As human communities spread out across our planet, these two learned behaviors have become ever more pliant, allowing previously uninhabitable regions to become habitable. Each new challenge spurred the development of new solutions. As time has progressed, the technology with which we provide for ourselves has advanced. Yet some things were just handed to us. Regardless of where humans have established communities, the basics of life support - air and water - have been provided by Earth's ecosystem. Humans learned to manipulate local portions of Earth's biosphere - but they did not have to create it de novo.
We now look forward to the prospect of leaving our world to visit others. As we do, we'll have to take a step that our ancestors did not face: we'll have to bring am artificial biosphere - a life support system - with us - one that we can sustain - and can sustain us in exchange. Some proposed life support systems rely heavily upon biological components, others depend mostly on physical and chemical processes. Most use both.
Earth's first greenhouse
Perhaps the first true attempt at creating an artificial life support system on Earth was the creation of greenhouses. While the exact date of the first greenhouse is not known, one of the earliest documented greenhouses was a "specularium" constructed around 30 A.D. for the Roman emperor Tiberius. You see, Tiberius wanted to eat cucumbers year round and his staff needed a way to meet his menu choices. Since flat glass had yet to be invented, a greenhouse-like structure was constructed out of a myriad of small translucent sheets of mica.
The immediate predecessors of today's greenhouses emerged in Europe in the 1600's. At first, their use was mostly for the propagation of tropical plants (orange trees) in colder, harsher environments. Over time, they grew in size and saw use as "solariums" whereby people could gain a warm respite from winter. The Victorian Age in England saw the construction of vast glass enclosed structures whose construction closely resembles those in use today.
Over time, as heating, cooling, irrigation, nutrition, disease management, and lighting technology advanced, greenhouses showed their utility for something other than just defeating winter: increasing the yield of a given patch of land well beyond its 'natural' carrying capacity by maximizing the optimal conditions for plant growth.
Looking back at concepts first visualized half a century ago, and iterated and refined ever since, virtually every Mars exploration scenario includes a greenhouse of some sort. A glance back at Wily Ley's "The Exploration of Mars" illustrated by Chesley Bonestell (1956) and Sir Arthur C. Clarke's "The Exploration of Space" illustrated by R.A. Smith (1951) feature paintings of domes on Mars with plants inside.
To date, spacecraft life support systems have been wholly physico-chemical in nature i.e. machines scrub the air, clean the water, and deal with waste without the intervention of biological processes that do this within Earth's biosphere. These systems also do not produce anything needed for human nutrition other than clean water. Nor do they recycle all wastes. Much is still thrown overboard.
In space, the more material you discard, the more you have to bring with you. The longer the trip, the more Herculean the logistics requirements become - to say nothing of the size of the rockets required. Human missions to Mars will most certainly have two main design characteristics: some sort of fuel generation on Mars - often referred to as "In Situ Resource Utilization (ISRU) and a closed life support system that includes plants and microorganisms (bioregenerative). The most likely way to grow plants on Mars? A greenhouse.
Greenhouses isolate plants from adverse conditions outside while providing optimal growing conditions inside. Given the harsh conditions outside on the surface of Mars, this will be quite a challenge. People are already working on the problems.
Building a functional greenhouse on Mars should be possible. However, significant attention will need to be paid to the rather extreme - even hostile conditions that exist in comparison to what greenhouse designers face on Earth.
Mars' orbit is more elliptical than Earth's. As such, the intensity of sunlight it gets ranges from 52% down to 37% of the irradiance you'd get just if you were just outside Earth's atmosphere (before the atmosphere dampens it). Given that Mars normally has clear skies (except for the occasional dust storm) the net amount of light available to plants on Mars may well be better - and more reliable - than available in the most optimal agricultural regions on Earth. A close approximation of Mars light levels is a greenhouse on Earth whose outside is in need of a good cleaning.
Earth and Mars have nearly identical axial inclinations with respect to the sun (23.5 Vs 25 degrees) and days of almost identical length (24.0 Vs 24.7 hours) such that plants used to seasonal and daily light cycles will have little problem adapting to Mars.
