Mars presents arguably the best location to for humans to make the first steps in colonizing space, not least of all for its abundant reserves of water.
Mars has a considerable but modest atmosphere, so that its inhabitants probably enjoy a situation in many respects similar to our own. – William Herschel, 1784 address to the Royal Society
One year ago, on November 26th, an Atlas V Rocket took to the skies from Cape Canaveral, Florida, sending on an escape trajectory 4 metric tons of scientific payload. As the rocket booster lurched ponderously towards the sky under 136,000 kg of thrust, one more robotic explorer joined the ranks of those who would map the solar system on behalf of mankind – first and foremost, to see what’s there, and second, to see if we are alone. Half a billion kilometers later, last month the Mars Science Laboratory – nicknamed Curiosity – touched down successfully on the Martian surface, to carry out its eponymous mission of exploration and discovery. Its primary objectives, to study the climate and geology, to search for signs of life, and thirdly, to further analyze the presence and role of water on Mars.
The history of the discovery of water on Mars is at once a history of our capacity for exploration as well as a history of the human imagination. Since ancient times the Planets, meaning wanderers in Greek, have captured our attention for their ability to move across the night sky whereas all other stars remain fixed. Planets were not known to be anything other than points of light until Galileo’s telescope revolutionized our perspective in 1609, which revealed that planets are places, as real as the Earth. Surface features of Mars were first mapped by Dutch Astronomer Christiaan Huygens in 1659, who also discovered Saturn’s rings. Huygens observed a dark spot on the surface of Mars and thought it water, and so named it Syrtis Major, after the Roman name for the Bay of Sidra on the coast of Libya. In reality, it is a series of valleys and impact craters, however in 1719 Maraldi observed that Mars has polar ice caps much like the earth, which have since been found to be mostly water ice.
Water on Mars first gained international attention in 1877, when Giovanni Schiaparelli noted the appearance of straight lines on its surface. He called these Canali, Italian for grooves, which was mistranslated into Canals in English. British astronomer William Herschel jumped upon the idea of canals on Mars, and wrote science fiction for the public describing an ancient and noble race constructing massive irrigation projects in a desperate bid to channel water from the polar caps to the desertifying latitudes. Canali were shown to be optical illusions as more powerful telescopes developed, but the seed of wonder at Mars was instilled in the population at large and remained rooted in science fiction and public imagination for generations to come.
Exploration and Evidence
The Vallis Marineris would have taken tens of thousands of cubic kilometers of flowing water to carve
In the modern era of space exploration Mars is by far the most visited of the other worlds, some 50 missions have been attempted yet only 21 have been successful. Of those attempts, Americans have launched 20 missions, the Russians 19, the European Space Agency (ESA) has sent two 2, and British, Japanese, and Chinese one each. The first spacecraft to orbit another planet, Mariner 9, entered Martian orbit in 1971 on a mission of surface photography. For the first time in history the detailed terrain of another planet was seen by human eyes: river networks, valleys, volcanoes, even weather fronts and fog. Though arid and lifeless, it was unmistakably similar to the Earth. Subsequent orbiters have substantiated these first impressions with detailed maps showing evidence for rainfall patterns, seasonal variations in ice coverage and massive aquifer (underground lakes) outflows. The largest of these outflows carved a canyon 25 kilometers wide and hundreds of meters deep, requiring a flow of water 10,000 times that of the Mississippi River.
Networks of valleys resemble those made on Earth by rainfall patterns
Surface missions to Mars began in 1976 with the Viking probes, which reported further evidence for water flooding, deep valley erosion, and estimated a soil water content as high as 1% by volume. Mobile surface robots, named Rovers, landed on Mars in 1997, twice in 2004, and again in 2012. Of these rovers two are currently operational, Opportunity and Curiosity. Rovers both qualified and elaborated on the initial findings from the orbiters and landers. Namely, atmospheric pressures are far too low and cold for surface liquid water, but highly salty water might exist in liquid form meters below ground. Additionally, there is evidence for hot springs bringing material up to the surface, and last month Curiosity sent back photographs of smoothly worn pebbles like the kind found in riverbeds showing that, at some time in the past, Mars had rivers that flowed for hundreds of years. Most notably, in the southernmost and northernmost one third of the planet, the top 10 meters of soil is as much as 50% water ice by volume, meaning a person’s daily water needs can be satisfied by a small bucket worth of Martian soil.
How much water?
An estimate of the total water on Mars, carried out by the Odyssey orbiter mission in 2001, is roughly twice the volume of Lake Michigan, or about 10,000 cubic kilometers of water. For comparison, 10,000 cubic kilometers was the estimate for total Arctic sea ice in 2010. The finding that Mars has as much water as there is Arctic ice on Earth might surprise some who imagine Mars as a dry, arid place, but evidence points to there being even greater amounts of water in Mars’s past.
