MERCURY
A true colour image of Mercury.
(NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
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Formation Mercury is the smallest and the innermost planet of our Solar System. It is only slightly larger than our Moon. Like the other terrestrial (rocky) planets, Mercury is thought to have formed from the swirling Solar Nebula about 4.5 billion years ago.
Orbital Characteristics One orbit around the Sun takes 88 Earth days, meaning that Mercury travels about 47 km per second. Its orbit is highly elliptical (egg-shaped)- it is 47 million km. from the Sun at its closest, and 70 million km at its furthest. One complete day/night cycle on Mercury lasts 176 Earth days, nearly equal to two complete orbits of the planet around the Sun. Mercury’s tilt is only 1/30th of a degree, hence it does not experience seasons like on most other planets. Craters near the poles are therefore in constant deep shadow, and could contain water ice.
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A colour-enhanced image of the Caloris Basin. This is not how the planet would appear to the human eye! Diameter is 1550 km
(NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington. Image reproduced courtesy of Science/AAAS)
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Structure, Atmosphere and Magnetosphere Mercury appears to contain about 70% metallic and 30% silicate ("rocky") substances. The silicate material forms the outer crust and mantle, overlaying an iron core. The core accounts for over one half of Mercury’s volume, compared to 17% for the Earth’s core. Several theories have been suggested to account for the unusually large iron core, but further data is needed.
Mercury’s surface looks a lot like our Moon. It is heavily cratered, indicating that Mercury has been geologically inactive for a very long time. Most of the craters are named after authors, artists and musicians. The largest impact crater is the “Caloris Basin”. At 1550 km wide, it is one of the largest impact basins in the Solar System. There is a region called the “Weird Terrain” 180 degrees opposite the Caloris Basin. It may have formed at the same time from the shock wave that resulted from the impact.
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Mercury has two distinct regions of relatively flat plains and rolling hills. These regions strongly suggest ancient volcanic activity. Similarly, the distribution of various types of minerals suggest that Mercury was volcanically active in the distant past.
Mercury has no true atmosphere. It does have an “exosphere” composed of atoms that are knocked loose from the surface by micro-meteorite impacts or blasted from it by the solar wind (a stream of energetic charged particles coming from the Sun). Because of Mercury’s lack of atmosphere and its proximity to the Sun, it has the largest range of surface temperatures in the Solar System, from -173 to +427 Celsius. Mercury has a weak magnetic field; its strength is about 1% of Earth’s. It is thought to result from the rotation of the iron core, and it does interact with the solar wind, resulting in a magnetosphere.
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Colour enhanced video reveals the geological differences in surface minerals. Note the Caloris Basin and the "Weird Terrain" on the opposite side.
(NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
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Observation of Mercury Mercury is never more that 27 degrees from the Sun. It is therefore seen only during the morning or evening twilight. Because it lies between the Earth and the Sun, it shows phases, like our Moon. Its brightness varies greatly, depending on its phase and its location along its highly elliptical orbit. When observing Mercury, great care must be taken to avoid looking at the Sun because of the high risk of permanent eye damage.
Mercury was known to many ancient cultures. Its earliest recorded observation was in the 14th century BCE. Ancient cultures called Mercury by different names depending on whether it appeared in the morning or the evening twilight. By about 350 BCE, ancient Greeks realized these were one and the same object.
Moons - Mercury has no moons
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Missions A mission to Mercury is an extreme technical challenge for a number of reasons. First, the spacecraft would have to function within the intense heat and radiation of the nearby Sun. Second, the close proximity to the Sun results in huge gravitational forces that must be overcome. Third, Mercury travels at a speed of 47 km per second, meaning the spacecraft would have to expend a lot of fuel simply to “catch up” to Mercury and to enter and maintain orbit.
There have been only two completed missions to Mercury. Both were led by NASA. Mariner 10 (1974-1975) made three approaches to Mercury, photographing its surface and capturing data relating to magnetism. Messenger (2008-2015) achieved an elliptical orbit around Mercury. It mapped the surface in high resolution and gathered a broad range of geological data. A further mission (BepiColombo) by the European Space agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) was launched in 2018. It completed the first of six fylbys of Mercury in October 2021, and it is expected to enter orbit in 2025. |
Mariner 10 - First interplanetary probe to visit Mercury (1974-1975).
(image courtesy of NASA)
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