Mars—Beyond the Red Horizon (14): Chapter 3 Basic Properties of Mars and Earth Analogies (1)
This chapter is not merely an enumeration of physical data. The question posed here is to what extent Mars is continuous with Earth, and where it diverges. To understand Mars is nothing less than to understand the structure of its similarities to and differences from Earth.
3.1 Planetary Size and Internal Structure—The Fate of Mass Difference
The radius of Mars is approximately 3,390 kilometers. This is about 53 percent of Earth's radius (approximately 6,371 kilometers). Its volume is about 15 percent of Earth's, and its mass is a mere 11 percent (precisely 6.4×10²³ kilograms, compared to Earth's 5.97×10²⁴ kilograms).
This mass difference is not trivial. It determined the fate of Mars.
A planet's internal structure depends heavily on its mass. When a planet is formed, the collision of planetesimals releases enormous energy, leaving the entire planet in a molten state—a magma ocean. Subsequently, heavy elements (iron, nickel) sink to the center to form a core. Lighter elements (silicon, oxygen) form the mantle.
Like Earth, Mars also possesses a layered structure consisting of an iron-based core, a silicate mantle, and a crust. The InSight lander, which touched down in 2018, observed Martian seismic activity (marsquakes) and estimated the internal structure. The results showed that the radius of the Martian core is approximately 1,830 kilometers (about 54 percent of the Martian radius), which is nearly the same ratio as Earth's core (radius of approximately 3,480 kilometers, about 55 percent of Earth's radius).
However, there is a decisive difference. That is the cooling rate.
For a planet to cool means that its internal heat is released from the surface into outer space. A planet's heat capacity is proportional to its volume, while its surface area is proportional to the square of its radius. In other words, the smaller the planet, the larger the ratio of surface area to volume, and the easier it is to cool.
Mars is smaller than Earth. Therefore, Mars cooled faster than Earth.
And this difference in cooling rate became the divergence point between Mars and Earth.
3.2 Loss of Magnetic Field—Loss of Protection
Earth still possesses a magnetic field today. This magnetic field is generated by the convective motion of Earth's core—the dynamo effect. Earth's core consists of a solid inner core and a liquid outer core. In the liquid outer core, iron convects, generating electric currents and forming a magnetic field.
This magnetic field protects Earth from the solar wind. The solar wind is a stream of high-energy charged particles (protons, electrons) emitted from the Sun. If there were no magnetic field, the solar wind would strike the atmosphere directly, stripping away light elements (hydrogen, helium, oxygen) into outer space.
Mars also once had a magnetic field.
Local magnetic anomalies remain in the Martian crust. In particular, the ancient crust of the southern hemisphere records strong magnetization. This is evidence that Mars once possessed a global magnetic field.
But today, Mars has no global magnetic field.
The time when the magnetic field disappeared is estimated to be about 4 billion years ago. Why did it disappear? It is because the Martian core cooled and convection ceased. Small Mars cooled faster than Earth. Once the core began to solidify, the dynamo effect could no longer be maintained.
The loss of the magnetic field was fatal for Mars. It meant the loss of the shield that protected its atmosphere.
3.3 Atmospheric Escape—To a Planet That Cannot Breathe
Earth's atmospheric pressure at sea level is approximately 101.3 kilopascals (1 atm). The average atmospheric pressure on Mars is approximately 0.6 kilopascals. It is a mere 0.6 percent of Earth's.
The Martian atmospheric composition is about 95 percent carbon dioxide, about 2.7 percent nitrogen, and about 1.6 percent argon. Oxygen is a mere 0.13 percent.
This thin atmosphere is the result of Mars's inability to retain its atmosphere.
Whether a planet can retain an atmosphere depends on the relationship between the planet's gravity and the thermal velocity of atmospheric molecules. Light molecules (hydrogen, helium) move at high speeds, and if they exceed the planet's escape velocity, they escape into outer space. Heavy molecules (carbon dioxide, nitrogen) are relatively easier to retain.
The escape velocity of Mars is approximately 5 kilometers per second. This is about half of Earth's escape velocity (approximately 11.2 kilometers per second). Mars is less capable of retaining an atmosphere than Earth.
But that is not all. The loss of the magnetic field accelerated atmospheric escape.
The MAVEN (Mars Atmosphere and Volatile Evolution) probe, which was placed into Martian orbit in 2013, observed how the Martian atmosphere is being lost to outer space. The solar wind strikes the Martian atmosphere directly, carrying ionized atmospheric molecules away into outer space.
According to MAVEN's observations, Mars continues to lose about 100 grams of atmosphere per second even today. That is about 3 tons per year. While this is a negligible amount compared to the current thin atmosphere, it becomes a massive amount on the scale of billions of years.
Mars once had a much thicker atmosphere. It is estimated that the atmospheric pressure of early Mars may have been tens to hundreds of times higher than it is today. But having lost its magnetic field, Mars was defenseless against atmospheric stripping by the solar wind and gradually lost its atmosphere.
As a result, Mars became a planet that cannot breathe.
(Published March 5, 2026)