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Above Image: Red soil on the surface of Mars © Public Domain

The dirt on Martian soil

Most people do not think twice about soil, but we could not live without it. The plants we eat, and the plants that feed the animals we eat, depend on soil. Plant roots need soil for physical support. Plants also get nutrients from soil, which they need for healthy growth and development. Future astronauts on a long-term mission to Mars will want to grow some food crops to add to their diet of packaged food (see Space Food). The soil on Mars is not the same as the soil on Earth…is a Martian garden possible?

The composition of soil on Earth

Soil on Earth contains mineral matter, organic matter, air and water (See Figure 1). Soil also hosts a range of living things, from burrowing animals to earthworms to microbes. Microbes, such as bacteria and fungi, are very important for healthy soil and therefore healthy plant growth. For example, they decompose organic matter to recycle nutrients (see below). Certain soil bacteria are essential for nitrogen fixation, the process that converts atmospheric nitrogen (an essential plant nutrient) into a form that plants can use.

Mineral matter is made up of small particles that come from the weathering, or breakdown, of rocks on the Earth’s surface. Soil on Earth is about 45% mineral matter. Mineral particles in soil are called clay, silt or sand, depending on their size. The relative amount of each particle determines the properties of the soil. Soils that are high in clay have small mineral particles and can hold a lot of water and nutrients. In contrast, sandy soils have large mineral particles and cannot hold a lot of water or nutrients. The size of silt mineral particles is between clay and sand, so silt-rich soils hold a medium amount of water and nutrients.

Organic matter is anything that contains carbon and comes from a living thing. Soil organic matter includes animal waste products, dead bacteria and fungi, and decomposing leaves, plants and animals. Earthworms and soil microbes break down organic matter. This releases nutrients for reuse by growing plants. Soil on Earth is made up of about 5% organic matter.

Air is the specific mixture of gases that makes up the atmosphere of Earth. There is also air in spaces between soil components. The main components of air in the atmosphere are nitrogen and oxygen, with a small amount of carbon dioxide. Air in the soil has less oxygen and more carbon dioxide than air in the atmosphere because plant roots and microbes use oxygen and produce carbon dioxide during respiration.

Water in soil is also found in the spaces between soil components. Together, air and water make up about 50% of soil; the amount of air versus water changes according to how wet the soil is.

The composition of soil on Mars

Martian soil does not have all of the same components of Earth’s soil. In fact, the soil on Mars is almost entirely made up of mineral matter, with small amounts of water (See Figure 1). Since living things do not exist on Mars, its soil does not have organic matter. There is no air in Martian soil, since air is specific to Earth’s atmosphere. Mars’ atmosphere is 96% carbon dioxide, with other gases in very small amounts, but it is 100 times less dense than Earth’s atmosphere (see Earth vs. Mars). Because Mars’ atmosphere is so thin, there is very little atmospheric gas in the Martian soil.

Martian Soil is Really Regolith

Soil, by definition, includes organic matter. Since there is no organic matter on Mars, there is technically no soil. The proper term for the surface material of Mars is regolith, which is a broad term for the loose material that covers the surface of some planets (Earth, Mars, Mercury) and Earth’s moon. Soil is a type of regolith. Scientists commonly refer to Martian “soil” despite this technical difference.

The mineral matter in Martian soil comes from weathered volcanic rock. It has clay and silt-sized particles, but it is overall a sandy soil. There is also a thin surface layer of very small dust particles. The soil has a reddish colour because it contains a lot of iron oxides (rust). It is similar to iron-rich volcanic soils on Earth. In fact, NASA has made a Mars soil simulant that comes from volcanic soil in Hawaii. In contrast to Earth, the soil on Mars is relatively homogenous (the same everywhere), because global dust storms move the soil around the planet. (See Figure 2)

Earth has a lot of water in oceans, lakes and rivers, and precipitation is common. In contrast, Mars is extremely dry. It has solid water ice, but very little liquid water. No water-based precipitation reaches the planet’s surface. (See Earth vs. Mars) This means that the soil on Mars is also extremely dry, containing just 2% water (See Figure 1). Scientists think that the water in Martian soil is absorbed from vapour in the planet’s atmosphere.

NASA’s Curiosity rover is studying samples of Martian soil and rock to learn more about its composition and properties.

Could plants grow in martian soil?

