User:Robertinventor/Mars Surface Life

Present
Present day life on Mars could occur kilometers below the surface in the hydrosphere, or in subsurface geothermal hot spots, or it could occur on or near the surface. The permafrost layer on Mars is only a couple of centimeters below the surface. Salty brines can be liquid a few centimeters below that but not far down. Most of the proposed surface habitats are within centimeters of the surface. Any life deeper than that is likely to be dormant. Water is close to its boiling point even at the deepest points in the Hellas basin, and so cannot remain liquid for long on the surface of Mars in its present state, except when covered in ice or after a sudden release of water. The Mars atmosphere varies in pressure over geological timescales, and may have been able to host liquid water in the recent geological past.

So far, NASA has pursued a "Follow the water" strategy on Mars and has not searched for biosignatures for life there directly since Viking. The observations by Phoenix in 2008 of potential drops of liquid brines forming on its legs lead to a renewed interest in the potential habitability of the surface of Mars. Nilton Renno and his team recently found a way that these droplets could form rapidly when salt and ice touch each other so may have formed when salt and ice from the surface got thrown up onto the legs during the landing. . Since then, experiments have lead to many suggestions for potential habitats on the surface of Mars. However, though liquid water is now confirmed to occur there in brine layers, its not yet known whether any of the liquid water on Mars is habitable. This depends on factors such as the exact mix of salts and the local conditions on Mars.

Dormant subsurface life
Curiosity measured ionizing radiation levels of 76 mGy a year. . This level of ionizing radiation is sterilizing for dormant life on the surface of Mars. However, it varies considerably in habitability depending on its orbital eccentricity and the tilt of its axis. If the surface life has been reanimated as recently as 450,000 years ago, which is possible, then our rovers on Mars could find dormant but still viable life at a depth of only one meter below the surface, according to an estimate in the paper that published the Curiosity ionizing radiation measurements.

Habitability factors for non dormant surface life
Modern researchers do not consider that ionizing radiation is a limiting factor in habitability assessments for present day non dormant surface life. The level of 76 mGy a year measured by Curiosity is similar to levels inside the ISS. . In the 2014 Findings of the Second MEPAG Special Regions Science Analysis Group, their conclusion was:


 * "From MSL RAD measurements, ionizing radiation from GCRs at Mars is so low as to be negligible. Intermittent SPEs can increase the atmospheric ionization down to ground level and increase the total dose, but these events are sporadic and last at most a few (2–5) days. These facts are not used to distinguish Special Regions on Mars."

Here a SPE is a Solar Proton Event (solar storm) and a GCR is a Galactic Cosmic Ray. A "Special Region" is a region where Earth life could potentially survive.

UV radiation
On UV radiation, the report finds


 * "The martian UV radiation environment is rapidly lethal to unshielded microbes but can be attenuated by global dust storms and shielded completely by < 1 mm of regolith or by other organisms."

Perchlorates
Though the superoxidizing conditions are harmful to some microbes, there are many microbes that actually metabolize perchlorates on Earth. See Perchlorates - Biology. Nowadays perchlorates on Mars are generally thought as boosting habitability. Even when Phoenix discovered perchlorates in 2008, NASA said that the perchlorates do not rule out life on Mars. For a modern view on them, Cassie Conley, planetary protection officer for NASA is quoted in the New York times as saying: :

""The salts known as perchlorates that lower the freezing temperature of water at the R.S.L.s, keeping it liquid, can be consumed by some Earth microbes. “The environment on Mars potentially is basically one giant dinner plate for Earth organisms,” Dr. Conley said.""

Recurrent Slope Lineae - potentially habitable
These features form on sun facing slopes at times of the year when the local temperatures reach above the melting point for ice. The streaks grow in spring, widen in late summer and then fade away in autumn. This is hard to model in any other way except as involving liquid water in some form, though the streaks themselves are thought to be a secondary effect and not a direct indication of dampness of the regolith. Although these features are now confirmed to involve liquid water in some form, the water could be either too cold or too salty for life. At present they are treated as potentially habitable, as "Uncertain Regions, to be treated as Special Regions"

The "Special Regions" assessment says of them:


 * "Although no single model currently proposed for the origin of RSL adequately explains all observations, they are currently best interpreted as being due to the seepage of water at > 250 K, with $$a_w$$ [water activity] unknown and perhaps variable. As such they meet the criteria for Uncertain Regions, to be treated as Special Regions. There are other features on Mars with characteristics similar to RSL, but their relationship to possible liquid water is much less likely"

They were first reported in the paper by McEwan in Science, August 5, 2011. . They were already suspected as involving flowing brines back then, as all the other models available involved liquid water in some form. Finally proven pretty much conclusively to involve liquid water in some form, possibly habitable if temperatures and salinity are right - after detection of hydrated salts that change their hydration state rapidly, reported in a paper published on 28th September 2015 along with a press conference. The brines were not detected directly, because the resolution of the spectrometer isn't high enough for this, and also the brines probably flow in the morning. MRO is in a slowly precessing sun synchronous orbit inclined at 93 degrees (orbital period 1 hr 52 minutes). Each time it crosses the Mars equator on the sunny side, South to North, the time is 3.00 pm, in the local solar time on the surface, all year round. This is the worst time of day to spot brines from orbit.

Subsurface
Although Mars soils are likely not to be overtly toxic to terrestrial microorganisms, life on the surface of Mars is extremely unlikely because it is bathed in radiation and it is completely frozen. The radiation environment on the surface, as recently determined by Curiosity rover, "is so high that any biological organisms would not survive without protection." Therefore, the best potential locations for discovering life on Mars may be at subsurface environments that have not been studied yet. The extensive volcanism in the past possibly created subsurface cracks and caves within different strata where liquid water could have been stored, forming large aquifers with deposits of saline liquid water, minerals, organic molecules, and geothermal heat – potentially providing a habitable environment away from the harsh surface conditions.

Surface brines
Although pure liquid water does not appear at the surface of Mars, there is conclusive evidence of hydrated perchlorate brine flows on recurring slope lineae, based on spectrometer readings of the darkened areas of slopes. Astrobiologists are keen to find out more, as not much is known about these brines. Some geologists think that brines may provide a potential habitat for terrestrial salt and cold-loving microorganisms (halophile psychrophilic). Several biologists argue that although chemically important, thin films of transient liquid brine are not likely to provide suitable sites for life, as the activity of water on salty films, the temperature, or both are less than the biological thresholds across the entire Martian surface and shallow subsurface.

The damaging effect of ionizing radiation on cellular structure is one of the prime limiting factors on the survival of life in potential astrobiological habitats. Even at a depth of 2 meters beneath the surface, any microbes would probably be dormant, cryopreserved by the current freezing conditions, and so metabolically inactive and unable to repair cellular degradation as it occurs. Also, solar ultraviolet (UV) radiation proved particularly devastating for the survival of cold-resistant microbes under simulated surface conditions on Mars, as UV radiation was readily and easily able to penetrate the salt-organic matrix that the bacterial cells were embedded in. In addition, NASA's Mars Exploration Program states that life on the surface of Mars is unlikely, given the presence of superoxides that break down organic (carbon-based) molecules on which life is based.