Evidence for the distribution of perchlorates on Mars

Abstract

Various Mars missions have detected Cl atoms, chlorides and perchlorates in martian surface materials. The global soils, in particular, always contain significant levels of observable Cl. Direct evidence points to this Cl being in the form of both chlorides and perchlorates, and possibly also chlorates and other oxychlorines. The most widespread measurements have been of Cl atoms, and cannot discern the chemical form. However, from separate evidence of perchlorate obtained at high latitudes (Phoenix lander) and low latitudes (Curiosity rover), it is likely that perchlorates are widespread, albeit in varying proportions relative to the total amount of ubiquitous Cl.

Full text

Evidence for the distribution of perchlorates
on Mars
Benton C. Clark
1
and Samuel P. Kounaves
2
1
Space Science Institute, 4750 Walnut, Boulder, CO 80301, USA e-mail: bclark@spacescience.org
2
Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, MA 02155, USA
Abstract: Various Mars missions have detected Cl atoms, chlorides and perchlorates in martian surface
materials. The global soils, in particular, always contain significant levels of observable Cl. Direct evidence
points to this Cl being in the form of both chlorides and perchlorates, and possibly also chlorates and
other oxychlorines. The most widespread measurements have been of Cl atoms, and cannot discern the
chemical form. However, from separate evidence of perchlorate obtained at high latitudes (Phoenix lander)
and low latitudes (Curiosity rover), it is likely that perchlorates are widespread, albeit in varying proportions
relative to the total amount of ubiquitous Cl.
Received 3 June 2015, accepted 3 August 2015
Key words: chloride, Cl, habitability, Mars, oxychlorine, perchlorate, salts, sulphate, water
Introduction
One of the seminal discoveries made on the first landed mission
to Mars was the unusual amount of Cl in the compositional
profile of soils (Clark et al. 1982), virtually the same at the
two widely separated Viking landing sites and at concentra-
tions far higher than in typical terrestrial soils or the lunar rego-
lith. Subsequent measurements have shown the same unusual
Cl concentrations at multiple, dispersed landing and rover sites
(Gellert & Clark 2015). The general interpretation has been
that the Cl must be in the form of one or more chloride salts
(Clark & van Hart 1981;Yenet al. 2006), in analogy with
its widespread occurrences on Earth. Not until the wet chemis-
try experiments on the Phoenix mission in 2008, some 32 years
after Viking, it was determined that not only there is chloride
but also that a significant fraction of the Cl can be in the form
of one or more perchlorates in the soil (Hecht et al. 2009;
Kounaves et al. 2014b). Speculation that perchlorate may be
widespread across Mars seems to be borne out by the more re-
cent evidence for oxychlorines from evolved gas analysis on the
Curiosity rover mission (Ming et al. 2014).
The Viking missions also detected one or more unexpected
oxidized species in the soils. The first evidence was the release
of O
2
gas upon humidification or wetting of the samples
(Oyama & Berdahl 1977). It was also found that a portion of
amino acids in the nutrient of the Labelled Release experiment
in the Biology investigation became oxidized, a possible indica-
tor of metabolic activity (Levin & Straat 1981) but also pos-
sibly due to an indigenous oxidant in the soils (Klein 1978).
These were clues that perhaps some or all of the chlorine and
bromine compounds in martian soil are in oxidized forms
(Clark et al. 2005).
Other oxidized forms of chlorine such as chlorate, chlorite
and hypochlorite may be present, either as intermediates in
the progression of oxidation of Cl or as end-products
(Fig. 1). The presence and variety of oxygen-rich forms of
chlorine on Mars is important for a variety of reasons, includ-
ing (a) as additional evidence for oxidative processes in the
martian environment; (b) as agents to mimic metabolic activity
and detection of life; (c) as reactants to destroy organic
molecules; (c) as sinks for H
2
O; (d) as strong freezing-point
depressants, enabling the formation of brines at very low tem-
peratures; and (e) with the potential for reacting with other
constituents during heating of soil, and thereby confounding
the results of in situ thermally-evolved gas analysis
investigations.
Detection of Cl on Mars
A variety of techniques are available for the detection of Cl
atoms, chemicals and minerals, ranging from X-ray to
gamma ray spectroscopy; from wet chemistry to thermal gas
evolution; and from reflection spectroscopy in the near and
mid-infrared (IR) wavelength regions to laser-induced optical
emissions. Each has, of course, its own strengths and limita-
tions, including specificity, accuracy and spatial resolution.
