Many years will pass before we are able to grow street trees on Mars, if ever. This article is for entertainment and to find out what trees readers would like to see in Mars habitats.
Planting street trees from our home planet in the first settlement on the new world of Mars will surely provide a psychological benefit to the new Martian population. Trees will provide a contrast to the built structures needed to protect people and to the surrounding rocky but beautiful Mars landscape, they will give a leafy aspect in the Mars city streets and may give a pleasant reminder of Earth, that feeling of home appreciated by many travelers and migrants.
Now planting on Mars isn’t without some difficulty, the Martian regolith contains perchlorates, ClO4-, a salt from perchloric acid, detected by past missions to Mars at about 3 to 4 times the concentration of earth soil and while not necessarily being toxic to plants, it is extremely toxic to humans. It impairs the thyroid gland and growing plants in it would be very risky because of the risk of human exposure to the regolith dust. Amendments can be made to the regolith however, such as leaching out the highly soluble perchlorates with water or disassociating ClO4- to make oxygen or combining ClO4- with ammonium to make rocket fuel (Davila et al., 2013).
Further to the difficulties on Mars, there is no ozone layer to protect against ultraviolet rays from the sun, which will result in pretty bad sunburn for plants and people. There is also no magnetic field emanating from Mars, as there is on Earth because of our rotating molten outer core, to protect against the solar radiation.
But these matters can be overcome, NASA has developed ‘Plant Pillows’ (NASA, 2016) for production of fresh food during long space flights, these could be used as the equivalent of large pots that are used now in some cities. Habitat domes could be designed to block the sun’s radiation by using thick walls and roofs made from the regolith and generate our own visible light, especially the blue wave lengths for plants and the few adjoining wavelengths of ultraviolet and infrared light necessary for life. Or maybe we need more protection from the harsh Martian environment and move underground into lava tubes. Lava tubes are formed by molten lava flows cooling and solidifying on the outside but staying molten and flowing on the inside, leaving a hollow tube when the molten lava has flowed out.
Once we have made a suitable habitat what street trees will we plant on Mars? At first, we may want trees that also produce food. Three trees that are drought and heat hardy and will grow in shallow soils are the coconut palm Cocos nucifera, which provides high fat content fruit and it is synonymous with travel and new lands. It is called a cosmopolitan plant because its origin source is unknown and it occurs naturally throughout the tropics. The date palm Phoenix dactylifera provides a delicious fruit and has been widespread throughout the Mediterranean since ancient times. Another Mediterranean tree is the olive Olea europaea which provides a high oil content and is very popular and nutritious fruit. The leaves of the olive also contain oils and it is very flammable, this of course will have to be considered if grown in a habitat dome, possibly a separate lava tube just for an olive grove.
When we expand into a larger settlement, we may want street trees that are more ornamental. In my home country Australia, like all countries, we have many species of street tree, and in Adelaide near where I live, three trees are widely used, the lilac flowered Jacaranda mimosifolia, possibly one of the most beautiful trees with fern like foliage and in summer a mass of blue flowers; the ornamental Manchurian pear Pyrus ussuriensis with a spring flush of white blossom and striking bronze leaves in autumn; the golden-rain tree Koelreuteria paniculata with its large pinnate leaves, showy bright yellow flowers in summer and its display of inflated bladder like seed pods which gives its common name (Lord, 1978). These are among many but I list them here because they are hardy, attractive in flower and form and don’t grow too large.
As our Mars settlement grows into a City, we may look for taller trees to grace our streets and create boulevards. I have chosen 3 species that are widely used around the world. The London plane Platanus x acerifolia, believed to be a cross between P. orientalis and P. occidentalis is a majestic tree and probably the tree that is most tolerant of atmospheric pollution. It has been grown in London for over 200 years through the coal and anthracite burning years. The Linden tree Tilia cordata has glossy green, heart shaped leaves and flowers in early summer attracting many bees. It is widely planted in Europe and North America. The common name Linden is derived from the Latin word lentus meaning flexible and refers to the trees wood which is used for intricate carvings. The Maidenhair tree Ginkgo biloba has beautiful two lobed leaves that turn a golden yellow in early autumn and persist on the tree well into winter, a trait referred to as marcescence (Lord, 1978). Ginkgo biloba is also of taxonomic interest, it is the only remaining extant species of the Division of the Plant Kingdom Ginkgophyta that was prominent in the mid-Mesozoic (160–100 million years ago), I have heard them called dinosaur food trees. Ginkgo, cycads and conifers are commonly called gymnosperms, they produce ovules and pollen in cones. Most other trees are in the Division Magnoliophyta, are flowering plants and are commonly referred to as angiosperms, they produce seed within fruit (Knox et al., 2014).
Notice that, apart from the first 3 productive trees, these species are deciduous, even the Jacaranda drops its leaves in the dry season, and being deciduous is one of the reasons that they are widely planted in cities. The particulate pollution in the air in cities collects on the surface of leaves and one way for the plant to rid itself of these particles is to drop their leaves and grow new leaves. Hopefully particulate pollution will not be a problem in the habitats on Mars.
Deciduous trees often drop their leaves in response to shorter day lengths but generally even trees growing under artificial street lighting will drop leaves eventually, possibly in response to cold temperatures. On Mars lighting in habitat domes can be set to imitate seasons and provide suitable conditions for the deciduous trees.
Gravity on Mars is about a third of Earth’s (Mars, 2020), will this be enough for plant shoots to grow up and roots to grow down? Plant growth, both root and shoot growth, is affected by gravity and this effect is called gravitropism. Plant apical buds also grow towards the light so this should also happen on Mars particularly with artificial light. Root cap cells sense gravity by using starch granules which gather on the lower side of each cell and as such act as statoliths, cellular gravity sensors, the elongation zone behind the root cap then starts to bend down (Knox et al., 2014) and the roots grow down in response to gravity.
So, in conclusion, I leave you with this thought. Will the street tree roots grow down on Mars and will the deciduous trees drop their leaves? Let’s go and find out!
Davila, A. F., Willson, D., Coates, J. D., & McKay, C. P. (2013). Perchlorate on Mars: a chemical hazard and a resource for humans. Int. J. Astrobiol, 12(04), 321–325.
Knox, B., Ladiges, P., Evans, B., Saint, R., (2014). Biology: An Australian Focus (5th Ed.). NSW. Australia.: McGraw-Hill Education. Book.
Lord, E. E. (1978). Shrubs and Trees for Australian Gardens (4th Ed.). Adelaide, S.A.: Lothian Publishing Co Pty Ltd. Book.
Mars NASA Exploration Program (2020) https://mars.nasa.gov/all-about-mars/facts/