner 10.
Messenger – In the year 2008 on 14 January, the Messenger probe passed within 200 km of the planet’s surface and obtained photos of almost half of the hemisphere that had been unseen by Mariner 10. The second fly mission of Messenger was followed in the same year. The Messenger executed its last and final fly on 29 September 2009 which further slowed down the space probe. Then in the year 2011, Messenger entered Mercury’s orbit and began a year’s study. Messenger’s mission was extended in the year 2012, and the same year it was confirmed that the condensed material in permanently shadowed craters near the poles was water ice covered by a layer of dark organic compounds.
