Mars, often called the Red Planet, stands out in our solar system as the planetary body that shares the most physical characteristics with Earth. Although Mars is smaller, colder, and has a much thinner atmosphere, its fundamental physical mechanics and geological history echo our own world, revealing how similar initial conditions can lead to profoundly different evolutionary paths. By examining these similarities, scientists gain deeper insight into the processes that shape terrestrial planets and the potential conditions for past or present habitability beyond Earth.
How Time and Climate Are Governed
The rotational mechanics of Mars closely mirror those of Earth. A single rotation on Mars, known as a sol, takes approximately 24.6 hours, making a Martian day nearly the same length as an Earth day, which is 23.9 hours. This near-identical duration is a foundational similarity, directly impacting any future human presence by providing a familiar light-dark cycle.
The planet’s seasons are also governed by a similar physical mechanism to Earth’s: axial tilt. Mars is tilted on its axis by about 25 degrees relative to its orbital plane, remarkably close to Earth’s tilt of 23.4 degrees. As Mars orbits the Sun, this results in different hemispheres receiving varying amounts of direct sunlight throughout the Martian year, causing distinct seasonal changes.
However, the Martian year is nearly twice as long as Earth’s, meaning each of its seasons lasts for an extended period. Because Mars has a more elliptical orbit than Earth, its distance from the Sun varies more significantly, which affects the intensity and duration of its seasons. For example, the southern hemisphere experiences shorter, warmer summers and longer, colder winters, due to its proximity to the Sun during its summer and distance during its winter.
Shared Geological Structure and Surface Features
Mars exhibits a layered internal structure similar to Earth’s, composed of a core, mantle, and crust. The Martian crust is primarily made of basaltic rock, which is a common volcanic rock on Earth. Geophysical measurements suggest Mars likely has a solid inner core and a liquid outer core, although the planet’s smaller mass meant it cooled and lost its global magnetic field much earlier than Earth.
The surface of Mars is covered in massive geological formations that are structurally analogous to Earth’s largest features. The planet hosts enormous shield volcanoes, such as Olympus Mons, which is the largest volcano in the solar system and structurally similar to massive shield volcanoes found on Earth, like those in Hawaii. These Martian volcanoes grew so large because the planet lacks the active plate tectonics that redistribute magma sources and prevent the buildup of single, massive structures on Earth.
Another spectacular comparison is the Valles Marineris, a massive canyon system that spans over 4,000 kilometers, a length comparable to the continental United States. While the formation mechanisms differ—Valles Marineris likely formed through a combination of crustal extension, faulting, and water erosion—its structure provides a terrestrial-like feature on a grander scale. Evidence of past volcanic activity also has striking parallels on the Martian plains, confirming a shared history of igneous processes.
The Presence and History of Water
The most compelling similarity between early Earth and ancient Mars is the history of liquid water on the surface. Orbiters and rovers have identified numerous geomorphic features that appear to have been shaped by flowing water billions of years ago. These include vast, dried-up river valleys, dendritic networks of channels that resemble terrestrial river systems, and sedimentary layers consistent with ancient lakebeds and deltas.
Chemical analysis of Martian rock by rovers has confirmed the presence of hydrated minerals, such as clays and sulfates, which only form in the sustained presence of water. These findings suggest that Mars once possessed a warmer, wetter environment, potentially supporting a thicker atmosphere that allowed liquid water to be stable on the surface. This hydrological history indicates that conditions on early Mars may have resembled those on early Earth, making it a prime target in the search for past microbial life.
Today, the water on Mars primarily exists in frozen forms, trapped in its polar ice caps and beneath the surface as permafrost. Furthermore, dark streaks called recurring slope lineae, which appear on steep slopes during warmer seasons, suggest the intermittent flow of contemporary, salty liquid water. This evidence confirms that water, though mostly frozen, remains an important component of the Martian system, connecting its deep past to its present state.