Mars: Facts, Size, Temperature, Moons, and Atmosphere

Mars is the fourth planet from the Sun, a rocky world known for its reddish surface, towering volcanoes, ancient river valleys, and thin atmosphere. Often called the Red Planet, Mars has many features that make it surprisingly familiar to Earth, including polar ice caps, seasons, weather, and a day that lasts almost as long as ours. Yet its cold climate, low atmospheric pressure, and lack of stable liquid water on most of its surface make it a very different place.

Mars is smaller than Earth, with about half Earth’s diameter and roughly 38% of its surface gravity. Its atmosphere consists primarily of carbon dioxide, and its average surface temperature is about −85°F (−65°C). Two small moons, Phobos and Deimos, orbit the planet. Evidence from spacecraft and robotic rovers indicates that Mars once had rivers, lakes, and environments that may have been suitable for microbial life. Whether life ever existed there remains an open scientific question.

Key facts about Mars

Mars occupies an important place in the solar system because it preserves evidence of a more active and potentially habitable past. Its physical characteristics also make it a major target for planetary exploration.

Mars follows an elliptical orbit around the Sun, meaning its distance from the Sun changes throughout the year. Its axis is tilted by about 25 degrees, close to Earth’s tilt of approximately 23.5 degrees. This similarity produces familiar seasonal patterns, although Martian seasons last much longer because a Martian year is nearly twice as long as an Earth year.

How big is Mars compared with Earth?

Mars is considerably smaller than Earth. Its diameter is approximately 53% of Earth’s, and its volume is only about 15% of Earth’s volume. If Earth were a basketball, Mars would be closer to the size of a large softball.

Despite its smaller size, Mars has a substantial surface area because it is a spherical planet. Its total surface area is about 56 million square miles (145 million square kilometers), roughly comparable to the total land area of Earth. This makes Mars a useful comparison for understanding how much terrain scientists can explore on another planet.

Mars has a mass of approximately 10.7% of Earth’s mass. Its lower mass means its gravitational pull is much weaker than Earth’s, although its surface gravity also depends on the planet’s radius. An object that weighs 100 pounds on Earth would weigh about 38 pounds on Mars, assuming the same object and ignoring minor differences in how weight is measured.

This lower gravity would affect human exploration. Astronauts could move more easily against gravity than they can on Earth, but the long-term effects of living in Martian gravity on human bones, muscles, and other body systems remain uncertain. Mars has not yet hosted a human visitor, so these effects have not been directly studied in people living on its surface.

Mars also has a much lower average density than Earth. Both planets contain rocky material and metallic cores, but their internal structures and proportions differ. Mars has a crust, a rocky mantle, and a core composed mainly of iron and other elements. Measurements of seismic waves from a lander have helped scientists investigate its interior and improve estimates of the sizes and properties of these layers.

Why is Mars red?

Mars gets its characteristic color from iron-rich minerals in its surface rocks and dust. When iron reacts with oxygen, it can form iron oxides, compounds similar to the rust that develops on iron exposed to air and moisture on Earth. These minerals give much of the Martian landscape its reddish and brownish appearance.

The process that produced the planet’s iron oxides is more complicated than ordinary rusting on Earth. Mars has experienced volcanic activity, weathering, and interactions between rocks and water over billions of years. Scientists continue to investigate how the planet’s iron minerals formed and how much water was involved.

Fine dust spreads across large areas of the surface and often becomes suspended in the atmosphere. This dust gives the Martian sky a butterscotch or yellowish-brown appearance under typical conditions. Near the Sun, the sky can look different because dust scatters sunlight in ways that depend on particle size, lighting, and viewing conditions.

Mars is not uniformly red. Its surface includes dark volcanic rocks, pale mineral deposits, sandy plains, impact craters, and regions that appear brown, gray, or nearly black. Images taken by landers and orbiters reveal a more varied landscape than the planet’s familiar red appearance suggests.

What is the temperature on Mars?

Mars is extremely cold compared with Earth. Its average surface temperature is about −85°F (−65°C), but actual temperatures vary widely with latitude, season, elevation, time of day, and local weather.

Near the equator, temperatures can rise above freezing during the warmest part of a sunny afternoon. At night, however, temperatures can plunge far below freezing because the atmosphere is too thin to retain much heat. Near the poles, winter temperatures can fall to around −195°F (−125°C) or lower.

Mars experiences these dramatic changes partly because its atmosphere is so thin. Earth’s atmosphere helps redistribute heat through winds and weather systems, while greenhouse gases trap some outgoing heat and reduce the rate at which the surface cools. Mars has carbon dioxide in its atmosphere, but the atmosphere’s low pressure limits its ability to retain and transport heat.

