Among other features, life’s creation and development depends on a source of water to play a critical role.Why do astrobiologists think that liquid water is the most important requirement for life?Life, whether on earth or alien in nature, must have a water-based foundation to create intelligence. Life must have a solvent such as water as well as at least one elemental unit for structure and function such as carbon. Solvents in their liquid are essential to allow chemical reactions and transporting materials (Houtkooper, 2007, p147).
In terms of biochemistry, water has a range of uses in creating life. Water is essentially the best solvent in terms of supporting life as it is the most abundant and it happens in its liquid state at a higher temperature array than other suitable elements (Houtkooper, 2007, p148). As it has strong hydrogen bonds, it then has a strong surface tension, encouraging the developing of membranes and the accumulation of prebiotic compounds. (Houtkooper, 2007, p148)
Water also forms bonds that are non-covalent with other elements, in Earth’s chemistry this supports the 3D structure of proteins (Stevenson, 2015, p67). Moreover, it engages in electron transport reactions which are “the key method of energy production in Earth’s chemistry” (Stevenson, 2015, p67). Evidently, biochemistry is more dynamic in an environment that is warmer, with there being more energy available to drive growth via biochemical reactions (Stevenson, 2015, p67).
Therefore, with such an environment it is more likely that organisms can prosper, reproduce and then evolve faster. There are other solvents such as hydrogen fluoride that can also engage in electron transport reactions as the liquid state is between -80 and 20 degrees Celsius at 1 atmosphere (Stevenson, 2015, p67). Therefore, this is a higher temperature range compared to other solvents that are also abundant in the universe that are not water. Statistically, these points make the greatest case for liquid water being the most important requirement for creating environments that support intelligent life. Other solvents and their biochemistries are definitely able to create and sustain life, however, they are limited to lower energy environments with slower rates of evolution and diversity.
What is the evidence for the past presence of liquid water on Mars? Evidence of water on Mars is a result of a long and complex history of environmental events. During the first part of Martian history volcanic events and asteroid impacts primarily maintained the prehistoric CO2 atmosphere. Therefore, it is assumed that the average yearly temperature was to be above freezing temperature (McKay, 2002, p57). Moreover, the pressure of the Martian atmosphere was to have exceeded one atmosphere (atm), combining all these features would’ve provided the ample environment for liquid water to be widespread. This period that had abundant CO2 in the atmosphere concluded due to the formation of carbonate, forcing the temperate and pressure to decline (Friedman, 2002, p96). Whilst this caused average temperatures to decline, peak temperatures still climbed above freezing, meaning ice covered lakes similar to those of the McMurdo Dry Valleys of Antarctica could’ve provided for the presence of liquid water (Wharton, 2002, p146).
Further along Martian history both the average and highest temperatures were below freezing, meaning the only liquid water would’ve been brief, and in small amounts. And in the end, closer to what we see as Mars today, the pressure decreased to near the triple point of water (where “the material can coexist in all three phases (solid, liquid and gas) in equilibrium”) (Wharton, 2002, p180).
This means water would no longer have been present on the surface of Mars. Findings of the several space crafts has increased the likelihood of Mars holding liquid-water in its past atmosphere. Unlike the present Martian atmosphere, a much denser earlier atmosphere provided the adequate conditions for a stable source of liquid water. Imaging from the Mariner 9 revealed the first actual evidence of past water on Mars, showing river beds, canyons and evidence of water erosion, fogs and water deposition (Baird, 1976, p194).
Images of valleys that branched out into rivers and channels confirmed that liquid water was pivotal to creating these structures (Baird, 1976, p194). The Viking orbiters discovered several geological shapes that form via the presence of water. River valleys showed evidence of flood eroding materials such as bedrock and drifting large distances (Toulmin, 1976, p202). The southern hemisphere contains branched networked of valleys, meaning that rain impacted the ground once and travelled along it. Moreover, the sides of volcanoes resemble volcanoes in Hawaii that have been exposed to rain (Toulmin, 1976, p207).
