Mercury: Solar System’s Smallest Planet

Mercury, the smallest and closest planet to the Sun in our Solar System, undergoes the most significant temperature changes among all planets due to its minimal atmosphere, which fails to trap heat effectively. Learn in detail about the planet mercury in this article.


  • Mercury, the closest planet to the Sun, orbits at an average distance of 35 million miles, with Earth trailing at 48 million miles.
  • Despite its small size, Mercury’s surface is marked by numerous impact craters, resembling Earth’s Moon.
  • Extreme temperatures on Mercury fluctuate drastically, ranging from 800°F (430°C) to -290°F (-180°C).

Formation of Mercury

  • Formed around 4.5 billion years ago from swirling gas and dust under gravitational pull, Mercury developed a rocky core, mantle, and crust like other terrestrial planets.

Physical Characteristics

  • Mercury’s diameter is 4,880 kilometers, about the size of a celestial pebble compared to Earth.
  • Its mass is approximately 0.055 times that of Earth, contributing to its second-densest status among planets.
  • Impact craters dot Mercury’s surface, revealing its vulnerability to cosmic collisions.
Position of Mercury
Position of Mercury

Orbit and Rotation

  • Mercury completes an orbit around the Sun every 88 Earth days, making it the fastest planet in the solar system.
  • Its rotation on its axis takes 58.7 Earth days, resulting in a day-night cycle longer than its year.
  • Mercury’s distance from the Sun varies from 29 million to 43 million miles due to its highly elliptical orbit.

Extreme Temperatures

  • Surface temperatures range from 100 K to 700 K (-173 °C to 427 °C), with highs of 800°F (427 °C) and lows of -269 °F (-173 °C).
  • Mercury’s lack of atmosphere causes the most extreme temperature swings in the solar system.

Geological Evolution

  • Over billions of years, Mercury has gradually shrunk, losing about 9 miles in diameter as its iron core cools.
  • Its surface bears the scars of numerous meteor impacts, resembling Earth’s Moon.

Impact Craters

  • The Caloris Basin, a massive crater spanning 1,550 km in diameter, is a reminder of Mercury’s violent past.
  • This ancient scar formed over 3.8 billion years ago and could engulf part of Europe.
Surface of Mercury
Surface of Mercury

Mercury’s Orbit

  • Mercury’s proximity to the Sun shortens its orbit to just 88 Earth days.
  • Named after the Roman god of commerce and communication, Mercury’s orbit mirrors its mythological namesake’s agility.


  • Mercury’s atmosphere, an ultra-thin exosphere, consists mainly of oxygen, sodium, hydrogen, helium, and potassium.
  • According to NASA, its composition includes 42% oxygen, 29% sodium, 22% hydrogen, 6% helium, and 0.5% potassium, along with potentially small quantities of argon, carbon dioxide, water, nitrogen, xenon, krypton, and neon.
  • Its atmosphere is minimal and fleeting due to atoms blasted off its surface by solar radiation and micrometeoroid impacts.

Temperature Fluctuations

  • Without an atmosphere to regulate heat, Mercury experiences extreme temperature changes, ranging from scorching days to freezing nights.

Facts about Mercury

Size0.38 Earths
Diameter4,880 kilometers
Mass3.3011 × 10^23 kg (0.055 Earths)
Aphelion (Farthest from Sun)0.466 Astronomical Units (1 AU = distance to Sun)
Perihelion (Closest to Sun)0.3 Astronomical Units
Gravity0.38 times Earth’s gravity
Orbital Period (1 Year)88 Earth days
Rotational Period (1 Day)58.7 Earth days
Surface Temperature100 K to 700 K (−173 °C to 427 °C)
Atmosphere Composition42% oxygen, 29.0% sodium, 22.0% hydrogen, 6.0% helium, 0.5% potassium, traces of other elements
Surface FeaturesHeavily cratered terrain, scarps, ridges
Geological ActivityTectonic activity, volcanic features
Magnetic FieldWeak magnetic field
Factsheet of Mercury


  • Mariner 10, launched in 1973, provided the first close-up images of Mercury, confirming its lack of atmosphere.
  • MESSENGER, launched in 2004, orbited Mercury from 2011 to 2015, uncovering evidence of water ice in polar craters and revealing its complex geological features.

Historical and Mythological Significance

  • Named after the Roman god Mercury, the planet has held a significant place in mythology and astronomy, symbolizing speed and agility.
  • Known by various names across cultures, Mercury’s celestial dance has inspired awe for millennia.

Modern Discoveries

  • Recent findings from MESSENGER revealed a vast valley over 620 miles long, shedding light on Mercury’s geological history.
  • Ongoing studies continue to unravel its mysteries, providing new insights into the dynamics of our solar system.
  • Harsh environment presents significant challenges for human exploration, requiring robotic missions and advanced technologies.

Frequently Asked Questions

Q: What is Mercury?

A: Mercury is the smallest and innermost planet in our solar system, situated closest to the Sun.

Q: How Far is Mercury from the Sun?

A: On average, Mercury orbits about 35.98 million miles (57.91 million kilometers) from the Sun.

Q: How does Mercury’s Size Compare to Earth?

A: Mercury is much smaller than Earth, with a diameter of approximately 4,880 kilometers (3,032 miles), slightly larger than Earth’s Moon.

Q: How Long is a Day on Mercury?

A: A day on Mercury (one rotation on its axis) takes about 59 Earth days.

Q: What is Mercury’s Atmosphere Like?

A: Mercury has a very thin atmosphere, known as an exosphere, composed primarily of oxygen, sodium, hydrogen, helium, and potassium.

Q: Does Mercury Have Moons?

A: No, Mercury does not have any moons orbiting around it.

Q: What Causes Mercury’s Extreme Temperature Fluctuations?

A: Lack of atmosphere results in extreme temperature variations, with daytime temperatures reaching up to 800°F (430°C) and nighttime temperatures dropping to -290°F (-180°C).

Q: How Fast Does Mercury Orbit the Sun?

A: Mercury orbits the Sun at an average speed of about 47.87 kilometers per second (29.66 miles per second), making it the fastest planet in our solar system.

Q: What is the Surface of Mercury Like?

A: Mercury’s surface is heavily cratered, resembling Earth’s Moon, due to impacts from meteoroids and comets over billions of years.

Q: How Many Spacecrafts Have Explored Mercury?

A: Only two spacecraft have been sent to study Mercury: Mariner 10 in 1973 and MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) in 2004.

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Binod G C

I'm Binod G C (MSc), a PhD candidate in cell and molecular biology who works as a biology educator and enjoys scientific blogging. My proclivity for blogging is intended to make notes and study materials more accessible to students.

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