Mars is the fourth planet from the sun and has a distinct rusty red appearance and two unusual moons. The Red Planet is a cold, desert world with a very thin atmosphere. But the dusty, lifeless (as far as we know it) planet is far from dull.

Phenomenal dust storms can grow so large they engulf the entire planet, temperatures can get so cold that carbon dioxide in the atmosphere condenses directly into snow or frost, and marsquakes — a Mars version of an earthquake — regularly shake things up.

Befitting the Red Planet's bloody color, the Romans named it after their god of war. In truth, the Romans copied the ancient Greeks, who also named the planet after their god of war, Ares.

Other civilizations also typically gave the planet names based on its color — for example, the Egyptians named it "Her Desher," meaning "the red one," while ancient Chinese astronomers dubbed it "the fire star."

The bright rust color Mars is known for is due to iron-rich minerals in its regolith — the loose dust and rock covering its surface. The soil of Earth is a kind of regolith, too, albeit one loaded with organic content. According to NASA, the iron minerals oxidize, or rust, causing the soil to look red.

MARS' SURFACE

The planet's cold, thin atmosphere means liquid water likely cannot exist on the Martian surface for any appreciable length of time. Features called recurring slope lineae may have spurts of briny water flowing on the surface, but this evidence is disputed; some scientists argue the hydrogen spotted from orbit in this region may instead indicate briny salts. This means that although this desert planet is just half the diameter of Earth, it has the same amount of dry land.

The Red Planet is home to both the highest mountain and the deepest, longest valley in the solar system. Olympus Mons is roughly 17 miles (27 kilometers) high, about three times as tall as Mount Everest, while the Valles Marineris system of valleys — named after the Mariner 9 probe that discovered it in 1971 — reaches as deep as 6 miles (10 km) and runs east-west for roughly 2,500 miles (4,000 km), about one-fifth of the distance around Mars and close to the width of Australia.

Scientists think the Valles Marineris formed mostly by rifting of the crust as it got stretched. Individual canyons within the system are as much as 60 miles (100 km) wide. The canyons merge in the central part of the Valles Marineris in a region as much as 370 miles (600 km) wide. Large channels emerging from the ends of some canyons and layered sediments within suggest that the canyons might once have been filled with liquid water.

Mars also has the largest volcanoes in the solar system, Olympus Mons being one of them. The massive volcano, which is about 370 miles (600 km) in diameter, is wide enough to cover the state of New Mexico. Olympus Mons is a shield volcano, with slopes that rise gradually like those of Hawaiian volcanoes, and was created by eruptions of lava that flowed for long distances before solidifying. Mars also has many other kinds of volcanic landforms, from small, steep-sided cones to enormous plains coated in hardened lava. Some minor eruptions might still occur on the planet today.

MARS' MOONS

The two moons of Mars, Phobos and Deimos, were discovered by American astronomer Asaph Hall over the course of a week in 1877. Hall had almost given up his search for a moon of Mars, but his wife, Angelina, urged him on. He discovered Deimos the next night, and Phobos six days after that. He named the moons after the sons of the Greek war god Ares — Phobos means "fear," while Deimos means "rout."

Both Phobos and Deimos are apparently made of carbon-rich rock mixed with ice and are covered in dust and loose rocks. They are tiny next to Earth's moon, and are irregularly shaped, since they lack enough gravity to pull themselves into a more circular form. The widest Phobos gets is about 17 miles (27 km), and the widest Deimos gets is roughly 9 miles (15 km). (Earth's moon is 2,159 miles, or 3,475 km, wide.)

Atmospheric composition (by volume) According to NASA, the atmosphere of Mars is 95.32% carbon dioxide, 2.7% nitrogen, 1.6% argon, 0.13% oxygen and 0.08% carbon monoxide, with minor amounts of water, nitrogen oxide, neon, hydrogen-deuterium-oxygen, krypton and xenon.

Magnetic field Mars lost its global magnetic field about 4 billion years ago, leading to the stripping of much of its atmosphere by the solar wind. But there are regions of the planet's crust today that can be at least 10 times more strongly magnetized than anything measured on Earth, which suggests those regions are remnants of an ancient global magnetic field.

MARS' POLAR CAPS Vast deposits of what appear to be finely layered stacks of water ice and dust extend from the poles to latitudes of about 80 degrees in both Martian hemispheres. These were probably deposited by the atmosphere over long spans of time. On top of much of these layered deposits in both hemispheres are caps of water ice that remain frozen year-round.

Additional seasonal caps of frost appear in the wintertime. These are made of solid carbon dioxide, also known as "dry ice," which has condensed from carbon dioxide gas in the atmosphere.

Mars is much colder than Earth, in large part due to its greater distance from the sun. The average temperature is about minus 80 degrees Fahrenheit (minus 60 degrees Celsius), although it can vary from minus 195 F (minus 125 C) near the poles during the winter to as much as 70 F (20 C) at midday near the equator.

MARS MISSIONS AND RESEARCH

The first person to observe Mars with a telescope was Galileo Galilei, in 1610. In the century following, astronomers discovered the planet's polar ice caps. In the 19th and 20th centuries, some researchers — most famously, Percival Lowell — believed they saw a network of long, straight canals on Mars that hinted at a possible civilization. However, these sightings proved to be mistaken interpretations of geological features.

A number of Martian rocks have fallen to Earth over the eons, providing scientists a rare opportunity to study pieces of Mars without having to leave our planet. One of the most controversial finds was Allan Hills 84001 (ALH84001) — a Martian meteorite that, according to a 1996 study, likely contains tiny fossils and other evidence of Mars life. Other researchers cast doubt on this hypothesis, but the team behind the famous 1996 study have held firm to their interpretation, and the debate about ALH84001 continues today.

The Soviet Union also launched numerous Red Planet spacecraft in the 1960s and early 1970s, but most of those missions failed. Mars 2 (1971) and Mars 3 (1971) operated successfully but were unable to map the surface due to dust storms. NASA's Viking 1 lander touched down on the surface of Mars in 1976, pulling off the first successful landing on the Red Planet. Its twin, Viking 2, landed six weeks later in a different Mars region.

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