Soil on Mars

Red soil on the surface of Mars

Red soil on the surface of Mars (NASA)

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Learn how soil is different on Mars than on Earth, and what that means for growing plants on the red planet.

Most people don’t think twice about soil, but we couldn’t live without it. The plants that we, and other animals eat, depend on soil. Plants and their roots need soil for physical support. They also need  from soil for healthy growth and development.

Plants get 16 essential nutrients from air, water or soil. This includes large amounts of nitrogen, phosphorus and potassium. They also need calcium, magnesium and sulphur as well as small amounts of iron, boron, manganese, molybdenum, chlorine, copper, and zinc.

It is pretty likely that astronauts on long-term missions to Mars will want to grow their own food to add to the prepackaged food that they bring with them. But will such a Martian vegetable garden even be possible?

The Composition of Soil on Earth

Soil on Earth contains mineral matter, organic matter, air and water.

Mineral Matter

Soil on Earth is about 45% mineral matter. Mineral matter is made up of small particles that come from the weathering, or breakdown, of rock on the Earth’s surface. Mineral particles in soil are called  or , depending on their size.

The amount of each particle determines the properties of the soil. Soils that are high in clay have small mineral particles. They can hold a lot of water and nutrients. Clay feels smooth when dry and sticky when wet. Sandy soils have large mineral particles. They can’t hold a lot of water or nutrients. Sand feels rough and gritty. Silt particles are between the size of clay and sand particles. So, silt-rich soils hold an average amount of water and nutrients. Silt feels smooth and powdery.

Soil texture triangle showing the names for different types of soil. The type of soil is determined by the ratio of clay, silt and sand
Soil texture triangle (Let's Talk Science based on an image by Mikenorton [CC BY-SA 3.0] via Wikimedia Commons).
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Shown is a colour graphic illustrating the names for soil according to the percentages of clay, silt, and sand it contains.

The illustration is triangular. The triangle is divided into 12 sections of different shapes and sizes. Each section has a different colour and is labelled with a different name. The three sides of the triangle are each marked with the numbers from 0 to 100 at regular intervals. 

The left edge is labelled Clay %, with an arrow pointing up. The names of the sections along this edge are: Sand at 0 to 10%, Loamy Sand at 10 to15%, Sandy Loam at 15 to 20%, Sandy Clay Loam at 20 to 35%, Sandy Clay from 35 to 55%, and Clay from 55 to 100%.

The right edge of the triangle is labelled Silt%, with an arrow pointing down. The names of the sections along this edge are: Clay at 0 to 40%, Silty Clay at 40 to 60%, Silty Clay Loam at 60 to 73%, Silty Loam at 73 to 87%, and Silt from 87 to 100%.

The bottom edge of the triangle is labelled Sand %, with an arrow pointing to the left. The names of the sections along the edge are: Silt at 0 to 20%, Silty Loam at 20 to 60%, Sandy Loam at 50 to 70%, Loamy Sand at 70 to 96%, and Sand from 96 to 100%.

There are two sections in the centre of the triangle, which don't touch any edges. The first is Clay Loam, just under the large Clay section. The second is Loam, just under the Clay loam section.

Organic Matter

Organic matter is something that contains carbon and comes from the remains of living things. Organic matter in soil includes animal waste, dead bacteria and fungi, as well as rotting leaves, plants and animals. Earthworms and soil  break down organic matter in soil. This releases nutrients for reuse by growing plants. Soil on Earth is about 5% organic matter.

Earthworms breaking down organic matter
Earthworms breaking down organic matter (Source: Ben185 via iStockphoto).
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Shown is a colour photograph of a tangle of worms in and on soil. 

The worms are plump and shiny, with a ribbed texture. They are almost white on one side and various shades of red, terra cotta, orange and warm yellow on the other. The soil is a rich dark brown colour. Small pieces of twigs and grass are visible mixed in the soil.

Air

 is the specific mixture of gases that makes up the . There is also air in the spaces between the particles that make up soil.

Did you know?

Air is 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide. It also has small amounts of other gases.

