Mars habitability analogue environments on Earth are environments that share potentially relevant astrobiological conditions with Mars. These include sites that are analogues of potential subsurface habitats, and deep subsurface habitats.
A few places on Earth, such as the hyper-arid core of the high Atacama Desert and the McMurdo Dry Valleys in Antarctica approach the dryness of current Mars surface conditions. In some parts of Antarctica, the only water available is in films of brine on salt / ice interfaces. There is life there, but it is rare, in low numbers, and often hidden below the surface of rocks (endoliths), making the life hard to detect. Indeed, these sites are used for testing sensitivity of future life detection instruments for Mars, furthering the study of astrobiology, for instance, as a location to test microbes for their ability to survive on Mars, and as a way to study how Earth life copes in conditions that resemble conditions on Mars.
Other analogues duplicate some of the conditions that may occur in particular locations on Mars. These include ice caves, the icy fumaroles of Mount Erebus, hot springs, or the sulfur rich mineral deposits of the Rio Tinto region in Spain. Other analogues include regions of deep permafrost and high alpine regions with plants and microbes adapted to aridity, cold and UV radiation with similarities to Mars conditions.
Precision of analogues
Mars surface conditions are not reproduced anywhere on Earth, so Earth surface analogues for Mars are necessarily partial analogues. Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly. There are no full-Mars simulations published yet that include all of the biocidal factors combined.
Ionizing radiation. Curiosity rover measured levels on Mars similar to the interior of the International Space Station (ISS), which is far higher than surface Earth levels. Atmosphere. The Martian atmosphere is a near vacuum while Earth's is not. Through desiccation resistance, some life forms can withstand the vacuum of space in dormant state. UV levels. UV levels on Mars are much higher than on Earth. Experiments show that a thin layer of dust is enough to protect microorganisms from UV radiation. Oxidizing surface. Mars has a surface layer which is highly oxidizing (toxic) because it contains salts such as perchlorates, chlorates, cholorites, and sulfates pervasive in the soil and dust, and hydrogen peroxide throughout the atmosphere. Earth does have some areas that are highly oxidizing, such as the soda lakes, and though not direct analogues, they have conditions that may be duplicated in thin films of brines on Mars. Temperature. Nowhere on Earth reproduces the extreme changes in temperature that happen within a single day on Mars. Dry ice. The Mars surface consists of dry ice (CO2 ice) in many areas. Even in equatorial regions, dry ice mixed with water forms frosts for about 100 days of the year. On Earth, although temperatures on Earth briefly get cold enough for dry ice to form in the Antarctic interior at high altitudes, the partial pressure of carbon dioxide in Earth's atmosphere is too low for dry ice to form because the depositional temperature for dry ice on Earth under 1 bar of pressure is and the lowest temperature recorded in Antarctica is , recorded in 2010 by satellite.
These partial analogues are useful, for instance:
Testing life detection equipment which may one day be sent to Mars Studying conditions for preservation of past life on Mars (biosignatures) Studying adaptations to conditions similar to those that may occur on Mars As a source of microbes, lichens etc. that can be studied as they may exhibit resistance to some conditions present on Mars.
Atacama Desert
The Atacama Desert plateau lies at an altitude of 3,000 metres and lies between the Pacific and the Andes mountains. Its Mars-like features include
Hyper arid conditions Cold compared to most arid deserts because of the altitude High levels of UV light (because it is relatively cloudless, also the higher altitude means less air to filter the UV out, and the ozone layer is somewhat thinner above sites in the southern hemisphere than above corresponding sites in the northern hemisphere) Salt basins, which also include perchlorates making them the closest analogues to Martian salts on Earth.
Yungay area
The Yungay area at the core of the Atacama Desert used to be considered the driest area on Earth for more than a decade, until the discovery in 2015 that Maria Elena South is drier. It can go centuries without rainfall, and parts of it have been hyper-arid for 150 million years. The older regions in this area have salts that are amongst the closest analogues of salts on Mars because these regions have nitrate deposits that contain not only the usual chlorides, but also sulfates, chlorates, chromates, iodates, and perchlorates. The infrared spectra are similar to the spectra of bright soil regions of Mars.
The Yungay area has been used for testing instruments intended for future life detection missions on Mars, such as the Sample Analysis at Mars instruments for Curiosity, the Mars Organic Analyzer for ExoMars, and Solid3 for Icebreaker Life, which in 2011, in a test of its capabilities, was able to find a new "microbial oasis" for life two metres below the surface of the Atacama desert. It is the current testing site for the Atacama Rover Astrobiology Drilling Studies (ARADS) project to improve technology and strategies for life detection on Mars.
