Mars is the fourth planet from our Sun, as well as the second-smallest major planet in our Sun’s family after Mercury. In English, Mars is named for the Roman god of war because of its rusty-red hue. This reddish coloring comes courtesy of the large quantities of iron oxide on the Martian surface, and it is unique among the astronomical bodies visible to the unaided human eye. Mars is a solid, terrestrial planet, that displays only a thin atmosphere. It also possesses surface features reminiscent both of the impact craters of Earth’s Moon and the polar ice caps, valleys, and deserts of Earth.
The Martian days and seasons are also comparable to those of our own planet. This is because the rotational period as well as the tilt of the rotational axis relative to the ecliptic plane are similar for both sister worlds. Mars also hosts Olympus Mons, the largest volcano and tallest known mountain in our entire Solar System. Another surface feature, named Valles Marineris, is one of the largest canyons in our Sun’s familiar family of planets, moons, and smaller objects. The smooth Borealis basin, located in the northern Martian hemisphere, covers 40% of the planet and is thought to be a gigantic impact scar left by a huge crashing object. Mars also is circled by a duo of tiny moons, Phobos and Deimos, which are irregularly shaped and resemble potatoes. The two little moons are considered to be captured asteroids.
The first observations of Mars were made by ancient Egyptian sky-watchers. By 1534 BCE, these very early astronomers were already familiar with the retrograde motion of the Red Planet. By the time of the Neo-Babylonian Empire, the Babylonian sky-watchers were making regular records of the positions of planets, as well as systematic studies of their behavior. In the case of the Red Planet, the ancient astronomers discovered that it made 42 circuits of the zodiac every 79 years. These ancient astronomers even devised mathematical methods in order to make minor corrections in regard to the predicted positions of the planets in our Solar System. The ancient sky-watchers called the planets “wandering stars.”
In the fourth century BCE, the ancient Greek philosopher Aristotle noted that Mars disappeared behind Earth’s Moon during an occultation. This indicated that the Red Planet was farther away from Earth than our Moon. The Greek astronomer, Ptolemy–who lived in Alexandria, Egypt–tried to determine the orbital motion of Mars, and his collective works and model on astronomy were presented in his multi-volume collection, under the title Almagest. The Almagest was the authoritative work on Western astronomy for the next four centuries.
Ancient Chinese astronomers were also familiar with Mars no later than the fourth century BCE. In the fifth century CE, the Indian astronomical work titled Surya Siddhanta presented a measurement of the estimated diameter of the Red Planet. In East Asian cultures, Mars is usually called the “fire star”–based on the Five Elements: wood, water, earth, metal, and fire.
In the 17th century, the astronomer Tycho Brahe measured the diurnal parallax of Mars that Johannes Kepler had used to make early calculations of the distance of Mars from Earth. When the earliest telescopes, used for astronomical purposes became available, the diurnal parallax of Mars was determined to make this measurement in 1692. However, these early measurements were flawed because of the poor quality of the telescopes.
Mars hasn’t always looked the way we see it today. The Red Planet suffered a catastrophic tilt billions of years ago. Before this tilt occurred, the Martian poles were not situated where they are now.
There are current investigations assessing the past habitability potential of the Red Planet, as well as the possibility of extant life. Future astrobiology missions are currently being planned. These missions include the Mars 2020 and Rosalind Franklin rovers. Liquid water cannot pool on the Martian surface today–except at the lowest elevations for brief periods–because of the low atmospheric pressure, which amounts to less than 1% of Earth’s.
Dozens of crewless spacecraft, including rovers, orbiters, and landers, have been sent to Mars by the United States, Europe, India, and the Soviet Union. These missions observed the Red Planet’s surface, climate, and geology. For the past twenty years, cameras in orbit around Mars have dispatched back to our planet a cornucopia of revealing pictures of the “fire star”.