The document summarizes research on the formation of manganese oxides on early Mars. Laboratory experiments found that oxyhalogen species like chlorate and bromate can oxidize dissolved manganese(II) ions in Mars-like fluids at much faster rates than molecular oxygen. Bromate in particular drove substantial manganese oxidation within weeks. The precipitates formed were identified as nsutite, a manganese oxide mineral similar to dark coatings observed on Mars. This suggests oxyhalogens may have been a more plausible oxidant for manganese than oxygen in the geochemical conditions of early Mars, indicating an active halogen cycle.

1. Nature Geoscience
naturegeoscience
https://doi.org/10.1038/s41561-022-01094-y
Article
Formationofmanganeseoxidesonearly
Marsduetoactivehalogencycling
Kaushik Mitra 1,3
, Eleanor L. Moreland 1,4
, Greg J. Ledingham1
& Jeffrey G. Catalano 1,2
InsituroverinvestigationsonMarshavediscoveredmanganeseoxidesas
fracture-fillingmaterialsatGaleandEndeavourcraters.Previousstudies
interpretedthesemineralsasindicatorsofatmosphericoxygenonearly
Mars.Bycontrast,weproposethattheoxidationofmanganesebyoxygen
ishighlyunlikelybecauseofexceedinglyslowreactionkineticsunder
Mars-likeconditionsandthereforerequiresmorereactiveoxidants.Herewe
conductkineticexperimentstodeterminethereactivityoftheoxyhalogen
specieschlorateandbromateforoxidizingdissolvedMn(ii)inMars-like
fluids.Wefindthatoxyhalogenspecies,whicharewidespreadonthe
surfaceofMars,inducesubstantiallygreatermanganeseoxidationrates
thanO2.Fromcomparisonsofthepotentialoxidationratesofallavailable
oxidants(includingreactiveoxygenspeciesperoxideandsuperoxide),
wesuggestthattheoxyhalogenspeciesarethemostplausiblemanganese
oxidantsonMars.Inaddition,ourexperimentsprecipitatedthemanganese
oxidemineralnsutite,whichisspectrallysimilartothedarkmanganese
accumulationsreportedonMars.Ourresultsprovideafeasiblepathwayto
formmanganeseoxidesunderexpectedgeochemicalconditionsonearly
Marsandsuggestthatthesephasesmayrecordanactivehalogencycle
ratherthansubstantialatmosphericoxygenation.
ConcentratedoxidizedmanganesedepositsonMarsoccurinveinsor
fractureatGale(>25 wt%)(refs.1–3
)andEndeavour(>2.3 wt%)(refs.4,5
)
craters.Thesemineralsserveasindicatorsofpastredoxconditionsthat
promoted abiotic oxidation of Mn(ii). Detailed understanding of the
redox state of aqueous solutions, the oxidants that facilitated Mn(ii)
oxidationandtheprocessesformingmanganeseoxidesarecriticalto
geochemically constrain past environmental conditions and provide
essential insight into the habitability of early Mars.
Manganese oxides occurring in fracture fills, such as in Gale and
Endeavourcraters,requiretransportinafluidandchemicalfractiona-
tion from iron. Weathering of ferromagnesian silicates (for example,
olivine)istheanticipatedsourceofdissolvedMn(ii).Althoughmolecu-
lar oxygen (O2) is currently considered the primary Mn(ii) oxidant
on Mars1,2,6,7
, slow reaction kinetics makes O2 an implausible Mn(ii)
oxidant.Paststudiesat25 °CdemonstrateincrediblyslowMn(ii)oxi-
dationrateswithnoobservableoxidationfor>7 yratpH 8.4in0.2 bar
O2 (ref. 8
). In addition, pO2 in the present Martian atmosphere is low
(~10−5
bar)9
althoughitperiodicallyreached0.001to0.05 baronearly
Mars6
. The rate of Mn(ii) oxidation by O2 decreases linearly with pO2
(refs.8,10–13
),furtherslowingreactionratesonMars.
