Mars is the fourth planet from the Sun, and has been known since ancient times for its reddish color. Mars is named after the Roman god of war, and takes that name from its color. Perhaps because its bloody hue, its close proximity to Earth, and its seasonally-changing surface features, Mars has played a larger role in human culture and mythology than any other planet.
Since ancient times, Mars has been identified with battle, blood, courage, fierce dedication, aggression and victory. The Greek name for Mars is Ares. The names chosen in modern times for Mars's two tiny moons, Phobos ("fear") and Deimos ("terror"), refer mythologically to the horsemen of Ares, and make an astronomical link to the ancient Apocalyptic Horsemen.
In Norse mythology and wider Germanic paganism, Tiw or Tyr was a one-handed god associated with combat and pledges. The second day of the work week, Tuesday, gets its name from an Old English word meaning "Tiw's day". The name is based on Latin dies Martis, "Day of Mars"; compare: French mardi, Spanish martes, and Italian martedi.
Orbit and Observation
Mars orbits the Sun at about 1.5 times the average distance of the Earth, with a an orbital period of 687 days. Mars's orbit eccentricity (0.0934) is about six times greater than the Earth's, so its distance from Earth varies widely - from 35 million miles (59 million km) at a "favorable" opposition near the orbit's perihelion, to 248 million miles (399 million km) at superior conjunction near its aphelion. Because of this, Mars varies greatly in its apparent size, from 3.5 to 25 arc seconds, and in brightness from magnitude -2.9 to +1.7.
Four views of Mars from the Hubble Space Telescope. (NASA)
In a small telescope, Mars shows many of the surface features that sparked the imagination of science fiction writers. Prominent white polar caps are visible, as are odd dusky markings on its surface. These markings show that Mars rotates once every 24 hours and 37 minutes - so its day is almost the same length as Earth's. Mars also has an axial tilt very similar to Earth's, and has seasons like the Earth. The polar caps shrink and expand during the Martian summer and winter, and the dark patterns on its surface also display seasonal changes. Mars has an atmosphere with sparse clouds, and exhibits occasional dust storms which sometimes grow to cover the entire planet's surface for a few weeks.
A Martian global dust storm, photographed by the Hubble Space Telescope. (NASA)
Spacecraft Exploration
Once we began to study Mars with telescopes, that planet's intriguing similarities with Earth became apparent. It was the American astronomer Percival Lowell in the early 1900s who first proposed that Mars was the home of a dying civilization, which built canals to carry water across its rusty deserts, from the polar ice caps to cities along the equator. That vision was popularized in the novels of H. G. Wells and Edgar Rice Burroughs, and captured the popular imagination, but it ultimately proved to be false.
This globe of Mars and its canals was prepared by Eugene Antoniadi in 1894 and redrawn by Lowell Hess in 1956.
The first spacecraft mission to fly by Mars (Mariner 4, in 1965) revealed a vast, barren wasteland of craters. Mars's atmosphere was only 1% as dense as Earth's at the surface, and composed of 95% carbon dioxide (with small percentages of nitrogen and argon). There was no protective ozone layer, and no magnetic field to shield the surface from deadly solar radiation. Its surface was cold - the average temperature is -81° F (-63° C), with a minimum of -200° F (-140° C), and a maximum of 68° F (20° C) on the warmest days at the equator.
These conditions made the surface of Mars completely inhospitable to life as we know it. The large number of craters seemed to indicate that Mars was a dead world, geologically speaking, as well.
Viking 1 photographed craters near the edge of Mars' Hellas impact basin, below a thin atmosphere. (NASA)
However, subsequent spacecraft exploration showed that Mars was perhaps not quite such a dead place after all. Mariner 9, which entered orbit around the planet in 1971 and mapped its surface in high detail for the first time, revealed the presence of huge volcanos and vast canyon systems. The tallest of the volcanoes, Olympus Mons ("Mount Olympus"), is about 17 miles (27 km) high - about three times the elevation of Mount Everest! - and is 340 miles (550 km) across at its base. Olympus Mons is both the largest volcano and the tallest mountain in the solar system, and is aptly named for the home of the gods.
