Where is the water on Mars?
We know from samples of Martian regolith obtained by Viking 1 and 2 that, globally,
there is a lot of total water contained in all of the regolith on Mars. The regolith in these samples were heated in an oven to \(500C°\). This caused water, initially frozen inside of the regolith, to be ejected in its gaseous state. This caused the weight of the regolith to decrease by \(1\)%. Thus, when the amount of water contained in these samples was measured, the regolith must have been \(1\)% water and \(99\)% other stuff. But the regolith measured in these samples was exceptionally dry and lost a lot of water content during the time interval when the regolith was extracted and stored before being cooked to \( 500C°\). We think that the average regolith on Mars is more like \(3\)% water. But that's just the amount of water stored in Martian regolith, on average. The northern latitudes of Mars are much wetter than the southern latitudes—thus, in these regions, the regolith contains much more water than the average water-content of Martian regolith. Also, some of the regolith is rich in water-contained minerals (i.e. gypsem) containing up to \(20\)% water. But the regolith isn't the only source of water on Mars. We think that there are also subsurface oceans of liquid water heated by Mars' geothermal energy. The depths of some of these oceans beneath Mars' surface is sufficiently small to be extracted. Other water sources on Mars include biles, permafrost, and frozen ice.
Methods for extracting water from Mars
"Truck, oven, and slag pile system for extracting water from Martian soil."\(^{[2]}\) Artwork by Michael Carroll.
In this lesson, we'll discuss some of the techniques which can be used to extract water from various different sources on Mars. All of the techniques used to extract water have one thing in common: they all involve heating the water in order to eject it (from the material it is frozen inside of) in its gaseous state. As discussed in the article, Terraforming and Colonizing Mars, one of the groups from the first hundred humans to arrive on Mars landed near Vistatas Borealis, a region rich in wet regolith and permafrost. One technique used by that group of Martian pioneers to extract water from the regolith would likely be the following: a dump truck would dump regolith onto a conveyor belt (see illustration above); the conveyor belt then transports the regolith to an oven powered by a \(100KWe\) (kilowat-electric) reactor. The oven would cook the regolith to \( 500C°\), hot enough to release water from that regolith in its gaseous state. The water vapor would then be collected in a condenser and subsequently stored inside of large containers. But not all of the power generated by the reactor would get converted into electricity to power the oven; the overwhelming majority of this power would get converted into waste heat. This waste heat could be used to cook additional regolith and used to extract even more water. Such a system could extract \(14,000\text{ }kg/day\) of liquid water. Much of this water would be kept, but some of it would be used to extract oxygen and hydrogen from. The amount of water and oxygen extracted thyrough this process would be more than sufficient to sustain this small group of Martians living near Vistatas Boreallis.
But other groups would prefer not such a sedintary life and would explore the vast Martian terrains. But doing so, they would need to be able to produce their own water and oxygen using Mars' indiginous resources. A very promising technique for doing just that would be to haul around a transparent domed tent using a vehicle. Humans would live inside of the vehicle and water would be extracted from regolith enclosed by the domed tent. Reflectors, which "follow" the Sun, would be used to concentrate sunlight onto the regolith heating it up to very high temperatures. Since the domed tent is a "closed environment," this would initiate a runaway greenhouse effect causing the regolith and interior of the domed tent to get hotter and hotter. The water vapor outgassed from the regolith could be condensed and stored for subsequent use as a source of drinkable water and breathable oxygen. Assuming that the water is extracted from regolith contained \(3\)% water (which is likely an underestimate) and the diameter of the hemisphereical domed tent is \(25m\), this method could be used to extracted \(224kg\) of water every eight hours—more than enough.
“One of the chief advantages of microwaves is that it will penetrate the soil, and so would greatly minimize if not eliminate requirement to dig.”
We believe that there are expansive regions of frozen permafrost near Vistatas Borealis. Extracting water from permafrost would be far less fruitful than getting your water straight from the Martian dirt, but there is a way to do it. In the novel Green Mars, Martian humans built 50 nuclear reactors across Vistatas Borealis and the northern lattitudes of Mars. One method would involve vehicles microwaving the permafrost. Such vehicles would have skirts attached to there bottoms to "trap" microwaved water vapor. This water vapor would then be condensed and subsequently stored. The electrical power requirements to power the microwaves would be enormous but could be met by attaching long cables between the vehicles and nuclear reactors. Each vehicle could extract \(2,200\text{ kilos}\) of water per day from the frozen bodies of permafrost. Such nuclear reactors could also be used to extract water from nearby subsurface water oceans.
The rover above is capable of releasing frozen water from regolith by heating it using microwaves. Credit:
SUTD/Gilmour Space Corporation
The methods we have discussed in this lesson for extracting water from Mars are some of the more efficient techniques that I have come across. There are many far less efficient techniques which have also been proposed that we won't cover here. We would likely use several different techniques for water extraction on Mars depending on the situation. In subsequent lessons, we'll analyze other industrial processes which we'll need to use to obtain other resources which are very important (and a lot of the times, essential) for long-term human survival including plastics, ceramics, glass, brick and mortar, and iron and steal just to name a few.
This article is licensed under a CC BY-NC-SA 4.0 license
References
1. Hsu, Jeremy. (2008, August). How to Mine Martian Water. Retrieved from https://www.space.com/5748-martian-water.html.
2. Zubrin, Robert. The Case for Mars: the Plan to Settle the Red Planet and Why We Must. Free Press, 2011.