With half the diameter of Earth, Mars has much lighter gravity (one-third of Earth norm) and a much thinner atmosphere. Humans cannot survive unaided on its surface.
The air on Mars, such as it is, is about 1 percent the density of air on Earth. Martian air is composed of more than 95 percent carbon dioxide. Since Mars lacks a substantial magnetic field like the Earth's, the Red Planet cannot deflect harmful radiation that comes from space.
It's also much colder on Mars than on the Earth. On Mars, the average temperature is about 210 Kelvin, or about minus 81 degrees Fahrenheit (minus 64 degrees Celsius). On Earth, however, the average temperature of the planet as a whole is about 290 Kelvin, or 62 degrees F (16.8 C).
Mars has two tiny potato-shaped moons, Phobos and Deimos. They are 14 miles (22.5 kilometers) and 8 miles (13 km) across, respectively.
Colonists on Mars may need to draw up a new kind of clock or calendar. A days on Mars - scientists call them "sols" - are slightly longer than Earth days. One Martian sol is about 24.63 hours, while an Earth day clocks in at 23.94 hours.
The Martian year is also longer than that of the Earth because it takes Mars longer to orbit the sun. One year on Mars lasts 687 days, compared to Earth's 365 days.