Mars is the last terrestrial planet and the fourth planet in our solar system. It is the most likely target for a manned space mission. Ancient civilizations were fascinated by it and honored Mars as a god; for the Romans and Greeks, it was the god of war.
Due to iron oxide (rust), Mars has a reddish color and therefore earned the nickname 'the Red Planet.' It is also the only planet in our solar system with two moons.
To date, over 50 space probes have been sent to Mars, although not all were successful. The Soviet/Russian space agency, in particular, faced many challenges with their missions. The first successful flyby was achieved by the American spacecraft Mariner 4 in July 1965. The first mission to make contact with the Martian soil was the Soviet spacecraft Mars 2; however, it did not send back usable data as the spacecraft was destroyed upon landing.
Currently, an array of spacecraft is orbiting Mars, including the European Mars Express and the American space probes Mars Odyssey, and Mars Reconnaissance Orbiter. Additionally, two rovers, Curiosity and Perseverance, are exploring its surface.
Surface
Mars has only a quarter of the surface area of Earth and only one-tenth the mass, though its surface area is approximately equal to that of Earth's dry land.
It is probable that Mars was once very similar to Earth and only due to the weak force of gravity lost its water and thick atmosphere. However, the thermal emission spectrometer on the Global Surveyor found no detectable carbonate signatures in surface materials at scales ranging from three to ten kilometers (two to six miles) during its six-year Mars mapping mission. But recent data on mineral deposits from the OMEGA instrument on board the European Mars Express probe confirm the Red Planet's humid past.
In addition, the Rovers found traces of ancient water, but the quantity is not clear. Additionally, Mars orbiters (especially the Mars Odyssey and later the Mars Express spacecraft) found "dirty ice" underground at the north and south poles of the planet. Although the visible polar caps consist of frozen carbon dioxide (dry ice).
Since Mars was volcanically more active in the north than in the south in the past, traces of meteorite impacts are mainly in the south. For example, the Lowell Crater, which was named after the American astronomer Percival Lowell. On the other hand, in the north are the largest volcanoes of our solar system, including the largest, Olympus Mons.
Periodically, great dust storms engulf the entire planet. The effects of these storms are dramatic, including giant dunes, wind streaks, and wind-carved features.
Another characteristic feature of Mars is the largest canyon in our solar system, the Valles Marineris, often referred to as the Mariner Valley.
Orbit
A sol (or solar day) on Mars is very close to Earth's day: 24 hours, 39 minutes, and 35.244 seconds, but a year on Mars is nearly twice as long as on Earth. Mars moves in an elliptical orbit around the sun and has an eccentricity of 0.0935 and an inclination of 1.85°.
Atmosphere
Mars' atmosphere is thin: the atmospheric pressure on the surface is only 750 pascals, about 0.75% of the average on Earth. However, the scale height of the atmosphere is about 11 km, somewhat higher than Earth's 6 km. The atmosphere on Mars is 95% carbon dioxide, 3% nitrogen, 1.6% argon, and contains traces of oxygen and water.
In 2003, Earth-based telescopes detected traces of methane for the first time, which were confirmed by both NASA and ESA in 2004. The presence of the unstable gas methane in the atmosphere indicates that there must be “methane sources” on Mars, or at least that they existed hundreds of years ago. Possible sources include active volcanoes, comet impacts or even methane-producing microorganisms. The methane is not evenly distributed, but shows a pattern of slightly higher concentrations.
Life on Mars?
One of the most exciting questions in science today is whether life could develop on Mars. The evidence, however, remains inconclusive. On the one hand, it was discovered that the liquid water on Mars must once have had a high salt content, which would have made it difficult for microbes to form. At a NASA press conference in 1996, David S. McKay and other scientists claimed to have found traces of fossil bacteria in the meteorite ALH 84001. However, the scientific community remains divided over whether these structures are truly biological in origin. [1]
Tectonics
Unlike on Earth, where larger (continental) plates drift against each other, plate tectonics does not occur on Mars. Instead, hot lava rises from the interior and presses against the crust. The InSight Lander made the first-ever detection of quakes on the Red Planet. Its onboard seismometer measured over 1,300 seismic events and its onboard seismometer detected a magnitude 4 marsquake in 2021 that scientists later determined to be caused by one of the biggest meteoroid impacts on Mars.
Crust & Core
The InSight probe found out that the crust is somewhat thinner than expected, about 15 to 25 miles (25 to 40 kilometers) thick, comprising three internal layers. In addition, Mars has a mantle that extends 969 miles (1,560 kilometers) below the surface and a molten core, but it is not solid iron.
Magnetic field
Unlike the Earth, Mars does not have a global magnetic field. This was confirmed in 1997 with the arrival of the Mars Global Surveyor probe, equipped with a magnetometer.
Moons
Asaph Hall, an American astronomer, supported by his wife in his search for Martian moons, discovered Deimos in August 1877 and Phobos six nights later.
Ninety-four years later, NASA's Mariner 9 spacecraft provided a much clearer view of the two moons from its orbit around Mars. On Phobos, it discovered a dominant crater 10 km (6 miles) wide—nearly half the width of the moon itself—named Stickney, after Angelina Stickney's maiden name.
Mars' moons are among the smallest in the solar system. Phobos is slightly larger than Deimos, and orbits only 6,000 km (3,700 miles) above the Martian surface—no known moon orbits its planet more closely. It circles Mars three times a day, while the more distant Deimos takes 30 hours for each orbit. Phobos is gradually spiraling inward, drawing about 1.8 meters closer to the planet each century. Within 50 million years, it will either crash into Mars or break up and form a ring around the planet.
The Face of Mars
In 1976, images from the Viking orbiter caused a stir because they appeared to show the features of a human-like face. It was not until 2001 that the Mars Global Surveyor probe produced a new, sharper image, clearly showing that this was just a rock formation.
Canals and the invasion from Mars
Since the 18th century, people have firmly believed that Mars is home to life. The assumption was supported by the observations of astronomers Percival Lowell and Giovanni Schiaparelli, who believed they had spotted artificial canals on Mars and called them canali, giving birth to the myth of Martian canals. The firm belief in “Martians” led to a mass panic on the east coast of the USA on October 30, 1938, as Orson Welles published a radio play on CBS based on the book “War of the Worlds” by H.G. Wells, in which people were confronted with an invasion from Mars. However, because many radio listeners missed the introduction explaining the fictional nature of the play due to a concurrent baseball broadcast, they mistakenly believed they were experiencing an actual extraterrestrial invasion.
But that was not all, in the 1950s astronomers discovered a strange sign on Mars that looked like an “M” and had a diameter of several hundred kilometers. Many commentators at the time suggested that the “M” stood for Mars and that a Martian civilization wanted to draw attention to itself, while others thought that it was not an “M” but a “W” and that this could be a declaration of war. However, the mysterious sign soon disappeared and in all likelihood it was only caused by one of the frequent sandstorms on Mars, which only spared the tops of four large volcanoes, temporarily revealing the vague outline of an “M” or “W”.