Experiments conducted on Mars have also been successfully repeated in this region. In 2003, a group led by Chris McKay repeated the Viking Lander experiments in this region and got the same results as those of the Viking landers on Mars: decomposition of the organics by non-biological processes. The samples had trace elements of organics, no DNA was recovered, and extremely low levels of culturable bacteria. This led to increased interest in the site as a Mars analogue.
Although hardly any life, including plant or animal life, exists in this area, the Yungay area does have some microbial life, including cyanobacteria, both in salt pillars, as a green layer below the surface of rocks, and beneath translucent rocks such as quartz. The cyanobacteria in the salt pillars have the ability to take advantage of the moisture in the air at low relative humidities. They begin to photosynthesize when the relative humidity rises above the deliquescence relative humidity of salt, at 75%, presumably making use of deliquescence of the salts. Researchers have also found that cyanobacteria in these salt pillars can photosynthesize when the external relative humidity is well below this level, taking advantage of micropores in the salt pillars which raise the internal relative humidity above the external levels.
Maria Elena South This site is even drier than the Yungay area. It was found through a systematic search for drier regions than Yungay in the Atacama Desert, using relative humidity data loggers set up from 2008 to 2012, with the results published in 2015. The relative humidity is the same as the lowest relative humidity measured by Curiosity rover.
A 2015 paper reported an average atmospheric relative humidity 17.3%, and soils relative humidity a constant 14% at depth of 1 metre, which corresponds to the lowest humidity measured by Curiosity rover on Mars. This region's maximum atmospheric relative humidity is 54.7% compared with 86.8% for the Yungay region.
The following living organisms were also found in this region: Actinomycetota: Actinobacterium, Aciditerrmonas, and Geodermatophilus Pseudomonadota: Caulobacter and Sphingomonas Bacillota: Clostridiales Acidobacteriota: Acidobacterium 16 new species of Streptomyces, 5 of Bacillus and 1 of Geodermatophilus.
There was no decrease in the numbers of species as the soil depth increased down to a depth of 1 metre, although different microbes inhabited different soil depths. There was no colonization of gypsum, showing the extreme dryness of the site.
No archaea was detected in this region using the same methods that detected archaea in other regions of the Atacama Desert. The researchers said that if this is confirmed in studies of similarly dry sites, it could mean that "there may be a dry limit for this domain of life on Earth."
McMurdo Dry Valleys in Antarctica
These valleys lie on the edge of the Antarctic plateau. They are kept clear of ice and snow by fast katabatic winds that blow from the plateau down through the valleys. As a result, they are amongst the coldest and driest areas in the world.
The central region of Beacon Valley is considered to be one of the best terrestrial analogues for the current conditions on Mars. There is snowdrift and limited melting around the edges and occasionally in the central region, but for the most part, moisture is only found as thin films of brine around permafrost structures. It has slightly alkaline salt rich soil.
Don Juan Pond Don Juan Pond is a small pond in Antarctica, 100×300 metres, and 10 cm deep, that is of great interest for studying the limits of habitability in general. Research using a time-lapse camera shows that it is partly fed by deliquescing salts. The salts absorb water by deliquescence only, at times of high humidity, then flows down the slope as salty brines. These then mix with snow melt, which feeds the lake. The first part of this process may be related to the processes that form the Recurring Slope Lineae (RSLs) on Mars.
This valley has an exceptionally low water activity (aw) of 0.3 to 0.6. Though microbes have been retrieved from it, they have not been shown to be able to reproduce in the salty conditions present in the lake, and it is possible that they only got there through being washed in by the rare occasions of snow melt feeding the lake.
Blood Falls
This unusual flow of melt water from below the glacier gives scientists access to an environment they could otherwise only explore by drilling (which would also risk contaminating it). The melt water source is a subglacial pool of unknown size which sometimes overflows. Biogeochemical analysis shows that the water is marine in source originally. One hypothesis is that the source may be the remains of an ancient fjord that occupied the Taylor valley in the tertiary period. The ferrous iron dissolved in the water oxidizes as the water reaches the surface, turning the water red.
Its autotrophic bacteria metabolize sulfate and ferric ions. According to geomicrobiologist Jill Mikucki at the University of Tennessee, water samples from Blood Falls contained at least 17 different types of microbes and almost no oxygen. An explanation may be that the microbes use sulfate as a catalyst to respire with ferric ions and metabolize the trace levels of organic matter trapped with them. Such a metabolic process had never before been observed in nature. This process is of astrobiological importance as an analogue for environments below the Glaciers on Mars, if there is any liquid water there, for instance through hydrothermal melting (though none such has been discovered yet). This process is also an analogue for cryovolcanism in icy moons such as Enceladus.
Subglacial environments in Antarctica need similar protection protocols to interplanetary missions.