Mars

Mars is the 4th planet from the Sun, and one of the 5 planets visible with the unaided eye – it's impossible to say who actually discovered Mars. When Mars is closest to the Earth, it becomes one of the brightest stars in the sky, appearing bright and red. Because of this bright red color, ancient peoples associated Mars with blood and war. The ancient Romans named Mars after their god of War, the same persona as the Greek god Ares. The symbol for Mars also represents the god of War.

Mars orbits the Sun at an average distance of 228 million km, which is about 1.5 times the distance of the Earth to the Sun. Earth and Mars can get as close as 54.6 million km apart, making Earth the closest planet to Mars.

The size of Mars makes it the third largest terrestrial planet, after Earth and Venus. The diameter of Mars is 6,792 km across. This is about half the diameter of Earth. The mass of Mars is even lower; the planet has a mass of 6.42 x 10²³ kg, which gives it a mass of only 10% compared to Earth. This lower mass gives Mars a much lower gravity. If you could stand on the surface of Mars, you would experience about 38% the force of gravity that you experience on Earth. The volume of Mars is only 15% the volume of Earth.

Like all of the planets in the Solar System, Mars formed approximately 4.6 billion years ago when the solar nebula collapsed. Scientists determined the age of Mars by studying meteorites from Mars that landed on Earth millions of years ago.

The composition of Mars, is similar to the Earth. The surface is mostly volcanic in origin, with vast plains of basaltic rock – this gives Mars its red color. The density of Mars is 3.94 g/cm³, which means that it probably does have a core of iron below the surface, but scientists don't think it rotates, like we have on Earth, and so, the Mars lacks a significant magnetic field. Without a magnetic field, Mars has no protection from the solar wind and radiation from space – this would make it almost impossible for life on Mars to exist at the surface; but it could be underground.

Water ice in the northern polar region of Mars. Credit: ESA/DLR/FU Berlin (G. Neukum).

A day on Mars is just a little longer than a day on Earth: 24 hours, 39 minutes and 35 seconds. And a Martian year is 1 year, 320 days and 18 hours. Like Earth, Mars is tilted with respect to the Sun. The tilt of Mars is 25-degrees, which gives Mars seasons, very similar to what we have on Earth. Of course, the temperatures on Mars can dip down to lower than -100 degrees C in the winter in the poles. Even in the summers, the hottest Mars gets is only about 20-degrees C in the heat of the day.

Known as the "Red Planet", Mars appears as a red world in telescopes, and even from the surface of the planet. Unlike Earth, it's a dry, dusty world without moisture and almost no atmosphere. The atmosphere of Mars has only 1% the pressure of Earth's atmosphere. Mars has no oceans or seas; instead, the world is covered with vast deserts of jagged rocks, sand, and mountains. The red-orange look to the planet comes from the higher proportion of iron (III) oxide – also known as hematite, or rust – in the Mars dust. Even with the thin atmosphere, Mars has weather. Mars can have dust devils and planet wide dust storms that make the surface impossible see clearly.

Mars is dry today, but scientists think that the planet probably had oceans and rivers in the past; there was water on Mars. The shorelines of ancient oceans and lakes are visible from orbit, as well as huge tributary channels, where river systems drained to the lakes. But this water dried up billions of years ago; the water was probably lost to space, with the planet's atmosphere. NASA's Mars Exploration Rovers have found multiple lines of evidence that show that large bodies of water were present on the surface of Mars for long periods of time.

The surface of Mars is heavily cratered. Most of the Mars craters were formed billions of years ago, which indicates that the planet's surface hasn't been heavily weathered for a long time. Mars has some of the most extreme surface features in the Solar System. One of these is Olympus Mons, the largest mountain in the Solar System; and the largest volcano on Mars. This shield volcano towers 26 km above the surrounding terrain. It's believed that plate tectonics on Mars shut down billions of years ago, so the hotspot under Olympus Mons just kept releasing lava. Another amazing feature on the surface of Mars is the Valles Marineris valley, which stretches 4,000 km around the planet, and can be 7 km deep in places. And recently caves were discovered on Mars.

Mars has two irregular, asteroid-like moons: Phobos and Deimos. Phobos (approximately 22 km across) orbits Mars at an average distance of 9,377 km; completing a single orbit in 7 hours and 39 minutes. Because Phobos orbits Mars faster than the planet rotates on its axis, the orbit of Phobos is decaying. In the next 10 million years or so, Phobos will be torn apart by the gravity of Mars; and then the pieces will crash onto the surface. Deimos measures 11.4 km across, on average, and orbits at a distance of 23,460 km from the surface of Mars. Mars has no rings, and only two moons.

Apart for Earth, of course, Mars is the most explored planet in the Solar System. Several orbiters are currently circling the planet, captured images from space – such as the famous Mars face – and rovers and landers are on the surface sampling the soil, examining rocks, and searching for evidence of past and present water. For example, NASA's Mars Polar Lander recently searched for ice on Mars at the planet's northern pole. Spacecraft take about a year to travel from Earth to Mars, and the launch window opens up every two years, so you can expect more missions will be on their way soon. Of course, it's really hard to land on Mars.

Because the rotation of Mars is similar to Earth, as well as the tilt and relatively warm temperatures in the Martian summers, many space enthusiasts think that Mars would be the ideal planet to explore and eventually colonize. As technology improves, future Mars colonists might even be able to terraform Mars, thickening its atmosphere, allowing plants to grow, and making it more habitable for human life.