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Relative similarity to Earth

The Earth is much like its "sister planet" Venus in bulk composition, size and surface gravity, but Mars' similarities to Earth are more compelling when considering colonization. These include:

The Martian day (or sol) is very close in duration to Earth's. A solar day on Mars is 24 hours 39 minutes 35.244 seconds. (See timekeeping on Mars.) Mars has a surface area that is 28.4% of Earth's, only slightly less than the amount of dry land on Earth (which is 29.2% of Earth's surface). Mars has half the radius of Earth and only one-tenth the mass. This means that it has a smaller volume (~15%) and lower average density than Earth. Mars has an axial tilt of 25.19, compared with Earth's 23.44. As a result, Mars has seasons much like Earth, though they last nearly twice as long because the Martian year is about 1.88 Earth years. The Martian north pole currently points at Cygnus, not Ursa Minor. Mars has an atmosphere. Although it is very thin (about 0.7% of Earth's atmosphere) it provides some protection from solar and cosmic radiation and has been used successfully for aerobraking of spacecraft. Recent observations by NASA's Mars Exploration Rovers, ESA's Mars Express and NASA's Phoenix Lander confirm the presence of water ice on Mars. Mars appears to have significant quantities of all the elements necessary to support Earth-based life.[1] Based on all evidence, existing environmental conditions can be considered antagonistic or fatal to all Earth-based life.

Differences from Earth

The surface gravity on Mars is 38% of that on Earth. It is not known if this is enough to prevent the health problems associated with weightlessness.[2] Mars is much colder than Earth, with a mean surface temperature of 63 C and a low of 140 C.[citation needed] The lowest temperature ever recorded on Earth was 89.2 C, in Antarctica. There are no standing bodies of liquid water on the surface of Mars. Because Mars is further from the Sun, the amount of solar energy reaching the upper atmosphere (the solar constant) is less than half of what reaches the Earth's upper atmosphere or the Moon's surface. However, the solar energy that reaches the surface of Mars is not impeded by a thick atmosphere and magnetosphere like on Earth. Mars' orbit is more eccentric than Earth's, exacerbating temperature and solar constant variations. The atmospheric pressure on Mars is ~6 mbar, far below the Armstrong Limit (61.8 mbar) at which people cannot survive without pressure suits. Since terraforming cannot be expected as a near-term solution, habitable structures on Mars would need to be constructed with pressure vessels similar to spacecraft, capable of containing a pressure between a third and a whole bar.

The Martian atmosphere consists mainly of carbon dioxide. Because of this, even with the reduced atmospheric pressure, the partial pressure of CO2 at the surface of Mars is some 15 times higher than on Earth. It also has significant levels of carbon monoxide. Mars has a very weak magnetosphere, so it deflects solar winds poorly.