Pioneer Ice Mining

When the first Mars pioneer steps down from the NASA Space Exploration Vehicle or the Mars Colonial Transporter will he/she rush to dig for gold in the red landscape?  No, he will not.  Will he/she search immediately for platinum, plutonium, or molybdenum, the highly valued metals on Earth?  No, he will not.  He will set up his equipment to collect the most precious material to be found on Mars: water.  

As of 2014 we know that water is abundant on Mars.  For example, when the Mars rover Pathfinder set down on the surface using its thrusters, it swooshed aside a layer of regolith dust and behold, there was ice beneath the lander.(1)  When the Mars Reconnaissance Orbiter surveyed the landscape near Hellas Planetia, the martian plains near the giant crater Hellas, then behold, it observed glaciers.(2)  When the Mars Science Laboratory tested the surface regolith on the seemingly dry surface of Gale Crater, behold, there was ice water in the amount of 2 percent.(3)

Ice Below Phoenix

 

The problem of Mars water is not its quantity but its availability.  If a dry land farmer from Earth were to plant his seeds in the martian soil and wait for rain, he would grow fetid before he grew crops;  It has not rained on Mars for at least 3 billion years.  All the water is trapped in very cold ice in the top surface layer, the cryosphere.  Water will never come to a Mars pioneer in the form of rain so he will have to go after it.  He will have two ways of collecting the water ice: well mining or block mining. 

Diamond Bit ChainsawBlock mining is the easier of the two methods.  It starts with locating the colonial settlement near a large source of ice that has been identified ahead of the landing of colonists.  The ice may be identified by means of observation from orbit or by robotic landers that confirm the presence of ice on the ground.  The source  will be a glacier or a deposit near the surface.  A place in permanent shade from the sun is a place that is likely to contain ice, perhaps under a light layer of regolith.  A source of high concentration of ice may be mined by simply cutting the ice mass into easily handled blocks and carrying them to a storage vault.  Of course this ice will represent dirty water from the presence of regolith, but no bacterial contamination.  A second process will be required to treat the water before it can be used.

 One type of equipment that will be used to cut ice into blocks is a diamond-bit chain saw. 

On earth, a handheld chainsaw is commonly used to cut ice into blocks or to carve ice sculptures, but why use a diamond bit?  The reason is that Mars ice will not be pure water ice, but will contain random chunks of basaltic stone and regolith.   A normal chain saw bit will dull rapidly if it encounters rock of any kind, but most basaltic rock will have a hardness of 7 on theQuarry Saw Mohs scale, about the same as quartz (3). Slicing Mars ice will be comparable to cutting hard stone on Earth, so stone-cutting tools with imbedded polycrystalline diamond bits will be necessary.  Even this type of bit may wear out fairly rapidly. (4)  

On Earth, diamond-bit chain saws are used in stone quarrying operations.  The adjacent figure shows a chainsaw mounted onto a metal frame that allows the saw to rotate in a vertical or horizontal direction.  The frame is laid onto a roller track, thus allowing the saw to make a series of block cuts by moving the frame down the track.  Such a fixture could be handled by an operator inside a Mars EVA vehicle (MEV) equipped with mechanical arms or a front end loader to realign the track and frame.  (MEVs are the subject of another article.)  For this type of ice mining a MEV truck will be useful to bring the ice blocks to a storage facility.  It may be necessary to keep the ice covered to avoid direct exposure to the sun; otherwise rapid boil-off could lose the water/ice into the atmosphere.  

Ice Storage and Treatment

 An ice and water storage facility represents an opportunity to build a useful structure from native materials.  Of course fuel tanks and other containers from earth will be easily converted into storage tanks for liquids such as water, liquid oxygen, hydrogen, and liquid methane.  But tanks can also be constructed by the same block building technique that was describe in Pioneer House Building.  However, instead of a long horizontal building, a vertical structure with an arched roof will be put together.  The same stone blocks used for the arched roof of a house will be deployed to build an arched tank top.

Ice Treatment and Storage

To begin, the location for the ice/water tank should be laid out in conjunction with the planned village.  Proximity to the village will limit the amount of pipes required to deliver water to it.  If the water is to be treated, a second tank should also be constructed to receive the treated water.  A treated water tank will be analogous to an elevated water tank on earth that stores water for a community.  Water will be supplied to the village through a water distribution system of pipes and valves.  

One consideration in locating the tank is the method of insulation.  This will almost always be regolith which means gathering and placing regolith around the completed tank(s).  To facilitate the piling of regolith around the tank, it may be located in a natural depression, such as a crater.    Alternatively, a hole could be dug to accommodate tank construction at a lower level.  A second consideration will be the method for filling the tank and discharging the liquid contents.  If material in solid form (ice) is to be deposited into the top of the tank, then a ramp made of regolith and stone will be constructed to allow a colonist to dump ice blocks into the tank.  

