March 17, 2026
Mercury Facts: Everything You Need to Know About the Closest Planet to the Sun
Mercury is the smallest planet in our solar system and the closest to the Sun. Named after the swift-footed Roman messenger god for its rapid orbit, Mercury races around the Sun in just 88 Earth days — faster than any other planet. Despite being nearest to our star, Mercury is not the hottest planet (that title belongs to Venus and its runaway greenhouse effect). Instead, Mercury is a world of extremes: scorching days above 400 °C, freezing nights below −180 °C, and a surface so battered by impacts that it resembles Earth's Moon. With an oversized iron core, a faint magnetic field, and virtually no atmosphere, Mercury is one of the most enigmatic worlds in our cosmic neighborhood. This complete guide covers everything we know about Mercury: its size, diameter, surface area, physical features, temperature extremes, and the missions that have revealed its secrets — all drawn from NASA's Planetary Fact Sheets and decades of spacecraft observations.
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Mercury Fact File
How Big Is Mercury?
Mercury is the smallest planet in our solar system, and it is not even close to the next largest. Its equatorial diameter is 4,879 km (3,032 miles) — only about 38% the width of Earth. To put that in perspective, you could fit roughly 18 Mercury-sized spheres inside Earth's volume. Mercury is only marginally larger than Earth's Moon, which has a diameter of 3,475 km. In fact, two moons in our solar system — Jupiter's Ganymede (5,268 km) and Saturn's Titan (5,150 km) — are larger than Mercury, though Mercury is considerably more massive than either due to its dense iron core.
The surface area of Mercury is 74.8 million km² (28.9 million square miles) — roughly twice the area of Africa or about 15% of Earth's total surface area. Its mass is 3.301 × 10²³ kg, approximately 5.5% of Earth's mass. Despite being the smallest planet, Mercury is the second-densest planet in the solar system (after Earth), with a density of 5,427 kg/m³. This extraordinary density hints at Mercury's defining internal feature: an enormous iron core. Surface gravity is 3.7 m/s² — about 38% of Earth's, coincidentally the same surface gravity as Mars. A 70 kg person would weigh only 26.6 kg on Mercury.
Mercury by the Numbers
Mercury orbits the Sun at an average distance of 57.9 million km (0.387 AU), making it roughly two-and-a-half times closer to the Sun than Earth. A Mercurian year is remarkably short: just 87.97 Earth days to complete one full orbit. But here is where things get strange — a single solar day on Mercury (sunrise to sunrise) lasts 176 Earth days, meaning a "day" on Mercury is twice as long as its year. This bizarre mismatch occurs because Mercury rotates very slowly on its axis (once every 58.65 Earth days) while orbiting the Sun quickly.
Mercury's orbit is the most eccentric of any planet, with an eccentricity of 0.2056. This means its distance from the Sun varies dramatically — from 46.0 million km at perihelion to 69.8 million km at aphelion. At its closest to the Sun, Mercury receives about 2.3 times more solar radiation than at its farthest. Mercury's orbital inclination of 7 degrees is also the highest of any planet, tilting its orbital plane well above the ecliptic. Meanwhile, its axial tilt is essentially zero (0.034 degrees) — the smallest in the solar system — meaning Mercury has no seasons whatsoever.
Mercury's Extreme Temperatures
Mercury holds the record for the most extreme temperature range of any planet in the solar system. The Sun-facing side can soar to 430 °C (806 °F) — hot enough to melt lead, tin, and zinc. Yet the night side plunges to −180 °C (−292 °F), cold enough to freeze most gases. That is a swing of approximately 610 °C (1,100 °F) between day and night, dwarfing the temperature variation on any other world.
A common misconception is that Mercury, as the closest planet to the Sun, must be the hottest. In reality, Venus is hotter (average surface temperature of 465 °C) because its thick CO&sub2; atmosphere traps heat through a runaway greenhouse effect. Mercury has virtually no atmosphere, so it cannot retain heat — energy from the Sun radiates back into space almost immediately once the surface rotates into shadow. Another widespread myth is that Mercury is tidally locked, always showing one face to the Sun. Radar observations in 1965 disproved this, revealing that Mercury is in a 3:2 spin-orbit resonance: it rotates exactly three times for every two orbits around the Sun. This unique resonance means every point on Mercury's surface eventually experiences both blazing sunlight and deep-space cold.
