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Perchlorate on Mars: A chemical hazard and a resource for humans


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Perchlorate (ClO 4 −) is widespread in Martian soils at concentrations between 0.5 and 1%. At such concentrations, perchlorate could be an important source of oxygen, but it could also become a critical chemical hazard to astronauts. In this paper, we review the dual implications of ClO 4 − on Mars, and propose a biochemical approach for removal of perchlorate from Martian soil that would be energetically cheap, environmentally friendly and could be used to obtain oxygen both for human consumption and to fuel surface operations.
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Perchlorate on Mars: a chemical hazard
and a resource for humans
Alfonso F. Davila
, David Willson
, John D. Coates
and Christopher P. McKay
Carl Sagan Center at the SETI Institute, 189 Bernardo Avenue, Suite 100, Mountain View, CA 94043-5203, USA
Space Sciences and Astrobiology Division, NASA Ames Research Center, Moffett Field, CA 94035, USA
Department of Plant and Microbial Biology, 271 Koshland Hall, University of California, Berkeley, CA 94720, USA
Abstract: Perchlorate (ClO
) is widespread in Martian soils at concentrations between 0.5 and 1%. At such
concentrations, perchlorate could be an important source of oxygen, but it could also become a critical
chemical hazard to astronauts. In this paper, we review the dual implications of ClO
on Mars, and propose
a biochemical approach for removal of perchlorate from Martian soil that would be energetically cheap,
environmentally friendly and could be used to obtain oxygen both for human consumption and to fuel
surface operations.
Received 8 May 2013, accepted 10 May 2013
Key words: hazards, ISRU, Mars, oxygen, perchlorate (ClO
Perchlorate (ClO
) has been directly detected at two landing
sites on Mars at concentrations between 0.5 and 1%: at the
Phoenix landing site at 68°N (Hecht et al. 2009) and at Gale
Crater at 4.5°S (Glavin et al. 2013). In addition, perchlorate
has been inferred at the two Viking landing sites, 22.5°N and
48.3°N (Navarro-Gonzalez et al. 2013). Measured abundances
of ClO
at each of these sites match total abundances of
Chlorine measured from orbit using the Gamma Ray
Spectrometer on board Mars Odyssey (Fig. 1), suggesting
that ClO
could be globally distributed on the planet, in top
tens of centimetres of the regolith. This is consistent with
models advocating an atmospheric origin of ClO
et al. 2010). The amount of ClO
in the surface regolith of
Mars is signicant compared with soils on Earth, where typical
concentrations are three to four orders of magnitude lower
than on Mars.
Since its discovery on Mars, ClO
has become the focus of
interest due to its possible role in destroying organics in
thermal stage of analytical instruments sent to Mars to detect
organics (Navarro-González et al. 2010). Quinn et al.(2013)
have shown that ionizing radiation decomposes ClO
in the formation of hypochlorite, other lower oxidation state
oxychlorine species and production of O
gas that remains
trapped in the salt crystal. They suggest that ionization
processing of ClO
alone can explain the Viking LR and
GEX results. Perchlorate could also lead to transient,
metastable brines by way of deliquescence, even under current
climate conditions (Rennó et al. 2009; Zorzano et al. 2009),
and therefore play a role in the meagre hydrological cycle on
Mars. In addition, ClO
can be used as a terminal electron
acceptor by a variety of prokaryotes (cf. Coates & Achenbach
2004), which has potential implications for habitability of
Martian soils.
Aside from its scientic implications, ClO
is also of
considerable interest with respect to the exploration of Mars
by humans. NASA has identied key strategic knowledge gaps
(SKGs) that need to be addressed before humans can be sent
to the planet (MEPAG 2010). Two key SKGs are potential
hazards to humans and the existence of resources that can
support human and robotic operations. Perchlorate could play
a central role in both instances: it could be an important source
of oxygen both for life support and to fuel surface operations,
but it could also become a critical chemical hazard for
astronauts. The possible implications of ClO
on Mars as a
hazard and as a resource could become a key aspect in design
and implementation of future missions, particularly since the
highest concentrations might occur in equatorial regions
(Fig. 1), where humans are more likely to land. Here, we
review the dual implications of ClO
on Mars, and suggest an
approach to ClO
utilization that would minimize the hazard
and maximize its use as a resource.