Air in soil has less oxygen and more carbon dioxide than air in the atmosphere. This is because plant roots and microbes use oxygen and produce carbon dioxide during cellular respiration.

Water 

Water is also found in soil spaces. Together, air and water make up about 50% of soil. The amount of air versus water changes depending on how wet the soil is.

Soil also hosts a range of living things. These range from burrowing animals like moles and earthworms to microorganisms. Microorganisms like  and  are very important for healthy soil. This means they are also important for healthy plant growth. For example, microbes decompose organic matter to recycle nutrients.

Certain soil bacteria are essential for nitrogen fixation. Nitrogen is an essential nutrient for plants, but they can’t use the nitrogen in our atmosphere. Nitrogen fixation is the process that converts atmospheric nitrogen into a form that plants can use.

The Composition of Soil on Mars

Martian soil does not have all of the same parts as Earth’s soil. In fact, the soil on Mars is almost entirely made up of mineral matter. There are also small amounts of water.

The average composition of soil on Earth and Mars
The average composition of soil on Earth and Mars (Let’s Talk Science using data from the University of Illinois and space.com).
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Shown are two colour graphs that illustrate the composition of soil on Earth and soil on Mars.

Both graphs have an arc shape, curving from the bottom edge on the left to the bottom edge on the right. Each graph is filled with coloured sections labelled with percentages. A legend below each one indicates what each colour represents, and its percentage.

The first section of the left graph is blue and labelled 50%. It represents the percentage of air and water in soil. The second section is grey and labelled 45%. It represents the  mineral matter in soil. The third section is brown and labelled 5%. It represents the organic matter in soil.

The first section of the right graph is blue and labelled 2%. It represents the percentage of air and water in soil. The second section is grey and labelled 98%,. It represents the mineral matter in soil.

Martian soil has no organic matter. This is because living things, as we know them, have not lived on Mars. There is also no air in Martian soil, since air is specific to Earth’s atmosphere. Mars’s atmosphere is 96% carbon dioxide, with other gases in very small amounts. It is also 100 times less dense than Earth’s atmosphere. Because Mars’s atmosphere is so thin, there is very little atmospheric gas in Martian soil.

Misconception Alert

Soil, by definition, includes organic matter. Since there is no organic matter on Mars, there is technically no soil. What Mars does have is regolith. This is a broad term for the loose material that covers the surface of some planets like Earth, Mars, Mercury. It also covers Earth's moon and some asteroids. It includes soil, but it can also include loose material without organic matter. Scientists commonly refer to Martian regolith as “soil” despite the fact that they are technically not the same.

The mineral matter in Martian soil comes from weathered volcanic rock. It has clay and silt-sized particles, but it’s made mostly of sand. There is also a thin layer of very small dust particles on the surface. The soil has a reddish colour because it contains a lot of iron oxides, or rust.

Martian soil is similar to iron-rich volcanic soils on Earth. In fact, NASA has made simulated Martian soil using volcanic soil from Hawaii. Unlike on Earth, the soil on Mars is quite  . This means that it is similar everywhere. This is because very large dust storms move soil around the planet all the time.

Soil samples from three different sites on Mars
Soil samples from three different sites on Mars (Source: NASA/JPL-Caltech).
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Shown are four colour photographs, arranged as a single image, showing soil samples from Mars.

The first photograph is labelled A. It shows smooth red toned soil, scattered with small grey rocks. Two trenches lined with a paler material extend diagonally across the photo.

The second photograph is labelled B. This shows dark brown soil around a trench of pale tan material that curves in a V shape across the photograph.

The third photograph shows a rover with two outstretched mechanical arms in the foreground. The soil in the background is red toned with dark brown, dotted with many sharp-looking rocks stretching into the distance.

The fourth photograph is a close-up view of dark tan soil that looks smooth near the bottom, and crumbly near the top.

Earth has a lot of water in oceans, lakes and rivers. , like rain and snow, is also common on Earth. Mars, on the other hand, is extremely dry. It has ice made of solid water, but very little liquid water and no water-based precipitation reaches the planet’s surface. This means that the soil on Mars is also extremely dry. It contains just 2% water. Scientists think that the water in Martian soil comes from water vapour in the planet’s atmosphere.