The planet’s distance from the Sun also matters. Mars receives less solar energy per unit area than Earth because it orbits farther away. Its elliptical orbit creates additional seasonal variation, particularly in the southern hemisphere, where summer occurs when Mars is relatively close to the Sun. Southern summers can therefore be warmer and more intense than northern summers.

Elevation creates further temperature differences. Higher regions generally have lower atmospheric pressure, which affects the local thermal environment. Dust storms can also change surface temperatures by absorbing sunlight in the atmosphere and altering how much solar energy reaches the ground.

These conditions would make human exploration challenging. Equipment, habitats, spacesuits, and life-support systems would need to withstand severe cold and rapid temperature changes. Any long-term human settlement would require reliable insulation, heating, and protection from the planet’s harsh environment.

What is the atmosphere of Mars made of?

The atmosphere of Mars is composed primarily of carbon dioxide, which makes up about 95% of its gaseous content. Nitrogen accounts for roughly 2.7%, and argon makes up about 1.6%. Small amounts of oxygen, carbon monoxide, water vapor, and other gases are also present.

Although carbon dioxide is a greenhouse gas, the Martian atmosphere is too thin to keep the planet warm in the way Earth’s atmosphere does. Surface atmospheric pressure averages around 6 millibars, less than 1% of Earth’s average sea-level pressure. The exact pressure varies with elevation, season, and the amount of carbon dioxide frozen at the poles.

This low pressure has important consequences for water. Under typical Martian surface conditions, liquid water is unstable and can rapidly evaporate or freeze. Pure liquid water can exist only under a limited range of pressure and temperature conditions, and the low atmospheric pressure on most of Mars makes persistent surface liquid water difficult to maintain.

The atmosphere also offers little protection against harmful radiation from space. Earth’s global magnetic field and relatively thick atmosphere help shield its surface from charged particles and high-energy radiation. Mars lacks a comparable global magnetic field today, and its thin atmosphere provides much less shielding. This is one reason that radiation protection would be essential for future human missions.

Mars once had a thicker atmosphere, according to multiple lines of geological and atmospheric evidence. Ancient river channels, lake deposits, and minerals formed in the presence of water suggest that surface conditions were more favorable for liquid water in the distant past. Scientists believe that Mars lost much of its atmosphere over time, partly because solar wind stripped atmospheric particles away and because the planet’s lower gravity made retaining a thick atmosphere more difficult.

The loss of atmospheric protection contributed to Mars becoming colder and drier. However, the planet’s climate history is complex, and scientists continue to investigate how changes in volcanic activity, atmospheric chemistry, water availability, and orbital conditions shaped its evolution.

Does Mars have weather and dust storms?

Mars has weather systems despite its thin atmosphere. Winds move dust across the surface, clouds form under suitable conditions, and seasonal changes affect atmospheric circulation. Water-ice clouds can develop at different altitudes, while carbon dioxide clouds form in particularly cold regions of the atmosphere.

Dust is one of the most influential parts of Martian weather. Strong winds can lift fine particles from the ground and carry them over large distances. Local dust storms occur regularly, and larger storms sometimes expand until they cover much of the planet. These planet-wide events can last for weeks or months.

Dust storms do not produce the same physical effects as hurricanes on Earth. Although winds on Mars can reach high speeds, the atmosphere is so thin that the force exerted by a given wind speed is much lower than it would be in Earth’s denser atmosphere. Even so, airborne dust can obscure visibility, coat machinery, and reduce the amount of sunlight reaching solar-powered spacecraft.

Dust also affects the planet’s temperature. Suspended particles absorb and scatter sunlight, changing the distribution of heat between the atmosphere and surface. This can alter atmospheric circulation and help sustain large storms under the right conditions.

The Martian water cycle differs greatly from Earth’s. Water exists mainly as ice in the polar caps, underground deposits, and small amounts of atmospheric vapor. Water vapor can move through the atmosphere and condense into ice clouds or frost, but Mars does not have an active global cycle of rainfall, rivers, and oceans like Earth.

How long is a day and year on Mars?

A Martian day, called a sol, lasts about 24 hours and 37 minutes. This is only slightly longer than an Earth day, making Mars one of the more familiar planets in terms of daily timekeeping. The similarity is useful for planning robotic missions because many lander and rover operations can follow schedules that are close to a 24-hour cycle.

A year on Mars lasts 687 Earth days, or about 1.88 Earth years. Mars takes longer to complete an orbit because it travels around the Sun at a greater average distance than Earth does.

Its axial tilt of about 25 degrees produces four seasons. However, each season lasts much longer than its Earth equivalent, and the seasons are not all the same length because Mars travels at different speeds along its elliptical orbit.