Perchlorate on Mars: a chemical hazard to humans
Perchlorate salts are very soluble in water, and the ClO
ion is
kinetically inert to reduction, and has little tendency to absorb
in minerals or organic surfaces, which make it a very persistent
compound in the environment and also in solution. Perchlorate
is a health concern because it can impair proper functioning
of the thyroid gland, by competitively inhibiting the uptake
of iodine ions, thereby hindering hormonal output (Fig. 2)
(cf. Smith 2006). Thyroid hormones are responsible for
regulating mammalian metabolism; a long-term reduction in
iodide uptake in an adult can ultimately result in thyroid
hyperplasia, goitre, decreased metabolic rates and slowing of
the function of many organ systems. The competitive effect of
on iodine uptake is reversible once ClO
International Journal of Astrobiology, Page 1 of 5
doi:10.1017/S1473550413000189 © Cambridge University Press 2013
ceases. Once ingested, ClO
is rapidly absorbed and has
a short residence time in the human body (ca. hours). The
reference dose (RfD) for ClO
is 0.7 μgkg
of body weight
per day (i.e. Brown & Gu 2006). This is the daily oral exposure
that is to remain without an appreciable risk of deleterious
effects during a lifetime, and corresponds to drinking water
equivalent level of 24.5 μgl
. The possible deleterious effects
of ClO
are still unclear, particularly with regard to long-term
exposure (ATSDR 2008), which only emphasizes the need to
understand the potential of ClO
as a hazard to humans on
Mars before the rst manned mission.
The persistence of ClO
in the environment, and its possible
widespread distribution on the Martian surface, make it a
global hazard to humans on the planet. The main routes of
exposure of astronauts to ClO
on Mars would be through
direct inhalation of dust into the respiratory system, ingestion
of contaminated water and ingestion of foods grown in the
presence of ClO
. Incorporation through direct skin contact
is less likely. Exposure to ClO
through inhalation is not a
serious problem on Earth, where concentrations are typically
low, but it could become a major concern on Mars. Like the
Moon, an important fraction of the Martian surface is covered
in dust. Dust became one of the main hazards to astronauts
on the Moon largely due to the abrasive nature of lunar dust
particles, which could cause lung damage. Inhalation of dust
particles <5 μm in size was of particular concern, because
particles of this size cannot be expelled by lung mucus. Aside
from abrasiveness, mobile fraction of Martian dust may
contain up to 1% ClO
or more, and inhalation of a few
milligrams of dust would already surpass the RfD, if
perchlorate were quickly absorbed into human blood circula-
tion. Astronauts could breathe airborne dust from their dusty
spacesuits after extra-vehicular activity (EVA), as occurred
with astronauts on the Moon during the Apollo missions, and
exposure to ClO
could also be critical during dust storms.
Contrary to the Moon, and due to the presence of perchlorate,
all dust particle sizes on Mars are a potential human hazard.
Managing ClO
exposure on Mars would be in many ways
no different than managing for example, uranium, lead or
general heavy metal contaminated areas in modern mines
where dust suppression, dust extraction and regular blood
monitoring is employed. The primary dust suppression method
in mines is water spray dust suppression systems (Xie et al.
2007). These could be employed in airlocks in the form of ne
fog sprays to clean dust particles <1 μm. Water sprays using
ultrasonic generated droplets that match the target dust
particles (Xie et al. 2007) ensure dust affects the droplets.
Dust particles smaller than water droplets do not affect the
droplets but ow around them in the airstream above the
boundary layer, and generating water droplets suited for all
dust sizes would be a challenge. A wash down spray could also
be employed to clean suits and equipment with dust deposits.
Perchlorate dust would quickly go into solution in this water
environment and be drained away. A separated process could
be used to recycle water for the sprays and to decompose ClO
into usable O
(see below). Vacuum systems with air-purged
lters are also used in the mining industry in particular for
habitable spaces. This includes, electrostatic cleaners or High
Efciency Particular Air lters (HEPA) technology and can be
applied to habitats on Mars. These practices can be coupled
with appropriate spacesuit technology specic to the type of
exploration being undertaken. Regular monitoring toxicity
levels in astronaut blood, as per many mining practices can be
employed to manage individual exposure risks.
