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Key Facts & Information
- Mercury is the closest planet to the Sun but, perhaps surprisingly, it does not have the highest temperatures. It is the second densest planet of the Solar System, but also the smallest planet. The structure of Mercury makes it the most similar planet to Earth. For a comparison, Mercury’s size is about a third of Earth, and Earth has a density of 5.51 g/cm³.
- In ancient times, Mercury was taught as being two different objects in the sky: The Morning Star and The Evening Star. Venus was also mistakenly believed to be two different things.
- Mercury is the fastest planet, completing a trip around the Sun in 88 Earth days. It orbits around the Sun with a speed of about 29 miles or 47 kilometers per second.
- Surface Temperature: -108°C
- Orbit Period: 87.97 Earth days
- Known Rings: None
- Known Moons: None
- Equatorial Circumference: 15,329 km
- Equatorial Diameter: 4,879 km
Discovery & Formation
- One of the earliest known recorded observations of Mercury is the Mul.Apin tablets. It is believed that these observations were made by an ancient Assyrian astronomer around the 14th century BC. The name used in these tablets is translates as “the jumping planet”.
- The Babylonians called the planet Nabu, after the messenger to the gods in their mythology. The ancient Greeks knew the planet as Hermes while the Romans named it Mercury and it remained as such to this day.
- Mercury was first observed with the help of a telescope in the early 17th century, by Galileo Galilei. His crude telescope didn’t manage to capture Mercury’s phases. This would later be observed by astronomer Giovanni Zupi in 1639, who discovered that the planet had similar phases like Venus and the Moon.
- It is theorised that Mercury was formed about 4.5 billion years ago when gravity pulled swirling gas and dust together to form the small planet. Its small size but enormous core is thought to be the result of a collision with another giant object that stripped much of its surface.
Distance, Size & Mass
- Mercury is the closest planet to the Sun, at a distance of 57.91 million kilometers/35.98 miles or 0.4 AU away. It takes sunlight 3.2 minutes to travel from the Sun to Mercury.
- Mercury has a radius of 2,439 km or 1,516 miles, and a diameter of 4,879 km or 3,032 mi. It has a mass of about 3.285 × 10^23 kg or about 5.5% that of Earth.
- Despite being the smallest planet in the Solar System, it is the second densest planet after Earth, with a density of 5.43 g/cm³. For comparison, Mercury’s size is about a third that of Earth, and Earth has a density of 5.51 g/cm³.
Orbit & Rotation
- Mercury’s highly eccentric, egg-shaped orbit takes the planet as close as 29 million miles or 47 million kilometers, and as far as 43 million miles or 70 million kilometers from the Sun.
- It takes a trip around the Sun every 88 days thus 1 orbit/year is the equivalent of 88 Earth days. Mercury travels through space at nearly 29 miles or 47 kilometers per second, faster than any other planet.
- Radar observations in 1965 proved that the planet has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun.
- Mercury spins slowly on its axis and completes one rotation every 59 Earth days. Mercury travels in an elliptical orbit, slowing down when it’s farther from the Sun, and accelerating as it draws closer.
Surface And Geology
- Very similar in appearance to Earth’s moon, Mercury’s surface is scarred by many impact craters from comets or meteoroids.
- These craters also indicate that the planet has been geologically inactive for billions of years.
- It is believed that Mercury was heavy bombarded by comets and asteroids during and shortly after its formation. During this bombardment, its entire surface suffered even more because it lacks an atmosphere, that would have slowed impacts down.
- The largest known crater is the Caloris Basin, with a diameter of 1,550 km or 963 miles.
- Another interesting feature of Mercury’s surface is the numerous compression folds or rupes that criss-cross plains. A theory suggests that as Mercury’s interior cooled, it contracted and its surface began to deform, creating wrinkle ridges.
- Other factors indicate that this shrinking and geological activity may be present to this day. The volcanic system on Mercury is quite complex. Though its exact age is hard to pinpoint, it is speculated to be billions of years old.
- Temperatures on the surface of Mercury are both hot and cold. During the day, temperatures on the surface can reach up to 800 degrees Fahrenheit/430 degrees Celsius. Because the planet has no atmosphere to retain that heat, nighttime temperatures on the surface can drop to -290 degrees Fahrenheit/-180 degrees Celsius. These changes in temperature are the most drastic in the entire Solar System.
- Structure. Mercury is a terrestrial planet having three main layers: a core, mantle and crust. Mercury’s crust has no tectonic plates and its iron core is enormous, making up 85% of the planet’s radius, while Earth’s inner and outer core account for about 55%.
- Because of the core’s unusual size, it influences Mercury’s overall size by causing it to shrink. Similar to Earth, its mantle consists of silicates, but it only measures 500-700 km thick.
- Mercury consists of about 70% metallic and 30% silicate material leading to its high density and thus placing it as the second densest planet. This density also indicates that its core is huge and rich in iron. Mercury’s crust is estimated to be around 35 km or 22 miles thick.
- Atmosphere – Exosphere. Because of its proximity to the Sun, Mercury’s gravity is strongly affected. It is too small and hot for its gravity to retain any significant atmosphere over long periods of time. The surface temperature of Mercury ranges from 100-700K (−173 to 427°C;−280 to 800°F) at the most extreme places.
- Thus Mercury does not have an atmosphere, but it does have a thin exosphere. Its exosphere is made up of oxygen, sodium, hydrogen, helium and potassium that are all whipped up from the planet’s surface by the solar winds.
- Although the daylight temperature at the surface is generally extremely high, observations strongly suggest that ice / frozen water exists on Mercury.
- Magnetosphere. Even if it is small and has a slow 59-day-long rotation, Mercury has a significant and apparently global magnetic field. It has been estimated that this magnetic field has 1.1% the strength of Earth’s.
- Mercury doesn’t have any known satellites even though many objects much smaller than Mercury have. It is believed that moons form in the same time as their parent planets.
- Another theory why Mercury doesn’t have a moon is based on its closeness to the Sun. The Sun’s greater gravity force would overcome that of Mercury and pull away any objects around it. Overall, Mercury’s closeness to the sun prevents it from ever having a satellite.
Did You Know?
- From the surface of Mercury, the Sun would appear more than three times as large as it does when viewed from Earth, and the sunlight would be as much as seven times brighter.
- NASA’s spacecraft Mariner 10 was the first mission to explore Mercury in 1974-1975.
- Mercury, like several other planets and the brightest stars, can be seen during a total solar eclipse.
- Mercury doesn’t experience any seasons and is the most cratered planet in the solar system.