Mars orbits the Sun at a semi-major axis of approximately 227.94 million kilometers, or 1.524 astronomical units.¹⁰ Because its orbital eccentricity of 0.0934 is significantly more pronounced than Earth's 0.0167, the distance between Mars and the Sun varies substantially over its year, from 206.65 million kilometers at perihelion to 249.26 million kilometers at aphelion. Traveling at an average orbital velocity of 24.07 kilometers per second along an orbital plane inclined 1.85 degrees to the ecliptic, Mars completes one sidereal revolution every 686.98 Earth days (1.88 Earth years). Its synodic period—the interval between successive oppositions when Earth passes directly between Mars and the Sun—averages 779.94 days, or roughly 26 months, which dictates the launch windows for interplanetary spacecraft.¹¹

As Mars travels around the Sun, it completes one sidereal rotation on its axis every 24 hours, 37 minutes, and 22 seconds (24.6229 hours). Its solar day, referred to by planetary scientists as a "sol," lasts 24 hours, 39 minutes, and 35 seconds (24.6597 hours), making a Martian year equal to 668.6 sols. The rotational axis of Mars is tilted at an obliquity of 25.19 degrees relative to its orbital plane, closely resembling Earth's axial tilt of 23.44 degrees. Consequently, Mars experiences four distinct seasons, though they last nearly twice as long as terrestrial seasons and vary noticeably in length and intensity between the northern and southern hemispheres due to the planet's elliptical orbit.¹²¹³

In physical dimensions, Mars has an equatorial radius of 3,396.2 kilometers and a polar radius of 3,376.2 kilometers, yielding a flattening ratio of 0.00589. Its volume is approximately 15.1 percent of Earth's, and its mass is 6.4169 × 10²³ kilograms, or 10.7 percent of Earth's mass. Because its interior is enriched in lighter chemical compounds and compressed under less gravitational force, Mars has a mean density of 3.934 grams per cubic centimeter, compared to Earth's 5.513 grams per cubic centimeter. The resulting surface gravity at the Martian equator is 3.72 meters per second squared—roughly 38 percent of Earth's gravity—and its escape velocity is 5.03 kilometers per second.

Like Earth, Mars underwent planetary differentiation early in its 4.6-billion-year history, separating into a dense metallic core, a silicate mantle, and an outer rocky crust. For decades, the exact dimensions and physical state of the Martian interior were estimated only through gravitational moment-of-inertia measurements and geochemical analyses of Martian meteorites found on Earth, known as SNC (shergottite, nakhlite, and chassignite) meteorites.¹⁴ Direct seismological investigation became possible between 2018 and 2022 through NASA's InSight lander, which recorded seismic waves from hundreds of "marsquakes" and meteorite impacts.¹⁵¹⁶ Initial seismic inversions in 2021 indicated a liquid core of iron, nickel, and light elements—such as sulfur, oxygen, carbon, and hydrogen—with a radius of 1,800 to 1,850 kilometers.¹⁷ Subsequent 2023 analyses by researchers including Henri Samuel and Amir Khan revised the liquid core radius downward to between 1,650 and 1,700 kilometers, proposing that it is enveloped by a 150-kilometer-thick basal layer of molten silicate magma at the bottom of the mantle. In September 2025, a team led by Huixing Bi analyzing deep core-transiting (PKKP) and boundary-reflecting (PKiKP) seismic phases identified direct evidence of a crystallized solid inner core with a radius of approximately 613 ± 67 kilometers.¹⁸

Surrounding the core is a silicate mantle roughly 1,240 to 1,880 kilometers thick, overlaid by a rigid crust composed largely of iron-rich basaltic rock containing magnesium, aluminum, calcium, and potassium. Unlike Earth, contemporary Mars exhibits no active horizontal plate tectonics; its crust forms a single stagnant lid averaging between 10 and 50 kilometers in depth. Because the crust does not recycle into the mantle, ancient surface features and volcanic edifices remain preserved over billions of years. Although Mars lacks a global dipolar magnetic field today, localized regions of its ancient southern crust retain strong remnant magnetization, demonstrating that convection in the planet's core drove an active magnetosphere during its first several hundred million years before cooling shut the dynamo down.¹⁹