Another critical aspect of ClO
as a chemical hazard to
astronauts is its possible presence in ground ice. The Phoenix
Lander detected ClO
in the regolith at polar latitudes down to
the ice table (Hecht et al. 2009), but no data exist regarding its
Fig. 1. Equatorial and mid latitude distribution of Cl within the top 1 m of Mars measured by the Gamma Ray Spectrometer onboard Mars
Odyssey (from Keller et al. 2006). The global concentration of Cl is similar to the measured concentration of ClO
at two landing sites
(Px = Phoenix; C = Curiosity), suggesting that ClO
could be globally distributed. V1-Viking 1; V2 =Viking 2; O =Opportunity; S =Spirit;
2 Alfonso F. Davila et al.
concentration within ground ice. Given the persistence of ClO
in water, extraction of ground ice for human consumption
would be compromised if ClO
were present at concentrations
similar to those in the dry regolith or higher. Similarly, the use
of extracted water for food growth would also be compromised
because ClO
can bio-accumulate in the tissue of vegetables
(Ha et al. 2011). Knowledge of the chemical composition
of ground ice on Mars with respect to ClO
would be critical
to assess whether ground ice can be used as a resource for
humans, or whether ClO
removal prior to use would be a
At the time of writing ClO
is the only Cl-oxyanion that has
been found on Mars. However, studies on Earth show that
chlorate (ClO
) co-occurs with ClO
in all environments,
often at equimolar concentrations. While the possible effects of
on human health are far less understood, they cannot be
disregarded in the event of a human mission, unless further
investigations suggest otherwise. In addition, as mentioned
above, ionizing radiation can decompose small quantities of
into other Cl-oxyanions, such as ClO
and ClO
et al. 2013), which are much more reactive and can be the
cause of other health concerns such as respiratory difculties,
headaches, skin burns, loss of consciousness and vomiting.
These more reactive species might also be cause for concern
with regard to the corrosion of astronaut suits, instruments and
other materials. As such, in preparation for human exploration
it is important to fully characterize the composition of the
Martian regolith, and specially its most mobile fraction,
with respect to ClO
and other reactive Cl-oxyanions, such
as ClO
, ClO
, ClO
gas and ClO
Perchlorate removal from Martian dust and regolith
could be done in a number of ways, but in the next section
we propose a mechanism for removal that would be
energetically cheap, environmentally friendly and could be
used as a source of oxygen both for human consumption and
for surface operations.
Fig. 2. Top. Perchlorate as a hazard. ClO
can impair proper functioning of the thyroid gland, by competitively inhibiting the uptake of iodine
ions, thereby hindering hormonal output. Bottom. Perchlorate as a resource. Perchlorate can be biochemically degraded into innocuous Cl
usable O
by means of concentrated extracts of naturally occurring enzymes. Data from Coates and Achenbach (2004)
Perchlorate on Mars 3
Perchlorate on Mars: a useful resource for humans
The ClO
ion consists of a central chlorine atom surrounded
by a tetrahedral array of four oxygen atoms. Owing to its
strong oxidizing power at higher temperatures, ammonium
perchlorate (NH
) is predominantly used as an energetic
booster or oxidant in solid rocket fuel. The most benecial use
of ClO
on Mars would be as a source of O
for human
consumption and to fuel surface operations. For example,
humans breathe or consume 550 litres of oxygen per day.
Based on the amounts of ClO
measured in Martian regolith, a
daily supply of oxygen for one astronaut could be obtained by
complete dissociation of ClO
contained in 60 kg of regolith
(40 litres).
More importantly, mining out oxygen from ClO
Martian regolith could be done cleanly and with minor
alterations to the regolith, taking advantage of existing
microbial biochemical pathways for perchlorate metabolism.