OwingtoequilibrationwiththeCO2-rich(0.5 bar)atmosphere,the
pHofatmosphericallyconnectedaqueoussystemsonMarswasweakly
acidic (pH < 6.3) during late Noachian to early Hesperian periods14
(SupplementaryDiscussionsection1).Althoughwater–rockreaction
mayneutralizetheacidityfromatmosphericCO2 andgeneratealkaline
systems in the Martian subsurface15
, these processes consume all O2
while conditions are still acidic (Extended Data Fig. 1). The role of O2
in manganese oxide formation on Mars is further challenged by the
Received: 24 January 2022
Accepted: 26 October 2022
Published online: xx xx xxxx
Check for updates
1
Department of Earth and Planetary Sciences, Washington University, St Louis, MO, USA. 2
McDonnell Center for the Space Sciences, Washington
University, St Louis, MO, USA. 3
Present address: Department of Geosciences, Stony Brook University, Stony Brook, NY, USA. 4
Present address: Department
of Earth, Environmental and Planetary Sciences, Rice University, Houston, TX, USA. e-mail: catalano@wustl.edu

2. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
durations.ThesolubilityofO2 islow,andifthesystemwereclosedafter
initialO2 entrythenafullysaturated1-cm-widefracturecouldgenerate
a MnOOH coating only 20 nm thick for pO2 of 0.02 bar and 3 nm thick
for pO2 of 0.003 bar. These are orders of magnitude too thin to have
producedtheconcentrationsobservedviaX-rayspectrometry1–4
.There
arethussubstantialkineticandthermodynamicbarrierstoO2 having
servedastheoxidantthatproducedfracture-fillingmanganeseoxide
depositsonMars.
Abundant oxychlorine species (perchlorate and chlorate) have
beenfoundubiquitouslyonthesurfaceofMars19,20
andwereprobably
present since ~4 billion years ago (Ga) (ref. 21
). Chlorate (ClO3
−
) can
readily oxidize dissolved iron(ii) orders of magnitude faster than O2
orviaultravioletphoto-oxidationunderawiderangeofMars-relevant
conditions,includinginacidicfluidswheretheseotheroxidationpath-
waysareexceedinglyslow22–24
.ChlorateoxidizesMn(ii)at~160 °C(ref.
25
) but previous work has not studied such reactions under ambient
conditions.AssessmentofpotentialMn(ii)oxidationratesattempera-
tures relevant to Mars through analogy with Fe(ii)24
are not possible
because these species display reaction rates with other oxidants that
varywidely,althoughMn(ii)oxidationratesaregenerallyslower11
.
Oxychlorine-forming processes on Mars also produce oxybro-
mine compounds since halogens share similar chemistry19
. Mars is a
halogen-rich planet26
, with chlorine and bromine concentrations in
bulkMarsabout4timesgreaterthanonEarth26,27
.Bromineconcentra-
tions on the Martian surface are 1–10% of the total chlorine concen-
tration27
. Bromine was detected in all samples analysed by the Spirit,
Opportunity5
andCuriosityrovers3,27
,withthehighestconcentrations
in the manganese-rich Dillinger member at Gale crater3
. In addition,
bedrockshows~80%enrichmentinbrominecomparedwithGalecrater
soil28
. Bromine volatilizes and oxidizes faster than chlorine, thereby
experiencing more rapid production–consumption cycles on Mars
andoccurspredominantlyasbromate(BrO3
–
)(refs.19,29
).Whilebromate
hasbeenshowntooxidizedissolvedMn(ii)atapproximatelypH –0.5
(ref. 30
), no studies have explored its reactivity with Mn(ii) under less
extreme chemical conditions. In this Article, we show that the rate of
Mn(ii) oxidation by oxyhalogens is orders of magnitude faster than
by O2 in Mars-like conditions and that manganese oxides indicate an
activehalogencycleonMars.
Oxidationofmanganesebyoxyhalogensin
Mars-relevantfluids
We conducted laboratory experiments to determine the reactivities
of the oxyhalogen species, chlorate and bromate, towards oxidizing
dissolved Mn(ii). The experiments were conducted in magnesium
chlorideandmagnesiumsulfatebackgroundsolutionsatpH ~3,5and
7 to emulate Mars-like fluids. We studied the rate of Mn(ii) oxidation
inkineticexperimentsbymonitoring[Mn(ii)]andpHinsystemscon-
taining individual oxidants, either bromate or chlorate, and in mixed
systemscontainingbothoxidants(seeMn(II)oxidationbyoxyhalogen
species for details).
