Mars, often called the Red Planet, has captivated humanity for centuries. Despite its prominent place in our solar system, Mars is surprisingly small, significantly less massive than Earth. This disparity in size prompts a fundamental question: why is Mars so diminutive? Understanding its compact stature provides insights into the processes that shaped our solar system’s early history.
Mars’s Unique Stature
Mars has a diameter of approximately 6,779 kilometers, roughly half the size of Earth (12,742 kilometers). Its mass is only about 10.7% that of Earth’s. While Earth, Venus, and Mercury are terrestrial planets, Mars is notably smaller within this group. As the fourth planet from the Sun, orbiting beyond Earth, its position played a significant role in its developmental trajectory.
Planetary Origins
Planets, including Mars, form from a vast disk of gas and dust known as a protoplanetary disk. Within this disk, microscopic dust particles collide and stick together, forming larger aggregates. This process, known as accretion, leads to kilometer-sized planetesimals. These planetesimals grow by attracting more material, accumulating into larger protoplanets. Over millions of years, these protoplanets sweep up debris, evolving into the planets we observe today.
Jupiter’s Dominance
A leading hypothesis for Mars’s stunted growth centers on Jupiter’s gravitational influence, the solar system’s most massive planet. Early in the solar system’s history, Jupiter underwent an inward and then outward migration, often referred to as the “Grand Tack” model. As Jupiter moved closer to the Sun, its immense gravity disrupted planetesimal distribution in the inner solar system, scattering much available building material. This migration cleared out much of the protoplanetary disk where Mars was forming.
After its inward journey, Jupiter migrated back outward, further perturbing the nascent asteroid belt and material between its orbit and the Sun. This dynamic movement “starved” the Martian region of raw materials for growth. Consequently, Mars had a limited supply of planetesimals to accrete, preventing it from reaching a size comparable to Earth or Venus. The timing and extent of Jupiter’s migration appear to be a primary factor in Mars’s relatively small final mass.
Impact of Its Scale
Mars’s smaller size has profound implications for its geological and atmospheric evolution. A smaller planetary body possesses a greater surface area to volume ratio, leading to more rapid heat loss from its interior. This faster cooling caused Mars’s molten core to solidify much earlier than Earth’s. The cessation of convection within its core meant the planet lost the dynamic processes that generate a global magnetic field, which shields against the solar wind.
Without a protective magnetic field, the solar wind could directly interact with and strip away Mars’s atmosphere over billions of years. This atmospheric loss led to a decrease in surface pressure and temperature, preventing liquid water from remaining stable on its surface. The planet’s current cold, dry state and limited habitability are direct consequences of its initial, smaller scale.