X-ray based systems
The X-ray fluorescence (XRF) method of analyzing rocks and
soils is quite sensitive and accurate for measuring the relatively
high concentrations of Cl atoms in soils and sediments on
Mars. From the 2.62 keV K-alpha-line fluorescence emission
of Cl, nearly every sample of material analysed on Mars by
in situ missions contained detectable chlorine. Regolith fines
that encompass the global soil unit range from 0.4 to 0.8 wt.%
Cl (Yen et al. 2014). Although some cases of Cl may be simply
due to contamination by soil or airfall dust, there are ample ex-
amples of exposed interiors of rocks (via grinding) with igneous
International Journal of Astrobiology, Page 1 of 8
doi:10.1017/S1473550415000385 ©Cambridge University Press 2015

compositional profiles that nonetheless have Cl at levels higher
than in terrestrial analogs or martian meteorites.
The three plots of Fig. 2 are histograms showing the fre-
quency of occurrence of different concentrations of Cl atoms
as measured by the Mars Exploration Rover (MER) and
Mars Science Laboratory (MSL) rover missions. It is note-
worthy that the preponderance of Cl concentrations cluster be-
tween about 0.5 and 1.5 wt.% Cl for all three sites of
exploration, although the MSL data at this point in its mission
show more cases of higher levels of Cl than the MER rovers
measured in Gusev crater and Meridiani Planum. Values
above 2.5 wt.% Cl are relatively rare. This is surprising in
view of the abundance of a wide variety of evidence for aque-
ous processes at all three sites because all the salts of Cl that are
geochemically plausible based on the availability of appropri-
ate cations are highly soluble, even at temperatures below the
freezing point of pure H
2
O. In particular, in the multi-layered
sediments of the ubiquitous Burns Formation in Meridiani
Planum there are much higher concentrations of sulphates
(3–5x higher than in soils and elsewhere on Mars), and though
many of the inferred sulphates are highly soluble, the Cl con-
centrations are not strongly enhanced (Clark et al. 2005), with
the upper layers of the Burns Formation containing almost the
same concentrations typical of soils, and the lower layers with
only double that. The correlation between S and Cl in the
Burns Formation is very weak, whereas in global soils ob-
served at all landing sites, there are strong correlations between
the two elements (e.g. Yen et al. 2014).
Although X-ray fluorescence detects only the presence of
atoms, the X-ray diffraction patterns from the CheMin instru-
ment on the MSL rover Curiosity can, in principle, detect vari-
ous mineralogical forms of Cl. However, the results to date
have not revealed the presence of Cl minerals, with the excep-
tion of akaganeite (Vaniman et al. 2014). This lack of detection
of salts may be due to the relatively low concentration of the Cl
minerals compared with the much more abundant minerals
made up by the major elements. Alternatively, it could be
that a significant portion, or all, of the Cl is in X-ray
amorphous forms, such as sub-micron particles, or disordered
material, or as thin rinds adsorbed or chemisorbed on individ-
ual silicate grains. Finally, there could be crystalline species but
in multiple forms such that none is at a high enough concentra-
tion to be revealed by CheMin’s X-ray diffraction patterns,
compared with the patterns of the other constituent minerals.
These forms could include not only multiple cation chemistries
combined with Cl-bearing phases, but also different hydration
states (stoichiometric numbers of H
2
O molecules of crystalliza-
tion) occurring in different crystallographic forms.
Gamma ray and neutron techniques
Spectroscopy from orbit of energetic gamma rays stimulated by
cosmic rays and their secondary particles allows Cl atoms to be
detected (Keller et al. 2006;Diezet al. 2009). Systematic planet-
wide measurements of Cl have been made by direct detection of
the 1.95, 5.60 and 7.78 MeV gamma emissions by the Gamma
Ray Spectrometer (GRS) on the MarsOdysseyorbiter. The spa-
tial resolution of this technique is large,with *300–400 km f oot-
print on the surface. The neutron spectrometer portion of GRS
has a similar footprint and also can infer the presence of chlorine
atoms by their strong absorption cross-sections for thermal neu-
trons, relative to other rock- and soil-forming major and minor
elements (Diez et al. 2009).
