Want to stay on top of all the space news? Follow @universetoday on TwitterMercury is the closest planet to the Sun. Astronomers have accumulated a wealth of knowledge about the planet, but not nearly as much as what is known about planets closer to us. There have been a few missions and there is one currently in flight. There are a few special features of Mercury that should be discussed and that is what you find in the following paragraphs. All of these definitions and references are in relation to our solar system. When it is stated that Mercury is the smallest planet, it means that it is the smallest planet in our Solar System.
Mercury is one of only four terrestrial planets. It is the smallest planet(equatorial radius of 2,439 km). It70% metallic and 30% silicates. It has the second highest density(5.421 g/cm3), only slightly less than Earth’s(5.3g/cm3). Based on its density, scientists can infer that its core must be large and rich in iron(42% of the planet’s volume). Recent research suggests the planet has a molten core surrounded by a 500–700 km thick mantle consisting of silicates. The Mariner 10 mission returned data that leads scientists to believe that the crust is 100–30 0km thick. Mercury’s core has a higher iron content than that of any other major planet. A widely accepted theory is that Mercury originally had a metal-silicate ratio similar to common meteorites and a mass approximately 2.25 times what it has currently, but an ancient impact with a planetesimal about 1/6 that mass and several hundred kilometers across stripped away much of the original crust and mantle.
One distinctive feature of Mercury’s surface is the numerous narrow ridges, some are several hundred kilometers in length. Accepted theory holds that these were formed as Mercury’s core and mantle cooled and contracted after the crust had already solidified. Its surface is similar to the Moon; showing extensive mare-like plains and heavy cratering. This seems to indicate a lack of geological activity for billions of years. Our knowledge of Mercurian geology is been based on the flyby of Mariner in 1975, so it is the least understood terrestrial planet. The MESSENGER mission is currently sending back data that is increasing that knowledge. A new crater with radiating ‘arms’ called The Spider has recently been discovered. The planet is known to also have highlands, mountains, plains, escarpments, and valleys. Many of these features would have been formed during a brief period of volcanic activity billions of years ago or by bombardment during the early formation period of the Universe. The late heavy bombardment period may have contributed as well.
The largest known crater is Caloris Basin. It has a diameter of 1,550 km. The impact that created the basin would have been so powerful as to have caused lava eruptions and leave a concentric ring over 2 km tall surrounding the impact crater. At the antipode(direct opposite on other side of planet) is a large region of unusual, hilly terrain known as the “Weird Terrain”. One theorists believes that shock waves generated during the Caloris impact traveled around the planet and converged at the basin’s antipode to create this feature. The resulting high stresses fractured the surface.
These are not all of the special features of Mercury by far. I am stopping short to encourage you to research further by looking over NASA’s website for additional facts. Also, new MESSENGER data could add to the list of features at any moment.
We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury.