The Time Now > Astronomy > Mercury

The planet Mercury

MESSENGER spacecraft images have revealed portions of Mercury never seen by human eyes. © NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury is the smallest planet in the Solar System (Pluto, once the smallest planet is now named as a dwarf planet), and the planet with the most none concentric orbit among the eight planets. Mercury is also the planet that is nearest the sun, sitting 46 million kilometers (in its nearest position to the sun) to 70 million kilometers (in its farthest position to the Sun). Despite its distance and position to the sun, Mercury has a very peculiar surface temperature; an extremely cold 100 Kelvin during the night and extremely hot 700 Kelvin during the daytime (which is second only to Venus). Mercury is an amazing sight for many planet watching folks on Earth, making appearances occasionally in the dawn and night sky, although it may be harder to see compared to Venus due to its proximity to the Sun.

Geological surface

Geological surface of Mercury
Geological surface of Mercury. © NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/U. S. Geological Survey/Arizona State University

Mercury’s surface looks a lot similar to the Earth’s Moon when it comes to appearance. There are numerous large craters scattered throughout its surface, along with mare-like plains spread all around. This is an indication that the planet has been geologically dormant for billions of years. The craters were caused by numerous asteroids and comets that crashed on Mercury’s surface, which followed its creation around 4.6 billion years ago. Due to its lack of atmosphere, the impacts were powerful, since there was nothing to slow the asteroids or the comets as they hit the planet. The surface features of Mercury are named after various sources – names that pertain to people are limited only to those who have already died – from artists, painters, musicians and authors. One good thing to note though, is that because much of the information about Mercury’s surface is being based on the flyby missions in 1975 (with the spaceship Mariner), there is still much that we do not know about it, making it the least understood among the four rocky planets.

Geological structure

Geological structure of Mercury compared to Earth
Geological structure of Mercury compared to Earth. © Adapted from a figure by NASA/Johns Hopkins Univ. Appl. Phys. Lab./Carnegie Inst. Washington

Mercury is a part of the four terrestrial planets (along Venus, Earth and Mars) in the Solar System. It is the smallest planet (the equatorial radius of the planet is 2,439.7 kilometers); in fact, it is even smaller than Titan and Ganymede, the two largest satellites (or moons) in the Solar System. Mercury’s composition is approximately 70% and 30% silicate substances. The core is believed to have an abundance of iron because of its high density (scientists estimate that among all the planets in the Solar System, Mercury has the highest concentration), which is 5.427 g/cm3 (which is only second to Earth’s). As for its internal structure, many geologists believe that Mercury’s core comprises around 42% of the planet’s volume, making it around 1,800 kilometers in radius. The mantle, which consists of silicate materials, surrounds the core and is around 500 to 700 kilometers thick. Finally, the crust of the planet is estimated to be around 100 to 300 kilometers thick. There are numerous narrow ridges along the surface which stretch several hundred kilometers along the surface. These are believed to have been formed when Mercury’s core and mantle were cooled and tightened along with the solidification of the crust.

Theories on Mercury’s formation

Formation of Mercury
Artistic creation illustrating the formation of Mercury. © NASA/JPL-Caltech

There are several theories on how Mercury’s surface and geological structure was formed. One theory subscribes to an impact event that caused the planet to shrink in size and mass. Many scientists believe that Mercury is smaller than it originally should have been, having much of its crust and mantle stripped away by the impact caused by a planetesimal that is 1/6 of Mercury‘s original mass. This theory is known as the giant impact hypothesis, which scientists also believe to be the cause of the Moon’s formation. Another theory stipulates that during the formation of the Solar System, Mercury was twice the size it is today. Back then, the sun was also being formed, with its nebula violently soaring across space until it finally stabilized. As the protosun was slowly forming into the sun that we know today, the temperatures fractured and vaporized a large portion of Mercury’s surface, leaving only half of its size. Another theory speculates that nebula from the sun caused drag on the particles that were forming the planet, causing the lighter particles to be lost.

Exosphere (atmospheric and geological conditions)

Atmospheric and geological conditions of Mercury
Atmospheric and geological conditions of Mercury. © Courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

As stated earlier, the surface temperature of Mercury is at extremes (100 Kelvin at night, 700 Kelvin by day); this is caused by the absence of a strong atmospheric coating around the planet that is responsible for reducing the effects of the Sun (the atmosphere may have also been vaporized due to the intense heat from the Sun). During the day, depending on where Mercury is on its orbit, surface temperatures average 550 Kelvin (during its aphelion, the farthest point of a planet to the Sun during its orbit) to 700 Kelvin (during its perihelion, the nearest point of a planet to the Sun during its orbit). At night, surface temperatures average 110 Kelvin; this is due to the absence of an active core, which is unable to hold atmosphere strongly and allows heat to escape from the planet. Because of this, many geologists believe that there is ice on Mercury. In fact, recent observations on the planet’s surface indicate several patches of white on Mercury’s surface, indicating frozen water. Aside from the inactivity of Mercury’s core, the planet’s size also has to do with its ability to hold a proper amount of atmosphere. Mercury is too small and has little mass to be able to produce gravity that is strong enough to hold atmosphere. Because of this, the surface is violent and unstable – atoms of different particles found on the surface of the planet are always lost and replenished.

Planetary rotation and orbit

Mercury has by far the most none concentric orbit among the eight planets in the Solar System; as stated earlier, in its aphelion, Mercury is as far as 70 million kilometers away from the Sun; at its perihelion, Mercury is as near as 46 million kilometers. It takes 87.969 Earth days to complete an orbit (meaning a year in Mercury only takes around 88 days here in Earth). One interesting fact about Mercury though, is despite its small size, it has a very slow rotation (possibly due to the inactivity of its core): it takes 58.7 Earth days for Mercury to complete its rotation (meaning for one day in Mercury to pass) it will take more than 58 days.

Planetary rotation and orbit of MercuryPlanetary rotation and orbit of Mercury. © NASA