On September 26th, 2013, NASA announced that the Curiosity rover had discovered liquid water on the surface of Mars. The water is not freely accessible, like on Earth, but is bound to other minerals in the soil. 2% of Martian soil, by weight, is water.
Curiosity made this discovery by drying small soil samples in an oven to 835 Celsius, which removes water and volatiles from the sample. Probes then measure the composition of the fumes coming off the samples. In addition to water, sulphur dioxide, carbon dioxide, oxygen, and perchlorates (which are toxic to humans) were also released from the soil.
A similar process is used on Earth to measure water content in soils in geotechnical engineering.
Evidence of water was first detected on Mars by the probe Mariner 9 in 1971, which imaged geological features created by erosion, including canyons and river beds. No actual water features were detected.
The absence of liquid water on the surface of Mars was confirmed in 1997, when the Pathfinder probe measured temperatures on Mars and found that it ranged from -8 Celsius to -78 Celsius: too cold for liquid water to exist on the surface, unless it was mixed with salts which would lower the freezing point. Furthermore, the daily pressure was found to be 0.007 atm, which would cause ice and water to rapidly sublimate or evaporate unless insulated by soil.
In 2003, the Odyssey probe found that Mars had an ice content of 0.5 kg ice per 1 kg soil near the poles, using gamma ray spectrometry, chemically bound to minerals below the planet surface.
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