TheresultsshowthatMn(ii)oxidationbychlorate,althoughther-
modynamically favourable (Extended Data Table 2), is not observed
on the timescales of weeks to months (Supplementary Fig. 1). The
negligiblepHchangeintheseexperimentsindicatesthat<0.1%ofthe
dissolvedMn(ii)oxidized.Bycontrast,solutionscontainingbromate
displayed extensive Mn(ii) oxidation within six to eight weeks (Fig. 2
andExtendedDataTable3).ThepHofthesystemsdecreasedtobelow
2 in all experiments, regardless of the initial pH (Fig. 2). Mn(ii) in sul-
fatesystemsdisplayedsloweroxidationthaninchloridesystemsdue
to greater complexation of Mn(ii), decreasing the concentration of
the free Mn2+
ion (Supplementary Fig. 2). In all experiments, fuming
brownish-red gas volatilized from the solutions upon de-capping the
reactors,implyingincompletebromatereductionbyMn(ii),forming
Br2 rather than Br−
. This also suggests a redox-mediated volatiliza-
tion mechanism for bromine on Mars. Batch experiments containing
exceedinglyslow(andoftenunmeasured)kineticsofMn(ii)oxidation
belowpH ~6(refs.8,10–12,16,17
)(ExtendedDataTable1).Applicationofan
existingratemodel11
showsthatpotentialpO2 conditionsonearlyMars
(0.05to10−5
bar(ref.6
))requirehundredstomillionsofyearstooxidize
manganese (Fig. 1). Focusing specifically on the period of Gale crater
formation, sedimentation and exhumation, peak O2 concentrations
ranged from 0.003 to 0.02 bar (ref. 6
). These O2 levels yield a half-life
(t1/2)ofMn(ii)oxidationof840–5,600 yratpH 6and7,600–51,000 yrat
pH 5(seeSupplementaryDiscussionsection2forfurtherdiscussion).
If the pH was as low as 4, which is plausible given the occurrence of
akaganeiteintheunitsurroundingtheGalecraterfractures18
,thenman-
ganese oxidation by O2 was not even thermodynamically favourable.
Furthermore,thesesubsurfacefracturesystemswouldhaveneededto
remainhydrologicallyconnectedtotheatmosphereforthesefulltime
4.0 4.5 5.0 5.5
pH
6.0 6.5 7.0 7.5
10
8
10
7
10
6
10
5
10
4
10
3
10
2
10
1
10
0
t
1/2
(yr)
t
1/2
(yr)
4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
pH
pH
limit
for
0.5
bar
CO
2
10
–5
10
–2
0.03
O2
(bar)
Water
availability
Rhodochrosite
saturation
10
–4
10
–3
0.05
pH
limit
for
0.5
bar
CO
2
10
–5
10
–2
0.03
Water availability
Rhodochrosite
saturation
10
–4
10
–3
0.05
a
b
O2
(bar)
10
8
10
7
10
6
10
5
10
4
10
3
10
2
10
1
10
0
Fig.1|TimescalesofMn(ii)oxidationbyoxygen. a,b,Thehalf-life(inyears)for
Mn(ii)(1 mmol l−1
)oxidationbyO2 plottedasafunctionoftheinitialpHat25 °C.
Allsimulationswerebufferedwith0.5 barCO2 andO2 settothespecifiedpartial
pressures.CalculationsconsideredbothfixedpHsystems(a)andfree-driftpH
systemsinwhichthepHisallowedtovaryinresponsetoH+
generationduring
Mn(II)oxidation(b).TheverticallinesatpH 6.34demarcatethepHabove
whichcarbonatemineralsmustbepresent.ThedashedverticallineatpH 5.56
demarcatesthepHofrhodochrositesaturation,abovewhich1 mmol l−1
Mn(ii)
isnotfullysoluble.Thehorizontallinedemarcatesthemaximumtime(1million
years)expectedforliquidwateravailabilityonMars6
.Conditionsforwhich
thermodynamicslimitMn(ii)oxidationto<50%orforwhichMn(ii)oxidation
cannotoccur(SupplementaryTable1)arenotrepresentedinthefigure.

3. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
100 mmol l−1
Mn(ii) and 10 mmol l−1
bromate in chloride-rich systems
showed considerable Mn(ii) oxidation after ~650 days at pH 3.48 and
7.53 (Extended Data Table 4), which is consistent with the expected
kinetic behaviour of the reaction if the rate is first order with respect
tobromateconcentration31
.
