A recent study has shown that the red dust on the surface of Mars, in combination with surface conditions of intense solar ultraviolet and cosmic radiation, is probably one of the most sterilizing environments imaginable. These new results cast some very cold water on the fervent hopes of some planetary scientists for indigenous martian life. Some have extended the list of potential worries for future explorers to problems these conditions might pose for martian agriculture. But none seem to be particularly concerned about the toxic effect Mars’ red dust might pose for the occasional visitor, let alone settlers.
The martian dust contains significant amounts of perchlorate—a chemical compound made up of one chlorine and four oxygen atoms. Perchlorate is found naturally in various salts; on Mars, it is probably joined with magnesium and sodium. This substance is highly reactive—aluminum perchlorate is one of the compounds in solid rocket propellant. The high reactivity of perchlorate means that interactions with other chemical substances are almost certain, which in turn means that perchlorate in martian soil is a chemical hazard to living organisms—not only for microbial life and plants, but to humans as well.
Experience gained during the Apollo program taught us that dust can be a problem for the unprepared. Lunar dust is the smallest grain size fraction of the lunar regolith—particles smaller than 40 microns, finer than talcum powder but with much greater hardness. Extremely abrasive, this dust can make most moving equipment parts immobile. During the short duration of the Apollo missions (the longest stay on the surface was three days), the crew simply put up with the inconvenience of coping with fine, abrasive dust, but longer stays will require that steps be taken to mitigate its negative effects.
Although lunar dust is physically abrasive, it is largely inert chemically. Testing done on the first lunar samples at the Lunar Receiving Laboratory exposed seeds and germinating plants to lunar regolith. As expected from the chemical composition of the regolith, the plants continued to thrive despite repeated and prolonged exposure to lunar dust. While actual growth experiments were not conducted (largely because lunar material was allocated in extremely small amounts, to maintain sample integrity), we have no reason to suppose that the fine lunar regolith cannot support vigorous plant growth, provided that some key nutrients like phosphorous and nitrogen (naturally present in extremely low quantity on the Moon) are added to the soil.
The situation on Mars, however, appears to be different. The soil of Mars is composed predominantly of clay minerals—weathering products of igneous rocks created in the presence of liquid water, resulting in dust grains that are fine and relatively soft. Thus, martian soil is probably not physically abrasive like the lunar regolith (although care will still need to be taken to keep moving parts as clean as possible). The problem lies with the highly reactive (and probably toxic) chemistry of the smallest particles in martian soil.
One aspect of the lunar experience relevant to the issue of future Mars surface exploration is the ubiquity of dust and how it coats, covers and invades all pieces of equipment, up to and including the human body. On the Moon, these phenomena did not result in any long-term ill effects. A broken fender on the lunar rover during the Apollo 17 mission sprayed lunar dust over the crew and their suits, stressing the heat rejection properties of the suits and equipment on the lunar rover. Fine dust coated the fittings of air hoses in the suit, impairing the crew’s ability to get good seals to prevent leaks. Although the astronauts inhaled minute amounts of lunar dust when they re-pressurized the LM cabin (they said that it smelled like gunpowder), there were no ill effects to crew breathing and health. Although silicosis (similar to black lung disease) might result from long-term exposure to the fine lunar dust, in terrestrial settings such effects (without mitigating efforts) would require years of exposure to develop.
This may not be the case on Mars. The highly reactive chemistry of perchlorates in the soil could make martian dust not simply an annoyance, but a dangerous hazard. Corrosive chemicals within dust grains suspended in air can be inhaled and could seriously and permanently damage lung and esophageal tissue. It may be possible to mitigate contamination through dust management and clever engineering. For example, we’ve found that most of the Moon’s dust grains are magnetic—a result of the deposition of vapor-phase metallic iron coatings, which allows for the removal of virtually all of the dust using strong magnetic cleaning. This technique will probably not be possible for the martian dust, which formed from chemical weathering on Mars and does not possess the vapor-deposited iron of the lunar dust grains.
The new results about martian soil strongly suggest that the Red Planet may not be the welcoming “second Eden” for humanity that is commonly portrayed. Even if we are able to somehow mitigate the toxic effects of the soil (for example, through chemical treatment), such an approach may not be easy enough to warrant the effort. Certainly, the scenario in the book and film “The Martian,” in which one simply plants pieces of seed potato, adds excrement and water, then harvests a locally grown food source, just isn’t plausible. The toxic effects of martian soil might be dealt with for short visits by exploring crews, but long-term human habitation and “colonization” of Mars is an entirely different proposition.
To successfully journey beyond Low Earth Orbit, we must provision ourselves using the vast resources of space—extracting resources beyond Earth will present many challenges as we master the skills necessary to work in new environments. This journey begins on the Moon—the staging ground, supply station and classroom for our coming voyage into the universe.