Physical Features of Mercury
Mercury's surface is dominated by impact craters, making it resemble Earth's Moon at first glance. The most prominent feature is the Caloris Basin, one of the largest impact craters in the solar system. At approximately 1,550 km (960 miles) in diameter, Caloris is so enormous that it could swallow the entire state of Texas with room to spare. The impact that created it was so violent that it sent shockwaves through the planet's interior, creating a region of chaotic, jumbled terrain on the exact opposite side of Mercury known as the "Weird Terrain" or Chaotic Terrain.
One of Mercury's most distinctive features is its network of lobate scarps — enormous cliff-like ridges that stretch for hundreds of kilometers across the surface. Some of these scarps rise over 3 km above the surrounding plains. Scientists believe they formed as Mercury's interior cooled and contracted over billions of years, causing the crust to wrinkle and buckle like the skin of a drying apple. MESSENGER data revealed that Mercury has shrunk by about 7 km in radius since its formation — far more than previously estimated. These scarps cut across craters of all ages, indicating that Mercury may still be contracting today.
Despite its airless, Sun-scorched nature, Mercury harbors a surprising secret: water ice in permanently shadowed craters near its poles. Because Mercury's axial tilt is nearly zero, the floors of deep craters at high latitudes never see sunlight. Temperatures in these eternal shadows drop below −170 °C, cold enough to preserve water ice for billions of years. Both Earth-based radar observations and MESSENGER's neutron spectrometer confirmed the presence of ice deposits, estimated at 100 billion to 1 trillion metric tons of frozen water.
Mercury's Oversized Iron Core
Mercury's interior is unlike any other planet in the solar system. Its iron core occupies about 85% of the planet's radius — proportionally far larger than any other rocky planet. For comparison, Earth's core occupies about 55% of its radius. Mercury's core is roughly 2,020 km in radius, leaving only a thin mantle (roughly 400 km thick) and an even thinner crust (about 35 km). If Mercury were the size of a baseball, the core would be the size of a softball crammed inside it.
Why Mercury has such a disproportionately large core remains one of planetary science's open questions. The leading hypothesis is that a giant impact early in Mercury's history blasted away much of its original mantle and crust, leaving behind the dense iron interior. Alternative theories suggest that intense solar radiation early in the solar system's life vaporized Mercury's lighter outer layers, or that the solar nebula's high-temperature environment near the young Sun simply favored iron-rich material during Mercury's formation.
Despite its small size, Mercury's partially molten iron core generates a weak but detectable magnetic field — about 1.1% the strength of Earth's. Mercury is the only other rocky planet besides Earth with a global magnetic field generated by an active dynamo. This field is strong enough to deflect the solar wind and create a small magnetosphere, though it provides far less shielding than Earth's. The magnetic field is offset significantly toward the north pole, a peculiarity that scientists are still working to explain.
Mercury's Thin Exosphere
Mercury does not have a true atmosphere in any conventional sense. Instead, it possesses an extremely thin exosphere — a tenuous collection of atoms that barely clings to the planet's surface. The surface pressure is approximately 10¹&sup5; times less than Earth's sea-level pressure, essentially indistinguishable from a laboratory vacuum. Atoms in Mercury's exosphere are so sparse that they are more likely to escape into space or fall back to the surface than to collide with each other.
This exosphere is constantly being replenished by three processes: solar wind sputtering (charged particles from the Sun knock atoms off the surface), micrometeorite impacts (tiny space rocks vaporize surface material on contact), and photon-stimulated desorption (ultraviolet sunlight energizes surface atoms enough to escape). The exosphere contains traces of oxygen, sodium, hydrogen, helium, potassium, calcium, and magnesium. Sodium and potassium atoms form a visible tail that extends away from the Sun — essentially a faint comet-like feature that can be observed through telescopes from Earth. Without a real atmosphere, Mercury has no weather, no wind, no erosion from air, and no protection from the relentless bombardment of solar radiation and micrometeorites.
Exploring Mercury: MESSENGER and BepiColombo
Mercury is one of the least-explored planets, largely because reaching it is extraordinarily difficult. Counterintuitively, it takes more energy to send a spacecraft to Mercury than to Pluto. A craft falling toward the inner solar system gains tremendous speed from the Sun's gravity and must brake hard to enter Mercury's orbit rather than flying past. Only two missions have visited Mercury. Mariner 10 (1974–1975) was the first, completing three flybys and mapping about 45% of Mercury's surface. It discovered the planet's magnetic field and confirmed the extreme temperature variations.