The Cl abundances (Boynton et al. 2007), as normalized to
Pathfinder results, mostly range from 0.3 to 0.7 wt.%, close to
Fig. 1. The variety of plausible oxidation states for Cl on Mars.
Pathways for progressive oxidation may be balanced by certain
processes favouring the loss, rather the gain of oxygen, such as ionizing
radiation.
Fig. 2. Histograms of chlorine abundances for three rover missions
(throughsol 3750 for Meridiani, and sol 854 at Gale crater).
2Benton C. Clark and Samuel P. Kounaves

the values for global soils. Lower average values (as low as
0.2 wt.%) tend to occur more often in southern latitudes,
where bedrock may be more common. The highest values
(up to 0.8 wt.%) occur in equatorial regions.
IR spectroscopy
Near-IR spectroscopy with the Phoenix mission camera was
used to identify patches of higher concentrations of hydrated
perchlorate (Mg- or Ca-) around the lander. The absorption
band at 967 nm (Cull et al. 2010) is prominent for these per-
chlorates. These authors acknowledge that several zeolites
also produce this feature at this wavelength, as do some phyl-
losilicates and hydrated chlorides (bischofite), but provide
various arguments why they conclude that the patches are
perchlorates.
Also, from orbit the spectrum in the thermal IR region, as
measured by the THEMIS instrument (also on the Odyssey
spacecraft), has been interpreted as indicating the presence of
chlorides. The absorption normally present in the region of
1260–900 cm
−1
(7.9–11.1 µm) range is spectrally elevated by
higher chloride-specific emissivity (Osterloo et al. 2008,
2010). Spatial resolution of these observations is *3×6km
2
.
Maps of these chloride concentrations show a preponderance in
low albedo areas in the southern hemisphere, at latitudes mostly
equatorward of 45°.Occurrences can be patchy, with a tendency
to occur in topographic lows relative to nearby terrain, such as
crater floors, and to be light-toned and exhibiting polygonal
fracturing. In a critical review by Jensen & Glotch (2011),
it was concluded that although certain sulphide minerals
could partially mimic the spectral slope, there were sufficient
differences between the spectra to rule them out, including
the observation that alteration in the region is also indicated
by the detection of phyllosilicates adjacent to the deposits in
many cases.
Recently, Ojha et al.(2015) have discovered that four exam-
ples of the special type of gullies which change form with time
(termed the Recurring Slope Lineae) show evidence of oxy-
chlorines in their IR spectra, as measured by the Compact
Reconnaissance Imaging Spectrometer for Mars instrument
on the Mars Reconnaissance Orbiter. These implicated salts in-
clude Mg- and Na-perchlorate, and Mg-chlorate. As discussed
below, oxychlorine salts in solution can significantly reduce the
freezing point of water and hence produce brines that can flow
under cold ambient conditions on Mars.
Laser-induced breakdown spectroscopy (LIBS)
The ChemCam (CCAM) instrument on the Curiosity rover in-
cludes a telescopic micro-imager and a LIBS system which ana-
lyses individual spots on rocks and soils as small as 300 µm in
diameter (Wiens et al. 2012). Although the sensitivity is
not high, there is evidence of detection of Cl using the emission
line at 837.8 nm. Laboratory studies underway (A. Cousin
2015 personal communication) have shown that there are im-
portant matrix effects that will require different calibrations for
rocks and soils. A search has also been made for detecting the
molecular emission line of CaCl
2
, so far without success.
Wet chemistry analysis
Definitive identification and quantification of the perchlorate
cation (ClO
4
−
) in the Phoenix landing site soil was accom-
plished by two ion selective electrode sensors flown in the
Wet Chemistry Laboratory (WCL) on the Phoenix polar lan-
der (Kounaves et al. 2010). One electrode originally labelled as
a‘nitrate’sensor (often referred to as a ‘Hofmeister’sensor), is
known to be three orders-of-magnitude more sensitive to per-
chlorate than nitrate (i.e. it is also a perchlorate sensor). The
other, a sensor specific for detection of calcium (Ca
2+
), is nor-
mally not responsive to anions, except to perchlorate, which
causes a unique negative bias in its response to Ca
2+
. These two
sensors definitively measured and identified *2.7 mM ClO
4
−
in
all three Phoenix soil samples, equivalent to *0.6 wt.% in the
soil. The response of the Hofmeister sensor to ClO
4
−
was so
overwhelming that other species to which it responded would
have to have been present in the soil at physically impossible
amounts. It is also possible that the solution contained chlorate
at concentrations *3–6 mM, which would have been masked
by the sensor’s greater sensitivity to perchlorate. The unique
behaviour of the Ca
2+
sensor in the presence of ClO
4
−
also al-
lowed identification of the ClO
4
−
parent salts as a 3 : 2 mixture
of Ca(ClO
4
)
2
and Mg(ClO
4
)
2
(Kounaves et al. 2014b; Quinn
et al. This Issue).
