Humans will be able to reside on new planets in forthcoming years thanks to progression in science and technology, starting with the nearest planets and moving on to the more remote ones. A few missions for human settlement on Mars are happening presently, for example, the Mars-One project, which has already been bankrupted, or newly proposed schemes from NASA.
In this article, the chance to build and use underground structures on Mars for sheltering humans against the harsh climate and hazardous emissions are examined. Main locations, such as Arsia Mons, for primary settlement on Mars, were inspected under suitable circumstances. Numerical analysis for tunneling to produce underground spaces was directed bearing in mind the diverse spatial orientation of key discontinuities such as faults and joints in Martian gravity. It was concluded that tunneling Martian rocks near the surface is not vulnerable to main failures and unsteadiness. It was determined that underground structures rank among the best options for housing humans on Mars and sheltering them from the harsh temperature by creating insulation and forming an adequate depth of burden rock mass against hazardous cosmic and solar rays.
NASA has issued a new proposal, and additionally, Space X’s president has shown some indication for combating the risk of human extinction by developing a Falcon heavy rocket. Currently, in diverse countries, a few percentages of GDP income are consumed in research and development for a broad range of science and technology. Consider that disbursements of around 1/10,000 of an R&D budget are spent on investigations about dominating other planets, and housing humans on them seems very rational. This allowance not only creates a basis for gaining expertise in related sciences, but it can also speed up settlements on other planets to rescue the human race from potential elimination on Earth.
Now, landing human-prepared robotic machines on Mars, conquering the planet via stabilizing bases, and inhabiting it, has turned into a hot topic. Even awareness of mining the mineral resources of the Moon and, recently, comets, has led to some pronouncements and policy declarations by diverse governments. In this article, we tried to improve the likelihood of constructing human shelters under the Martian ground surface. Severe environment conditions occur on the Martian surface (-100° C or less) in some locations. On the other hand, a lack of construction material comparable to Earth’s for building shelter that provides liveable circumstances for humans is also problematic.
An idea of excavating surficial rocks and isolating quarried space by insulation material to create habitable heating in excavated spaces is a novel idea. Excavation of rocks might be done via diverse transportable equipment comprised of solar-, nuclear-, or electrical-powered machines. This machines may differ in size from handheld drillers to micro- or mini-sized tunnel boring machines (TBM)(See Figure 1). With developments in rocket technology such as the Falcon Heavy by Space X, shipping the necessary apparatus looks feasible.
Mars’ environmental settings and gravity
Gravity of Mars
It has been detected that the usual surface gravity acceleration of Mars is equivalent to 3.72076 m/s2, with an allowance for a global change and alteration range of around 0.059 m/s2 or 1.6%. The maximum quantity for MGM2011 gravity accelerations is situated at the lowermost of the Jojutla Crater (at the coordinates of 81.6°N, 169.3°W), which is located in the low-lying Northern Plains. The minimum amount is situated at the edge of Arsia Mons (at the coordinates of 8.4°S, 121.4°W) which is located at the southernmost of the Tharsis Shield Volcanos (Hirt et al., 2012).
Topography and temperature of Mars
The topography of Mars was measured by the Mars Orbiter Laser Altimeter, or MOLA, an instrument on the Mars Global Surveyor (MGS), launched in 1996. It took 4 1/2 years to finish the mapping mission. Geomorphologic research on Mars by orbiters presented that in the northern hemisphere of Mars, elevation stands low and flat, and few craters are present. Large shield volcanoes in the northern hemisphere exist. Noticeable dendritic channel shapes and huge flood channels, as well as cratered highlands, occur on the southern hemisphere and at the equator, a morphology which is totally different from the Northern Hemisphere (Bargery et al., 2011).
Study on the climate and temperature of Mars presented that the weather on Mars 3.5 billion years ago was warm and humid, similar to early earth. Through the interaction of Mars’ atmospheric carbon dioxide and water, carbonate rocks were shaped, using most of the CO2. No recovering process like Earth’s plate tectonics exists in Mars to transport carbon dioxide back into the atmosphere. Consequently, the Martian atmosphere is emaciated by very cold temperature, leading to a frozen water presence in the Martian poles in the shape of permafrost or confined at subversive depths.
