Diameter: 6,779 km (4,212 miles)
Mean distance from Sun: 227,943,824 km (141,637,725 miles)
Orbital period (year): 687 Earth days
Rotation period (day): 24.6 hours
Atmosphere: 95.3% carbon dioxide, 2.7% nitrogen, 1.6% argon,
tiny amounts of oxygen, carbon monoxide and water vapor
Mars was named for the Roman god of war.
Mars was the Roman god of war, whom the Greeks called Ares. Ares always went into battle with his sons Phobos (panic) and Deimos (terror). When American astronomer Asaph Hall discovered the two small Martian moons in 1877, he named them Phobos and Deimos.
Mars isn't bloody, it's rusty.
For a god of war, a planet the color of blood seems fitting. However the color comes from a layer of iron oxide – that's the chemical name for rust.
Mars doesn't have liquid water on the surface.
There isn't enough air pressure from the thin Martian atmosphere to keep water liquid on the planet's surface. Long ago Mars flowed with water, but it had a thick atmosphere then. There is still lots of water frozen on Mars. For example, if the southern icecap melted, there could be enough water to cover the whole planet with 11 meters (36 feet) of water.
Mars is much smaller than Earth.
The diameter of Mars is only half that of Earth and the force of gravity on Mars is only 38% of what we're used to. So a person weighing 100 lbs (45.5 kg) on Earth would only weigh 38 lbs (17 kg) on Mars. This might sound like fun, but the low gravity is one reason why Mars lost most of its atmosphere. You could run and jump, but not breathe.
Seasons on Mars are the most Earth-like of all the planets.
A planet has seasons because its axis of rotation is tilted. (For an explanation, click on the link below this article “Why Planets Have Seasons”.) Earth's axis is tilted 23 degrees and the Martian axis 25 degrees, so we have a similar pattern of seasons. Of course, Mars is a lot colder and its seasons are longer. It's one and half times farther away from the Sun than we are and its years are two Earth years long. If Mars still had a thick atmosphere, it would be warmer, because an atmosphere acts like a blanket.
The highest mountain in the Solar System is on Mars.
Olympus Mons is 27 km (17 miles) high, which is more than three times higher than Mount Everest. Mars also has the largest known canyon. Valles Marineris is about 4000 km (2500 miles) long. If the Grand Canyon in Arizona extended all the way to Florida, it still wouldn't be as long as Valles Marineris.
There weren't ever any giant canals on Mars.
Nineteenth-century Italian astronomer Giovanni Schiaparelli thought he saw long channels (canali in Italian) on Mars. This sounded like the English "canals," which are always man-made. A prominent American Percival Lowell was enchanted by the supposed great Martian civilization that had built the canals, so he built Lowell Observatory in Arizona to observe them. As telescopes got better, it was obvious that the canals were an illusion. However Clyde Tombaugh discovered Pluto at Lowell Observatory. Although we lost a Martian civilization, we gained an interesting dwarf planet.
Mars has the biggest dust storms in the Solar System.
The storms can be local and last a few days, but others cover the whole planet for months. Amateur astronomers can see really big ones through their telescopes. The Martian Rovers Spirit and Opportunity were caught in such a big storm in 2007 that NASA was afraid they'd be permanently damaged. Both rovers survived to continue their exploration, but sadly, Spirit stopped communicating on March 22, 2010 and her mission was formally ended on May 25, 2011.
The "stone face" in Cydonia is a natural rock formation.
In 1976 the Viking orbiter took a picture of the Cydonia area of Mars that included something that looked like a giant face. This was a trick of the light and shadows. You can see a more recent high-resolution picture of the same rock formation, along with the original Viking photograph.
We haven't found any life on Mars yet.
Many people still hope to find life on Mars even though ultraviolet radiation from the Sun would be fatal to any living thing at the planet's surface. Earth is protected from most of the dangerous radiation by a thick atmosphere and a magnetic field, but Mars doesn't have either of these. A future mission looking for life would be designed to dig deeply into the soil to take samples. If there is still bacterial life – or evidence of past bacterial life – we'd be most likely to find it underground.