Chloride was also measured directly with the WCL instru-
ment. Values ranged from 0.24 to 0.6 mM in solution, equiva-
lent to 0.03–0.05 wt.% in the soil (Kounaves et al. 2010).
Evolved gas analysis
The MSL rover Curiosity targeted Gale Crater, well over
5600 km from Phoenix’s landing site near Heimdall Crater.
Gale’s stratigraphy, mineralogy and landforms are starkly dif-
ferent from those that surrounded Phoenix. Yet, after four soil
analyses by the Sample Analysis at Mars (SAM) instrument,
the soils have been found to contain significant levels of an oxy-
chlorine. The first clear evidence for the presence of ClO
4
−
,or
another oxychlorine phase, came from the SAM analysis of the
Rocknest (RN) soil sample. Using both pyrolysis evolved gas
analysis (EGA) by quadrupole mass spectrometry and gas
chromatography mass spectrometry (GCMS), the samples
also released several chlorine-bearing hydrocarbons at the
same temperature where a rise in O
2
and HCl were detected.
The O
2
is assumed to have evolved from the decomposition
of a perchlorate or chlorate salt, with an equivalent concentra-
tion in the soil of 0.3–0.5 wt.%, Table 1 (Leshin et al. 2013).
The Curiosity rover then traversed the Yellowknife Bay forma-
tion to Sheepbed, the lowermost stratigraphic unit, where it
drilled two holes 3 m apart, designated John Klein (JK) and
Cumberland (CB). These mudstone samples (formed in an an-
cient lake bed) were also analysed by the EGA and GCMS.
Both releasedO
2
, assumed to be from ClO
4
/ClO
3
oxychlorines,
with that from the CB sample (equivalent to 1.3 wt.% Cl
2
O
7
)on
average about eight times that from JK (equivalent to 0.24 wt.%
Cl
2
O
7
), and bracketing that from RN (Ming et al. 2014). It should
be noted, though, that the CB sample contained on average three
times the total Cl of the JK sample as measured by APXS,
Distribution of perchlorates on Mars 3

1.41 wt.% Cl versus 0.53 wt.% Cl for CB and JK, respectively
(McLennan et al.2014, Table S5). The JK sample though had
two distinct peaks, implying a different O
2
source or possibly con-
sumption of O
2
by organics or other material, and which could
account for the lower abundance. Converting from the weight
percentages for the two species, the fraction of Cl atoms detected
by APXS which may be in the perchlorate form is *10% (for JK)
to 20% (for RN, CB). This and the Phoenix results demonstrate
that the ratio Cl(perchlorate)/Cl(total) varies with the sample.
Recently, Archer et al. (2015) also concluded this and pointed
out that this ratio is linearly and positively related to the total
Cl content, for the Gale crater samples.
Relative abundances of chemical and mineral forms
of Cl
Ever since the time of discovery of oxygen release by simply
wetting of martian soil (Oyama et al. 1977), the identification
of oxidants in soils has been an important question. Oxidative
processes involving atmospheric photochemical products
under contemporary martian environmental conditions have
been identified and continue to be studied (Hunten 1979;
Yung & DeMore 1999; Krasnopolsky 2006; Lefevre et al.