Regions of the Martian surface seem to have lost great amounts of water due to the “chaotic terrain” (where “a planetary surface area where features such as ridges, cracks, and plains appear jumbled and enmeshed with one another” (Britt, 2005, Space.com)) that causes channels to be created downstream. Still providing evidence for past water on Mars, the water activity in these regions was close to ten thousand times the Mississippi River. The Mars Pathfinder found that temperatures fluctuated on the Martian surface on the diurnal cycle. Warmest (-8 Celsius) in the afternoon and coldest (-78 Celsius) before sunrise, Martian extremes happened close to the ground and changed drastically in short periods of time (Kreslavsky, 2010, p29).
Therefore the Mars Pathfinder mission concluded that whilst it was too cold for liquid water to form and be maintained, if mixed with a range of salts, water could exist in the liquid form. The Pathfinder also confirmed that the pressure on Mars decreases daily from 6.75 millibars to under 6.7 millibars, meaning compared to Earth’s 1000 millibars, it is very low (Kreslavsky, 2010, p32). Ultimately, the pressures found by the Pathfinder would not allow for liquid water to be present, however if insulated by soil it could provide conditions for liquid water. In addition to this, the rock formations along channels and valleys suggest that rushing flood waters pushed the rocks so that they faced away from each other and that the flood waters rounded the pebbles (Kreslavsky, 2010, p33).
The Mars Odyssey has discovered amounts of water in its solid form over different and long ranging location on Mars. The high density of ice under the surface of Mars is formed in different chemical structures such as clay, however there are very low levels of water bound chemically in the soil on Mars (Arvidson, 2010, p41). Images from the Odyssey confirm the idea that Mars once maintained and stored serious amounts of liquid water on its surface. In addition to this, the Phoenix Lander found similar information to that of the Mars Odyssey, finding a few inches of ice below the Martian surface, roughly 8 inches deep (Arvidson, 2010, p42).
Ultimately, supported and confirmed by various voyagers to Mars, liquid water once flowed freely on it’s surface. Is it possible that there is still liquid water on Mars today?Whilst there is no liquid water on the surface of Mars at the present time, however there are large amounts beneath the surface. Presently, due to the Martian atmosphere (low pressure and temperature), water in its liquid form cannot be sustained apart from at the lowest points on the surface and only for a small time period (Head, 2008).
In 2006, observation from the Reconnaissance Orbiter showed deposits of gullies that were not present in 1996, giving evidence for a possible flowing brine of liquid water during warmer months on the Martian surface (Head, 2008). However, there is disagreement among the scientific community as the Martian surface obviously exhibits improper conditions- lower pressure, cold regions and the non-geothermal nature of Mars (Malin, 2006). Moreover, the gullies could’ve been created by dry grains or even softened by carbon dioxide, as the conditions are ideal for solid carbon dioxide (McEwen, 2011).
The carbon dioxide would melt in the summer and create liquid, which could then create the gully. Finally, the source of this particular water is unknown however (McEwen, 2011), which increases unreliability. However, dark streaks were noticed in 2011 that faded downslope during the warmest part of the Martian year and occurred cyclically (Wilson, 2018). It is consistent with salty water that would flow downslope and then after that, evaporate. Spectoscopic instruments have also made observations of salts that are hydrous in form at the same time of these recent gully formations (Wilson, 2018).
Moreover, these lineae have been confirmed as being produced by a slow flow of liquid brines (Wilson, 2018)that penetrate the soils, containing chlorate that is hydrated and perchlorate salt, that then have liquid water molecules. Whilst the source of these brines are still not known, it is supported that the liquid water is a result of water vapour condensation or dry flows that are granular (Wilson, 2018) . Therefore, whilst there is liquid water on today’s Mars, it is limited to traced of moisture in the Martian atmosphere, limiting the potential for life.