Planet Mercury

Mercury is only visible to the naked eye in the morning or twilight hours, a phenomenon that lead some ancient cultures to believe that Mercury was actually two different planets. Considering the wildly varying temperatures, the intermittent atmosphere and the fact that humans have only observed less than half of its surface, perhaps in a way it is.

A Look at Mercury

The closest of the four terrestrial planets to the sun, Mercury is also the smallest, yet surprisingly is the second most dense, after Earth. This is just one of the many puzzling facts that have helped scientists decipher the mysteries of Mercury over the years. Named by the Romans after their god of the same name, but this tiny planet has had many names; Hermes (Greek), Nabu (Babylonian), Budha (India), the water star (China) and Kokhav Hamah (Hebrew) which meant the star of the sun.

Mercury is closest to the sun and feels its impact in a great way. With no appreciable atmosphere, the sunlight on the surface of Mercury is about 6.5 times greater than the sunlight on the surface of Earth, and temperatures vary wildly, dipping to 90K and raging to 700K. Yet, deep down, at the bottom of some of Mercury’s many craters, the sunlight never reaches directly. Here it is believed that water may lie.

Our Best Look

In 1974, Mariner 10 passed by Mercury and began to record. From its incomplete mapping, we now know that about 40% of the planet surface is like, and the closest comparison is the Earth’s Moon. Like the Moon the surface of Mercury is riddled with craters, believed to be formed during the period of heavy bombardment some four million years ago. With such little atmosphere there was nothing to slow the impact of the craters. Some scientists believe the water they imagine to be at the bottom of some of these craters was actually deposited by the comets that created them.

An Iron Core

It is surmised that the reason Mercury is so dense despite its size has to be because of a core that is proportionally very large and rich in iron. Based on these ideas it has been determined that the core, likely molten, is 1,800 kilometres thick, surrounded by a 600 kilometres mantle and a thin crust of only 100 to 200 kilometres. It is further surmised that the reason for such a thin crust and mantle is a deep impact scenario, similar to the theory of how Earth’s Moon was created. Another theory proposes that Mercury’s proximity to the sun’s solar winds have blown away material through accretion.

Our Next Look

There are two planned visits to Mercury, one on the way and one set to launch in the upcoming future. The first, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) will arrive at Mercury in January of 2008, following up with two more passes in October 2008 and September 2009. After these three passes the probe will enter Mercury’s orbit in March of 2011.

Next up will be BeppiColumbo, a joint mission between Japan and Russia that will launch in 2013 and reach Mercury in 2019. Both missions plan on examining the planet in full detail, getting a good look at the entire surface, seek out clues regarding the incredible density of the planet, and determine once and for all if there is indeed water on Mercury.