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This is a question of great interest in astrobiology. Does Mars in its present state have any potential habitats for native microbes, lichens, or other living organisms, either on or near the surface, or deep underground?

This article focuses on the potential habitats referred to in Objective B of NASA's first Mars Science Goal:

Goal I: determine if Mars ever supported life Objective A: determine if environments having high potential for prior habitability and preservation of biosignatures contain evidence of past life. Objective B: determine if environments with high potential for current habitability and expression of biosignatures contain evidence of extant life.

Life on Mars covers the more general topic of habitats for life through the entire history of Mars, with a brief summary for present day life.

A related question is, are there any regions of the Mars surface where Earth life could potentially survive? These are called "Special regions" in Planetary protection discussions and require special precautions to be taken for robotic missions. It's possible that extant Martian life can survive in habitats that Earth life can't tolerate and vice versa.

From the Viking landers in 1976 through to the Phoenix measurements in 2008, conditions on Mars seemed so inhospitable that many scientists believed that water "does not and cannot exist on the surface of Mars today" (to quote NASA Astrobiology magazine).. There are only five regions where the atmospheric pressure is high enough for liquid fresh water can form, Amazonis, Chryse and Elysium Planitia, in the Hellas Basin and the Argyre Basin. However, even in those places, it is close to its boiling point of 10 °C and would soon evaporate. Also there may be little by way of a source of ice to melt to form water as ice exposed to the atmosphere is not long term stable in the equatorial regions within around ± 30° of the equator.

This view of the Martian surface as totally sterile and inhosptiable to present day life changed in 2008 with the Phoenix lander, the first and so far only spacecraft to land successfully in the martian polar regions, near the north pole. It observed droplet-like features that formed on its landing legs, and also detected the presence of geologically recent surface or near surface brines indirectly, through isotopic measurements of oxygen in the atmosphere that had been exchanged with surface water. Perchlorates and other salts act as an antifreeze, and some mixes of salts can stay liquid on Mars at temperatures within the habitability range of life. In December 2013, Nilton Renno and his team using the Michigan Mars Environmental Chamber were able to simulate the conditions at the Phoenix landing site and to simulate the Phoenix leg droplets. They formed salty brines within minutes when salt overlaid ice. The team concluded that suitable conditions for brine droplets may be widespread in the polar regions. There are many other suggestions of potential habitats covered in this article.

If these droplet microhabitats exist, there are many additional challenges. The Mars soil (regolith) and dust contains between 0.5 and 1% of reactive perchlorates. The UV radiation is rapidly lethal to microbes, unless shielded, and the ionizing radiation will sterilize any dormant life within 500,000 years on the surface of Mars.

However, these conditions may not make the surface completely uninhabitable to microbial life. NASA's Mars Exploration Program Analysis Group concluded based on the Curiosity rover's RAD measurements that 500 years of ionizing radiation would kill only 90% of even the most radiation-sensitive bacterium such as E. coli. The UV is easily blocked by about 0.3 mm of surface soil,a millimeter of dust, or protective pigments such as melanin, parietin and usnic acid. The perchlorates, though harmful to some forms of life, are metabolized by others. Cassie Conley, the NASA planetary protection officer from 2006 to 2018, said of the perchlorates, “The environment on Mars potentially is basically one giant dinner plate for Earth organisms,”.

There are brines in equatorial regions too. Curiosity detected evidence of a temporary brine layer beneath the dune surfaces as it drove over them, although this particular layer is thought to cycle every day between phases when it is too cold, at least, for Earth life, or else too salty. Nilton Renno has speculated that life could take advantage of it using a biofilm to create microhabitats. Then there are seasonal dark streaks, the Recurrent Slope Lineae, that form in spring, extend and broaden through summer and autumn, and then fade away. Some of these form along the walls of the Valles Marineres in equatorial regions, and there's a site on Mount Sharp as well, not far from Curiosity. There is some evidence that they may be associated with liquid brines below the surface, although dust cascades seem to be involved as well.

Other potential habitats for present day life include lakes formed in the higher latitudes after cometary or meteorite impacts, or as a result of geothermal heat or volcanic activity. These may remain liquid for centuries, or up to a few thousand years for the largest impacts, with the heat trapped by an insulating layer of ice. Also there are suggestions that Mars may have a deep hydrosphere, a liquid layer below its cryosphere, a few kilometers below the surface. Deep rock habitats on Earth are inhabited by life so if this layer exists, it may also be habitable on Mars.. In July 2018, a lake was discovered by radar, 20 kilometers across, and 1.5 kilometers below the ice of the Southern polar plain of Mars Planum Australe. It is not yet known if this lake is habitable to Earth life.

There are, as yet, no confirmed habitats for Earth life on or beneath the surface of Mars. However there are several planned and proposed spacecraft missions to search for these potential habitats There are many instruments designed by astrobiologists to search directly for this life on Mars. The Urey instrument and Life Marker Chip separately got into the manifest for ExoMars but were later de-scoped. The first and only dedicated astrobiology missions to Mars were the two Viking landers, Viking 1 and Viking 2 in 1976. Many of the potential habitats covered in this article were first proposed after 2008.

By the nature of the topic area of this article, it focuses on the few areas where life may be possible, even if it is just an occasional droplet forming when salt falls on ice, or an occasional seep of a few millimeters thickness of salty brines in narrow streaks on occasional hill slopes from spring through to autumn. Most features and processes on Mars are not thought to be associated with life.