Is there dirt or its structural equivalent on Mars?

Question

One potential solution to an initial permanent colony on Mars would be a series of modules with dirt floors and walls and slightly convex, transparent roofs. Assuming the interior temperature was artificially maintained at a human comfortable 22 C, it would take significant time for the walls and floors to come close to thermal equilibrium. However, once they did, they would provide nearly infinite thermal insulation on all sides, except the top. Their large thermal inertia would also be helpful in mitigating Mar's large diurnal temperature cycles. Lastly, they would block the constant bombardment of galactic cosmic rays (GCRs) on all sides, except the top.

Considering these aspects, the idea of human habitat modules built with dirt floors and walls (possibly covered with some sort of masonry) seems worth exploring further. The problem is, what we think of as dirt on Earth, usually contains a significant component of organic detritus. Assuming the existence on Mars of regolith, sand, clay, and bedrock, if you tried to dig a big hole, what would you be digging in? It would seem that a combination of sand and clays as a dirt, or dirt substitute, could be structurally sound. So my question is: is there dirt or its equivalent on Mars? Maybe a better question might be "How can you create a large, deep hole in the ground on Mars?".

Answer 1

The fine regolith on Mars is regarded as being the closest equivalent to an Earth like soil. It contains sand and dust. Clay deposits have been found. The two could be mixed to produce a more graded material with moisture retaining properties.

Technically martian soil is not dirt, but some may refer to it as either dirt or soil. If differs from soil on Earth because no martian soil has yet been found to contain biotic materials, such as humus and microbes. Additionally, most martian surface soil lacks water.

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Edit following questions in comment to the first version of this answer.

Earth and Mars have had different geological histories. At one time water was present on the surface of Mars, now it is not. Earth has had a longer history of water in its surface and this is one of the reasons for the differences in geological history.

Water still plays an importance role in shaping Earth geology, as does weathering. Much of Earth's soil is the result of weathering: heat, water - rain, rising and falling water table, ponding, submersion, desiccation, rehydration, ice, chemical reaction with atmospheric gases, with liquid acids, microbial activity. Earth also has plate tectonics and volcanic activity.

All these continually renew the surface of the Earth. Mars lacks much of this. The renewal of it's surface is significantly slower and different to Earth's.

Like the Moon, meteorite impacts have a significant affect on martian regolith. When a meteorite impacts Mars, a crater is created and material is ejected from the crater. The size distribution of the ejected material varies from dust sized particles to clasts that can be boulder sized. All this gets mixed up and is part of the regolith.

It may be possible to find regions where the upper part of the regolith resembles a dry version of soft Earth soil that is easily dug with an excavator or shovel. Apart from sand dunes, I would expect most martian regolith to be a varied mixture of particles ranging in size of silt to boulders, much like stone deserts on Earth.

If you want to get something like a "soft soil" on Mars some preparation might be required. Dig up a lot regolith and use mechanical screens to sort the material into stockpiles of material of different sizes. Mix the small size materials into a uniform mix and backfill the hole.