2008; Clancy et al. 2013). Two recent studies however indicate
that the formation of oxychlorines may be predominately and
globally occurring on Cl- bearing mineral surfaces. The first
used a one-dimensional photochemical model to calculate the
deposition rates of oxyanions and perchlorate from Mars’at-
mosphere, and concluded that the modelled formation of per-
chlorate via purely gas-phase oxidation of volcanically-derived
chlorine is insufficient by several orders of magnitude to ex-
plain the *0.6 wt.% ClO
4
−
measured by Phoenix (Smith
et al. 2014). The second experimental investigation showed
that ClO
4
−
and ClO
3
−
can be produced photochemically on
Cl-minerals without the presence of atmospheric chlorine or
aqueous conditions, most likely due to SiO
2
and metal-oxides
acting as photocatalysts, generating O
2
−
radicals from O
2
which
react with chloride (Carrier & Kounaves 2015).
A key question is whether the speciation of Cl among chlor-
ide and the various oxychlorines is a constant fraction of total
Cl in the martian global soils, in Cl-containing coatings, and in
sediments with enrichments in Cl. Without in situ experiments,
such as the WCL on Phoenix, these determinations are very
difficult. Elemental analysis has been performed on all landed
missions except Phoenix, but does not discriminate between
molecular forms of Cl. Remote sensing is difficult for oxychlor-
ines, and the reported chloride deposits actually anti-correlate,
in general, with the Cl trends revealed by GRS.
On Earth, perchlorate is rare relative to chloride: the relative
concentration of Cl in chloride to Cl in perchlorate is very high
and also quite variable by location, from 5 × 10
2
to over 1 × 10
6
,
with the lowest values occurring in the most arid location, the
Atacama desert (Jackson et al.2015). On Mars, the relative
concentrations of perchlorate are generally much higher, as
discussed above. This may be related to the greater importance
of photochemical processes and the more extremely desiccated
environment on Mars, as compared with Earth.
Another important and not fully resolved question is which
cations are involved in the Cl salts. Major candidates include
Na, Mg and Ca, and could involve Fe as well. The specific
form of the salt affects its physicochemical properties, such
as freezing point depression. The cations identified so far in-
clude Mg and Ca, with indications of a preponderance of
the latter at both the Phoenix site (Kounaves et al. 2014b)
and the Gale site (Ming et al. 2014). It also should be recog-
nized that binary salts often exist in nature, such as combina-
tions of Na, Mg, and/or Ca with anions such as Cl and
sulphate. Several whose occurrences have been significant on
Earth have mineral designations, such as tachyhydrite
(Ca, Mg, Cl), tatarskite (Ca, Mg, S, Cl), D’Ansite (Na,
Mg, S, Cl and replacements of Mg with Fe or Mn), kainite
(K, Mg, S, Cl), sulphohalite (Na, S, Cl), and tatarskite
(Ca, Mg, S, Cl). Most of these and the simple salts as well,
are hydrated. Many more salt complexes are possible and the
martian environment may uniquely favour assemblages that
do not occur on Earth, especially for salts of oxychlorines.
Oxychlorines in martian meteorites
Aside from the Phoenix measurement, the only other direct
measurement of perchlorate in martian material has been
made in the EETA79001 Mars meteorite, recovered in
Antarctica in 1979 at the Elephant Moraine ice ablation region.
One of the largest, at 7942 g, it has been dated to *170 Myr,
with an ejection age of *0.65 Myr, and terrestrial age of
*12 kyr (Jull & Donahue 1988). EETA79001 is composed of
three lithologies: (A) a primary basaltic host of medium-grained,
feldspathic pyroxenite; (B) a coarser-grained basalt similar to
‘A’but free of olivine megacrysts, and (C) several shock-melted
glass pockets and glass-filled veins (McSween & Jarosewich
1983). Within ‘A’is a unique inclusion of white material com-
prised mainly of calcium carbonate (Martinez & Gooding
1986).
Ion chromatography and isotopic analyses of cutting fines
(‘sawdust’), that included the material from the inclusion, re-
vealed the presence of approximately 0.6 ppm ClO
4
−
, 1.4 ppm
ClO
3
−
and 16 ppm NO
3
−
(Kounaves et al. 2014a), with molar
ratios of the NO
3
−
to ClO
4
−
of *40 : 1 and Cl
−
to ClO
4
−
of 15 : 1.
As shown in Fig. 3, these ratios are very different than those for
the Antarctic Dry Valley soils and ice near where the meteorite
was recovered (10 000 : 1 and 5000 : 1, respectively). More
Table 1. Derived concentrations of ClO
4
and Cl in MSL soil
and rock samples
EGA/GCMS
wt.% ClO
4
APXS
wt.% Cl (±30%)
Rocknest (RN) 0.3–0.5
a
0.61
b
Cumberland (CB) 0.95 (±0.4)
c
1.41
d
John Klein (JK) 0.13 (±0.05)
c
0.53
d
a
Leshin et al. (2013).
b
Blake et al. (2013).
c
Ming et al. (2014).
d
McLennan et al. (2014).
