This view is one of the first from the MESSENGER probe's Oct. 6, 2008 flyby of Mercury.

Mercury is the closest planet to the sun. As such, it circles the sun faster than all the other planets, which is why Romans named it after their swift-footed messenger god.

The Sumerians also knew of Mercury since at least 5,000 years ago. It was often associated with Nabu, the god of writing. Mercury was also given separate names for its appearance as both a morning star and as an evening star. Greek astronomers knew, however, that the two names referred to the same body, and Heraclitus, around 500 B.C., correctly thought that both Mercury and Venus orbited the sun, not Earth. [Latest Photos: Mercury Seen by NASA's Messenger Probe]

Mercury's physical characteristics

Because the planet is so close to the sun, Mercury's surface temperature can reach a scorching 840 degrees Fahrenheit (450 degrees Celsius). However, since this world doesn't have much of a real atmosphere to entrap any heat, at night temperatures can plummet to minus 275 F (minus 170 C), a temperature swing of more than 1,100 degrees F (600 degree C), the greatest in the solar system.

Mercury is the smallest planet — it is only slightly larger than Earth's moon. Since it has no significant atmosphere to stop impacts, the planet is pockmarked with craters. About 4 billion years ago, an asteroid roughly 60 miles (100 kilometers) wide struck Mercury with an impact equal to 1 trillion 1-megaton bombs, creating a vast impact crater roughly 960 miles (1,550 km) wide. Known as the Caloris Basin, this crater could hold the entire state of Texas. Another large impact may have helped create the planet's odd spin.

As close to the sun as Mercury is, in 2012, NASA's MESSENGER spacecraft discovered water ice in the craters around its north pole, where regions may be permanently shaded from the heat of the sun. The southern pole may also contain icy pockets, but MESSENGER's orbit did not allow scientists to probe the area. Comets or meteorites may have delivered ice there, or water vapor may have outgassed from the planet's interior and frozen out at the poles. [Related: First Photos of Water Ice on Mercury Captured by NASA Spacecraft]

As if Mercury isn't small enough, it not only shrank in its past but is continuing to shrink today. The tiny planet is made up of a single continental plate over a cooling iron core. As the core cools, it solidifies, reducing the planet's volume and causing it to shrink. The process crumpled the surface, creating lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high, as well as Mercury's "Great Valley," which at about 620 miles long, 250 miles wide and 2 miles deep (1,000 by 400 by 3.2 km) is larger than Arizona's famous Grand Canyon and deeper than the Great Rift Valley in East Africa.

"The young age of the small scarps means that Mercury joins Earth as a tectonically active planet with new faults likely forming today as Mercury's interior continues to cool and the planet contracts," Tom Watters, Smithsonian senior scientist at the National Air and Space Museum in Washington, D.C., said in a statement.

Indeed, a 2016 study of cliffs on Mercury's surface suggested the planet may still rumble with earthquakes, or "Mercuryquakes." This could mean that Earth is not the only tectonically active planet, the authors of the research said.

In addition, in the past, Mercury's surface was constantly reshaped by volcanic activity. However, another 2016 study suggested Mercury's volcano eruptions likely ended about 3.5 billion years ago.

Mercury is the second densest planet after Earth, with a huge metallic core roughly 2,200 to 2,400 miles (3,600 to 3,800 km) wide, or about 75 percent of the planet's diameter. In comparison, Mercury's outer shell is only 300 to 400 miles (500 to 600 km) thick. The combination of its massive core and abundance of volatile elements has left scientists puzzled for years.

A completely unexpected discovery made by Mariner 10 was that Mercury possessed a magnetic field. Planets theoretically generate magnetic fields only if they spin quickly and possess a molten core. But Mercury takes 59 days to rotate and is so small — just roughly one-third Earth's size — that its core should have cooled off long ago.

"We had figured out how the Earth works, and Mercury is another terrestrial, rocky planet with an iron core, so we thought it would work the same way," Christopher Russell, a professor at the University of California, Los Angeles, said in a statement.

An unusual interior could help to explain the differences in Mercury's magnetic field when compared to Earth. Observations from MESSENGER revealed that the planet's magnetic field is approximately three times stronger at its northern hemisphere than at its southern. Russell co-authored a model that suggests that Mercury's iron core may be turning from liquid to solid at the core's outer boundary rather than the inner.

"It's like a snow storm in which the snow formed at the top of the cloud and middle of the cloud and the bottom of the cloud too," said Russell. "Our study of Mercury's magnetic field indicates iron is snowing throughout this fluid that is powering Mercury's magnetic field."

