Humans have long looked to Mars with curiosity, imagining a world that might one day sustain life beyond Earth. Water is a fundamental requirement for all known life forms and an indispensable resource for future human exploration. Water on Mars presents both promising opportunities and significant challenges for human consumption. While evidence confirms water exists on Mars, its form and composition mean it is not readily drinkable. This reality shapes strategies and technologies for future missions, highlighting the ingenuity required to make Martian resources usable for astronauts.
Water’s Presence on Mars
Water on Mars exists predominantly as ice, found in various locations. Significant amounts are locked within the northern and southern polar ice caps. The northern polar cap, primarily water ice, is about 1,000 kilometers in diameter and contains approximately 1.6 million cubic kilometers of ice, enough to cover the cap to a depth of 2 kilometers if melted. The southern polar cap, while smaller at 350 kilometers in diameter, also contains substantial water ice beneath a permanent layer of carbon dioxide ice.
Beyond the poles, substantial subsurface water ice, often called permafrost, is detected across much of Mars. More than 5 million cubic kilometers of ice are estimated to be at or near the surface, enough to cover the entire planet to a depth of 35 meters if uniformly spread. Water ice concentrations exceed 20% poleward of 60° latitude, with evidence suggesting its presence at shallower depths closer to the equator. Robotic missions have provided direct proof, with the Phoenix lander uncovering solid water ice at 68° north latitude.
While liquid water is generally unstable on the Martian surface today, geological features provide clear evidence of its extensive past presence. Ancient riverbeds, lakebeds, and deltas suggest a warmer, wetter environment billions of years ago. Minerals like clays and sulfates, which form only in the presence of liquid water, have been found by rovers like Perseverance, confirming water flowed as recently as 2 to 2.5 billion years ago. Trace amounts of water vapor are also present in the thin Martian atmosphere, varying with season and location, and forming cirrus-like water-ice clouds.
Why Martian Water is Not Drinkable
Despite the abundant ice on Mars, it is not suitable for direct human consumption due to several factors, including chemical contamination, its physical state, and a lack of sterility. A primary concern is perchlorates, naturally occurring salts found in Martian soil. These compounds are toxic to humans, particularly affecting the thyroid, and cannot be removed by heating or boiling. Martian ice and soil are often mixed with other dissolved salts and particulate matter, rendering extracted water unpalatable and potentially harmful without extensive treatment.
The extreme physical conditions on Mars present additional challenges. Most water exists as ice because the average surface temperature is well below freezing, around -58°C. Even if the ice were to melt, the extremely low atmospheric pressure (averaging only 6 to 7 millibars, less than 1% of Earth’s sea level pressure) means liquid water would rapidly boil and sublimate into vapor before it could be easily collected or stored. This makes accessing and maintaining liquid water on the surface incredibly difficult.
Furthermore, Martian water lacks the natural protection found on Earth, contributing to a non-sterile environment. The planet’s thin atmosphere offers minimal shielding against solar and cosmic radiation, degrading water quality. Dust and fine particulate matter are ubiquitous on Mars, frequently lifted by winds and dust storms. This particulate matter would contaminate exposed water sources, requiring rigorous filtration for human use.
Processing Martian Water for Human Use
Making Martian water drinkable requires advanced extraction and purification technologies. One method involves digging up water-rich soil or ice and heating it. Techniques include baking soil in ovens or using microwave beams, which efficiently heat water molecules to vaporize ice. The resulting vapor can then be collected and condensed into liquid.
Once extracted, water undergoes a multi-stage purification process to remove contaminants. Distillation is a leading candidate, involving vaporizing and re-condensing water, leaving impurities like salts and perchlorates behind. Vacuum membrane distillation, operating at lower pressures, is explored for efficiency on Mars. Reverse osmosis is another proven method, forcing water through a semi-permeable membrane to filter dissolved substances, including perchlorates, bacteria, and viruses. Ion exchange resins can also target and remove negatively charged contaminants like perchlorates.
These extraction and purification processes demand significant energy, generated on Mars through solar or potentially nuclear sources. Such systems are part of In-Situ Resource Utilization (ISRU). ISRU aims to use local Martian resources to produce consumables like water, oxygen, and rocket propellant, reducing the need to transport heavy supplies from Earth. This approach is essential for establishing a sustainable human presence on Mars, enabling longer missions and greater independence from Earth.