4Benton C. Clark and Samuel P. Kounaves

interestingly, the nitrate oxygen and nitrogen isotopic ratios for
the meteorite were found to give a δ
15
Nof−10.5 ± 0.3‰and a
δ
18
O of +51.6 ± 0.7‰. If the nitrate had been acquired from the
ice which it was encased and transported through, the δ
15
N and
δ
18
O should be the same or similar to the δ
15
N (+250‰) and
δ
18
O (+30‰) of the Antarctic Dome-C ice and especially to
the nearby Miller Range (MIL) blue ice which is similar to
that where EETA79001 had been recovered. As can be seen
in Fig. 4, the Dome-C ice values range from a δ
15
N of +350
to −10‰, while the MIL ice has a δ
15
N of about +102‰
and δ
18
O of about +43‰. If the meteorite had been contami-
nated with salts from the blue ice, the δ
15
N values should be the
same or similar. These values and the location of the salts with-
in the meteorite, make it difficult to reconcile with terrestrial
contamination, leading to the conclusion that the salts are of
martian origin. In conjunction with discoveries of perchlorate
by Phoenix and Curiosity, as well as the evidence for oxidants
at the Viking sites, these findings support the hypothesis that
ClO
4
−
is ubiquitous on Mars. In addition, the presence of ClO
3
−
suggeststhe possible presence of other highlyoxidizing oxychlor-
ines such as ClO
2
−
or ClO
−
, produced both by ultraviolet oxida-
tion of Cl
−
(Kang et al.2009;Catlinget al.2010; Schuttlefield
et al.2012; Kounaves et al. 2013; Carrier & Kounaves 2015)
and the effects of ionizing irradiationof ClO
4
−
(mimickingexpos-
ure to galactic cosmic rays, Quinn et al. 2013). The locations and
age of the material analysed also suggests that perchlorate has
been present on Mars for much of its history.
Detection of other halogens
Because the elements comprising the halogen group share some
chemical characteristics, it is of interest to consider whether they
too may be present at unusually high concentrations and whether
they are also susceptible to oxidation. For example, bromine is
ubiquitous and at unexpectedly high levels on Mars. It has
been suggested that Br may be present, atleast in part, also in oxi-
dized states, such as bromates and perbromates (Clark et al.
2005). Bromine abundances are, however, highly variable on
Mars and can range from less than one Br atom per 100 Cl
atoms, to ten times that amount. Why the Br/Cl ratio should
be so variable, especially considering that the Br content of
soils is at trace levels to begin with, may be related to the relative
partitioning into oxybromines and bromides, especially com-
pared with its analog chlorine species. These various salts have
different hydration levels, solubilities and eutectic points, and
hence different mobility’s under martian environmental condi-
tions. Sensors for measuring soluble Br
−
and I
−
were included
in the Phoenix WCL, but neither was detected at the respective
sensor’s limits of detection of 5 × 10
−5
mol l
−1
, equivalent to
100 ppm in the soil samples (Kounaves et al.2010).
Fluorine has been detected via its molecular emission by
Ca-F (Forni et al. 2014). Fluorate and perfluorate salts may
also exist on Mars, and their detection and quantification,
combined with analogs of chlorine and bromine, may shed fur-
ther light on the mechanisms which produced them. To date,
none of the available spacecraft instrumentation techniques
have demonstrated the identification of F-containing salts.
Potentially limiting its usefulness as a diagnostic for salt, F
can also occur in many other minerals not subject to dissol-
ution under aqueous conditions.
Sources of chlorine
The explanation of why Cl is so ubiquitous in global soils has
long been thought to reflect the origin of Cl as gases released by
magmatic activity, especially as a product of the volcanism that
Fig. 3. A ternary plot for the concentrations of ClO
4
−
,NO
3
−
and Cl
−
in
the Beacon (•) and University (■) Antarctic Dry Valleys (ADV), and
the EETA79001 Mars meteorite (▲). The ADV soils show 2–3 orders
of magnitude more NO
3
−
and Cl
−
compared with the EETA79001.
