New Mars Forums

It may turn out that substantial water will eventually be found beneath the surface of the moon.  Go just 1m down and temperature is a stable -20°C even at the equator.  So any water at this depth will not be mobile.  Any that migrates through the interlocking maze of mineral grains, will be trapped at this depth.  Any small cracks and crevices in the surface may also serve as cold traps.

Regarding the reduction of Mars iron oxide using hydrogen.  This would be a good way of producing extremely strong bricks.  Graded fine regolith is richer in iron oxide than most Earth soils.  Graded fines could be compressed into a mold producing a green brick.  The green brick is then baked in an oven at temperatures of 1000°C for a period of several hours in a hydrogen atmosphere.  The iron oxide woukd be steadily reduced by the H2.  This results in long chain oxide molecules with partial oxidation states.  This polymerised iron oxide makes the brick stronger than steel (in compression).

One material that we have examined in the past is cast basalt.  This has compressive strength of 300-500MPa.  This is about the same as the tensile strength of low alloy steel.  From the data in the attached article, I estimate that between 1.5-2MJ of heat are needed to transform 1kg basalt from a room temperature solid to a castable liquid.  Basalt has density 3t/m3, whereas low alloy steel is 7.8t/m3. 
http://www.rmag.soil.msu.ru/articles/478.pdf

Low alloy steel has an energy cost of about 30MJ/kg if produced from ore rather than recycled metal.  On Mars, it will all be produced from ore for a long time to come.  This means that 1 cubic metre of cast basalt has about 2.5% of the energy cost of the equivelant volume of steel.  We also need a lot less equipment to produce cast basalt.  An electrically heated furnace and a set of moulds to cast the ceramic members.

Cast basalt tiles could then be glued together using a thin film of epoxy resin to produce a tibrel vaulted roof.  If the tiles are polished after casting, then the epoxy film could be as thin as 0.1mm thick.  The vault would then be covered in a thick berm of compressed graded regolith.

Addressing the point made by SpaceNut.  Ultimately, it is transportation of people, goods and wastes, that limits the geographical size of a city.  I found this article to be enlightening.
https://www.d5render.com/posts/walkable … ples-guide

For an entirely pedestrian city to be practical, it must be possible to walk from any one part to any other part in a reasonable amount of time.  This is one of the reasons why preindustrial cities tend to be quite small and compact.  Ancient Rome is a notable exception.  But in general, inhabitants must be able to access all of the amenities of the city in easy walking distance of where they live.  In practice, this tends to result in pedestrian urban areas being very built up, with narrow streets and compact terrace houses.  Amsterdam, Venice and the North African medina towns demonstrate this.

The grand medina in Fez, houses some 150,000 people, along with thousands of commercial businesses and small manufacturers, all on just 1 square mile of land.  Population density is 550 people per hectare, which amounts to some 18 square metres per person.  What makes this even more incredible, is that the mud based buildings are rarely higher than two stories.  The arrangement works by making the most efficient use of space.  Street space in most cities is actually greater than that devoted to buildings.  But pedestrian streets can be as small as 2' wide and it is common for streets not to exceed 3' in width.  This is enough for people to pass each other.  The houses in Fez are also small, as they were built in more minimalist times.  On Mars, we can do better than the medieval builders of Fez.  We will be building cities in dry enclosures on a planet with only 2/5 of the gravity.  So we can safely build 3-4 storey structures out of rammed soil bricks and mortar bound stone, without needing impractically thick walls of the ground floor.  With stronger materials, we can build higher.  We can even have streets on multiple levels.  That would be impractical with cars, but can work in a pedestrian city.  The low gravity of Mars allows us to free up space for street restaurants, cafes and small gardens, whilst maintaining very high population density.

Another option occured to me when walking around the Dutch cities.  On Earth, areas with high rainfall need a sloped roof, to prevent the weight of water from overburdening roof supports.  On Mars, we will be building our cities under frames covered with rock and soil.  Essentially, artificial caves.  There will be no precipitation unless we deliberately introduce it.  Gravity is only 2/5 that of Earth.  It should be possible to build all structures with flat roof space.  This roof space can be developed as a greenspace, provided that soil is not too thick and heavy.  This would provide an open enironment that is above the cramped streets below.  In this way, we can effectively stack a garden on top of our city.  A place where people can walk and enjoy greenery.  The plants will contribute to cleaning and freshening the air.  This would be impractically heavy on Earth.  But the lower gravity of Mars could allow it.

