Things about Mercury

 What we know about the Smallest Planet

Mercury is the closest planet to the sun, spinning around our parent star at an average of 36 million miles away. It circles the Sun faster than all the other planets. However, Mercury's orbit is not a perfect circle. The planet can pass as close as 29 million miles and as far as 43 million miles. The planet completes one orbit around the sun in just 88 Earth days. One Mercury solar day (one complete rotation) equals 176 Earth days. It is also the smallest planet in the solar system, measuring just 3,032 miles wide at the equator. This makes it only slightly larger than Earth's moon. 

Because mercury is so small and co close to the sun, it is the most elusive of the five planets that are visible to the naked eye. You can only watch mercury at dawn and dusk, and it usually does not rise far above the horizon. But that's not the only time the tiny planet makes an appearance. Because of its position in the solar system, Mercury passes between Earth and the Sun 13 times each century in an event known as a 'transit'. During a transit of Mercury, observers on Earth can see the planet in silhouette as it seems to sweep across the sun's disk. 

Despite its proximity to our star, Mercury is not the hottest planet in the solar system. With no atmosphere to trap heat, the surface temperatures on Mercury can swing from 426 degrees celsius during the day to -198 degrees celsius at night. Mercury may even have reservoirs of ice sitting deep inside the permanently shadowed craters at its poles. By contrast, the surface of hazy Venus sits at a sweltering 470 degrees celsius year-round, making it the hottest planet in our solar system. 

Lack of atmosphere alose means that Mercury's surface is filled by numerous impact craters since incoming meteors don't encounter any friction that would cause them to burn up. According to NASA, the atmosphere of Mercury is a 'surface-bound exosphere' essentially a vacuum. It contains 42% oxygen, 29% sodium, 22% hydrogen, 6% helium, 0.5% potassium, with possible trace amounts of argon, carbon dioxide, water, nitrogen, xenon, krypton, and neon. Seen via telescopes and spacecraft, Mercury looks like a battered world covered in overlapping basins, soaring cliffs, and occasional smooth plains. 

Mercury is the second densest planet after Earth, with a huge metallic core of roughly 3,600 to 3,800 kilometers wide, or about 75% of the planet's diameter. In comparison, Mercury's outer shell is only 500 to 600 km thick. As if Mercury isn't small enough, it did not only shrink in its past but it's continuing to shrink today. 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. This process crumpled the surface, creating lobe-shaped scarps or cliffs, some hundreds of kilometers long and soaring up to 2 kilometers. For example, Mercury's 'Great Valley', which is about 1,000 kilometers long and 400 kilometers wide with a depth of 3.2 kilometers. It is larger and deeper than the Great Rift valley in East Africa. The combination of its massive core and abundance of volatile elements has left scientists puzzled for years. About 4 billion years ago, an asteroid approximately 100 km wide struck Mercury with an impact equal to 1 trillion megaton bombs. creating a vast impact crater roughly 1,550 km wide. Known as the Valoris Basin, this crater could hold the entire state of Texas. Another large impact may have helped create the planet's odd spin. 

Mercury's surface resembles that of the Earth's moon, scarred by many impact craters. Craters and features on Mercury are named after famous deceased artists, musicians, or authors, including children's author Dr. Seuss and the dance pioneer Alvin Ailey. Most of Mercury's surface would appear greyish-brown to the human eye. The bright streaks are called 'crater rays'. They are formed when an asteroid or comet strikes the surface. The tremendous amount of energy that is released in such an impact digs a big hole in the ground and also crushes a huge amount of rock under the point of impact. Some of this crushed material is thrown far from the crater and then falls to the surface, forming the rays. Fine Particles of crushed rock are more reflective than large pieces, so the rays look brighter. The space environment (dust impacts and solar wind particles) causes the rays to darken with time.

A completely unexpected discovery made by Mariner 10 was that Mercury possessed a magnetic field. Planets theoretically generate a magnetic field only if they spin quickly and possess a molten core. But Mercury takes 59 days to rotate and is so small, almost one-third Earth's size, that its core should have cooled off long ago. An unusual interior could help to explain the differences in Mercury's magnetic field compared to Earth. Although Mercury's magnetic field is just 1% the strength of Earth's, it is very active. The magnetic field in the solar wind (the charged particles streaming off the sun) periodically touches upon Mercury's field, creating powerful magnetic tornadoes that channel the fast, hit plasma of the solar wind down to the planet's surface.