Fig. 4. The δ
15
N and δ
18
OofNO
3
−
for EETA79001 in comparison
with those for Miller Range and Dome-C ice. The Dome-C data are
taken from Frey et al. (2009) and range in altitude from *2000 to
3300 m and depths of 0–30 and 30–60 cm (♦Dome-C ice 0–30 cm;
•Dome-C ice 30–60 cm; ▲Miller Range ice at 20 cm; ■
EETA79001).
Distribution of perchlorates on Mars 5

has so importantly shaped the surface of Mars (Baird & Clark
1981; Craddock & Greeley 2009). The content of Cl in igneous
rocks from Mars (SNC meteorites), 30 to 100 ppm (Lodders
1998), is less by factors of 40 to over 200 than in martian global
soils. Deep-seated parental magmas on Mars may, however,
contain as much as 0.3 wt.% Cl before devolatilization, greater
than their actual H
2
O content (Filiberto & Treiman 2009).
These magmas will lose much of their volatiles, whether
through volcanic releases and outgassing of extruded lava, or
indirectly through subsurface venting, fumerolic emissions, or
formation of hydrothermal systems.
Infall of meteorites and interplanetary dust particles is an-
other potential source but is not likely to have contributed sig-
nificant Cl to the martian regolith. Most meteorites contain
less than 0.03 wt.% Cl although some, such as the relatively
rare enstatite chondrites and the carbonaceous meteorites,
may contain up to 0.08 wt.% Cl (Mason 1971). However, the
S/Cl ratio across the broad range of meteorites is almost always
at least 100 : 1, whereas in the martian global soil the chlorine is
much higher and the S/Cl ratio (atom/atom) is typically 4 : 1 or
somewhat less. Thus, unless undiscovered large reservoirs of Cl
are found with low S concentrations, the contribution of me-
teorites to the regolith is small, in agreement with estimates
by Flynn (1996). Furthermore, the concentration of organic
compounds in martian soil is so small that the influx of carbon-
aceous meteorites and interplanetary dust particles (IDP)
should have been detectable if a significant fraction of the or-
ganic material or its relics had survived the oxidative pressure
of the environment.
Recycling of chlorines
Although there have been several attempts at modelling the
mechanism(s) of oxidation of chlorides on Mars, there has
been little attention to processes which may reverse these reac-
tions. It has been shown in experiments by Quinn et al. (2013),
however, that ionizing radiation can ultimately cause the loss
of O atoms from perchlorate (also resulting in species whose
reactivity is such that they can mimic the results of the various
Viking life-detection metabolism experiments). The unrelent-
ing bombardment of galactic cosmic rays and episodic energet-
ic solar particle events provides a continuous pathway for
converting chlorine valence states.
Another mechanism for reducing oxychlorines is heating to
*300°C. Such temperatures and higher can be attained with
magmatic activity, especially when volcanic extrusives contact
soils. Some of the target material during hypervelocity impact
is also transiently heated to very high temperatures, which can
induce decomposition reactions to evolve gases which quickly
escape the milieu. If the global soils are ancient, a significant
fraction may have been exposed to the impact heating events
of primary target material and the consequences of emplace-
ment of hot ejecta blankets, thereby causing the loss of O
from oxychlorines, and reversion to lower oxidation states or
HCl and chlorides.
Yet another mechanism is via reaction with organic com-
pounds, endogenous or exogenous. The organics imported
from asteroids, comets and IDPs (Flynn 1996) in one billion
years could populate the soil to a depth of 10 m with a concen-
tration of 0.2 wt.% (2000 ppm), approximately equal to the
current estimates of perchlorate concentration. The apparent
existence of some halogenated hydrocarbons (Navarro-
González et al. 2006; Freissinet et al. 2015) would be testimony
to the interaction of organics with inorganic Cl in soil, since
these compounds are rare in meteorites. Most of the organic
constituents of the exogenous influx are presumably lostto oxi-
dation by the perchlorates and/or photochemical mechanisms
(Hunten 1979; ten Kate 2010). In addition, endogenous
sources should not be ruled out. Photosynthetic pathways for
production of organics under contemporaneous martian con-
ditions has been demonstrated in the laboratory (Hubbard
et al. 1973). Furthermore, past conditions on Mars may have
been more reducing and hence much more favourable to
large-scale synthesis of organic compounds. If so, then some
organics could provide reducing power to react with oxidants
such as the oxychlorines.