Planetary geologists divide the history of Mars into three primary epochs named after prominent surface regions and defined by impact crater density.²⁰²¹ Following the formative Pre-Noachian period (4.5 to 4.1 billion years ago), the Noachian epoch lasted from roughly 4.1 to 3.7 billion years ago.²² During the Noachian, heavy asteroidal bombardment carved gigantic impact basins—including Hellas, Isidis, and Argyre—while widespread volcanic outgassing and episodic liquid water formed extensive valley networks and clay minerals (phyllosilicates). The Hesperian epoch, spanning approximately 3.7 to 3.0 billion years ago, saw a sharp decline in impact rates alongside massive flood basalt volcanism that resurfaced at least 30 percent of the planet, colossal catastrophic water outflows, and the deposition of sulfur-rich sulfate minerals. The Amazonian epoch, extending from roughly 3.0 billion years ago to the present, has been characterized by hyper-arid, freezing conditions, low impact rates, wind-driven erosion, glacial cycles, and intermittent volcanic activity.²³

The topography of Mars is defined by a striking global dichotomy between its northern and southern hemispheres.²⁴ The northern third of the planet consists of flat, low-lying volcanic plains known as the Vastitas Borealis, where the crust is thinner and impact craters are sparse, leading many researchers to hypothesize that the basin was formed by a single cataclysmic impact early in Martian history. In contrast, the southern two-thirds of the planet consist of ancient, heavily cratered highlands sitting several kilometers higher in elevation over a substantially thicker crust. Spanning 30 kilometers in total vertical topographic relief from its lowest basin to its highest peak, Mars exhibits far more extreme elevation differences than Earth.

Absent moving tectonic plates to carry crust over mantle hotspots, magma plumes on Mars erupted repeatedly through the same crustal vents for hundreds of millions of years, building the largest volcanic structures in the Solar System. The western equatorial hemisphere is dominated by the Tharsis bulge, a vast volcanic plateau thousands of kilometers wide that deformed the planet's spherical shape and supports three aligned shield volcanoes known as the Tharsis Montes. Just northwest of this chain rises Olympus Mons, a shield volcano roughly 27 kilometers (17 miles) high and 600 kilometers (373 miles) across—nearly three times the elevation of Mount Everest above sea level. Another major volcanic province, Elysium Planitia, lies further west.²⁵²⁶

Stretching eastward from the Tharsis region along the equator is Valles Marineris, a system of tectonic rift canyons more than 3,000 kilometers (over 2,000 miles) long, up to hundreds of kilometers wide, and plunging up to 7 kilometers deep. Formed largely as the crust fractured and collapsed under the immense crustal stresses of the rising Tharsis bulge, Valles Marineris spans a distance comparable to the width of the continental United States. In the southern hemisphere, the Hellas Planitia impact crater forms a basin more than 2,000 kilometers across and roughly 7 kilometers deep. Because Mars lacks oceans to provide a sea-level datum, planetary cartographers measure elevations relative to a reference equipotential surface (areoid) and measure longitude from a prime meridian defined as passing through the small crater Airy-0 within the larger crater Airy.²⁷

The Martian atmosphere is an extremely thin envelope of gas with an average surface pressure of about 6 millibars (0.6 kilopascals), less than one percent of Earth's 1,013-millibar sea-level pressure. By volume, it consists of approximately 95 percent carbon dioxide, 2.6 to 3 percent molecular nitrogen, and 1.6 to 2 percent argon, with trace concentrations of oxygen (about 0.15 percent), carbon monoxide, water vapor, and intermittent detections of methane. After the loss of the planet's global magnetic field billions of years ago, solar wind particles and ultraviolet radiation steadily stripped away the lighter upper atmosphere into space, transforming the climate.²⁸ Because the remaining air is too thin to retain solar heat effectively, surface temperatures average around −65 °C (−85 °F) and fluctuate wildly between a maximum of about 20 °C (68 °F) at the equator during summer afternoons and a minimum of −153 °C (−243 °F) at the winter poles.

Despite its low density, the Martian atmosphere is dynamically active. Fine iron-rich dust particles suspended in the air absorb sunlight, warming the atmosphere and giving the daytime sky a tawny or pinkish-red hue. Strong surface winds generate localized dust devils as well as seasonal dust storms that occasionally coalesce into planet-encircling tempests, lofting dust to altitudes of 50 kilometers and obscuring the entire surface for months. High-altitude clouds of water ice and carbon dioxide crystals also form, particularly over major volcanoes and polar regions.²⁹ During each hemisphere's winter, temperatures drop low enough that 25 to 30 percent of the global atmospheric carbon dioxide freezes directly out of the air onto the winter pole as a seasonal layer of dry ice, causing global atmospheric pressure to rise and fall annually as the frost sublimates back into gas each spring.