Chemistry: with regard to the raw ingredients plants and other forms of life require, Mars has shown itself to be amply supplied. The evidence gets better with every passing month and every new discovery from spacecraft orbiting Mars. While it is thin, Mars does have an atmosphere - one that can be mined for carbon dioxide. Its subsurface is apparently endowed with substantial amounts of frozen water - and perhaps carbon dioxide as well. While Martian dirt (in this case the term 'regolith' is much more appropriate) is likely devoid of many substances found on Earth, it should be of use in providing the basis for a growing substrate for plants. While nitrogen is not overly abundant, recycling it from human waste and other byproducts (with some augmentation from onboard supplies) should be sufficient.
Gravity: Mars' gravitational field is 0.38 of that on Earth. Experiments aboard spacecraft show that plants can grow (after a fashion) in microgravity and can reproduce. As such, it is unlikely that Mars gravity will be a huge problem. Moreover, with a reasonably substantial gravity field (as compared to Earth) certain operational aspects of greenhouse operations can be performed. In microgravity, density-driven phenomena do not exist. Heated (lighter) air will not rise and cooler (heavier) air will not sink. As such traditional ventilation systems would not work. On the surface of Mars, systems familiar to greenhouse operators on Earth can be readily employed albeit within a closed loop.
While lighting may be OK, gravity of no concern, and raw materials more or less abundant, there are other environmental factors that must be dealt with: radiation, gravity, temperature, and atmospheric pressure.
Radiation: this could be a significant problem. Mars has a very thin atmosphere that blocks very little of what radiation reaches the surface. Secondly, it does not have the massive magnetic field that serves to protect Earth from a constant onslaught of solar and galactic radiation. Recent results from the MARIE instrument on the Mars Odyssey spacecraft (now in orbit around Mars) show that cosmic-radiation levels on Mars' are higher than had been predicted before the spacecraft's arrival.
Mars is also bathed with high levels of ultraviolet light. Martian greenhouses will need to be able to filter this out much more effectively than they do on Earth. As for high energy cosmic rays and particles ejected during solar flares, additional shielding - including emergency shielding will need to be provided. It is uncertain with today's technology whether a true greenhouse could be built on the surface of Mars (or would be practical even if it could be built) that could withstand a constant onslaught of radiation and yet let light in for plants. This might lead to he building of growth chambers under a radiation shield comprised of regolith with light provided from collectors located on the surface. NASA-sponsored research has shown that low power LEDs (light emitting diodes) could find applicability as a low power option for illuminating Martian crops.
Temperature: Mars gets cold. Temperatures can range from -20°C down to -75°C or colder. It can also get warm. It is possible that close to the equator, temperatures could get as high as 20°C during the day. Since there is not much air on Mars, a conductive loss of heat from a greenhouse structure to the surrounding air would be low. However, the greenhouse would radiate heat to the sky at much the same rate as does the Martian surface. As such heaters will be needed to supplement any infrared radiation received from the sun. This will require a substantial amount of power.
Soft vs hard construction: There are two schools of thought with regard to how a Martian greenhouse could be built on the surface. With weight always a precious resource during a space mission, inflatable structures have long been touted as an obvious solution. Think of them as large cylindrical spacesuits that entire crews live within. Recent research with large inflatable modules - the so called "Transhab" concept at NASA Johnson Space Center show considerable promise. This concept is just fine for habitats. However, greenhouses have one critical requirement: letting light in. While materials do exist that can provide inflatable habitat on the surface (or in space), no such material exists yet that has both the structure and optical properties required. Add in the need to moderate heat loss and block harmful radiation, and the engineering challenge gets greater.
The other approach is the use traditional "rigid" materials. By removing a substantial amount of the structural support away from the transparent material, more emphasis can be put into creating materials that can let in light, keep out radiation, and keep heat loss to an acceptable level. One way to reduce some of the structural support required is to use a mixture of rigid and inflatable technologies. In addition, research at NASA Kennedy Space Center has shown that it might be possible to grow plants in a greenhouse at reduced atmospheric pressure. This would reduce the workload of the materials used to construct the greenhouse.
Even a little green goes a long way
There is also another factor to consider in the design and operation of a greenhouse on Mars: the human factor. One of the earliest uses of greenhouses on Earth was for purely psychological reasons. In addition to the refreshing novelty of eating summer foods in the depths of winter, having sunlight - and warm air touching your skin can be very therapeutic.