OxyhalogensproduceMn(iii/iv)oxidensutite
Batch experiments were conducted to investigate the mineral prod-
ucts formed when bromate oxidizes Mn(ii) in Mars-relevant fluids
(ExtendedDataTable5).Mn(ii)oxidationbybromateyieldsdarkbrown
to black manganese coatings inside the reactor walls (Extended Data
Fig. 3), which are confirmed by X-ray diffraction (XRD) to be the min-
eral nsutite (γ-MnO2) (Fig. 3a,d, Extended Data Fig. 2). Nsutite is a dis-
ordered, nanoscale intergrowth of the MnO2 polymorphs pyrolusite
andramsdelliteandcanincludeMn(iii)substitutionsforMn(iv).X-ray
photoelectronspectroscopy(XPS)resultsshowthatthesolidsformed
in sulfate fluids and at higher pH values generally contained greater
Mn(iii)contents(Fig.3b,eandSupplementaryTable3).Mixed-valent
Mn(iii/iv)mineralsarethereforemorelikelytoformthanpureMn(iv)
in sulfate-rich fluids common on Mars. The visible and near infrared
(VNIR) reflectance spectra of the precipitates have characteristically
lowreflectancevalues(Fig.3c,f).Thespectraaresimilartothemanga-
nese oxide rock coatings at Endeavour crater4
(Extended Data Fig. 4).
TheslightlybrighterspectraonMarsareprobablycausedbythepres-
ence of magnesium sulfate or other salts intermixed with manganese
oxides.Nsutiteadsorbsmetals(forexample,Pb,Co,Cu)32
similarlyto
othermanganeseoxides33,34
.Itsformationisthusnotinconsistentwith
thecorrelationofMnwithCuandNiatGalecrater(Stephen)1,2
,between
Ni and Mn at Endeavour crater (Pinnacle Island and Stuart Island)4
.
Comparisonofplausiblemanganeseoxidantson
Mars
HomogeneousMn(ii)oxidationbyO2 hasexceedinglyslowkineticsbelow
pH~7(ref.35
),withexperimentsreportinglittletonooxidationovertime-
scales of months to years at 0.2 bar O2 and 20 °C (refs. 8,10–12,16,17,36
). The
order of Mn(ii) oxidation by O2 with respect to [OH−
] is ~2 (refs. 8,10–13
).
TheMn(ii)oxidationratethusdecreases~100×witheachunitdecreasein
pH13
,makingthereactionlessfavourableandprobablyunfeasibleatacidic
pH35
.Heterogeneousredoxreactionssuchassurfaceandelectrochemi-
cal catalysis promote faster Mn(iii/iv) mineral formation than during
oxidationinhomogeneoussolution16,17
.Studiesreport22%,51%and63%
Mn(ii)oxidationwithin60daysinthepresenceofmagnetite,two-line
ferrihydrite and goethite, respectively, at pH 7 and 0.2 bar O2 at 25 °C
(ref.17
).However,notethatsurfacecatalysisdependsonMn(ii)adsorp-
tiononmineralsurfaces,whichdropsprecipitouslybelowpH 6(ref.37
),
eliminatingthisoxidationpathwayunderweaklyacidicconditions.No
studieshavedemonstratedtheabioticoxidationofMn(ii)byO2 below
pH6atambienttemperaturesforeitherhomogeneousorheterogeneous
systems.ThehighpCO2 ofearlyMarsrequiresthatmostwatersincontact
withtheatmosphere,thesolesourceofO2,wouldhaveapH < 6(ref.14
).
TherateofMn(ii)oxidationbyO2,bothhomogeneousandsurface
catalysed, is slower than by bromate (Figs. 1 and 2 and Extended Data
0
20
40
60
80
100
0 15 30 45 60
0
1
2
3
4
5
6
7
0 15 30 45 60
0
20
40
60
80
100
0 15 30 45 60
0
1
2
3
4
5
6
7
0 15 30 45 60
0
20
40
60
80
100
0 15 30 45 60
0
1
2
3
4
5
6
7
0 15 30 45 60
(Mn(
II
))
(mmol
l
–1
)
(Mn(
II
))
(mmol
l
–1
)
(Mn(
II
))
(mmol
l
–1
)
pH
pH
pH
(Mn(II))
(Mn(II))
(Mn(II))
a
b
c
(Mn(
II
))
(mmol
l
–1
)
(Mn(
II
))
(mmol
l
–1
)
Time (d) Time (d) Time (d) Time (d)
(Mn(
II
))
(mmol
l
–1
)
Time (d) Time (d) Time (d) Time (d)
Time (d) Time (d) Time (d) Time (d)
d
e
f
(Mn(II))
(Mn(II))
(Mn(II))
0
20
40
60
80
100
0 15 30 45 60 0 15 30 45 60
0
20
40
60
80
100
0 15 30 45 60
0
1
2
3
4
5
6
7
0 15 30 45 60
0
20
40
60
80
100
0 15 30 45 60
0
1
2
3
4
5
6
7
0 15 30 45 60
0
1
2
3
4
5
6
7
pH
pH
pH
Fig. 2 | Rates of Mn(ii) oxidation by bromate. a–f, Dissolved Mn(ii) concentration and pH versus time in systems containing approximately 100 mmol l−1
Mn(ii)
with 100 mmol l−1
bromate in 100 mmol l−1
MgCl2 (a–c) at initial pH 2.95 (a) 4.77 (b) and 6.91 (c) and in 100 mmol l−1
MgSO4 (d–f) at initial pH 3.03 (d), 4.92 (e) and
6.87 (f) at 24 °C.

4. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
Table1).AssumingaratelawthatisfirstorderwithrespecttoMn(ii)and
bromate31
, it would take 120, 400 and 900 days to oxidize about 20%,
50% and 90% Mn(ii), respectively, in a system containing 24 mmol l−1
Mn(ii)and10 mmol l−1
bromateatpH 7at25 °CwiththepHdecreasing
to2.50,2.10and1.82,respectively.Bycontrast,onlyabout8%Mn(ii)is
oxidizedin120daysby0.2 barO2 undersimilarreactionconditions16
.
In heterogeneous systems containing ferrihydrite, a 30–50% drop in
Mn(ii) is observed within 120 days (Extended Data Table 1)16,17
. Note
that rates on Mars would be at least an order of magnitude slower
because of the lower pO2. For reference, 0.21 bar O2 would produce
Intensity
(counts) pHinitial
2.96
2.49
5.15
5.95
6.65
γ-MnO2
γ-MnO2
γ-MnO2
γ-MnO2
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
15 25 35 45 55 65
a
Intensity
(counts)
0
1,000
2,000
3,000
4,000
5,000
6,000
15 25 35 45 55 65
pHinitial
2.86
3.69
4.30
6.72
7.59
γ-MnO2
γ-MnO2
γ-MnO2
γ-MnO2
0
0.04
0.08
0.12
0.16
0.20
0.24
0.28
0.40 0.90 1.40 1.90 2.40
Wavelength (µm)
Reflectance
(+0.02)
(+0.06)
(+0.09)
(+0.16)
pHinitial
3.69
2.86
4.30
6.72
7.59
e
f
Reflectance
(+0.05)
(+0.10)
(+0.15)
(+0.20)
pHinitial
2.96
2.49
5.15
5.95
6.65
0
0.06
0.12
0.18
0.24
0.30
0.36
0.40 0.90 1.40 1.90 2.40
Wavelength (µm)
°2θ (Cu Kα) °2θ (Cu Kα)
b
c
d
0
500
1,000
1,500
2,000
45
47
49
51
53
pHinitial
3.69
4.30
7.59
Binding energy (eV)
Arbitrary
units
Mn(III)
Mn(IV)
Mn(II)
50.6:49.4:0
48.8:51.2:0
33.7:66.3:0
(IV):(III):(II)
0
500
1,000
1,500
2,000
45
47
49
51
53
2.96
5.15
5.95
Binding energy (eV)
Arbitrary
units
Mn(III)
Mn(IV)
Mn(II)
100:0:0
43:57:0
(IV):(III):(II)
76:24:0 pHinitial
Fig.3|MineralproductsofMn(ii)oxidationbybromate. a–f,TheXRD
patterns(a,d),XPSspectra(b,e)andVNIRreflectancespectra(c,f)ofthe
mineralsprecipitatedfollowingtheoxidationofdissolvedMn(ii)bybromate
inmagnesiumchloride(a–c)andmagnesiumsulfate(d–f)fluids.Patternsand
spectraareoffsetvisuallyforclarity.ThelabelsindicatetheinitialpHofthe
samples.SeeExtendedDataTable5forcompletesampledetails.Diagnostic
peaksintheXRDscans(a,d)arelabelledandindicatedbydashedlines.TheMn
3pXPSspectra(b,e)includemineralstandards(dottedlines)corresponding
toMn(iv),Mn(iii)andMn(ii).ThepercentagesofMn(iv),Mn(iii)andMn(ii)in
eachsampleareindicated(seeSupplementaryTable3forfittingparameters).