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) launched in 2004 and entered Mercury orbit in 2011 — the first spacecraft to do so. Over four years, MESSENGER mapped the entire planet in high resolution, confirmed water ice in polar craters, measured the magnetic field in detail, and discovered that Mercury's surface contains surprisingly high levels of volatile elements like sulfur and potassium. MESSENGER ended its mission in April 2015, deliberately crashing into Mercury's surface at over 14,000 km/h and creating a new 16-meter crater.
The next chapter in Mercury exploration is BepiColombo, a joint ESA–JAXA mission launched in October 2018. BepiColombo is currently on a complex seven-year journey involving nine gravity assists (one at Earth, two at Venus, and six at Mercury itself) and is scheduled to enter Mercury orbit in November 2026. It carries two orbiters: the Mercury Planetary Orbiter (ESA) will study the surface and interior, while the Mercury Magnetospheric Orbiter (JAXA) will investigate the magnetic field and magnetosphere. Together, they aim to answer the biggest remaining questions about this small but fascinating world.
Frequently Asked Questions About Mercury
▶What is the size of Mercury?
Mercury has an equatorial diameter of 4,879 km (3,032 miles), making it the smallest planet in our solar system. Its total surface area is 74.8 million km² — roughly twice the area of Africa. Mercury is only about 38% the diameter of Earth and just slightly larger than Earth’s Moon (3,475 km).
▶How big is Mercury compared to Earth?
Mercury is far smaller than Earth in every dimension. Its diameter is 38% of Earth’s, its mass is just 5.5% of Earth’s, and its surface gravity is 38% of Earth’s — coincidentally the same ratio as Mars. You could fit about 18 Mercury-sized spheres inside Earth’s volume. A 70 kg person would weigh only 26.6 kg on Mercury.
▶What is the diameter of Mercury in km?
The equatorial diameter of Mercury is 4,879.4 km (3,032 miles). For comparison, Earth’s equatorial diameter is 12,756 km — about 2.6 times wider than Mercury. Mercury is so small that two of Jupiter’s moons (Ganymede and Callisto) are actually larger than it.
▶Why is Mercury so hot and so cold?
Mercury experiences the most extreme temperature swings of any planet because it has virtually no atmosphere to retain heat. The sun-facing side can reach 430 °C (806 °F), hot enough to melt lead. But without an atmosphere to trap warmth, the night side plunges to −180 °C (−292 °F). This 610 °C range is the largest day-night temperature swing in the solar system.
▶What are the main physical features of Mercury?
Mercury’s major physical features include the Caloris Basin (a 1,550 km wide impact crater — one of the largest in the solar system), extensive lobate scarps (cliff-like ridges up to 3 km tall formed as the planet shrank), a heavily cratered surface resembling Earth’s Moon, and an enormous iron core that makes up about 85% of the planet’s radius.
▶Does Mercury have an atmosphere?
Mercury does not have a true atmosphere. Instead, it has an extremely thin exosphere — a tenuous envelope of atoms blasted off the surface by solar wind, micrometeorite impacts, and solar radiation. This exosphere contains traces of oxygen, sodium, hydrogen, helium, and potassium, but its total pressure is less than one-trillionth of Earth’s atmospheric pressure. It provides no insulation, no weather, and no protection from the Sun’s radiation.
Explore More
Want to test your knowledge of Mercury and the other planets? Our Solar System games challenge you to sort planets by properties, identify worlds from clues, and explore the inner and outer solar system. Mercury is just the beginning.
Sources
- NASA Planetary Fact Sheet. "Mercury Fact Sheet." nssdc.gsfc.nasa.gov.
- NASA Science. "Mercury: Facts." science.nasa.gov/mercury/facts.
- Johns Hopkins APL. "MESSENGER Mission to Mercury." messenger.jhuapl.edu.
- ESA Science & Exploration. "BepiColombo." esa.int/Science_Exploration/Space_Science/BepiColombo.
- Hauck, S. A. et al. (2013). "The Curious Case of Mercury's Internal Structure." Journal of Geophysical Research: Planets.