It has been known for several decades that some micro-
organisms can reduce ClO
under anaerobic conditions, and
more than 50 dissimilatory perchlorate-reducing bacteria
have been isolated in pure culture (Coates and Achenbach,
2004). The biogeochemical redox cycle of chlorine is well
understood (i.e. Coates & Achenbach 2006), and consists of
three key steps: (1) ClO
reduction; (2) chlorite dismutation
and (3) oxygen reduction. The rst enzymatic step of the
pathway, perchlorate reduction to chlorite, is performed by
perchlorate reductase (Pcr). The chlorite is subsequently
converted to chloride and oxygen by chlorite dismutase
(Cld). Finally, oxygen is reduced to water by an oxygen
reductase. The entire metabolic pathway converting perchlor-
ate to Cl
and molecular oxygen occurs in the periplasmic
space of the cell, owing to the toxicity of both chlorite and
oxygen (Coates and Achenbach, 2004). For our purposes, the
key biochemical step in this pathway is reduction of ClO
chlorite and dismutation of chlorite with resultant formation
of oxygen. Studies with washed whole-cell suspensions and
puried enzyme preparations demonstrated that Cld is highly
specic for chlorite, and alternative anions tested are not
substitute substrates for dismutation. Puried Cld has a specic
activity of 1928 μmol chlorite dismutated per mg of protein
per minute (Coates and Achenbach, 2004). Puried enzymes
involved in microbial ClO
metabolism could be the basis of
an automated system of oxygen generation from perchlorate in
Martian regolith. Based on the specic activity of Cld, 100 g of
puried enzyme could generate a daily supply of oxygen for
one astronaut in >1 h (Fig. 2). As a proof of concept, we have
developed a portable emergency O
system that can provide an
astronaut with 1 h of breathable O
based on soil perchlorate
decomposition catalysed by enzymes extracted from per-
chlorate reducing bacteria. The astronaut would collect ca.
6 kg of Martian regolith into a bag and add water, which would
dissolve and carry the highly soluble ClO
into a container
holding the Pcr and Cld enzymes. The O
produced could be
directly fed into the astronauts suit.
The biochemical extraction of oxygen from ClO
Martian regolith would be compliant with Planetary
Protection requirements, because it would be based solely on
puried enzymes, and not on introduced terrestrial species.
Once the oxygen was extracted, the regolith could be returned
to the surface free of ClO
, and in the case of ground ice, the
water would be suitable for human consumption or food growth.
Perchlorate on Mars has two opposing aspects, it poses a
serious risk to astronauts but can also be a life-saving resource.
As such, ClO
on Mars ought to be considered an SKG that
needs to be addressed prior to exploration of the planet by
humans. Inhalation of ClO
-bearing dust particles could be a
major concern, but mitigation technologies exist in the mining
industry that could be applied on Mars. Perhaps the most
efcient and cost-effective mechanism to mitigate the risk of
toxicity on Mars is by developing biochemical systems
that decompose ClO
into innocuous Cl
and usable O
based on concentrated extracts of natural enzymes. This way,
mitigation of ClO
toxicity could be coupled to in situ resource
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Perchlorate on Mars 5
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1] Isotopic studies indicate that natural perchlorate is produced on Earth in arid environments by the oxidation of chlorine species through pathways involving ozone or its photochemical products. With this analogy, we propose that the arid environment on Mars may have given rise to perchlorate through the action of atmospheric oxidants. A variety of hypothetical pathways can be proposed including photochemical reactions, electrostatic discharge, and gas-solid reactions. Because perchlorate-rich deposits in the Atacama desert are closest in abundance to perchlorate measured at NASA's Phoenix Lander site, we made a preliminary study of the means to produce Atacama perchlorate to help shed light on the origin of Martian perchlorate. We investigated gas phase pathways using a 1-D photochemical model. We found that perchlorate can be produced in sufficient quantities to explain the abundance of perchlorate in the Atacama from a proposed gas phase oxidation of chlorine volatiles to perchloric acid. The feasibility of gas phase production for the Atacama provides justification for future investigations of gas phase photochemistry as a possible source for Martian perchlorate.