ThedashedlineoverlyingtheVNIRspectra(c,f)indicatesthepositionofthe
1.92 μmabsorptionband.TheverticaloffsetsoftheVNIRspectraareindicatedin
parentheses.

5. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
<0.1%oxidationofMn(ii)undertheconditionsofourexperimentswith
oxyhalogensthatcompletelyoxidizeMn(ii)atpH ≤ 7(ExtendedData
Table 6). As demonstrated by the results, bromate can oxidize Mn(ii)
downtopH ~3whereMn(ii)oxidationbyO2 becomesabout108
×slower
(100× decrease in rate with each unit pH decrease). Unlike bromate,
chlorate produced no observable Mn(ii) oxidation in homogeneous
systems in this study. Such reactions could possibly be accelerated in
a heterogeneous system via surface catalysis, analogous to the faster
reactionbetweenMn(ii)andO2.
Arecentpaper6
suggestedO2 asaneffectiveMn(ii)oxidantonMars
during bolide impacts on the surface. Thermal and kinetic modelling
using extrapolated data36
showed that at 50–75 °C, dissolved Mn(ii)
can be readily oxidized (within ~tens of days) by 0.03 bar O2 in 0.5 bar
CO2 atpH 8.However,maintainingapH 8fluidisunachievableundera
0.5 barCO2 atmosphereasthiswouldrequireadissolvedbicarbonate
concentration in excess of 1 mol l−1
(See Supplementary Discussion
section 5 and Supplementary Table 4 for details). Dissolution of CO2
into alkaline fluids produced by water–rock reaction would decrease
the pH to between 4.56 and 6.95, depending on the temperature and
assumptionsofthereactionsbufferingpH.Owingtothesecond-order
rate dependence on [OH-
], the rate of reaction will decrease between
125× and 7,600,000×, requiring decades to millennia to appreciably
oxidize Mn(ii) using O2 as an oxidant. In addition, extrapolation of
the rate constant for Mn(ii) oxidation by O2 to 75 °C in the study6
was
potentially erroneous because of imprecise rate data at 25 and 37 °C,
as previously cautioned36
. Therefore, Mn(ii) oxidation by O2 during
thermal transients associated with bolide impacts6
is many orders of
magnitudeslowerthanestimatedinthenotedpreviousstudybecause
thepHconsideredisunattainableunderhighpCO2.
Other species are ineffective or implausible oxidants of Mn(ii)
in subsurface fracture systems. Hydrogen peroxide (H2O2) reduces
Mn(iii/iv)toMn(ii)34,38
.Whilethereactiveoxygenspeciessuperoxide
(O2
−
)(refs.39,40
)doesoxidizeMn(ii)41
,theresultingH2O2 mayeffectively
negatetheoxidationbyO2
−
.Otheroxychlorinespecies,suchaschlorite
(ClO2
–
),hypochlorite(ClO−
)andchlorinedioxide(ClO2)gasaswellasan
arrayofradicals,areprobablyproducedasshort-livedintermediateson
Mars42,43
andareknowntorapidlyoxidizedissolvedMn(ii)44,45
.However,
theirnon-detectiononMars20
indicatesthattheydonotaccumulateon
thesurface,unlikemoreoxidizedspecies,andtheirtransientnaturein
water46
suggeststhattheymaydecomposebeforereachingsubsurface
fracturesystemswheremanganeseoxidesformed1,2,4
.Notethatwedo
notdiscountthepotentialroleofthesereactiveoxychlorinespeciesin
oxidizingMn(ii)intheshallowsubsurfaceofMars.Thefinelydissemi-
natedmanganeseoxidesthatwouldresulthavenotbeenobservedto
date but warrant investigation as active missions and future sample
returnenableidentificationviamicroanalysis.However,bromateisthe
firstspeciesidentifiedthatplausiblyoccursonMarscapableofbeing
transported into subsurface fracture systems and oxidizing Mn(ii)
at measurable rates under neutral to acidic conditions. Oxyhalogens
are thus more likely the primary oxidants of manganese on ancient
Mars than O2, and manganese oxides in sedimentary units cannot be
interpretedasreliableindicatorsofearlyMartianatmosphericoxygen.