Indeed, many planetary scientists argue that expansive oceans were once a feature of Mars. The geography of Mars is dramatically split between Northern and Southern hemispheres. The entire North is thousands of meters lower than the South, thought to be due to a massive impact early in Mar’s history. Along the boundaries between South and North, numerous sites show evidence of ancient shorelines and erosion. This ocean hypothesis is also substantiated by the direction and level of erosion of valley networks, showing rain that fell close to the North and returned through rivers. An ocean matching the shorelines found would, in fact, cover 75% of the surface of Mars. Such an ocean would have had some 60,000,000 cubic km of water, an amount to fill the entire Mediterranean Sea 20 times over. If Mars once had water in such abundance, where is it today?
The history of Martian water
To understand where Mars’ water has gone, it’s necessary to understand two features of the Martian climate – its air pressure, and temperature. Currently temperatures average -40°C on Mars and atmospheric pressure is roughly 0.6% of Earth’s. As a result, any exposed water quickly freezes then evaporates directly into a gas, and once in the atmosphere is struck by radiation from the sun and split into Oxygen and Hydrogen. The Oxygen reacts with iron minerals in the Martian soil, rusting together and giving the planet its reddish color. The Hydrogen, being lightest of all elements, rises high into the atmosphere before being blown off into space by the pressure of the Sun’s light. Every second, some 5 – 6 kg of Martian atmosphere is forever lost to space. On Earth, a powerful magnetic field protects our atmosphere from the Sun’s radiation, and organic processes continuously renew the supply of highly reactive oxygen.
Over billions of years the supply of water on Mars has frozen, killing the planets ability to retain heat. As the water ice slowly separates into hydrogen and oxygen the hydrogen is blown away into space by solar winds, while oxygen binds to iron in the crust rusting the planet into its current reddish hue.
The water we see today on Mars is just a tiny remainder from the past, the leftovers of a world-wrapping ocean. Many scientists believe the Martian atmosphere was once as thick as the Earth’s, composed mainly of CO2. A thick atmosphere of CO2 is important for planetary water cycles, as the CO2 traps heat from the sun and the thick atmosphere allows water to be in liquid form, but also acts as a resevoir of heat preventing drastic swings in temperature from night to day. Over great periods of time, CO2 binds to rocks to form carbonate minerals. On Earth these carbonates are washed down from dry land onto oceanic plates, which slowly subduct beneath the continents and form volcanic mountain ranges through heat and friction. Volcanoes release this CO2 back into the atmosphere when they erupt, completing the CO2-rock cycle.
Mars, however, has only one-tenth the mass of the Earth, and therefore much less internally generated heat. Less heat means less active movement of material within the planet’s mantle, preventing the formation and movement of tectonic plates and thus the formation of new volcanoes. Without volcanic renewal of CO2, the atmosphere of Mars slowly settled into the ground, forever trapped in a static geology. As CO2 decreased the temperature dropped, eventually low enough for the great world ocean to freeze completely. When the ocean froze it turned white, and began reflecting most of the Sun’s light back into space. Temperatures plummeted, so much so that most of the remaining CO2 atmosphere froze into dry ice, and snowed itself on to the poles where it remains today. Without any atmosphere and hardly any heat the frozen world ocean began evaporating into space, its Hydrogen component blown away by stellar winds and the Oxygen rusting with iron in the soil, which gives the planet its ruddy hue. The Mars we see today is a frozen, airless shadow of its former self, the lifeblood of its hydrosphere lost to space or else absorbed into endless dunes of red sand
A one-way street?
As mentioned, the great majority of Martian water is locked up in the soil along with the entire CO2 atmosphere. Much of the atmosphere and water are in the form of ice crystals that, if heated, would readily evaporate back into gas. Once the CO2 evaporates into the air it again traps the Sun’s energy, raising the temperature and releasing more CO2, and so on in a reinforcing process. Warming Mars by as little as 7°C could kick-start the whole hydrosphere and climate, though the best method for doing so is hotly debated. Among the possibilities
• Hundreds of square kilometers of mirrors in orbit to reflect sunlight on to the poles, melting the CO2 ice and artificially keeping temperatures high
• Launching thousands of robotic factories that belch a continuous stream of pollution chemicals – precisely the same substances that are killing life on Earth could make Mars suitable for life in the future
• Drilling deep under the frozen soil and detonating hundreds or thousands of thermonuclear warheads to melt the permafrost. Over centuries radiation in these areas would fall to safer levels.
• Dragging asteroids made of frozen methane, nitrogen, and water ice into extremely close approach orbits. Even with a sparse atmosphere, speeds of 30km per second would quickly vaporize this material into a ready-made atmosphere.
Whatever the route may be, Mars has all the elements we need – carbon, iron, sulfur, phosphates, nitrogen, and an abundance of water. Mars is ready to receive us; the technology required is within our grasp. The only question remains is, are we ready for Mars?
Raising temperatures by as little as 7 K could release substantial CO2 deposits into the atmosphere, kickstarting a positive feedback loop that might eventually make Mars a habitable, warm, and water-laden planet once more.