The most comprehensive search for organics in the Martian soil was performed by the Viking Landers. Martian soil was subjected to a thermal volatilization process to vaporize and break organic molecules, and the resultant gases and volatiles were analyzed by gas chromatography-mass spectrometry. Only water at 0.1–1.0 wt% was detected, with traces of chloromethane at 15 ppb, at Viking landing site 1, and water at 0.05–1.0 wt% and carbon dioxide at 50–700 ppm, with traces of dichloromethane at 0.04–40 ppb, at Viking landing site 2. These chlorohydrocarbons were considered to be terrestrial contaminants, although they had not been detected at those levels in the blank runs. Recently, perchlorate was discovered in the Martian Arctic soil by the Phoenix Lander. Here we show that when Mars-like soils from the Atacama Desert containing 32 ± 6 ppm of organic carbon are mixed with 1 wt% magnesium perchlorate and heated, nearly all the organics present are decomposed to water and carbon dioxide, but a small amount is chlorinated, forming 1.6 ppm of chloromethane and 0.02 ppm of dichloromethane at 500°C. A chemical kinetics model was developed to predict the degree of oxidation and chlorination of organics in the Viking oven. Reinterpretation of the Viking results therefore suggests ≤0.1% perchlorate and 1.5–6.5 ppm organic carbon at landing site 1 and ≤0.1% perchlorate and 0.7–2.6 ppm organic carbon at landing site 2. The detection of organics on Mars is important to assess locations for future experiments to detect life itself.
Dust suppression in coal mines is a worldwide problem which has not been solved effectively. The application of negative pressure secondary dust removal (NPSDR) is a breakthrough in the coal mine safety field. In this paper, NPSDR technology and ultrasonic dust suppression systems are introduced. High pressure water is supplied to the NPSDR device which is mounted on the shearer. A negative pressure field is formed in the device. At the same time, the dusty air around the shearer drum will be sucked into, and purged from, the NPSDR device by the negative pressure field. An ultrasonic dust suppression system uses water and compressed air to produce micron sized droplets which suppress respirable coal dust effectively. The NPSDR technology can be used for shearer dust suppression while ultrasonic dust suppression can be applied in areas such as the transportation positions. These dust suppression methods have the following advantages: high efficiency, wide applicability, simple structure, high reliability and low cost.
Abstract Results from the Viking biology experiments indicate the presence of reactive oxidants in martian soils that have previously been attributed to peroxide and superoxide. Instruments on the Mars Phoenix Lander and the Mars Science Laboratory detected perchlorate in martian soil, which is nonreactive under the conditions of the Viking biology experiments. We show that calcium perchlorate exposed to gamma rays decomposes in a CO2 atmosphere to form hypochlorite (ClO(-)), trapped oxygen (O2), and chlorine dioxide (ClO2). Our results show that the release of trapped O2 (g) from radiation-damaged perchlorate salts and the reaction of ClO(-) with amino acids that were added to the martian soils can explain the results of the Viking biology experiments. We conclude that neither hydrogen peroxide nor superoxide is required to explain the results of the Viking biology experiments. Key Words: Mars-Radiolysis-Organic degradation-in situ measurement-Planetary habitability and biosignatures. Astrobiology 13, xxx-xxx.
The Phoenix Mars lander measured perchlorate as a key soluble anion in the soil at an abundance of ~1wt%. Here, we discuss how the perchlorate was likely formed from atmospheric oxidants acting on chlorine-bearing species in Mars' arid environment.
Considerable amounts of ecotoxicological data have been generated since the discovery of perchlorate contamination in the environment to assist in evaluating the potential for ecological exposures and subsequent effects. This chapter attempts to provide a synopsis of, and to interpret the available ecological data pertaining to perchlorate, and to identify areas in need of further study. Perchlorate is an oxidizing anion found as a contaminant in ground and surface waters as a result of the dissolution of perchlorate salts (ammonium, potassium, magnesium, and sodium). Perchlorate salts are highly watersoluble and, in aqueous solution, perchlorate is quite unreactive within the range of temperatures and pHs normally encountered in the environment. 1 Once in water, perchlorate can persist for many years, and water serves as a carrier. Because it is an anion, perchlorate does not adsorb to soils and its adsorption to minerals is weak and reversible.2.