Manganeseoxidesasasignatureofhalogen
cyclingonMars
Manganese oxides in fractures on Mars formed in weakly acidic flu-
ids.TheKimberlyformationthathoststhefracture-fillingmanganese
oxidesinGalecratercontainsakaganeiteandpossiblyjarosite18
,which
require pH values of 5 or lower to form. Crater rim fractures in Gale
craterthatcontainmanganeseoxidesalsocontainmagnesiumsulfates
andwereprobablyhydrologicallyconnectedtofluidsfromtheBurns
formation, which was also acidic. Mineralogical data are lacking for
the manganese oxide fracture-filling material, which inhibits further
evaluation of the formation pH. However, acidification associated
Groundwater table
HCl Cl
(Deposit on the surface)
Chloride/
bromide
salts
HCl, HBr
Fe(II)
Fe(III)
P
r
e
c
i
p
i
t
a
t
i
o
n
Mineral
transformation
Fe(III)
Br
Chlorate/
bromate
salts
Dissolutio
n
Groundwater interaction with basalt
F
e
(
I
I
)
a
n
d
M
n(II)-rich groundwater
M
n(II)
M
n(III/IV)
Precipitation
B
r
O
3
–
B
r
–
Chloride/
bromide
salts
E
v
a
p
oration
H2/CH4
H2O/CO2
CO3
2–
BrO3
–
Br–
?
Fractures
HBr
ClOx (ClO3
–
, ClO3
–
, ClO2)
Atmospheric pathways of oxyhalogen formation
Surficial pathways
of oxyhalogen formation
(for example, D, E, F)
ClO3
–
/BrO3
–
Oxyhalogen
brines
Tentative destruction of reduced atmospheric gases
Cl–
ClO4
–
/ClO3
–
/ClO2
Mn(III/IV) oxides
Fe(III) oxides,
sulfates
Halide salts recycled to
produce oxyhalogens
(deposited)
(B)
(A)
(C)
(G)
(for example, H, I)
(J)
(A) Ref.
51
(B) Ref.
52
(C) Ref.
56
(D) Ref.
53
(E) Ref.
54
(F) Ref.
55
(G) Ref.
22
(H) Ref.
57
(I) Ref.
58
(J) Ref.
50
BrO3
–
Cl–
/Br–
Fig.4|PotentialpathwaysofhalogencyclingonMars,includingreactions
withironandmanganese.HalogenreleasedfromtheinteriorofMarsserves
astheprimarysourceontheMartiansurface51
.Oxyhalogenspecies,especially
perchlorate,chlorateandbromate,areproducedbyatmospheric52
andsurface
processes53–55
.TheseaccumulateonthesurfaceofMarsuntildissolvinginwater
andbeingtransportedintostandingbodiesofwaterorpercolatingdownward
intothesubsurfaceasbrines.Oxyhalogensinstandingbodiesofwatermayserve
asoxidantsofdissolvedFe(ii)22
andformironoxidemineralsaswellaschloride
andbromide,whichuponevaporationcouldformhalidesalts56
.Oxyhalogen
brinesmayinteractwithdissolvedMn(ii)infractureandveinsandforminsoluble
manganese(iii/iv)oxides.Reducedgases(forexample,H2 andCH4)inthe
atmospheremaypotentiallyreactwithoxyhalogens,decomposingthesegases
andgeneratingchlorideandbromide50
.Thehalidesaltsformedfrommultiple
processesundergooxidationtorejuvenatetheoxyhalogenpool57,58
.

6. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
withanatmospherewithhighpCO2 demonstratesthattheseprobably
formed at pH 6.5 or lower.
Accordingtotheprecedingcomparisonofoxidantsandtheweakly
acidic pH conditions associated with the fracture-filling manganese
oxide deposits on Mars, oxyhalogen species produce far greater oxi-
dation rates than is possible with O2. This indicates that manganese
oxides in fractures on Mars are indicators of past halogen cycling
(Fig. 4). We propose that manganese oxides were formed when dis-
solved Mn(ii) was oxidized by downward-percolating oxyhalogen
brines in the Martian subsurface rather than by O2-saturated waters.