Whether by ionizing radiation, volcanic processing, impact
heating, or reaction with organic compounds, pathways exist
for interconversion in multiple directions of various oxychlor-
ines and chlorides.
Salts of halogens can be potent freezing point depressants.
Brines should readily form on Mars provided there is availabil-
ityofliquidwater,ice,frost,orapartialpressureofH
2
Ovapour
sufficiently high to induce deliquescence. As seen in Fig. 5,there
are important differences in the effectiveness of various plausible
salts (and acids). Combined with natural segregation processes
due to differences in freezing point depression and hence trans-
port by cold, aqueous processes, a cycling system for Cl could
exist on Mars, resulting in spatial segregation of different salts
and greater or lesser exposure to oxidizing mechanisms at the
Fig. 5. Freezing point depression for aqueous eutectic solutions of
the indicated cation salts with Cl and Br anions and oxyanions.
Dotted red line indicates a typical average subsurface temperature on
Mars at low to moderate latitudes. Several Cl salt brines do not freeze
until below the average temperature, and for soils near the surface,
temperatures often rise much higher. Note that although acids are very
strong freezing-point depressors, their chemical reactivity with
generally basic minerals would convert them to other species shown.
6Benton C. Clark and Samuel P. Kounaves

interface of the regolith with the atmosphere. These geochemical
segregation and cycling mechanisms may contribute to the
strong isotopic fractionation of Cl observed in some samples
in Gale crater (Farley et al. 2015, in preparation).
Future measurements
Several instruments comprising the science payload for
NASA’s 2020 Mars rover will have important detection cap-
abilities, especially for mineral constituents. The XRF analysis
is by the PIXL instrument, which can analyse the detailed
structure of rocks, soils and sediments at spatial increments
down to the 100 µm scale. It has good sensitivity for Cl and
with it the potential for identifying specific salt grains or miner-
als containing Cl. The SHERLOC Raman spectrometric
imager has the ability to identify perchlorate at low concentra-
tions. The SuperCam is an expanded version of ChemCam,
preserving all its capabilities but adding several others. As
these instruments progress through development for flight,
their calibration programs will now include a variety of
Cl-containing minerals, including not only chloride salts but
also the oxychlorine salts as well as chlorapatites, akaganeite,
lawrencite, molysite and so forth.
The European Space Agency’s 2018 ExoMars rover mission
includes an infrared spectrometer (ISEM), a vis-IR spectrom-
eter (MicrOmega), and the RLS Raman spectrometer, which
may have detection capabilities for Cl-containing minerals.
The payload of the 2020 Al Amal ‘Hope’mission by the
United Arab Emirates includes EMIRS, a remote-sensing IR
spectrometer, which could detect chloride enrichments that
cover large areas *300 km diameter.
Conclusion
The detection of chlorine abundances on Mars has been
possible by a wide variety of techniques, both from orbit
and on the surface. In almost all cases, this has been the
consequence of a broader range of the measurements which
only fortuitously included sensitivity to Cl atoms or minerals.
No instrument has been specifically designed or tailored for
investigating Cl or its forms, other than an electrode for detect-
ing chloride ions in the WCL instrument. Yet, the variety and
abundances of oxychlorines may be key to a deeper under-
standing of the martian physical-chemical environment, with
importance to the search for organics, evidence of previous
actions by liquid water, and the limits to life itself.
The global soil unit may have relative abundances of chlor-
ides and oxychlorines that are similar everywhere, due to the
widespread mixing caused by planet-wide dust storms.
Wherever there has been processing to enrich (or deplete) Cl
compared with that in the global soil, somewhat different
profiles would be expected, and the relative amount of perchlo-
rate could actually be enhanced in some circumstances. Per-
chlorates or other oxychlorines may exist wherever Cl is
detected. This is the simplest hypothesis based on the disparate
locations and geologic natures of the sites of exploration by
Phoenix, Curiosity and Viking.
Acknowledgements
We wish to thank the reviewers for their comments and
suggestions, especially by P. D. Archer Jr. This work
was made possible in part by support from NASA and
its JPL-led Mars missions, MER, MSL and Phoenix.
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