Soil is the weathered, mineral-based, loose particulate surface material of a planet. All soil types are partly comprised of fine particles of rock. It can vary greatly in composition, structure, and consistency. Some soil types, such as sand and regolith, consist primarily of rock grains and/or powders with no other component. However, other soil types can contain water, organic matter, and other material in proportions similar to that of rocky minerals. Arable soils have a relatively neutral pH with a balanced organic content allowing them to retain liquid water and nutrients while maintaining aeration. They can support living organisms on an agricultural scale and are essential to the life of many crop plants. Unfortunately, there are no known arable soils on Mars.
Five space probe landers have reached the Martian surface to analyse its soil.
Viking I & II Landers Mars Pathfinder (with Sojourner Rover) Mars Exploration Rovers (MER) A & B (Spirit & Opportunity)
All of these landers obtained X-ray Fluorescence (XRF) spectra of soil targets, and all employed shallow trenches to examine subsoil composition. The Viking landers also conducted gas content analysis, soil testing with chemical reagents, and tests for living microorganisms. The Mars Exploration Rovers have provided data about the greatest number of soil targets in the greatest variety of locations to date, obtaining thermal and high magnification images as well as XRF and Mossbauer spectra. The landing sites of successful probes have been on plains in the Martian equatorial regions and mid-latitudes of the northern hemisphere. Soil from Mars’s southern highlands and polar regions has not yet been sampled.
Martian Soil Characteristics
Due to continual transport by surface winds, the composition of Martian topsoil is relatively homogenized across the entire northern hemisphere of the planet. Its chemical components are typically similar to the ubiquitous atmospheric dust. Variation is observed in areas such as craters, rock outcrops, and other locations where weathering of bedrock introduces material other than transported dust. Martian soil is typically dry, with no liquid water content. However, it is not as desiccated as lunar regolith. Many Martian soil deposits are frequently exposed to water frost condensed from the Martian atmosphere. Under microscopic examination, some Martian soil samples exhibit cemented particles, crusting, and other evidence of exposure to traces of liquid water. Martian soil also contains hydrates and other minerals formed under exposure to water. However, it is effectively devoid of organic material.
The mineral content of Martian soil suggests that it would form a sandy, highly acidic, saline soil if enriched with enough water for plant growth. It typically lacks carbonates (resulting in a high inherent acidity), and no ready sources of carbonates appear available in the northern plains of Mars to enrich Martian soils using in situ resources. Its lack of humus and clays also combine to give it a high water flow rate with poor water retention. These factors make it useless for agricultural purposes in its native state. Untreated Martian soil is likely toxic to most crop plants and unsuitable for plant growth as either a fertilized soil or a hydroponic medium.
Consideration of Martian topsoil as an acid salt-inundated sandy soil suggests the following procedure for soil reclamation (conversion to arable soil):
The soil will first have to be processed mechanically.
Martian soil should be repeatedly soaked and rinsed over the course of several days using 3 m<math>^3</math> distilled water, at 30C and 1 ATM, per m<math>^3</math> of soil, with the rinse water recycled or otherwise disposed of after use. This will dissolve much of its salt and acid content. The salts in the soil act as a chemical buffer, and their removal along with the acidic ions would likely make the remaining soil sodic over time. Introduction of a less soluble buffer, such as calcium sulfate (gypsum), can prevent this. The gypsum can either be mixed in before the soil is rinsed, or, if some other unbuffered chemical remediation is needed (such as the addition of carbonates to further neutralize acid content in the event rinsing is insufficient), it can be added afterward and the soil soaked again and re-rinsed using only 1 m<math>^3</math> water per m<math>^3</math> of soil. Less than 10 kg of gypsum per ton of treated soil should be required.
The rinsed soil should then be dried at near atmospheric pressure (not freeze dried) and stored.
This processed soil can be used as a support medium for hydroponics methods, requiring only the periodic circulation of a fertilizer solution through the soil without the addition of organic matter or other soil components required to make an arable soil. This use will eventually re-salinate the soil, which would need to be replaced over time.
