Relative similarity to Earth
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.)The Earth is similar to its "sister planet" Venus in bulk composition, size and surface gravity, but Mars' similarities to Earth are more compelling when considering colonization. These include:
- 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°, similar to 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 like Earth's.
- Recent observations by NASA's Mars Reconnaissance Orbiter, ESA's Mars Express and NASA's Phoenix Lander confirm the presence of water ice on Mars.
Differences from Earth
- While there are kinds of micro-organisms and lichens that survive in extreme environmental conditions, including simulations that approximate Mars, plants and animals generally cannot survive the ambient conditions present on the surface of Mars.
- The surface gravity of Mars is 38% that of Earth. Although microgravity is known to cause health problems such as muscle loss and bone demineralization, it is not known if Martian gravity would have a similar effect. The Mars Gravity Biosatellite was a proposed project designed to learn more about what effect Mars' lower surface gravity would have on humans.
- Mars is much colder than Earth, with a mean surface temperature between 186 and 268 K (−87 °C and −5 °C). The lowest temperature ever recorded on Earth was −93.2 °C, in Antarctica.
- There are no standing bodies of liquid water on the surface of Mars.
- Because Mars is farther from the Sun, the amount of solar energy entering the upper atmosphere per unit area (the solar constant) is less than half of that entering the Earth's upper atmosphere. However, due to the thinner atmosphere, more solar energy reaches the surface.
- Mars' orbit is more eccentric than Earth's, increasing temperature and solar constant variations.
- Due to the relative lack of a magnetosphere, in combination with a thin atmosphere – less than 1% that of Earth's – Mars has extreme amounts of ultra-violet radiation that would pose an ongoing and serious threat.
- The atmospheric pressure on Mars is ~7.5 mbar, far below the Armstrong Limit (61.8 mbar) at which people can survive without pressure suits. The atmospheric pressure on Earth, at sea level, is 1,013 mbar, 135 times that of Mars. 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 300 and 1000 mbar.
- The Martian atmosphere is 95% carbon dioxide, 3% nitrogen, 1.6% argon, and traces of other gases including oxygen totaling less than 0.4%.
- Martian air has a partial pressure of CO2 of 7.1 mbar, compared to .31 mbar on Earth. CO2 poisoning in humans begins at about 1 mbar. Even for plants, CO2 much above 1.5 mbar is toxic. This means Martian air is completely toxic to both plants and animals even at the reduced total pressure.