At McMurdo Base in Antarctica, crews "over winter " and are left without fresh food supplies for 6 months at a time. A greenhouse (actually light-equipped growth chamber) is in operation at the base which, at its peak productivity, is able to provide everyone with fresh salads twice per week. However, there is another benefit to this greenhouse. People are regularly found sleeping in hammocks strung between rows of racks growing lettuce. A choice of music selections is available to visitors on CD and tape. A similar, but much smaller growth chamber is also in operation at South Pole station and is due for upgrading in the next year or so as a new station structure is built. People are prone to hang out in this greenhouse as well.
The first greenhouses to fly in space have been rather modest in size and have been used to understand some of the basic factors affecting plant growth in the microgravity of space. While horticulture in space is still in its infancy, cosmonauts aboard space station Mir often reported that the tending of experimental plants was a task they looked forward to as a means of relaxation.
Practicing Martian horticulture on Earth
In an attempt to examine some of the operational aspects of operating a greenhouse on Mars, an experimental greenhouse is slated for installation on Devon Island in 2002. Devon Island is a remote location in the Nunavut territory of Canada barely a thousand miles from Earth's north pole. The greenhouse is being donated by SpaceRef Interactive as part of the ongoing NASA-SETI Institute Haughton Mars Research project [see "Mars on Earth: The Haughton-Mars Research Project"]. Named for noted author (and NSS governor) Sir Arthur C. Clarke, the "Arthur Clarke Mars Greenhouse" is designed to serve over the next few years as a testbed for a variety of technologies needed to develop a greenhouse capability on Mars.
Gardening on Mars
Although the complexities of large scale human - tended greenhouses on Mars are still being addressed, some thought is being given to sending small demonstration projects to Mars. Chris McKay from NASA Ames Research Center has proposed sending a small greenhouse (aboard a future Mars lander mission) to the surface of Mars. This greenhouse would be capable of supporting only a couple of plants. McKay envisions the use of Carbon dioxide from Mars atmosphere as well as some Martian regolith to grow the plants. These materials could be provided as part of a larger ISRU experiment package.
As plants and Martian regolith are brought together the growth of the plants would serve as part of an indication of the presence (or absence) of toxic materials. Such a demonstration might also serve to dampen concerns about back contamination by any samples subsequently brought back to Earth from Mars. This would also allow a determination to be made as to what materials might need to be added to Martian regolith to support the growth of plants. McKay envision follow-on "growth modules" that could be sent to Mars in so as to provide more complex tests of systems that will eventually be needed by human crews.
Of course there is a next step beyond greenhouses and closed environments that many envision for Mars. A big step. This involves the creation of a self-sustaining ecosystem - for the entire planet. The most common term for this is "terraforming" which is often used to suggest the creation of 'another Earth'. Another, more appropriate term is "ecosynthesis" or the creation of an ecosystem. Since Mars is not Earth, any introduction of life from Earth, however successful, is likely to result in an ecosystem that is uniquely Martian.
Various concepts for terraforming have been proposed over the years. Some are slow and would take thousands of years. Others are more radical and might provide some level of results in much less time. Regardless of the time frame, all concepts include the use of living systems (plants and microorganisms) to hasten the terraformation of Mars and the sustainability of the ecosystem that is being created. Since even the hardiest forms of life on Earth would probably have a hard time living on Mars' surface, the requirement for some genetic manipulation and selective breeding is almost certain. Since Mars offers a unique set of physical environmental factors, some of which cannot be duplicated on Earth (gravity) it is likely that the organisms developed for use in terraforming would be develop on Mars - inside Martian greenhouses.
Looking back - and looking ahead
In his science fiction novels "Prelude to Space" and "The Sands of Mars" (written in the 1950's) Sir Arthur C. Clarke spoke of plants on other worlds - including plants that had adapted to the vacuum on the Moon and to the near vacuum on Mars. Always the visionary pragmatist, he imagined plants that don't even need a greenhouse! Indeed, He recently suggested, with a touch of his wry ironic humor, that he might have actually spied a few plants growing on the in the southern polar regions of Mars in some provocative pictures sent back from the Mars Global Surveyor. The geologists insist that these are not plants but non-biological phenomena.
Alas, the Mars we expect to visit probably doesn't have plants growing on the surface now. Perhaps, in a distant, warmer past it did. Perhaps it will once again in the future - with a little help. Perhaps we'll see Sir Arthur's Martian plants after all.
Note: A slightly shorter version of this article appears in the May/June 2002 issue of Ad Astra magazine