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

Abstract

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.

Full text

Perchlorate on Mars: a chemical hazard
and a resource for humans
Alfonso F. Davila
1,2
, David Willson
2
, John D. Coates
3
and Christopher P. McKay
2
1
Carl Sagan Center at the SETI Institute, 189 Bernardo Avenue, Suite 100, Mountain View, CA 94043-5203, USA
e-mail: adavila@seti.org
2
Space Sciences and Astrobiology Division, NASA Ames Research Center, Moffett Field, CA 94035, USA
3
Department of Plant and Microbial Biology, 271 Koshland Hall, University of California, Berkeley, CA 94720, USA
Abstract: 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.
Received 8 May 2013, accepted 10 May 2013
Key words: hazards, ISRU, Mars, oxygen, perchlorate (ClO
4
−
).
Introduction
Perchlorate (ClO
4
−
) 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
4
−
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
4
−
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
4
−
(Catling
et al. 2010). The amount of ClO
4
−
in the surface regolith of
Mars is significant 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
4
−
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
4
−
resulting
in the formation of hypochlorite, other lower oxidation state
oxychlorine species and production of O
2
gas that remains
trapped in the salt crystal. They suggest that ionization
processing of ClO
4
−
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
4
−
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 scientific implications, ClO
4
−
is also of
considerable interest with respect to the exploration of Mars
by humans. NASA has identified 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
4
−
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
4
−
on Mars, and suggest an
approach to ClO
4
−
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
4
−
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
ClO
4
−
on iodine uptake is reversible once ClO
4
−
exposure
International Journal of Astrobiology, Page 1 of 5
doi:10.1017/S1473550413000189 © Cambridge University Press 2013

ceases. Once ingested, ClO
4
−
is rapidly absorbed and has
a short residence time in the human body (ca. hours). The
reference dose (RfD) for ClO
4
−
is 0.7 μgkg
− 1
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
− 1
. The possible deleterious effects
of ClO
4
−
are still unclear, particularly with regard to long-term
exposure (ATSDR 2008), which only emphasizes the need to
understand the potential of ClO
4
−
as a hazard to humans on
Mars before the first manned mission.
The persistence of ClO
4
−
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
4
−
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
4
−
. Incorporation through direct skin contact
is less likely. Exposure to ClO
4
−
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
4
−
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
4
−
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
4
−
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 fine
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 flow 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
4
−
into usable O
2
(see below). Vacuum systems with air-purged
filters are also used in the mining industry in particular for
habitable spaces. This includes, electrostatic cleaners or High
Efficiency Particular Air filters (HEPA) technology and can be
applied to habitats on Mars. These practices can be coupled
with appropriate spacesuit technology specific 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
4
−
as a chemical hazard to
astronauts is its possible presence in ground ice. The Phoenix
Lander detected ClO
4
−
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
4
−
at two landing sites
(Px = Phoenix; C = Curiosity), suggesting that ClO
4
−
could be globally distributed. V1-Viking 1; V2 =Viking 2; O =Opportunity; S =Spirit;
P=Pathfinder.
2 Alfonso F. Davila et al.

concentration within ground ice. Given the persistence of ClO
4
−
in water, extraction of ground ice for human consumption
would be compromised if ClO
4
−
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
4
−
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
4
−
would be critical
to assess whether ground ice can be used as a resource for
humans, or whether ClO
4
−
removal prior to use would be a
requirement.
At the time of writing ClO
4
−
is the only Cl-oxyanion that has
been found on Mars. However, studies on Earth show that
chlorate (ClO
3
−
) co-occurs with ClO
4
−
in all environments,
often at equimolar concentrations. While the possible effects of
ClO
3
−
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
ClO
4
−
into other Cl-oxyanions, such as ClO
2
−
and ClO
−
(Quinn
et al. 2013), which are much more reactive and can be the
cause of other health concerns such as respiratory difficulties,
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
4
−
and other reactive Cl-oxyanions, such
as ClO
3
−
, ClO
2
−
, ClO
2
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
4
−
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
−
and
usable O
2
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
4
−
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
4
ClO
4
) is predominantly used as an energetic
booster or oxidant in solid rocket fuel. The most beneficial use
of ClO
4
−
on Mars would be as a source of O
2
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
4
−
measured in Martian regolith, a
daily supply of oxygen for one astronaut could be obtained by
complete dissociation of ClO
4
−
contained in 60 kg of regolith
(40 litres).
More importantly, mining out oxygen from ClO
4
−
in
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
4
−
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
4
−
reduction; (2) chlorite dismutation
and (3) oxygen reduction. The first 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
4
−
to
chlorite and dismutation of chlorite with resultant formation
of oxygen. Studies with washed whole-cell suspensions and
purified enzyme preparations demonstrated that Cld is highly
specific for chlorite, and alternative anions tested are not
substitute substrates for dismutation. Purified Cld has a specific
activity of 1928 μmol chlorite dismutated per mg of protein
per minute (Coates and Achenbach, 2004). Purified enzymes
involved in microbial ClO
4
−
metabolism could be the basis of
an automated system of oxygen generation from perchlorate in
Martian regolith. Based on the specific activity of Cld, 100 g of
purified 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
2
system that can provide an
astronaut with 1 h of breathable O
2
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
4
−
into a container
holding the Pcr and Cld enzymes. The O
2
produced could be
directly fed into the astronaut’s suit.
The biochemical extraction of oxygen from ClO
4
−
in
Martian regolith would be compliant with Planetary
Protection requirements, because it would be based solely on
purified enzymes, and not on introduced terrestrial species.
Once the oxygen was extracted, the regolith could be returned
to the surface free of ClO
4
−
, and in the case of ground ice, the
water would be suitable for human consumption or food growth.
Conclusions
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
4
−
on Mars ought to be considered an SKG that
needs to be addressed prior to exploration of the planet by
humans. Inhalation of ClO
4
−
-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
efficient and cost-effective mechanism to mitigate the risk of
ClO
4
−
toxicity on Mars is by developing biochemical systems
that decompose ClO
4
−
into innocuous Cl
–
and usable O
2
,
based on concentrated extracts of natural enzymes. This way,
mitigation of ClO
4
−
toxicity could be coupled to in situ resource
utilization.
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