Water on contemporary Mars exists almost exclusively as solid ice and trace atmospheric vapor, as the low atmospheric pressure typically causes exposed ice to sublimate directly into gas rather than melt.³⁰ Both poles are covered by permanent ice caps up to 3 to 3.5 kilometers thick, containing an estimated 1.6 million cubic kilometers of ice each.³¹ The northern cap (Planum Boreum), about 1,000 kilometers across in summer, consists of water ice covered by a 1-to-2-meter seasonal veneer of dry ice in winter, whereas the smaller southern cap (Planum Australe), 350 kilometers across, retains a permanent 8-meter-thick cap of carbon dioxide ice atop its water ice core. Beyond the poles, orbital radar and neutron spectroscopy from missions such as Mars Odyssey and Mars Express have identified more than 5 million cubic kilometers of subsurface permafrost and buried glaciers—enough to cover the planet in 35 meters of water if melted—including massive layered deposits up to 3.7 kilometers thick in the equatorial Medusae Fossae Formation.³² In 2018, radar reflections from the MARSIS instrument on Mars Express suggested the possible presence of a liquid subglacial brine lake 1.5 kilometers beneath the south pole, though other researchers contend the reflections are caused by clay minerals, saline ice, or constructive radar interference between dusty ice layers.

Mars has two small, irregularly shaped natural satellites, Phobos and Deimos, both discovered in August 1877 by American astronomer Asaph Hall at the United States Naval Observatory in Washington, D.C.³³ They are named after the mythological Greek twin sons of Ares (the counterpart to the Roman war god Mars), representing fear (Phobos) and terror or dread (Deimos). Because of their low masses and weak surface gravities, neither body is large enough to have pulled itself into a hydrostatic sphere; both are heavily cratered, covered in thick regolith, and display dark, reddish spectra similar to carbonaceous asteroids.

Phobos, the inner and larger moon, has a mean diameter of 22.2 kilometers and a mass of 1.08 × 10¹⁶ kilograms. Its most prominent geological feature is Stickney, an impact crater approximately 9.5 kilometers wide—nearly half the diameter of the moon itself—surrounded by networks of linear grooves. Phobos orbits in a nearly circular equatorial path at a semi-major axis of 9,377 kilometers from the center of Mars (just 5,989 kilometers above the Martian surface), completing one orbit in 7 hours and 39 minutes. Because Phobos revolves around Mars more than three times faster than the planet rotates on its axis, an observer on the Martian surface sees Phobos rise in the west, race across the sky, and set in the east roughly twice per sol. Tidal interactions between Phobos and Mars are causing the moon's orbit to decay inward by approximately 1.8 centimeters per year (nearly 2 meters per century); astronomers calculate that within 50 to 100 million years, Phobos will either impact the Martian surface or be torn apart by gravitational tidal forces to form a planetary ring.

Deimos, the outer and smaller moon, measures 12.6 kilometers in mean diameter with a mass of 1.5 × 10¹⁵ kilograms and presents a smoother surface than Phobos because its craters are partially filled with loose regolith. Orbiting outside synchronous altitude at a semi-major axis of 23,460 kilometers with an orbital period of 30 hours and 18 minutes (30.35 hours), Deimos rises in the east and moves so slowly relative to the rotation of Mars that it takes 2.7 sols to set in the west. Both satellites are tidally locked, always keeping the same face turned toward Mars. The origin of Phobos and Deimos remains an open scientific question: their dark, low-density compositions suggest they may be asteroids captured from the outer asteroid belt, whereas their nearly circular, coplanar equatorial orbits favor the hypothesis that they accreted in orbit from a disk of debris blasted off Mars by a giant impact.³⁴

Visible to the naked eye as a bright reddish-orange star that periodically undergoes retrograde loops across the night sky, Mars was tracked by ancient Egyptian astronomers over 4,000 years ago, who called it Her Desher ("the red one"), and by Babylonian, Greek, Chinese, and Roman skywatchers. In the early 17th century, Johannes Kepler used Tycho Brahe's precise naked-eye observations of Mars to deduce that planetary orbits are ellipses rather than circles, formulating his laws of planetary motion, while Galileo Galilei made the first telescopic observations of the planet. During the favorable opposition of 1877, Italian astronomer Giovanni Schiaparelli mapped linear markings across the Martian disk that he termed canali ("channels"). Mistranslated into English as "canals," these optical illusions were popularized by American astronomer Percival Lowell as evidence of an irrigation network built by a dying Martian civilization, inspiring decades of science fiction before spacecraft photography proved the features did not exist.