Mars is the most Earth-like world in our solar system and has long been a focus of scientific inquiry. Studying Mars allows scientists to engage in comparative planetology, gaining insights into the conditions that allow a planet to sustain liquid water and, potentially, life. This exploration focuses on the striking similarities in Mars’s day-night cycle, its capacity to hold water, and its large-scale geological structures.
Shared Orbital and Rotational Characteristics
The most immediate similarity between Mars and Earth is the length of their rotational periods, which governs the day-night cycle. A Martian solar day, known as a “sol,” lasts approximately 24 hours, 39 minutes, and 35 seconds, making it only about 40 minutes longer than an Earth day. This rotational rhythm is closer to our own than any other planet in the solar system.
The planet’s axial tilt, or obliquity, dictates the presence of seasons. Mars is tilted on its axis by about 25 degrees, a value close to Earth’s 23.4-degree tilt. This orientation means that, like Earth, Mars experiences four distinct seasons as it orbits the Sun. Because Mars has a longer orbit (687 Earth days) and a more elliptical path, Martian seasons are significantly longer and vary more in intensity than those on Earth.
Evidence of Past and Present Water
The history of water on Mars provides the strongest evidence for past Earth-like conditions and potential habitability. Ancient Mars, particularly during the Noachian period (4.5 to 3.5 billion years ago), possessed conditions that allowed vast quantities of liquid water to flow across its surface. Evidence for this past is visible in enormous outflow channels, extensive river valley networks, and sedimentary structures resembling lakebeds and deltas on Earth.
The morphology of these ancient Martian channels often shows dendritic, or branch-like, patterns characteristic of river deltas formed by sustained water erosion. Rovers have also identified mineral deposits, such as hydrated sulfates and clays, which require the presence of liquid water to form. While water has largely disappeared from the surface due to atmospheric loss, research suggests liquid water may have persisted in some form until about 2 billion years ago.
Today, water on Mars is primarily locked away in solid form, most notably in the massive polar ice caps and as subsurface permafrost. These ice caps expand and contract seasonally, mirroring the cycles of ice and snow on Earth. Significant quantities of ground ice have been detected just beneath the surface. While the thin atmosphere prevents stable liquid water on the surface now, evidence suggests intermittent flows of briny (salty) water may occur down steep slopes during warmer seasons.
Comparable Surface Geology and Features
Despite being only about half the diameter of Earth, Mars possesses a total surface area nearly equal to Earth’s dry land area. The planet’s surface features spectacular geological structures comparable to those found on Earth, including massive shield volcanoes and extensive canyon systems.
The most famous example is Olympus Mons, the largest volcano in the solar system, which stands about three times taller than Earth’s Mount Everest. The Valles Marineris canyon system stretches over 4,000 kilometers, making it ten times longer and much deeper than Earth’s Grand Canyon. These colossal structures formed differently than on Earth, which has active plate tectonics, but they represent similar large-scale structural processes.
The Martian crust shows similarities to Earth’s composition. Mars is a terrestrial planet with a differentiated structure, possessing a metallic core, a silicate mantle, and a crust. The Martian crust is largely basaltic, a volcanic rock that is the primary component of Earth’s oceanic crust. The planet’s distinctive reddish hue comes from the widespread oxidation, or rusting, of iron-rich minerals within this basalt and the fine dust covering the surface.