Preparation of an arable soil is more complex, and begins with the preparation of compost.
Preparation of compost for the reclaimed soil’s organic content should begin as soon as possible, preferably well before soil treatment. Compost material is preferably sorted by source, salt content and moisture content prior to processing, then shredded into pieces as small as possible. Compost piles can be wet or dry type. Wet compost piles use enough water to completely inundate the compost material and newly added material enters the reactor in a suspension or slurry. Wet piles are useful for liquid wastes and composting materials with a high salt content and/or a high ketone content. Wet piles tend to process material at a faster rate than dry piles. Dry compost piles add only enough water to maintain their internal humidity and rely on the moisture content of the compost material to provide water for processing. They are much simpler to operate than wet piles, can be constructed on site with no machining capacity, and require very little pre-processing of compost material. However, care should be taken to control the overall salt content of material added to a dry pile since a dry pile cannot regulate salt content like a wet pile. This can be accomplished with proper sorting of compost material. Either type of pile can be used for any type of compost if suitably designed, and either type can be designed for continuous processing. Compost piles require aeration and other environmental control. Wet piles require their own independent environmental control systems, but a dry pile design can operate using cabin life support.
Pile reaction rates can be controlled by changing the mixture of material introduced for composting. A given pile of any type can be run “hot” or “cold” by adjusting the compost mixture added. For optimum composting, a small amount of previously processed compost should be added back into the pile to enrich the pile with mold and bacteria populations able to conduct the entire decomposition process. Processed soil can be added to control reaction rate as well. The mixture should also be controlled to optimize the mixture of nutrients available to the mold and bacteria cultivated in the compost pile. A fertilizer blend may be needed for wet piles, and will prove useful for any hot pile.
The mold and bacteria cultivated in a compost pile can convert the bulk of compost mass to water vapor and carbon dioxide during composting. It is possible to reduce an average compost pile to less than 1% of its initial mass over time through continuous decomposition, removing almost all organic content. Optimum processing, however, leaves from 10% to 20% of the original mass as processed compost. The time required is on the order of a few weeks for a wet hot compost pile to more than a year for a dry cold compost pile. The processed compost should be ground up and graded to remove any remaining large pieces which did not decay during processing.
The resulting compost will be a coarse mixture of humus and partially decomposed material. It will support plant growth by itself, but will continue to rot away faster than it can be replaced by the plant material it supports. Its continued decomposition will place a load on environmental control systems as well. Processed Martian soil must be added to the compost in approximately equal volume to reduce its aeration rate, improve its water retention, provide support for rooting, and control its rate of decomposition. This will create a sandy soil with a somewhat chunky distribution of organic matter. The resulting soil may be slightly acid and/or saline as a result of trace minerals introduced by the compost, but can be further processed using water rinsing with addition of buffers and carbonates to adjust its pH and salt content.
The end result of this mechanical processing will be a poor quality sandy soil which is sufficiently arable to support some hardy crop plants. The soil quality can be further improved by the addition of pulverized clay, which helps maintain moisture content and nutrients through adsorption rather than the simple reduction of aeration. This allows crops to survive a wider range of moisture, nutrient, and salinity conditions.
Final improvement of the soil will require the action of growing plants over time. This can be speeded by planting annual crops, such as clover or legumes, with a rapid growth cycle. However, it typically takes at least several seasons.
Large scale reclamation of Martian soil to produce an arable soil will require years of processing, and is not a task suitable for a small Martian settlement. The toxicity and poor quality of Martian soil will likely make hydroponics a much more promising basis for initial Martian agriculture. However, soil reclamation is a desirable goal for larger colonies because the composting which makes it slower to conduct than hydroponics also provides a means of recycling colony wastes. Arable soils are also depleted at a slower rate than hydroponic media, are as easily worked, and require less chemical processing to create.