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.
Key Facts & Summary
- Since Mercury can be seen without the need of a telescope, many ancient civilizations saw the planet, and as such it is impossible to determine who discovered it first. However, it was first observed with the help of a telescope in early 17th century, by Galileo Galilei.
- Galileo’s crude telescope didn’t manage to capture Mercury’s phases, this would be observed later by astronomer Giovanni Zupi in 1639, and thus he discovered that the planet had similar phases like Venus and the Moon.
- In ancient times, Mercury was taught as being two different objects in the sky: The Mourning Star and The Evening Star. In Venus’s case, it was also mistakenly believed to be two different things.
- Venus spends most of its time away from Earth. This paradoxically makes Mercury the closest planet to Earth, a plurality of the time.
- Mercury was named after the Roman messenger god, because of its fast movements around the Sun.
- 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.
- Despite its closeness to the Sun, it is not the hottest planet, that title belongs to Venus but Mercury is the fastest planet, completing a trip around the Sun in 88 Earth days. This also makes one year on Mercury the equivalent of 88 Earth days, the shortest year of any planet.
- It orbits around the Sun with a speed of about 29 miles or 47 kilometers per second.
- Despite being the smallest terrestrial planet from the Solar System, and in fact the smallest of all the planets, it is the second densest planet in the Solar System, with a density of 5.43 g/cm³.
- For a comparison, Mercury’s size is about a third of Earth, and Earth has a density of 5.51 g/cm³.
- Mercury has a radius of 2.439 km or 1516 mi, and a diameter of 4.879 km or 3.032 mi.
- Mercury's axis has the smallest tilt of any of the Solar System's planets at about 1⁄30 degrees, while its orbital eccentricity is the largest of all known planets in the Solar System.
- Mercury's distance from the Sun is only about two-thirds or 66%, of its distance at aphelion, at its aphelion it is 0.44 AU away from the Sun.
- At its closest distance or perihelion, it is 0.30 AU away from the Sun.
- Mercury spins slowly on its axis and completes one rotation every 59 Earth days. One Mercury solar day or one full day-night cycle, equals 176 Earth days—just over two years on Mercury. (Reminder, one year on Mercury is 88 Earth days)
- Mercury does not have any known satellites or ring systems.
- Its surface is very similar to Earth’s Moon, implying that the planet hasn’t been geologically active for many years.
- Instead of an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and striking meteoroids. Mercury's exosphere is composed mostly of oxygen, sodium, hydrogen, helium and potassium.
- 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.
- Mercury's magnetic field is offset relative to the planet's equator. Though the magnetic field at the surface has just 1.1% the strength of Earth's, it interacts with the magnetic field of the solar wind to sometimes create intense magnetic tornadoes that funnel the fast, hot solar wind plasma down to the surface of the planet.
- Mercury rotates in a way that is unique in the Solar System. It is tidally locked with the Sun in a 3:2 spin-orbit resonance.
- Mercury and Venus orbit the Sun within Earth’s Orbit, this makes them inferior planets.
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 14th century BC. The name used in these tablets is translated as “the jumping planet”.
Some Babylonian records date back to the 1st millennium BC. They 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.
Credits cannot be given to a single civilization or person because the planet has always been easy to spot on the sky. What we can do is to give credit to those who first studied the planet by more “modern” means such as Galileo Galilei in the early 17th century, and Giovanni Zupi who in 1639, observed that the planet had phases just like Venus and the Moon.
It is theorized 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 theorized to be the result of a collision with another giant object that stripped much of its surface.
Distance, Size and 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 1516 mi, and a diameter of 4.879 km or 3.032 mi. It is about the size of the continental United States, slightly bigger. It has a mass of about 3.285 × 10^23 kg or about 5.5% that of Earth.
Despite being the smallest planet from the Solar System, it is the second densest planet in the Solar System, with a density of 5.43 g/cm³ after Earth. For comparison, Mercury’s size is about a third of Earth, and Earth has a density of 5.51 g/cm³.
Orbit and 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.
The diagram above illustrates the effects of the eccentricity, showing Mercury’s orbit overlaid with a circular orbit having the same semi-major axis. The resonance makes a single solar day on Mercury last exactly two Mercury years, about 176 Earth days.
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. The eccentricity of Mercury's orbit makes this resonance stable at perihelion, when the solar tide is strongest. The Sun is nearly still in Mercury's sky. The orbital eccentricity of Mercury in simulations varies chaotically, from zero or circular to more than 0.45 over millions of years because of the perturbations of the other planets.
More accurate modeling based on a realistic model of tidal response has demonstrated that Mercury was captured into the 3:2 spin–orbit state at a very early stage of its history, within 20 or 10 million years after its formation.
Mercury spins slowly on its axis and completes one rotation every 59 Earth days. But when Mercury is moving fastest in its elliptical orbit around the Sun, and it is closest to the Sun, each rotation is not accompanied by a sunrise and sunset like on most other planets. The morning Sun appears to rise briefly.
It then sets and rises again from some parts of the planet's surface. The same thing happens in reverse at sunset for other parts of the surface. Mercury travels in an elliptical orbit slowing down when it’s farther from the Sun, and accelerating as it draws closer.
The axial tilt is almost zero, with the best measured value as low as 0.027 degrees. This is significantly smaller than that of Jupiter, which has the second smallest axial tilt of all planets at 3.1 degrees. On average, Mercury is the closest planet to Earth, and to each of the other planets in the Solar System.
Surface and Geology
Very similar in appearance to Earth’s moon, Mercury’s surface is scarred by many impact craters from comets or meteoroids. Interestingly, many of these craters are named after famous deceased artists and authors. There are also extensive mare-like plains present and the 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 4.6 billion years ago, as well as during a possibly separate subsequent event called the Late Heavy Bombardment that ended 3.8 billion years ago.