Martian weather differs significantly throughout the yearly seasons compared to earth. Due to the tilt of its axis, the seasons in the southern hemisphere are harsher than northern hemisphere (NASA Website). The maximum temperature on Mars can reach nearly 70 degrees Fahrenheit (20 degrees Celsius) at the equator position throughout the summer at noon. The lowermost temperature can reach about -225 degrees Fahrenheit (-153 degrees Celsius) at the poles. In the latitudes situated in the middle sections, the average temperature can be almost -50 degrees Celsius and -60 degrees Celsius at nights, whereas at noon in summer it may extend to a maximum of 0 degrees Celsius. A screenshot from a movie of temperature variation on the Martian surface, which was organized by the Mars Odyssey THEMIS (Thermal emission imaging system) instrument and a NASA team at Arizona State University, has been shown in (Figure 2). A link to the movie can be found under the image (Mars Odyssey and THEMIS Team from ASU University).
Mars’ main rocks components and equivalent engineering properties
Mars, similar to earth, has a crust, mantle, and core. Current models display a core section that is mainly composed of iron, nickel, and about 15–17% sulfur (Kavner et al., 2001). Its core stays partially in fluid standing and has light elements concentrated more than the Earth’s core in nearly twice the quantity, and the core is enclosed by a silicate mantle (Fuller and Head, 2002). With the ancient Martian crust, crustal reprocessing did not occur as much as Earth’s owing to the absence of tectonic activity (Zuber, 2001). Data gained from remote sensing spectral data and, similarly, from meteorites on Mars displays that crustal and surficial materials remain mostly basaltic or devised from basaltic rocks mostly because of volcanic happenings (McSween et al., 2009).
The Mars Opportunity Rover found sedimentary rocks in the Meridiani Planum. Data from these findings display that the surface of Mars has been enclosed by Regolith and Basaltic soils. Some research about mechanical characteristics and bearing capacity of Mars simulant soils were done by analog research methods on analogous soils from earth (ElShafie et al., 2012). Outcomes from these investigations can be applied in finding the penetrability, stability, and traffic-ability on the surface of Mars. The surface frequently is covered by volcanic rocks. Nevertheless, sediments in the form of soil occur in the shape of low to high thickness layers.
No considerable information exists on the bedrock depth beneath soil layers. Yet, if the rocks are hard igneous rocks, miniature TBMs rather than shield machines or even blasting would be further useful. If in some places, rocks have been constituted from weak pyroclastic rocks, the drilling machines for the blasting should be fixed to the ground outward. Typically, water is applied to carry cuttings and to remove heat from the drill bits; consequently, a new strategy or technique is suggested for this problem.
Underground structures in Mars in place of early shelters
In equatorial locations, some natural caves from volcanic rocks (e.g. basalt) exist that can be developed or excavated further. The key opinion that was recommended earlier by Fogg (1997) and Cushing et al. (2007) is that within the Arsia Mons volcano (see Figure 3), Odyssey has revealed natural caves or lava tubes. Mars colonizers in early days could utilize these natural shelters to protect against radiation and micrometeorites. Presence of geothermal energy is concluded in equatorial regions near Arsia Mons (Fogg, 1997). It can be concluded that Arsia Mons is a good candidate for initial settlement where natural caves can be excavated additionally to deliver more space for human actions.
Natural materials that occur on Mars with additives or binders taken from the earth can be used to seal these spaces and provide insulation against the harsh low temperatures outside. Expansion of underground space can be performed by applying mini TBMs that can be carried with powerful rockets to Mars. These TBMs can be remotely controlled by experts on earth. In the circumstances that TBMs cannot be taken to Mars, portable drills can simply be taken to Mars and powered with electrical energy. However, productivity in excavating a large volume spaces remains small with these portable drills.