The discovery in 2007 by Earth-based radar observations that Mercury's core may still be molten could help explain its magnetism, though the solar wind may play a role in dampening the planet's magnetic field.

Although Mercury's magnetic field is just 1 percent the strength of Earth's, it is very active. The magnetic field in the solar wind — the charged particles streaming off the sun — periodically touches upon Mercury's field, creating powerful magnetic tornadoes that channel the fast, hot plasma of the solar wind down to the planet's surface.

Instead of a substantial atmosphere, Mercury possesses an ultra-thin "exosphere" made up of atoms blasted off its surface by solar radiation, the solar wind and micrometeoroid impacts. These quickly escape into space, forming a tail of particles.

One 2016 study suggested that Mercury's surface features can generally be divided into two groups — one consisting of older material that melted at higher pressures at the core-mantle boundary, and the other of newer material that formed closer to Mercury's surface. Another 2016 study found that the dark hue of Mercury's surface is due to carbon. This carbon wasn't deposited by impacting comets, as some researchers suspected — instead, it may be a remnant of the planet's primordial crust.

Mercury's orbital characteristics

Mercury speeds around the sun every 88 Earth days, traveling through space at nearly 112,000 mph (180,000 km/h), faster than any other planet. Its oval-shaped orbit is highly elliptical, taking Mercury as close as 29 million miles (47 million km) and as far as 43 million miles (70 million km) from the sun. If one could stand on Mercury when it is nearest to the sun, it would appear more than three times as large as it does when viewed from Earth.

Mercury is the closest planet to the sun and has a thin atmosphere, no air pressure and an extremely high temperature. Take a look inside the planet. (Image credit: Karl Tate, SPACE.com)

Oddly, due to Mercury's highly elliptical orbit and the 59 Earth-days or so it takes to rotate on its axis, when on the scorching surface of the planet, the sun appears to rise briefly, set, and rise again before it travels westward across the sky. At sunset, the sun appears to set, rise again briefly, and then set again.

In 2016, a rare transit of Mercury happened, where the planet crossed the face of the sun. Mercury's transit may have yielded secrets about its thin atmosphere, assisted in the hunt for worlds around other stars, and helped NASA hone some of its instruments.

Composition & structure

Atmospheric composition (by volume):

According to NASA, the atmosphere of Mercury is a "surface-bound exosphere, essentially a vacuum." It contains 42 percent oxygen, 29 percent sodium, 22 percent hydrogen, 6 percent helium, 0.5 percent potassium, with possible trace amounts of argon, carbon dioxide, water, nitrogen, xenon, krypton and neon.

Magnetic field: Roughly 1 percent the strength of Earth's.

Internal structure: Iron core roughly 2,200 to 2,400 miles (3,600 to 3,800 km) wide. Outer silicate shell about 300 to 400 miles (500 to 600 km) thick.

Orbit & rotation

Average distance from the sun: 35,983,095 miles (57,909,175 km). By comparison: 0.38 Earth's distance from the sun.

Perihelion (closest approach to sun): 28,580,000 miles (46,000,000 km). By comparison: 0.313 times that of Earth

Aphelion (farthest distance from sun): 43,380,000 miles (69,820,000 km). By comparison: 0.459 times that of Earth

Length of day: 58.646 Earth-days

Research & exploration

The first spacecraft to visit Mercury was Mariner 10, which imaged about 45 percent of the surface and detected its magnetic field. NASA's MESSENGER orbiter was the second spacecraft to visit Mercury. When it arrived in March 2011, MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) became the first spacecraft to orbit Mercury. The mission came to an abrupt end on April 30, 2015, when the spacecraft, which had run out of fuel, crashed onto the planet's surface. [First Photos of Mercury from Orbit]

In 2012, scientists discovered a group of meteorites in Morocco that they think could have originated from the planet Mercury. If so, it would make the rocky planet a member of a very select club with samples available on Earth; only the moon, Mars and the asteroid belt have verified rocks.

In 2016, scientists released the first-ever global digital-elevation model of Mercury, which combined more than 10,000 images acquired by MESSENGER to take viewers across the wide-open spaces of the tiny world. The model revealed the planet's highest and lowest points — the highest is found just south of Mercury's equator, sitting 2.78 miles (4.48 km) above the average elevation of the planet, while the lowest point resides in Rachmaninoff basin, the suspected home of some of the most recent volcanic activity on the planet, and lies 3.34 miles (5.38 km) below the landscape average.

Additional reporting by Nola Taylor Redd, Space.com contributor

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