The above sketch shows an example of an  arrangement of two tanks for ice and water treatment and storage.   In this figure a horizontal dotted line represents the original ground level before any construction begins.  Then a pit will be dug out for tank number one (left) and the excavated rocks and regolith will be set aside for further use.  Note that this excavation will only be feasible where the surface is amenable to digging and not where it consists of bedrock.  It also requires suitable equipment is such as a backhoe.  If only a front end loader is available, the original ground level may be the best elevation for tank 1. As shown in the figure, the second (storage) tank will be  elevated for the purpose of distributing water to the village by gravity flow.  This may be accomplished by siting the tank on a naturally occurring mound or by building a moundIce Truck of stone and regolith.  The higher tank elevation may also be achieved by siting the tank against the side of a hill; however, a tank placed on a hillside will be difficult to cover with regolith because of the great amount of fill required on the downhill side.

 As shown, tank 1 is used to receive ice blocks from an ice mining operation and for storing them for batch processing.  A door is installed near the top of tank 1 for loading ice from a conveyance (an MEV ice-truck ) into it.  Water and ice on Mars will not have any bacterial contamination to be treated, so treatment will consist of separating regolith from the water.  This could be important because regolith particles may have sharp edges that could be damaging if consumed.  The figure shows a sloping metal plate that forms a sedimentation basin for collecting regolith at the bottom of the tank.  It also provides a cavity where equipment and devices will be placed.  These devices will consist of a heater, an evaporator, and a pump.  The heater will serve two purposes.  First, it will raise the temperature of the tank so as to melt the ice.  Second, a small amount of melt water will be heated to boiling and the collected steam will be pumped to the second tank. 

 Note that the ground on Mars will always be cold and the steam in the line will likely become water once again.  To prevent the water/steam from freezing, these pipes will have to be continuously heated.  On Earth, water pipes exposed to possible freezing conditions are equipped with water pipe heating cable that is usually strapped to the pipe with utility (Duck) tape.  The same will be needed here.

Note that boiling and condensing the water is the most important treatment process to separate regolith particles from the water because it will achieve nearly nearly 100 percent removal.  Nevertheless, water filters will be advisable within the martian house as a further safety measure.Water Pipe Heat Cable

Also note that water and steam passing from tank 1 into tank 2 is likely to encounter very cold conditions in tank 2, at least before heating.  As the water and steam spurt from the pipe, temperature and pressure will drop rapidly, thus creating a snow blower effect that will soon fill tank 2 until it is remelted.  Water will then be stored and distributed into the community pipes in liquid form.  

This two-tank treatment and storage system will be a batch process because a certain amount of time will be required to fill the tank with ice.  Then a substantial amount of heat will be required to melt the ice, boil it, and drive it over to the next tank.  Before the next batch of ice is processed, the bottom of the tank and the bottom of the evaporator will need to be cleaned of sediment.  This will be accomplished by a person in a space suit who will be lowered into the tank by a cable; a bucket, shovel and hand tools will be required.  

Tanks 1 and 2 are designed to be built by the same procedures as those employed for building the Mars house. (See the article Pioneer House Building.)  The same size stones will be employed as well as he same  scaffold to construct an arched ceiling over each tank.  The standard size square tank will be 4 m by 4 m by 6 m high and will have a nominal capacity of 28,000 gallons.  The equipment compartment and ice and snow displacement could substantially reduce the effective storage volume of each tank.  On the other hand, tank size could be increased by lengthening the side walls or by building a taller tank, still using the same blocks and scaTo begin building a standard tank, the site will be prepared over a 5 m by 5 m area by removing rocks and boulders, breaking any jutting rocks, applying regolith, and by tamping to achieve a flat hard surface.   The floor of a water storage tank must be more robust than a house because of the heavy weight of water.  Thus the entire floor will be composed of foundation blocks rather than tile blocks, as were used in the Mars house.  Foundation blocks are  60 cm wide by 40 cm high by 100 cm long, the same as house foundation blocks. They will be positioned with block lengths in the same direction as the length of the wall.  (The direction of the walls of a square tank will be arbitrarily chosen.)   Each row will require 10 blocks for a floor width of 600 cm and 6 rows for a floor length of 600 cm. Every other row will require two half-width blocks to avoid cracks between blocks being aligned with cracks in the adjacent row.  As each row of blocks are laid down, the edges of blocks should beWater Storage Tank melted together by a methalox torch to hold them together during further construction.  

The scaffold with arch supports willset up directly upon the foundation/floor pad with the width of the scaffold in the same direction as the width of the floor.  The lower section of the scaffold, containing  three vertical beams, will be the same height as the vertical sidewalls, as shown in the adjacent sketch.  Oblique braces will also be longer to support the middle beam.  The upper arch support structure will be the same structure as used for house building.  As before, the ends of the walls will be set back 15 cm from the edges of the corresponding foundation blocks so that walls will be centered on foundation blocks.  Long oblique braces will also be required in the third dimension to support the scaffold during the laying of arch blocks.  Note that only one position of the scaffold is required to build a square tank.   