Modeling of tunneling in Martian rocks
A numerical analysis was performed to investigate the circular tunneling method in Martian rocks. The gravity of Mars and the influence of potential faults or joints in Martian rocks were considered in identifying the probability of rock failure in this numerical analysis. Rock stability graphs around Martian underground structures, which were modeled in a circular tunnel form, were acquired in this research. Stress distribution was calculated to acquire a basic understanding of the likelihood of rock failure and fault slip around a circular tunnel. Stress constituents were calculated using the 2-D analytical solution by Kirsch (Jaeger et al., 2009).
Primary vertical stress, p2, was assumed to be overburden pressure. Initial horizontal stress, p1, was assumed to be k times the vertical stress. The coefficient k was assumed to be 0.25, 0.5, 1.0, 1.5, or 2.0. The stress components were altered to principal stresses or normal and shear stresses on joint planes which incline at b from x to the y-axis. To see equations that were used in this calculation, readers can refer to the main article appeared in the journal Tunnelling and Underground Space Technology in 2017 (Underground structures in Mars excavated by tunneling methods for sheltering humans, authored by Morteza Sheshpari, Yoshiaki Fujii, and Takuya Tani). The cohesion to prohibit rock failure or joint slip was calculated assuming an internal friction angle of rock or friction angle of the joint plane at 30° and presented after normalized by the initial vertical stress (Figure 4). For more numerical analysis figures and graphs in different scenarios of K, readers can refer to the main article. Negative values in the figure denote the required tensile strength to prohibit tensile failure of a rock or joint opening.
Exposure to galactic cosmic rays and solar particle events
On other planets, great amounts of radiation from cosmic rays can harm the human body because of the absence of atmosphere and magnetic shielding, which occurs on Earth’s surface. Prolonged contact with galactic cosmic rays (GCR) and particles released in unexpected solar particle events (SPE) can cause cancer and death (Reitz et al., 2012). In space missions, particularly in flights outside low Earth orbit, astronauts are bombarded by both galactic cosmic radiation (GCR) and solar particle event (SPE) radiation. Data displays that past SPE radiation intensities could have occurred in deadly intensities for unprotected astronauts (Battersby, 2005). The GCRs and resilient SPEs from space produce gamma rays and neutrons that definitely can break molecular bonds in the Martian soil (Gifford, 2014). Neutron is the most penetrating radiation on Mars, and it can be halted via 5–10 t/m2 mass. In other words, 2–4 m of overburden rock can halt its penetration and infiltration.
Colonizing other planets is inevitable, and if it is not occurring now, it will soon, especially taking into account forthcoming natural or man-made disasters that can initiate the annihilation of humans on Earth. Dedicating a small portion of an R&D budget on diverse features of human colonization on other planets can initiate a method for settlement on close planets, moving forward over time. The occurrence of water in frozen form on Mars, its close distance, and recent close observations have nominated this planet as a good candidate for settlement, sustaining humans’ other main and basic conditions.
Diverse developing technologies in heavy-duty rockets, such as the Falcon Heavy by Space X, or in space elevators, for instance, the Obayashi corporation space elevator (scheduled to be built by 2050), and in the science of growing nutritional plants in low gravity, can cause this aim to be attainable easier. One of the most practical methods for housing humans on Mars is by using natural caves found on rocky formations of Mars, like the ones in Arsia Mons, and expanding them through underground excavation methods. These types of underground structures create an achievable and good shelter against the harsh environment of Mars and dangerous GCR and SPE radiation.
Transportable drilling or excavation tools powered by any energy type can be applied to develop current caves or build new ones in desired landing locations. These portable excavation tools can vary from handheld drillers to miniature or bigger-sized TBMs that can be lifted by heavy duty rockets. TBMs do not need fluid to excavate, and they can be remotely controlled. The numerical analysis which was described in this research displays that tunneling in circular form does not generate severe rock instabilities in Martian gravity and temperature, allowing for diverse circumstances of joint and fault occurrence. Minimum support is necessary for the excavated area and can be avoided by surveillance and engineering judgment.