Earth and Mars share a common origin in the solar system but exhibit a dramatic difference in scale. Understanding the physical relationship between the two worlds requires analyzing their linear dimensions, total mass, and the long-term planetary consequences of those metrics. This comparison provides insight into the fundamental requirements for a world to retain an atmosphere and sustain geological activity over billions of years.
Comparing Earth and Mars: Linear Size and Volume
Earth is significantly larger than Mars, a fact quantified by examining their diameters and volumes. Earth’s diameter is approximately 12,742 kilometers, while Mars’s diameter is roughly 6,779 kilometers, meaning the Red Planet is just over half the width of our home world. This linear difference is magnified dramatically when comparing the total space each planet occupies.
The volume of a planet scales with the cube of the radius, meaning a small difference in diameter results in a massive difference in capacity. Mars possesses only about 15% of Earth’s total volume; it would take about nine Marses to equal the volume of a single Earth. Although Earth’s surface area is more than twice that of Mars, the Red Planet’s surface area is comparable to the total dry land area of Earth due to the vast oceans covering 70% of our planet.
Difference in Planetary Mass and Gravitational Pull
The size disparity is compounded by a profound difference in mass, a measurement that incorporates both size and density. Earth is approximately ten times more massive than Mars. This massive difference is partly due to Mars having a lower average density, primarily because its core makes up a smaller fraction of its total size compared to Earth’s internal structure.
The total mass of a planet directly determines its gravitational pull. The surface gravity on Mars is only about 38% of Earth’s gravity, meaning an object weighing 100 pounds on Earth would only weigh 38 pounds on the Martian surface. This substantial reduction in gravity plays a role in the planet’s ability to retain its atmosphere.
The Consequences of Size: Atmosphere and Geological Activity
The smaller size and mass of Mars initiated a chain of planetary evolution that resulted in its current desolate state. A smaller planetary body possesses a higher surface area-to-volume ratio, causing it to lose its primordial internal heat much faster than a larger world like Earth. This rapid heat loss caused the molten iron core of Mars to cool prematurely, which led to the cessation of the planet’s core dynamo.
The shutdown of the dynamo, which occurred relatively early in Mars’s history, meant the loss of a global magnetic field, or magnetosphere, which acts as a shield against solar wind. Without this protective magnetic shield, charged particles from the sun were able to interact directly with the Martian atmosphere. Over billions of years, this solar wind interaction stripped away the atmosphere, leaving Mars with an atmosphere that has a surface pressure less than one percent of Earth’s. The low gravity and lack of atmospheric protection prevented liquid water from remaining stable on the surface, fundamentally altering Mars’s potential for sustained habitability.