After the sidewall stones and arch stones are laid, the end-walls will be constructed so that the outer edge of each end-wall is even with the ends of the sidewalls.  Each end-wall will be completely built of stone blocks with no gaps except for an opening for a door under the ceiling arch at one end, as shown in the previous figure of tank 1.  The door will be designed to be tight-fitting to its door frame, but it does not have to be airtight as do house portals.  The door should, however, be thick and insulated for temperature control.  After all blocks for the tank have been laid, the cracks inside the tank should be reinspected and brazed with a methalox torch, as needed.  

Well Mining

 A second method of obtaining water from natural sources on Mars will be well mining which will consist of drilling to install s well casing, melting ice at the bottom of the casing, and pumping meltwater to a storage tank.  The drilling component will be analogous to well drilling on Earth, but not the same.  This is because  the extreme cold of the subsurface of Mars ensures that no water will be found underground, only ice.  Drilling will be carried out in order to sink a 6-inch well casing that will allow a heater and a pump to be lowered into the ground.  This will require a drilling rig to be imported from Earth which will cause the initial cost to be higher than simple ice mining.  The ice around the heater will be melted and the melt water pumped into a storage tank.

Well-mining will be employed where ice mining is impractical.  In this article the location chosen for the new Mars village is assumed to be near a large source of water. This source could be above ground where ice mining is more feasible or below ground.  In the case of an above-ground source such as a glacier, the ice will be mixed with some regolith and rocks.   The concentration of water in the glacier needs to be at or above 90 percent, otherwise the chain saws cutting the ice into blocks will rapidly become dulled despite being diamond-tipped.  On the other hand, when the water source is located in the subsurface cryosphere the concentration of water needs to be greater than 30 percent . If the water concentration is found to be less than this, one could question whether the source is large.  When subsurface water concentration falls into the appropriate range then well mining can be put into operation. 

In the diagram of well mining shown in the sketch below certain details may be noted.  First, the size of the well casing (six inches) is judged to be the minimum size that can accommodate 1) a submerged well water pump and screen such as those used on Earth, 2) an accompanying electrical cord for the heater, 3) a water line that contains the meltwater pumped out of the well,  4) a water pipe heater cable to keep the water line from freezing, and 5) a heater with accompanying electrical cord.  Note that lightweight PVC pipe is utilized to save weight in shipping from Earth, except for the pipe section containing the heater and pump .  This section is built of steel to withstand the heat that is generated and the abrasion from sediment and water. 

Also note that theWell Mining Diagram storage tank in this diagram is built on bedrock and not on the cryosphere where the ice to be mined is located .  This is to avoid building over an area potentially subject to subsidence.  The design of the tank will be similar to tank 2 in the previous sketch rather than tank 1 because the incoming water will be liquid rather than ice.  The melt water is sprayed into the tank to avoid back-pressure from the weight of water pushing on the water in the income ing water pipe.   Water sprayed into the tank may also produce a snow blower effect unless the tank is preheated to maintain water temperature above the freezing point.  

Well-mining can be employed at a depth of about 50 meters, but the depth limit is really determined by the weight of equipment the colony sponsors are willing to transport from Earth.  Shallow drilling may be accomplished with a minimum of drilling devices, but deeper drilling requires heavier equipment. For every activity on Mars a balance must be found between the cost of transport and the effectiveness of the equipment imported from Earth.  On the other hand, well drilling at a depth of less than 50 meters introduces the potential danger of subsidence.  Note that melting underground ice will create a large cavity.  The size of that cavity may or may not be known depending on the sophistication of underground monitoring devices that can be brought from Earth.  A second unknown will be the strength of the ceiling of the underground cavern being created.  Some safety precautions may include the following:

            > Locate a colony/village on solid bedrock and not on the cryoshere where subsidence could occur due to mining.

            >  Maintain a minimum of equipment directly over a well mining shaft.

            >  Employ devices to monitor the extent of cavitation.

            >  Move the well mining operation from time to time.

            >  Mark off any area suspected of near-surface cavitation to preclude any unnecessary surface activities over it.

 

References

 

  1. A view of the ground surface under the Pathfinder lander with ice clearly showing through the dust.  I cant find my original reference for this photo.
  2. Jackson School of GeoSciences, University of Texas at Austin, Glaciers on Mars, geology.com, Accessed 4-01-2014.
  3. Wikipedia, Mohs Scale of Mineral Hardness, Accessed 3-30-2014.
  4. K. Zacny and G. Cooper, Coring basalt under Mars low pressure conditions, The International Journal of Mars Science and Exploration, Mars 3, 1-11, 2007;doi:10.1555/mars2007.2007.0001.