Scientists have been studying the Martian surface with spacecraft since 1965 when the Mariner 4 spacecraft collected the first images. Since then, there have been additional flyby missions, as well as orbiting and landed missions. You might recall the movie, The Martian, and the attempt by the character Mark Watney to grow crops to survive on Mars. We’re not close to sending humans to Mars, but we have collected a lot of information about Mars and its surface in the past several decades.
Mariner 4 collected images showing a desolate-looking planet with a cratered surface like that of the Moon. Later missions, however, demonstrated that ~3-4 billion years ago, Mars had abundant rivers and lakes of liquid water. In addition, we know now that the Martian surface has active wind-driven processes. Current observations show localized dust devils and Mars’ global-scale dust storms. Scientists have also determined that Mars has an active cryosphere with surface and near-surface water ice at mid-to-high latitudes.
The evidence for liquid water in Mars’ distant past and water ice today suggests soils may have formed on the Martian surface over its 4-plus-billion-year history. We know that Earth’s soils form through a complex, time-consuming process of biological, chemical, and physical processes. Organic matter – rich in carbon that provides nutrients for the abundant microbial life in Earth’s soils – is typically enriched in soils on Earth.
Mars is different, though. Active wind-blown sediments and ancient sedimentary rocks on Mars have very low abundances of organic molecules. So, when planetary scientists talk about “soils” on Mars, we don’t mean soil like you could see here on Earth. We ignore the need for this organic component and are typically referring to modern-day, unconsolidated sediments or “regolith.”
We have yet to unequivocally identify an ancient soil profile on Mars complete with distinct soil horizons. Measurements by orbiters, landers, and rovers show a variety of minerals that formed from interactions between rock and liquid water. These different “secondary minerals” tell us that the pH, temperature, and salinity of liquid water on ancient Mars changed over space and time.
Some discoveries by Mars rovers:
- Spirit found rocks and soils enriched in silica and/or sulfate. This suggests that acidic fluids interacted with these rocks and sediments. In the process, these fluids removed sodium, potassium, magnesium, and calcium.
- Opportunity and Curiosity found shapes in ancient sedimentary rocks that looked like salt crystals formed from evaporation. This suggests that some ancient surface and ground waters on Mars were saline.
- Curiosity has been studying extremely thin layers of sedimentary rocks primarily deposited by lakes and rivers 3.5 billion years ago. Using specialized equipment, we have identified secondary minerals that suggest some sediments were altered in acidic and saline liquid water. We have also found evidence that other sediments interacted with fresh water. We think these ancient environments where freshwater dominated would have been habitable to microbes if they ever existed on Mars.
Multiple landed missions have studied the composition of modern soils on the Martian surface. These soils are composed of basaltic igneous minerals, like what would be in Hawaii or Iceland. They have found materials that are like volcanic glass.
The Mars Phoenix Lander studied the soils in the Martian arctic. It was able to perform wet chemistry experiments by mixing scooped soils with liquid water. The equipment identified pH and dissolved cations and anions in the scooped soil samples. When Phoenix scooped into the surface, it found water ice just beneath the surface and found the soils had a slightly alkaline pH of 7.7. Phoenix also discovered that perchlorate, a chlorine-based anion, was the dominant anion from the wet chemistry experiments.
Studies of the Martian surface are critical for preparing for future human missions to the Red Planet. Water, in the form of water ice in the mid-to-high latitudes, will be an important resource for future humans on Mars. Water (as various forms) in secondary mineral structures also will be important. The water can be used for drinking and watering crops. It can also be separated into oxygen and hydrogen molecules that could be used as rocket propellant for the return trip to Earth.
Based on the composition of modern Martian soils, they would be appropriate media for plant growth with a few modifications. Perchlorate can be toxic to humans but is highly soluble in water. That means the Martian soil would need to be “washed” to remove the perchlorate.
Current Martian soils have some phosphorus, but very little nitrogen, which are essential nutrients for plants. Martian soils would have to be treated with phosphorus and nitrogen, and plant-essential microbes would have to be added to grow healthy plants on Mars.
Of course, plants will need to be grown in climate-controlled greenhouses. The Martian surface temperatures in Gale crater, near the equator, can swing between 32°F during the day to -94°F at night. The Martian atmosphere is also very thin – about one percent of the of Earth’s atmospheric pressure. There is little protection from galactic radiation, which would destroy any unprotected life on the surface.
The next NASA rover to Mars, named Perseverance, landed in Jezero crater on February 18, 2021. Perseverance will collect about twenty rock and soil samples for eventual return to Earth. Studying these returned materials in our laboratories on Earth will help us further characterize the composition of Martian rocks and soils and prepare for human missions to Mars.
Answered by Elizabeth Rampe, NASA
Dr. Rampe presented her finding at the ASA, CSSA, SSSA Annual Meeting held virtually in 2020. Click this link to listen to her talk.
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