During this bombardment, Mercury’s entire surface suffered even more because of its lack of atmosphere that would have slowed impacts down. It is believed that Mercury was volcanically active during this period.
Basins such as the Caloris Basin were filled with magma, producing smooth plains similar to the lunar marias found on the Moon.
The largest known crater is Caloris Basin, with a diameter of 1,550 km or 963 miles. About 15 impact basins have been identified on Mercury, with more to be revealed.
Two geologically distinct plains regions on Mercury have been identified. Gently rolling, hilly plains the regions between craters are Mercury’s oldest visible surfaces, predating the heavily cratered terrain.
These crater plains appear to have obliterated many earlier craters. Unlike lunar maria, the smooth plains of Mercury have the same albedo as the older inter-crater plains. Another interesting feature of Mercury’s surface is the numerous compression folds or rupes that crisscross plains. A theory suggests that as Mercury’s interior cooled, it contracted and its surface began to deform, creating wrinkle ridges and lobate scarps associated with thrust faults. These features have indicated that Mercury’s radius became smaller, shrinking in the range of 1 to 7 km or 4 miles.
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 but 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.
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 planets 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. The iron core has slowly cooled and contracted for about 4.5 billion years. By doing this, it pulled the surface inward, and thus has reduced the planet’s size between 1 – 7 km or 4 miles.
The planet consists of about 70% metallic and 30% silicate material leading to its high density and thus placing it as the second densest planet. It is believed that if the effects of the gravitational compression were to be factored out from both Mercury and Earth, Mercury would take the first place as the densest.
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 mi 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 to 700 K (−173 to 427 °C; −280 to 800 °F) at the most extreme places but it never rises above 180 K at the poles, due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles.
Thus Mercury does not have an atmosphere, but it does have a thin exosphere. The exosphere is traditionally the outermost layer of a planet’s atmosphere. Mercury’s 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 of Mercury is generally extremely high, observations strongly suggest that ice / frozen water, exists on Mercury. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102 K, far lower than the global average.
Water ice strongly reflects radar, and observations by the 70-meter Goldstone Solar System Radar and the VLA in the early 1990s revealed that there are patches of high radar reflection near the poles. Although ice was not the only possible cause of these reflective regions, astronomers think it was the most likely.
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. The strength at its equator is about 300 nT, and like that of Earth’s it is dipolar. The difference is that Mercury’s poles are nearly aligned with the planet’s spin axis.
It is speculated that the magnetic field is generated by a dynamo effect, similar to the magnetic field of Earth. This effect would be resulted from the circulation of the planet’s iron-rich liquid core. Particularly strong tidal effects caused by the planet's high orbital eccentricity would serve to keep the core in the liquid state necessary for this dynamo effect.
The magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. It interacts with the magnetic field of the solar wind to sometimes create intense magnetic tornadoes that funnel the fast, hot solar wind plasma down to the surface of the planet.
Extreme temperatures both cold and hot make it unlikely that life may develop there. The temperatures and solar radiation that characterize this planet are most likely too extreme for organisms to adapt to.
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 and in the case of Mercury, all the materials around it were used up by the planet leaving almost nothing left so that a moon could be formed.
Another theory suggests that Mercury couldn’t have a moon because of its closeness to the Sun. Because of this, the Sun’s greater gravity force would overcome that of Mercury and pull any objects around it, towards itself. Overall, Mercury’s closeness to the sun prevents it from ever having a satellite.
Future plans for Mercury
Because of Mercury’s closeness to Earth, it will always be a target for missions and further observations. The third spacecraft set to arrive on Mercury is called BepiColombo, and it is planned to arrive at Mercury in 2025.
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.
- NASA’s spacecraft MESSENGER was the first to orbit Mercury in 2008.
- Mercury’s apparent distance from the Sun as viewed from Earth never exceeds 28°.
- You could fit approximately 21,253,933 Mercuries inside the Sun.
- In ancient China, Mercury was known as “the hour Star” – being associated with the direction of the north.
- Modern Chinese, Korean, Japanese and Vietnamese cultures refer to the planet as “water star.”
- Hindu mythology used the name “Buddha graha” to refer to Mercury. This god was thought to preside over Wednesday.
- The god Odin or Woden of Germanic paganism was associated with the planet Mercury and also Wednesday.
- Maya people may have represented Mercury as an owl that served as a messenger to the underworld.
- Mercury can, like several other planets and the brightest stars, be seen during a total solar eclipse.
- Even though Mercury is a planet, it is even smaller than the largest natural satellites in the Solar System: Ganymede and Titan, albeit Mercury is more massive.
- Mercury's core has a higher iron content than that of any other major planet in the Solar System.
- Your weight on Mercury would be 38% of your weight on Earth as Mercury has a gravity of 3.7 m/s², while Earth has 9.807 m/s².
- The most cratered planet in the solar system is Mercury.
- NASA has mapped the entire surface of Mercury.
- It is believed that Mercury has a tail: it streams of particles sloughing off its surface.
- The naming and discovery of Mercury cannot be attributed to anyone.
- The crater Caloris Basin is large enough to fit in the state of Texas.
- An event called a “transit” occurs 13 times every century making it possible to see Mercury from Earth, as it crosses the Sun’s face.
- Mercury is about 50% larger in diameter than Earth’s Moon.
- It would take about 18 Mercury’s to match Earth.
- Mercury doesn’t experience any seasons.
- It is one of the most popular planets in our imagination.
- Mercury is about twice as large as Pluto.