Mars also has polar ice caps that change with the seasons. During winter, temperatures can become cold enough for atmospheric carbon dioxide to freeze onto the surface, forming seasonal deposits of dry ice. As spring arrives, some of this frozen carbon dioxide turns directly back into gas, a process known as sublimation.

The seasonal movement of carbon dioxide between the atmosphere and the polar regions changes atmospheric pressure across the Martian year. Water ice also contributes to the permanent polar deposits, although the proportions of water ice and carbon dioxide vary between regions and seasons.

What are the two moons of Mars?

Mars has two known natural satellites: Phobos and Deimos. Both are small, irregularly shaped objects, quite unlike Earth’s relatively large, nearly spherical Moon. Their names come from Greek mythology, where Phobos and Deimos represent fear and terror.

<PRIVATE_PERSON> is the larger and closer of the two moons. It measures about 17 miles (27 kilometers) across at its widest point and orbits Mars at an average distance of roughly 3,700 miles (6,000 kilometers) from the planet’s center. It completes an orbit in about 7 hours and 39 minutes, moving around Mars faster than the planet rotates.

Because <PRIVATE_PERSON> orbits so close to Mars, it rises in the west and sets in the east, unlike the usual east-to-west movement of the Moon across Earth’s sky. Its orbit is gradually shrinking, and tidal forces are drawing it closer to Mars. In the distant future, it may break apart under tidal stresses or collide with the planet.

Deimos is smaller and farther away. It is about 8 miles (13 kilometers) across at its widest point and orbits Mars at an average distance of approximately 14,600 miles (23,500 kilometers) from the planet’s center. It takes about 30 hours to complete an orbit.

Both moons have dark surfaces and heavily cratered terrain. Their shapes and properties have led scientists to consider whether they are captured asteroids or formed from material associated with an ancient impact on Mars. Their origins are not yet settled, and understanding them could reveal more about the early history of the Martian system.

Phobos and Deimos are too small to hold significant atmospheres. Their weak gravity would also make movement and landing operations very different from those on Mars itself. Even so, they could become useful destinations for future robotic or crewed missions because they may offer vantage points for studying Mars and its surrounding environment.

What is the surface of Mars like?

Mars has one of the most diverse landscapes among the rocky planets. Its surface includes vast plains, towering volcanoes, deep canyons, ancient impact craters, sand dunes, and deposits shaped by wind and ice.

Olympus Mons, a shield volcano, is the largest known volcano in the solar system. It rises roughly 13 miles (21 kilometers) above the surrounding plains, making it more than twice the height of Mount Everest above sea level. Its immense size reflects Mars’s long volcanic history and the fact that the planet lacks Earth’s kind of active plate tectonics, in which moving crustal plates continually change the locations of volcanic regions.

Valles Marineris is an enormous canyon system extending thousands of miles across the Martian surface. In places, it reaches depths of several miles. It is far larger than the Grand Canyon and likely formed through a combination of crustal stretching, faulting, collapse, and erosion rather than by a single river.

Impact craters are widespread because Mars has been struck by asteroids and other objects throughout its history. Some craters are relatively fresh, while others have been worn down by erosion or partly buried by sediments. Their distribution helps scientists estimate the relative ages of different surface regions.

The surface also contains ancient channels and features resembling river deltas. These structures provide strong evidence that flowing water once shaped parts of Mars. Minerals such as clays and sulfates, which can form or change in the presence of water, offer additional clues about past environments.

Today, wind is one of the dominant forces reshaping the surface. It moves sand, builds dunes, exposes rocks, and transports dust across the planet. Frost and ice also influence some landscapes, while occasional impacts continue to create new craters.

Was there ever water on Mars?

Evidence indicates that Mars once had liquid water flowing across its surface. Orbiters have identified ancient valley networks, branching channels, sedimentary deposits, and minerals associated with water. Rovers have examined rocks that formed in or were altered by ancient lakes and groundwater.

These observations suggest that parts of early Mars had environments where liquid water persisted long enough to carve landscapes and leave chemical traces. Gale Crater, explored by the Curiosity rover, contains sedimentary rocks that record the presence of an ancient lake system. Jezero Crater, explored by the Perseverance rover, contains an ancient river delta where sediments accumulated as water entered a lake.

However, evidence of ancient water does not mean that Mars once resembled modern Earth in every respect. Scientists are still determining how much water was present, how long surface water lasted, and whether it existed widely or mainly in particular regions and periods.

Much of Mars’s water appears to have disappeared from the surface over time. Some escaped into space after being broken down or carried into the upper atmosphere. Some became trapped in polar ice, underground ice deposits, and water-bearing minerals in the crust. The relative contributions of these processes remain subjects of active research.