NASA’s Curiosity rover is currently studying samples of soil and rock on Mars 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 . But the level of most nutrients is too low for healthy plant growth. This is why  would need to be added to Martian soil if people want to grow plants there.

On Earth, scientists have used a Mars soil simulant to grow several different food crops. Scientists in the Netherlands have grown ten crop plants. These included tomatoes, peas and rye. The plants grew best when freshly cut grass was added to the simulant as organic material.

Of course, Mars soil simulant isn’t really soil from Mars. It’s the closest match we currently have 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 use.

A small pile of martian regolith simulant JSC MARS-1A
A small pile of martian regolith simulant JSC MARS-1A (Source: Z22 [CC BY-SA 3.0] via Wikimedia Commons).
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Shown is a colour photograph of martian regolith simulant.

A small, heaped pile of reddish brown material sits on a pale grey surface. The material looks powdery. There is fine dust to the left of the pile, and tiny crumbs of material around the edges of the main pile.

Experiments using actual Martian soil will have to wait for future missions. Some day, Martian soil samples will be collected and brought back to Earth for experiments.

But, growing plants on Mars will need more than just fertilizer. Martian soil is harmful for plants and people. This is because it contains a lot of chlorine in molecules called perchlorates. These toxic molecules, which kill microorganisms, will need to be removed before the soil can be used to grow food crops on Mars. Read more about perchlorates in this in an article from Space.com.

The harsh Martian environment poses many other challenges to growing plants.

Challenges and Possible Solutions for Growing Plants on Mars
Challenge Possible Solution
Water
Plants need water, but there’s not much liquid water on Mars. And the water is too salty to grow plants.
Water will need to be sent from Earth, or collected and purified on Mars.
Temperature
Plants cannot survive the planet’s freezing temperatures.
Plants will need to be protected and heated.
Light
Mars gets less sunlight than Earth because it’s farther away from the Sun. Frequent dust storms also reduce the amount of sunlight that reaches the planet’s surface.
Plants will need artificial lighting to grow.

Future Mars explorers will need some kind of Martian greenhouse to recreate Earth-like conditions. But scientists are working to meet these challenges. Just imagine eating a fresh salad on the surface of Mars!

Learn More

Mars Curiosity rover: Mission updates (2020)
This page keeps track of Curiosity’s latest discoveries as it explores the surface of Mars.

ISRU Part IV: How to Grow Food on Mars (2020)
This article, from the Mars Society of Canada, has information about the science going on in order to grow plants on Mars.

Farming on Mars will be a lot harder than ‘The Martian’ made it seem (2020)
This article, from Science News, explains why growing plants in Red Planet soil will require adding nutrients and removing toxic chemicals.

Growing Plants in Space (2020)
This page explains the experiments NASA is conducting to find the best ways to grow food in space.

Mars 2020 Mission Perseverance Rover (2020)
Perseverance, a new rover, is scheduled to land on Mars in February 2021! This page follows its progress.

References

American Society of Agronomy, Crop Science Society of America, & Soil Science Society of America. (n.d.). The soils of Mars: Physical, elemental, and mineralogical properties.

Espace Pour la Vie in Montreal. (n.d.). Plants’ nutrient requirements. City of Montreal.

Florida Institute of Technology. (2016, October 17). Researchers explore Martian farming.

Langly, C. (2020, May 12). The dirt on soil conservation. Let's Talk Science.

Let's Talk Science. (2019, September 3). What is inside soil?

Let's Talk Science. (2019, September 21). What is soil?

NASA. (n.d.). Curiosity rover

NASA. (2015, October 5). Can plants grow with Mars soil?

Sharp, T. (2017, December 12). What is Mars made of? Composition of planet Mars.

Soil Science Society of America. (2014, November 21). January: Soils sustain life.

Soil Science Society of America. (2015, June 23). July: Soils are living!

University of Illinois. (n.d.). The great plant escape: Case 2 – facts of the case.