A composite of Olympus Mons from Viking Orbiter images. (NASA)
The deepest and longest canyon system on Mars is Valles Marineris (the "Mariner Valley"), and is up to 4 miles (7 km) deep, 120 miles (200 km) wide, and 3000 miles (5000 km) long. It is the largest known crevice in the solar system, and if placed on Earth, it would span the entire United States. Mars was clearly home to some significant geological activity in its past.
The central part of Valles Marineris; composite of Viking images. (NASA)
Of 38 launches from Earth in an attempt to reach Mars as of mid-2012, only 19 have succeeded - a failure rate of 50%. This high failure rate is informally called the "Mars Curse", and is sometimes facetiously used to "explain" recurring difficulties in reaching the Red Planet.
The "Face on Mars" photographed by the Viking orbiters in 1976 (top) turned out to be an illusion when seen in higher resolution (bottom) by the Mars Global Surveyor in 2001. (NASA)
Air and Water on Mars
More importantly, spacecraft images showed surface features that seemed to indicate the presence of water: channels, dry riverbeds, and flood plains. Features such as these look strikingly similar to features on Earth which have been created by liquid water. But on Mars, unlike Earth, liquid water is all but nonexistent today.
Currently, most of Mars's water is buried beneath the surface or frozen as ice in the polar caps. Mars's permanent polar caps are made of water ice; the seasonal expansion and contraction of the polar caps is actually due to the presence of carbon dioxide ice freezing out of the atmosphere - Mars's polar regions never become warm enough for water to melt.
Mars's north polar ice cap, seen by the Mars Global Surveyor. (NASA)
The first spacecraft to successfully land on Mars (Viking 1, in 1976) detected minute trace amounts of water vapor in the atmosphere. Later missions, particularly NASA's Spirit and Opportunity rovers which landed on Mars in 2004, confirmed that the Martian surface was once covered by abundant amounts of liquid water. The Opportunity rover photographed mineral formations, dubbed "blueberries", which could have only formed in the presence of liquid water.
"Blueberries" found by Opportunity indicate that the site was once under water. (NASA)
The Mars Phoenix mission, which landed near Mars's north polar region in 2008, may have photographed deposits of water ice directly under the lander itself. These ice deposits were revealed when the lander's rockets blasted away the overlying topsoil. Later, photographs showed what appeared to be droplets of liquid water condensing briefly on the lander's legs, before evaporating into the thin, dry atmosphere.
In September 2015, NASA confirmed that liquid Water flows on Mars today. The Mars Reconnaissance Orbiter (MRO) photographed dark, narrow streaks that ebb and flow over time. They darken and flow downhill during warm seasons, and fade in cooler seasons. A spectrometer on MRO detected hydrated salts on slopes where mysterious streaks are seen, corroborating that they are indeed formed by liquid water.
Diagram of the Martian atmosphere.
The cold, desolate planet’s atmosphere is so thin that water will freeze or evaporate away quickly. Ancient Mars had rivers and lakes, and so much have had a much denser, warmer atmosphere. But where did that atmosphere go? In 2015, NASA'a MAVEN (Mars Atmosphere and Volatile Evolution) probe revealed that most of Mars' atmosphere has been stripped away by the solar wind - a stream of charged atomic particles flowing from the Sun.
This problem is much worse for Mars than for Earth, because Mars, unlike Earth, has no magnetic field to deflect those particles. Mars is currently losing 10 tons of atmosphere per day, and 10 - 20 times more during solar outbursts.
Life on Mars?
The Viking landers of the 1970s carried experiments to detect the presence of life. They looked for organic compounds in the Martian soil; they introduced a liquid nutrient solution into the soil and looked for gases released by metabolizing organisms; and they traced the release of metabolic gases from nutrient solution labelled with radioactive carbon-14.
The first image ever returned from the surface of Mars, by Viking 1 in 1976. (NASA)
The first two experiments failed to detect any organic molecules or metabolic gas exchange. But the third experiment detected a steady stream of radioactive gases from the soil. Unfortunately, subsequent attempts failed to generate the same results, and today's majority opinion is that the Viking landers failed to conclusively detect life.