Today, water ice is known to exist in the polar regions and beneath parts of the surface. Orbital measurements, radar observations, and lander investigations provide evidence of buried ice at different latitudes. This ice is scientifically important because it preserves clues about the planet’s climate history and could potentially serve as a resource for future exploration.

Liquid water may occur under unusual local conditions, but stable bodies of exposed liquid water are not known to persist on the modern Martian surface. The planet’s low pressure and cold temperatures make widespread surface liquid water unlikely under present conditions.

Could Mars support life?

The possibility of life on Mars is one of the most important questions in planetary science. Scientists have not confirmed that life ever existed there, but evidence of ancient water and chemically diverse environments makes the planet a valuable place to investigate the conditions needed for life.

Life as we know it requires liquid water, a source of energy, and essential chemical ingredients. Some ancient Martian environments appear to have provided these conditions, at least temporarily. Lakes, river systems, and groundwater could have offered habitats where microorganisms might have survived if life developed or arrived there.

Modern Mars is much less hospitable. Its surface is cold, dry, and exposed to radiation, while its atmosphere offers little protection from the space environment. Nevertheless, underground environments may be more protected than the surface, and scientists continue to investigate whether conditions suitable for microbial life could exist in some subsurface locations.

Finding evidence of life would require more than discovering organic molecules. Organic compounds contain carbon and can be produced by biological and nonbiological processes. A convincing discovery would require multiple independent lines of evidence that distinguish possible biological activity from geological chemistry and contamination.

Robotic missions are helping scientists evaluate these possibilities. The Curiosity rover studies rocks, minerals, and environmental chemistry in Gale Crater. The Perseverance rover investigates ancient environments in Jezero Crater and collects carefully selected rock and soil samples for possible return to Earth through a future sample-return effort.

Even if Mars never supported life, understanding why it became less habitable would help scientists explain how rocky planets evolve. It could also improve our understanding of the conditions that make Earth suitable for life.

How do scientists explore Mars?

Scientists study Mars using orbiters, landers, and rovers, each of which provides a different perspective. Orbiters circle the planet to map its surface, measure its atmosphere, identify minerals, and investigate subsurface structures using radar and other instruments.

Landers descend to the surface and conduct measurements at fixed locations. Their instruments can monitor weather, examine soil, analyze rocks, and measure seismic activity. Although a lander cannot travel across the landscape, it can collect detailed data over extended periods.

Rovers add mobility. By traveling between rocks, slopes, and sediment deposits, they can compare geological settings and select targets for closer examination. Cameras provide detailed images, while spectrometers and other instruments help identify minerals and chemical compounds.

Mars exploration has revealed that the planet is geologically more complex than early observations suggested. Its landscape records volcanic activity, impacts, erosion, changes in atmospheric conditions, and interactions with water over billions of years. Each mission adds information that helps scientists reconstruct this history.

Robotic exploration also prepares the way for possible human missions. Engineers must understand radiation exposure, dust hazards, surface conditions, available resources, and the difficulty of landing and operating equipment in a thin atmosphere. Mars has enough gravity to influence spacecraft landings, but not enough atmospheric density for parachutes alone to slow heavy spacecraft safely through every stage of descent.

Human exploration would require systems that supply breathable oxygen, water, food, power, and shelter. Any long-duration mission would also need to manage radiation exposure, extreme temperatures, and the psychological and medical challenges of operating far from Earth.

Why Mars matters to science

Mars offers a natural laboratory for studying how rocky planets form, change, and sometimes lose the conditions that allow liquid water to persist. Its geological record preserves clues about processes that also shaped Earth, while its different climate history reveals how two neighboring planets can evolve along very different paths.

The planet’s ancient river valleys, lake deposits, and water-altered minerals provide evidence of a wetter past. Its thin atmosphere, cold surface, and polar ice caps reveal how dramatically environmental conditions can change over geological time. Its moons offer additional opportunities to investigate the formation and evolution of small bodies in the solar system.

Mars is also a practical focus for future exploration because it is relatively accessible compared with more distant planets. Yet reaching and exploring it safely remains a major engineering challenge, especially for human missions.

The central scientific questions remain compelling: How much water did Mars once possess? Why did it lose much of its atmosphere? Did life ever emerge in its ancient environments? And what can its history teach us about the long-term habitability of planets elsewhere in the universe?

Answering these questions will require careful measurements, continued robotic exploration, and the study of Martian rocks and soils. Mars is not simply a red world in the night sky. It is a record of planetary change, offering a rare opportunity to investigate how a planet can move from a potentially habitable past toward the cold, dry environment observed today.