Martian soil is toxic, due to relatively high concentrations of perchlorate compounds containing chlorine.[3] Elemental chlorine was first discovered during localised investigations by Mars rover Sojourner, and has been confirmed by Spirit, Opportunity and Curiosity. The Mars Odyssey orbiter has also detected perchlorates across the surface of the planet.
The NASA Phoenix lander first detected chlorine-based compounds such as calcium perchlorate. The levels detected in the Martian soil are around 0.5%, which is a level considered toxic to humans.[4] These compounds are also toxic to plants. A 2013 terrestrial study found that a similar level of concentration to that found on Mars (0.5 g per liter) caused:
a significant decline in the chlorophyll content in plant leaves,
reduction in the oxidizing power of plant roots
reduction in the size of the plant both above and below ground
an accumulation of concentrated perchlorates in the leaves
The report noted that one of the types of plant studied, Eichhornia crassipes, seemed resistant to the perchlorates and could be used to help remove the toxic salts from the environment, although the plants themselves would end up containing a high concentration of perchlorates as a result.[5] There is evidence that some bacterial lifeforms are able to overcome perchlorates and even live off them. However, the added effect of the high levels of UV reaching the surface of Mars breaks the molecular bonds, creating even more dangerous chemicals which in lab tests on Earth were shown to be more lethal to bacteria than the perchlorates alone.[6]