Spirit rover's landing site on Mars in 2004, with rover tracks leading away from the lander. (NASA)
Currently, the prevailing view is that chemical reactions with the Martian soil explain Viking's results. A sufficiently strong oxidizing molecule would react with the water Viking added to produce oxygen and hydrogen, and with the nutrients to produce carbon dioxide. The Martian soil, continuously exposed to ultraviolet light from the Sun, has built up a thin layer of a very strong oxidant. In 2008, the Phoenix lander detected perchlorate (a strong oxidizer) in the Martian soil, supporting the chemical interpretation of Viking's results. But as recently as 2011, some scientists argue for a biological interpretation, i.e. that Viking really did find life. It may be best to say that while Viking did not conclusively prove the existence of life on Mars, it did not conclusively disprove it, either.
Meteorites from Mars have landed on Earth, blasted from the Red Planet's surface by enormous asteroid impacts. Preserved in Antarctic ice, these meteorites are known to have originated on Mars, because the composition of gases trapped inside their porous interiors exactly matches that of the Martian atmosphere.
Electron microscope image of structures inside meteorite ALH84001. (NASA)
One particular Martian meteorite discovered in Antarctica in 1984, called ALH84001, became the subject of great controversy in 1996. That year, NASA scientists announced that amino acids and other organic compounds had been discovered inside ALH84001. They also showed images of microscopic structures inside the meteorite, resembling (but much smaller than) fossilized bacteria on Earth. These discoveries were first announced as solid evidence that life had actually arisen on Mars. But this conclusion was immediately disputed by other scientists, who argued that the evidence could also be explained by non-biological processes. The situation is unresolved, and in late 2009 some scientists reasserted that Martian meteorites still provide strong evidence of life on ancient Mars.
Crater walls surround the Opportunity rover's landing site, inside Mars's Meridiani Planum. (NASA)
Earth-based observations of Mars in 2003 revealed trace amounts of methane in the Martian atmosphere. Methane is quickly destroyed in the Martian atmosphere in a variety of ways, so this discovery indicates that some ongoing process is releasing the gas. Much of the methane in Earth's atmosphere is released by living organisms as they digest nutrients. However, other purely geological processes, like vulcanism and the oxidation of iron, also release methane. Right now, we don't have enough information to tell whether biology or geology - or both - are producing methane on Mars.
Curiosity snaps a self-portrait with Mount Sharp in the background. (NASA/JPL-Caltech/Malin Space Science Systems)
NASA's Mars Science Laboratory (MSL), nicknamed "Curiosity", landed on Mars in August 2012, inside Gale crater near Mars' equator - a site that had been selected from orbital photos which showed evidence of a habitable environment in the past. The rover sampled Mars’ atmosphere for methane between October 2012 and June 2013 - and detected none. A few months later, Curiosity detected a sudden burst of the gas, deepening the mystery.
During its first year on Mars, Curiosity began driving toward Mount Sharp, a sediment-covered, 3-mile-high peak at the center of Gale crater. Researchers confirmed that liquid water persisted on the crater floor millions of years ago. While Curiosity has not yet found any evidence of life, more results are expected as the complex data from NASA's most ambitious Mars mission have yet to be analyzed.
Sedimentary layers at the Base of Mount Sharp, Curiosity's eventual destination. (NASA/JPL-Caltech/MSSS)
The evidence is clear that Mars once had a much warmer, wetter past. There is a possibility that life once existed, or still exists, on Mars. Because of that possibility, Mars remains a primary target of our space exploration.
Moons of Mars
Mars has two tiny moons, Phobos and Deimos. Both moons were discovered by Asaph Hall in 1877. Both are tidally locked with Mars, always showing the same face to the planet; and both orbit Mars very close to the plane of its equator. Phobos and Deimos are both small rocky bodies, resembling asteroids. This has fueled speculation that they actually are captured asteroids.
Comparative sizes of Phobos (left) and Deimos (right), both imaged by Mars Global Surveyor. Note the large crater Stickney on Phobos. (NASA)
The main alternative hypothesis is that Phobos and Deimos accreted in their present positions, perhaps from material ejected by an impact on Mars - similar to the prevailing theory for the origin of the Earth's Moon.
