Mars Astrobiology: Recent Status and Progress - Planetary Exploration and Science: Recent Results and Advances - page 152

surface is dry and subfreezing, probably presenting an insurmountable obstacle
for living organisms. In addition, Mars lacks a thick atmosphere, ozone layer, and
magnetic field, allowing solar and cosmic radiation to strike the surface unimpeded.
The damaging effects of ionizing radiation on cellular structure are another one of
the prime limiting factors on the survival of life on the surface. Therefore, the best
potential locations for discovering life on Mars may be in subsurface environments.
Understanding water on Mars is vital to assess the planet's potential for harboring
life and for providing usable resources for future human exploration. For this reason,
“Follow the Water” was the science theme of NASA's Mars Exploration Program
(MEP) in the first decade of the twenty-first century. Discoveries by the 2001 Mars
Odyssey, Mars Exploration Rovers (MERs), Mars Reconnaissance Orbiter (MRO),
and Mars Phoenix Lander have been instrumental in answering key questions about
water's abundance and distribution on Mars. The ESA's Mars Express Orbiter has
also provided essential data in this quest. The Mars Odyssey, Mars Express, MER
Opportunity rover, MRO, and Mars Science Lander Curiosity rover are still sending
back data from Mars, and discoveries continue to be made.
On January 24, 2014, NASA reported that current studies on Mars by the
Curiosity and Opportunity rovers will now be searching for evidence of ancient life,
including a biosphere based on autotrophic, chemotrophic, and/or chemolithoau-
totrophic microorganisms, as well as ancient water, including fluvial-lacustrine
environments (plains related to ancient rivers or lakes) that may have been habitable.
The search for evidence of habitability, taphonomy (related to fossils), and organic
carbon on planet Mars is now a primary NASA objective (Various 2014 ).

8.3.1 Evidence from Rocks and Minerals

Today, it is widely accepted that Mars had abundant water very early in its history,
but all large areas of liquid water have since disappeared. A fraction of this water
is retained on modern Mars as both ice and locked into the structure of abundant
water-rich materials, including clay minerals (phyllosilicates) and sulfates. Studies
of hydrogen isotopic ratios indicate that asteroids and comets from beyond 2.5
astronomical units (AU) provide the source of Mars' water, which currently totals
6-27 % of the Earth's present ocean.

8.3.1.1 Water in Weathering Products (Aqueous Minerals)

The primary rock type on the surface of Mars is basalt, a fine-grained igneous rock
made up mostly of the mafic silicate minerals olivine, pyroxene, and plagioclase
feldspar. When exposed to water and atmospheric gases, these minerals chemically
weather into new (secondary) minerals, some of which may incorporate water
into their crystalline structures, either as H 2 O or as hydroxyl (OH). Examples of
hydrated (or hydroxylated) minerals include the iron hydroxide goethite (a common