An Introduction to Mars

Image of Mars. Credit: Smthsonian
Image of Mars. Credit: Smthsonian

Mars is the fourth planet from the Sun and the second smallest planet in the Solar System. Named after the Roman god of war, it is often described as the “Red Planet”, as the iron oxide prevalent on its surface gives it a reddish appearance.

Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of Olympus Mons, the second highest known mountain within the Solar System (the tallest on a planet), and of Valles Marineris, one of the largest canyons. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature. Mars has two known moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Martian trojan asteroid.

Until the first successful Mars flyby in 1965 by Mariner 4, many speculated about the presence of liquid water on the planet’s surface. This was based on observed periodic variations in light and dark patches, particularly in the polar latitudes, which appeared to be seas and continents; long, dark striations were interpreted by some as irrigation channels for liquid water. These straight line features were later explained as optical illusions, though geological evidence gathered by unmanned missions suggest that Mars once had large-scale water coverage on its surface. In 2005, radar data revealed the presence of large quantities of water ice at the poles and at mid-latitudes. The Mars rover Spirit sampled chemical compounds containing water molecules in March 2007. The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008.

Mars is currently host to five functioning spacecraft: three in orbit – the Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter; and two on the surface – Mars Exploration Rover Opportunity and the Mars Science Laboratory Curiosity. Defunct spacecraft on the surface include MER-A Spirit and several other inert landers and rovers such as the Phoenix lander, which completed its mission in 2008. Observations by the Mars Reconnaissance Orbiter have revealed possible flowing water during the warmest months on Mars.

Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude reaches −3.0, which is surpassed only by Jupiter, Venus, the Moon, and the Sun. Optical ground-based telescopes are typically limited to resolving features about 300 km (186 miles) across when Earth and Mars are closest because of Earth’s atmosphere.


Discovered By
Known by the Ancients
Date of Discovery
Orbit Size Around Sun (semi-major axis)
Metric: 227,943,824 km
English: 141,637,725 miles
Scientific Notation: 2.2794382 x 108 km (1.523662 A.U.)
By Comparison: 1.524 x Earth
Perihelion (closest)
Metric: 206,655,215 km
English: 128,409,598 miles
Scientific Notation: 2.06655 x 108 km (1.381 A.U.)
By Comparison: 1.405 x Earth
Aphelion (farthest)
Metric: 249,232,432 km
English: 154,865,853 miles
Scientific Notation: 2.49232 x 108 km (1.666 A.U.)
By Comparison: 1.639 x Earth
Sidereal Orbit Period (Length of Year)
1.8808476 Earth years
686.98 Earth days
By Comparison: 1.881 x Earth
Orbit Circumference
Metric: 1,429,085,052 km
English: 887,992,283 miles
Scientific Notation: 1.429 x 109 km
By Comparison: 1.520 x Earth
Average Orbit Velocity
Metric: 86,677 km/h
English: 53,858 mph
Scientific Notation: 2.4077 x 104 m/s
By Comparison: 0.808 x Earth
Orbit Eccentricity
By Comparison: 5.589 x Earth
Orbit Inclination
1.85 degrees
Equatorial Inclination to Orbit
Mean Radius
Metric: 3,389.5 km
English: 2,106.1 miles
Scientific Notation: 3.3895 x 103 km
By Comparison: 0.5320 x Earth
Equatorial Circumference
Metric: 21,296.9 km
English: 13,233.3 miles
Scientific Notation: 2.12969 x 104 km
Metric: 163,115,609,799 km3
English: 39,133,515,914 mi3
Scientific Notation: 1.63116 X 1011 km3
By Comparison: 0.151 x Earth
Metric: 641,693,000,000,000,000,000,000 kg
Scientific Notation: 6.4169 x 1023 kg
By Comparison: 0.107 x Earth
Metric: 3.934 g/cm3
By Comparison: 0.714 x Earth
Surface Area
Metric: 144,371,391 km2
English: 55,742,106 square miles
Scientific Notation: 1.4437 x 108 km2
By Comparison: 0.283 x Earth
Surface Gravity
Metric: 3.71 m/s2
English: 12.2 ft/s2
By Comparison: If you weigh 100 pounds on Earth, you would weigh 38 pounds on Mars.
Escape Velocity
Metric: 18,108 km/h
English: 11,252 mph
Scientific Notation: 5.030 x 103 m/s
By Comparison: Escape velocity of Earth is 25,030 mph.
Sidereal Rotation Period (Length of Day)
1.026 Earth days
24.623 hours
By Comparison: Earth’s rotation period is 23.934 hours.
Minimum/Maximum Surface Temperature
Metric: -87 to -5 °C
English: -125 to 23 °F
Scientific Notation: 186 to 268 K
Atmospheric Constituents
Carbon Dioxide, Nitrogen, Argon
Scientific Notation: CO2, N2, Ar
By Comparison: CO2 is responsible for the Greenhouse Effect and is used for carbonation in beverages.
N2 is 80% of Earth’s air and is a crucial element in DNA. Ar is used to make blue neon light bulbs.