I noticed a lot of houses did have cellars in Amsterdam.  Many of these had steps descending into them from street level and most appeared to me to have been converted into flats.  They appeared to be beneath the canal water level and presumably beneath the ground water level, which is never far beneath the surface in Holland.  When originally built, these cellars would have been used for storage.  Being beneath the water table would not necessarily have resulted in frequent catastrophic flooding, provided that evaporation balances the rate of seepage through the walls.  But it would have made these cellars rather damp places, vulnerable to fungal infestation.  To be habitable or suitable for storage of perishable items, tanking would be required, in addition to forced ventilation.  That can get expensive, as even small leaks can result in high humidity that fungus will take advantage of.

My own cellar in the UK has exactly this problem.  My house is 200 years old and was once a bakery.  Flour was stored in the damp cellar, hanging from iron hooks in the floor joists above.  I can only conclude that this arrangement was tolerable because flour was used quickly upon receipt, without giving it chance to rot.  I have made some improvements over the years.  A concrete floor.  I have repointed some of the original lime mortar within the granite walls with less permeable cement.  But damp is still a problem.

Some more pics of Amsterdam.

Utrecht (5)

Utrecht (3)

Some pictures from Utrecht.

Quite a lot to read through here, so I will comment again when I've had chance to read it all.  Gas turbine blades have always been made from high temperature nickel alloys.  Since the 90s, they have been grown as single crystals with mineral rods embedded to provide cooling channels by dissolving the rods in weak acid after casting.  So I'm not sure why the reference suggests that using nickel alloys is impractical or expensive.  It is standard aerospace practice.  Take any COTS GT and you find nickel alloy components.  For non-moving parts, steels can still be used at 700°C.  Strength will be reduced substantially and corrosion in hot CO2 will be more of a problem.  But is can be done.  There are specialist oxide dispersion strengthened mechanical alloys that were specifically developed for operation in this temperature range.

I will be back in Holland week after next.  Not somewhere I thought I would be going back to so soon, but it is where my son wants to go for holiday.  We are planning on visiting different places this time.  We are staying in Haarlem and will be getting the train to Utrecht, Delft and Den Hague.  I will take pictures as last time and post them here.

I have always found the Holland to be quite inspiring.  It is a place where about half of the land is reclaimed from the sea and much of the remainder was boggy marshland before humans terraformed it.  The sea is held back by a system of soil dikes.  Behind the dikes, water drains into ditches, and is pumped to sea level by pumps, originally wind driven.  This seems quite analogous to what we plan to do on Mars.  In that case, land will be recovered from vacuum by constructing a roof structure and covering with soil to counterbalance internal pressure.  The resulting habitable land will be relatively expensive.  Under the roof, the challenge will be to develop towns that are comfortable to live in despite high population density.  The urban architecture of pre-industrial Europe gives us solid examples of how to do that.

Much depends I think on how we construct space suits.  If we go with an MCP design, then you have an elastomer fabric covering skin.  Other, tougher garments can be worn over this.  As Robert noted, Mars dust grains are more rounded than lunar equivelent.  So I don't see that we need anything special compared to Earth based clothing.  On the moon, the situation is quite different.  That dust will destroy most fabrics quickly.  CNT or BNNT would appear to be necessary there.  Sharper dust will also be more toxic to the lungs if tracked back into the hab.

I have a GoreTex coat that is about 10 years old now and has seen heavy use.  It is still in good condition and is still reasonably waterproof.  That is a necessity in the northern parts of Britain.  The surface is easy to wipe down as it is relatively impermeable.  So it shoukdn't be difficult to keep clean on Mars.  The moon is a different case entirely.  It was noted that the original Apollo space suits were destroyed by a few days expusure to lunar dust.  I hate to think what it is going to do to astronaut lungs long term.  Will it be as bad as asbestos?