The good news is that plants can probably grow in Martian soil, with some help. All essential plant nutrients have been found in the soil on Mars or in Martian meteorites. However, the level of most nutrients is too low for healthy plant growth, so Martian soil will need to be improved with fertilizer.

On Earth, a Mars soil simulant has been used to grow several different food crops. Scientists in the Netherlands have grown ten crop plants, including tomato, peas and rye (a grain related to wheat). The plants grew best when freshly cut grass was added as organic material.

Of course, the Mars soil simulant is not really soil from Mars. It is the closest match to Martian soil that is currently available on Earth. Scientists at the Florida Institute of Technology are working to improve the simulant to make it more like soil on Mars. They are also growing lettuce in the simulant to figure out the best kind of fertilizers to add. Experiments using actual Martian soil will have to wait for future missions when soil samples can be returned to Earth.

Growing plants on Mars will require more than just fertilizing the soil. Martian soil is harmful for plants and people because it contains a lot of chlorine in molecules called perchlorates. These toxic molecules will need to be removed before using the soil to grow food crops.

The harsh Martian environment poses many additional challenges plant growth. Plants need water, but liquid water on Mars is scarce and too salty for plant growth. Water will need to be sent from Earth, or collected and purified on Mars. A Martian garden will also need protection from the planet’s freezing temperatures and modification of the atmosphere to be more like Earth’s. Mars receives less sunlight than the Earth because it is farther away from the Sun. Frequent dust storms further reduce the amount of sunlight that reaches the planet’s surface. This means that artificial lighting will also be needed to grow plants on Mars.

Some kind of Martian greenhouse will need to recreate Earth-like conditions to allow plant growth. Scientists are working to meet these challenges, to provide a fresh salad to Mars explorers!

Glossary

Air

The mixture of gases that surrounds Earth; the common name of Earth’s atmosphere. (Air is 78% nitrogen and 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and has trace amounts of other gases).

Clay

The smallest particle of mineral matter found in soil. Clay feels smooth when dry and sticky when wet. It has high nutrient and water holding ability.

Fertilizer

A substance added to soil to improve its ability to support plant growth. Fertilizer often contains the essential plant nutrients nitrogen, phosphorus and potassium.

Microbe

Typically single-celled organisms that cannot be seen without a microscope. Bacteria and certain fungi, such as yeast, are examples of microbes.

Mineral Matter

The component of soil that comes from the weathering of rocks on the surface of Earth. It is also the component of regolith that comes from the weathering of rocks on the surface of other planets, such as Mars.

Nitrogen Fixation

The process that converts atmospheric nitrogen gas (N2) into nitrogen compounds (such as ammonia, NH3) that can be used by plants. Soil bacteria are responsible for around 90% of the nitrogen fixation that occurs naturally (as opposed to industrial nitrogen fixation in fertilizer manufacturing).

Nutrient

A substance that an organism uses to survive and grow. Essential nutrients are nutrients that are required by an organism. Plants have 16 essential nutrients that are absorbed from air, water or soil. Plants need large amounts of nitrogen, phosphorus, potassium; medium amounts of calcium, magnesium, sulphur; small amounts of iron, boron, manganese, molybdenum, chlorine, copper, and zinc.

Organic Matter

The component of soil that contains carbon and comes from living things. Decomposing plants and animals are examples of organic matter in soil.

Regolith

The loose material that covers the surface of some planets (Earth, Mars, Mercury), Earth’s moon, and some asteroids. It includes soil, but is a broader term than also includes loose material without organic matter.

Respiration

The process used by plants and animals to get energy from sugar molecules. It is often called cellular respiration to distinguish it from breathing. Cellular respiration uses oxygen and sugar and produces carbon dioxide, water and heat.

Sand

The largest particle of mineral matter found in soil. Sand feels rough and gritty. It has low nutrient and water-holding ability.

Silt

The intermediate-sized particle of mineral matter found in soil (smaller than sand but bigger than clay). Silt feels smooth and powdery. It has medium nutrient and water-holding ability.

Simulant

A thing that resembles, or has similar properties to, something else (e.g., fake fur, artificial diamonds, etc.).

Water

A substance that is essential to all living things (as known on Earth); exists as liquid water, solid ice, or water vapour.

Weathering

The process that breaks down rocks into small particles of mineral matter. Weathering can be physical (e.g., freeze-thaw cycles), chemical (e.g., acid rain) or biological (e.g., plant roots).

References