How Does Earth Compare to Mercury?

Venturing through the cosmos, we often compare our Earth with its neighboring celestial bodies.

Among these is Mercury, a world that fascinates and puzzles astronomers and scientists alike. In this exploration, we will compare Earth—our lush, blue home—to Mercury, the smallest planet and innermost planet of the solar system.

Despite their differences, they share the same classification as terrestrial planets, offering a unique perspective on how varied planet profiles can be within our solar system.

Earth: A Closer Look

As the third planet from the sun, Earth is unique among the terrestrial planets for its liquid water ice, rich atmosphere, and abundant life.

It’s not the closest planet to the sun, a title held by Mercury, nor the hottest planet—thanks to Venus and its runaway greenhouse effect.

Yet, Earth’s average distance from the sun, at one astronomical unit, places it perfectly within the habitable zone, allowing for a stable space environment that fosters life.

Earth hospitable to life

Size and Structure

Earth, being significantly larger than Mercury, has a diameter of about 12,742 kilometers (7,918 miles), making it the largest of the inner planets after Venus.

Its substantial size contributes to a strong magnetic field and a complex internal structure, including a large iron core.

Related: How long would it take to reach Mercury?

Layers of Earth

  • Lithosphere: Earth’s solid surface, predominantly composed of silicate minerals.
  • Asthenosphere: Below the lithosphere, a semi-ductile region important for tectonic movements.
  • Outer Core: A liquid layer responsible for Earth’s magnetic field.
  • Inner Core: A solid center with temperatures rivaling those on the surface of the sun.

Atmosphere Composition

Earth’s atmosphere is rich and life-sustaining, starkly contrasting to Mercury’s virtually nonexistent atmosphere. Composed mainly of nitrogen (78%) and oxygen (21%), with traces of carbon dioxide and other gases, it supports a complex climate system.

Atmospheric Layers

  • Troposphere: The layer where we live and where weather occurs.
  • Stratosphere: Includes the ozone layer, which protects life from harmful ultraviolet rays.
  • Mesosphere: A region where many meteors burn up upon entry.
  • Thermosphere: Characterized by high temperatures and the auroras.
  • Exosphere: The outermost layer, where the atmosphere thins into space.

Mercury: An Overview

Mercury, often visible to the naked eye just before sunrise or after sunset, is a tiny planet that speeds around the sun faster than any other planet in our solar system.

Size and Composition

Mercury has a modest diameter of about 4,880 kilometers (3,032 miles), making it the smallest planet in our solar system and slightly larger than Earth’s Moon.

Despite its small size, Mercury’s mass is significant due to its dense composition, which includes a large iron core.

Mercury inner structure



Mercury’s surface is solid and heavily cratered, much like the moon, with numerous impact craters dotting its rocky surface.

The volume of Mercury is such that you could fit approximately 18 Mercury-sized planets into the Earth.

Atmosphere and Climate

Mercury’s atmosphere, or lack thereof, is composed of a thin atmosphere of trace gases, including oxygen, sodium, and hydrogen, which are constantly replenished by the solar wind.

Due to the lack of a significant atmosphere, there is a stark contrast in temperature between day and night. Daytime temperatures can soar to 430 degrees Celsius (800 degrees Fahrenheit), while nighttime can plummet to minus 180 degrees Celsius (minus 290 degrees Fahrenheit).

Geological Features

Caloris Basin, one of the largest impact basins in the solar system, is a major geological feature on Mercury’s surface.

Mercury’s core is disproportionately large for its size, making up a significant portion of the planet’s volume.

Fault scarps and rocky worlds give evidence of historical volcanic activity and geological dynamism.

Mercury’s surface also shows signs of contraction, or shrinkage, over time, likely due to the cooling and solidification of its large iron core.

Mercury's Caloris Basin
This mosaic of Caloris basin is an enhanced-color composite overlain on a monochrome mosaic featured in a previous post. The color mosaic is made up of WAC images obtained when both the spacecraft and the Sun were overhead, conditions best for discerning variations in albedo, or brightness.


Orbit and Rotation

Mercury orbits the sun once every 88 Earth days, making its year significantly shorter than an Earth year.

Its rotation period is quite slow compared to its orbit; Mercury takes 59 Earth days to complete one rotation on its axis, which leads to very long days and long nights.

The eccentric orbit of Mercury results in a unique phenomenon where the sun appears to rise, then briefly sets, and then rises again in some parts of the planet’s surface.

Comparing Earth and Mercury

From their sizes and compositions to their atmospheres and surface temperatures, let’s explore how these two terrestrial planets differ and what they might have in common.

How Big is Mercury Compared to Earth?

  • Diameter: Earth boasts a diameter of about 12,742 kilometers (7,918 miles), whereas Mercury has a diameter of approximately 4,880 kilometers (3,032 miles). This makes Mercury roughly 38% the size of Earth.
  • Mass: Earth has a mass of around six sextillion tons, significantly heavier than Mercury, which has a mass that’s about 5.5% of Earth’s. Mercury’s mass does not reflect its volume because of its dense composition.


Composition Differences

  • Inner Core: Both Earth and Mercury have a metallic core, with Mercury’s core being relatively larger compared to its size. Earth’s core is mainly iron, but it’s proportionally smaller.
  • Surface: Earth’s surface is dynamic and hospitable, supporting life and water in liquid form. Mercury’s surface, on the other hand, is barren, hosting numerous impact craters and no evidence of current volcanic activity or a significant atmosphere.
  • Terrestrial Features: Both planets are part of the terrestrial planets group, having solid surfaces composed of silicate minerals. However, Mercury’s proximity to the sun has led to extreme surface conditions, unlike Earth’s.

Atmospheric Differences

  • Composition: Earth’s atmosphere is a rich blend of nitrogen, oxygen, and other gases, sustaining life and water cycles. Mercury, contrastingly, has a very thin atmosphere, composed mostly of atoms blasted off its surface by the solar wind.
  • Climate: The presence of an atmosphere on Earth leads to a stable climate system with a greenhouse effect, while Mercury’s lack of an atmosphere results in dramatic temperature fluctuations.

By understanding these comparisons, we gain a greater appreciation for the unique characteristics that make Earth a cradle for life and Mercury an intriguing subject for scientific study.


Let’s explore some common questions about Mercury in comparison to Earth.

Can Mercury support life?

Currently, Mercury’s extreme temperatures and lack of atmosphere make it inhospitable for life as we know it.

How is Mercury different than Earth?

Mercury is smaller, has a harsher climate, no significant atmosphere, and experiences greater temperature extremes than Earth.