All Mars Colony renders are wrong. Learn why we live in deep mines.
Many articles have been written about the different dangers of living on Mars. The Mars colony is frequently presented as a backup solution for an asteroid collision with Earth. However, we might need to live deep underground there. And it might be a good way, how to source the energy needed.
Mars's atmosphere is very thin. Not just solar wind can penetrate it. Any rock can too.
If we are considering moving to Mars to avoid some planet killer asteroid wiping our civilization, we should make sure that our colony on Mars is not at even greater risk.
We can see small craters on the Mars surface as large as 13 feet across. So they are formed by objects about the size of a soccer ball. This size of meteor is harmless on Earth as they burn in high altitudes of our atmosphere. The meteorite needs to have a few meters in diameter to represent some risk at Earth. The real issue is much larger objects like the 10m+ meteor, which exploded over Chelyabinsk, Russia a few years ago. Earth meets with just a few objects of this size every year. However, there are about 100–10.000 smaller objects hitting Earth every year and so will Mars. That is a very high risk of even the smallest meteor hitting one of the agriculture domes on Mars would not only ruin a day but perhaps wipe out the whole colony. A settlement the size of the New York City metro area might see such an impact every 20 years.
You can stop solar flares with walls made of frozen water. But you can’t stop a small meteorite without tens of meters of dirt and ground. All visualizations of human settlement on Mars's surface are just wrong. Ground dome structures are fine for the scientific outpost. However in case of the permament seatlement, the safety concerns would demand living under the ground for tens of years or perhaps centuries. The lava tubes as shelters to start are more likely.
How to create an atmosphere at Mars. Fast.
The current Mars atmosphere is from the perspective of plants and animals almost vacuum. We can built a magnetic shield in space to protect it. However to re-introduce it might be a challange. We could try to build some mirrors in space or perhaps nuke the poles of the planet, as famously proposed by Elon Musk, to release some of the frozen CO2 into the atmosphere. However, some studies show that there might not be enough frozen CO2 to make a useful atmosphere on Mars. They estimated that a maximum of 7% of the Earth's sea-level pressure would be achieved.
That might barely be enough for some plants. Even with 70k Pa of pressure, (100 x increase from the current baseline) the water boiling temperature would be very low — less than 40 °C (104°F). We need to increase the pressure at least 10 times (7 kPa) to see liquid water on the planet's surface. Otherwise, it immediately boils or freezes into ice.
Perhaps we could find some ice-rich asteroids. Use their kinetic energy & hit Mars poles. Not only that it would convert much of CO2 into gas, but it could convert some of the ice into water gas to increase pressure and fastrack the green house efect. We would kill two birds with a single stone.
How many of these projectiles we would need? Mars has 144.37 million km2. Let’s say that we would need at least the amount of water equal to 5m of the water column to achieve sufficient pressure. That would mean that 721.000 km3 of water. That is about 1000 asteroids of the size that wipe out the dinosaurs. We would need to adjust the trajectories in such a way that they would hit Mars at the right place. If we could do it, we don’t need to worry about the accidental impact on Earth as we could simply divert such an asteroid.
Perhaps we could find a few such asteroids to melt CO2 at poles, but not likely hundreds. That sounds like a stretch for our current and near-future space capabilities. So the terraformation would be a very slow and gradual process using the existing Mars resource. So even a small penetration or pressure door malfunction would be a fatal error. Every mechanical system would fail once. It is just a matter of time even if properly monitored. So all necessary crops should be rather grasped in vertical underground gardens with multiple safety measures than instead of transparent domes on the ground.
Hence, humans would need to use a pressure suit for any walk on the Mars surface for centuries. Is there any other option? Yes, but we will need to build the colony deep, very deep into the Mars crust.
The colony in the deep underground system
On Mars, the surface pressure varies through the year as more C02 vaporizes. However, the pressure changes exponentially with the elevation. For example, the lowest place on Mars lies in the Hellas impact basin and Valles Marineris, which are both about 7 km below “Mars sea level”.
So the atmospheric pressure model built by NASA for Mars shows about twice as much pressure (1.33 kPa) than at the “sea-level” with (0.69 kPa). If we would dig a 10 km hole, the pressure would increase to 3.29 kPa at the bottom. That is almost 5x more than at sea level of Mars. We have gold mines on Earth 4km deep and the deepest underground drilling is about 12km.
So with Mars's lower gravity, we have already at our disposal technology to get to 20–30 km deep. That would immediately provide a pressure increase to 8.08 kPa or 19.88 kPa respectively (up to 20% of Earth sea-level pressure).
That is much closer to the long-survival limit for humans of 60 kPa, which is 4 km above the sea level on Earth. Hence we would need to increase the density of Mars atmosphere just about 3- 6 times. This seems feasible to achieve even by using the existing CO2 on Mars without the need for an intense asteroid bombardment.
So with the right atmospheric composition, it seems that we could just build a tunnel deep underground without the need for any door. The pressure would gradually increase to a sufficient level for farming, perhaps some simple breathers with oxygen for humans.
How deep can we dig at Mars?
As recently determined by the InSight probe, Mars’ core is surprisingly large with the crust approximately 15 kilometers or 47 kilometers thick. Beneath the crust comes the mantle with the lithosphere of more solid rock reaching 400–600 kilometers down — twice as deep as on Earth. We would need to deal with the increasing temperature in case we would like to make a deep colony. The mantle of Mars consists of liquid Iron-Nickel, which has a melting temperature of around 1,455 °C (similar to Earth). So the temperature gradient of 4° to 7°C/km expected within the upper of the crust seems reasonable.
Starting with the freezing -50°C ground temperature, the comfortable 25°C temperature would be at 11–20 km depth. We could either use the traditional The boring company technology. Or eventually leverage a modern concept of a robotic swarm, which would dig the underground cities on Mars for the human settlement.
To dig an extra 10–15 km to get the required atmospheric pressure, we would require to build a giant heat-exchanger to cool down the area dedicated for the colony. In other words, we would need a deep geothermal electric power plant. The liquid cold CO2 or even water would be circled down, heated, and vaporized. It would not only cool the ground down but convert the heat into a reliable geothermal energy source, perhaps the best option for Mars colony.
With the fast elevator, we would reach the Mars surface in 15–20 minutes. So any ground operations could be simply done. As Mars's atmosphere would be built and pressure would increase, the colony could just slowly move up. So it is closer to the surface without any risk of habitat unexpected decompression until the atmosphere can sustain life on its surface.
Conclusion
It seems that the Mars colony would look like more Trantor — a fictional planet inhabited by billions of people described by Asimov in his famous series of Foundation. It was the centrum of the human civilization without sky views. And it was powered completely by “planetary heatsinks” — a geothermal energy source from the planet’s inner mantle.