Halogensundergophotochemicaloxidationviavariousatmospheric
and surficial processes to produce oxyhalogens such as perchlorate,
chlorateandbromate.Subsequentreductionordecompositionofoxy-
halogensproduceschlorideandbromide,whichcanbephoto-oxidized
again,therebyformingahalogencycle.ThevolatilizationofBr2,formed
duringbromatereductionbyMn(ii),couldleadtolessbromineinthe
fluid,therebydecouplingmanganeseandbromineconcentrationsin
sedimentsuponfluidevaporation.Inaddition,Br–
issolubleandwould
concentrate in places where the fluid evaporated and not necessarily
where poorly soluble manganese oxides precipitate. Halogens are
inter-connected to the redox cycling processes of manganese and
irononMars21,47
asFe(ii)andMn(ii)canplayactiverolesindrivingthe
reductionportionofthehalogencycle22,23,48
.WhileFe(ii)hasmultiple
plausible oxidation routes on Mars beyond the oxyhalogens, Mn(ii)
oxidationisslowforallplausibleoxidantsexceptbromate.
Chlorine stable isotopes in apatites in Martian meteorites have
been interpreted to indicate an active chlorine cycle on Mars dating
back to ~4 Ga21
. It is unclear whether this cycling operated continu-
ously or was inhibited during periods with reducing atmospheres
containing H2 or CH4 (ref. 6
). Reactions between reduced gases and
oxychlorinespeciesdonotoccurinaqueoussystemswithoutmicro-
bial activity49
but have been observed with ultraviolet-irradiated
perchloratesalts50
.Chlorineandbrominecyclingaredrivenbysimilar
processesandthereforeoperatedintandem.Manganeseoxidemin-
erals directly observed in ancient sedimentary rocks1,2,4,5
by rovers
provideaninsiturecordofactivehalogencyclingonearlyMars.The
kinetically feasible production of manganese oxides proposed here
suggests a complex and active Martian halogen cycle and advocates
closer and more detailed geochemical study of halogens and oxyh-
alogen species on early Mars.
Onlinecontent
Anymethods,additionalreferences,NaturePortfolioreportingsum-
maries, source data, extended data, supplementary information,
acknowledgements,peerreviewinformation;detailsofauthorcontri-
butionsandcompetinginterests;andstatementsofdataandcodeavail-
abilityareavailableathttps://doi.org/10.1038/s41561-022-01094-y.
References
1. Lanza, N. L. et al. High manganese concentrations in rocks at Gale
crater, Mars. Geophys. Res. Lett. 41, 5755–5763 (2014).
2. Lanza, N. L. et al. Oxidation of manganese in an ancient aquifer,
Kimberley formation, Gale crater, Mars. Geophys. Res. Lett. 43,
7398–7407 (2016).
3. Berger, J. A. et al. Elemental composition and chemical evolution
of geologic materials in Gale crater, Mars: APXS results from
Bradbury landing to the Vera Rubin ridge. J. Geophys. Res. Planets
125, e2020JE006536 (2020).
4. Arvidson, R. E. et al. High concentrations of manganese and sulfur
in deposits on Murray Ridge, Endeavour crater, Mars. Am. Mineral.
101, 1389–1405 (2016).
5. Mittlefehldt, D. W. et al. Diverse lithologies and alteration events
on the rim of Noachian‐aged Endeavour crater, Meridiani Planum,
Mars: in situ compositional evidence. J. Geophys. Res. Planets 123,
1255–1306 (2018).
6. Wordsworth, R. et al. A coupled model of episodic warming,
oxidation and geochemical transitions on early Mars. Nat. Geosci.
14, 127–132 (2021).
7. Liu, Y. et al. Manganese oxides in Martian meteorites Northwest
Africa (NWA) 7034 and 7533. Icarus 364, 114471 (2021).
8. Diem, D. & Stumm, W. Is dissolved Mn2+
being oxidized by O2
in absence of Mn-bacteria or surface catalysts? Geochim.
Cosmochim. Acta 48, 1571–1573 (1984).
9. Trainer, M. G. et al. Seasonal variations in atmospheric
composition as measured in Gale crater, Mars. J. Geophys. Res.
Planets 124, 3000–3024 (2019).
10. Davies, S. H. & Morgan, J. J. Manganese (ii) oxidation kinetics on
metal oxide surfaces. J. Colloid Interface Sci. 129, 63–77 (1989).
11. Morgan, J. J. Kinetics of reaction between O2 and Mn(ii) species in
aqueous solutions. Geochim. Cosmochim. Acta 69, 35–48
(2005).
12. von Langen, P. J., Johnson, K. S., Coale, K. H. & Elrod, V. A.
Oxidation kinetics of manganese(ii) in seawater at nanomolar
concentrations. Geochim. Cosmochim. Acta 61, 4945–4954
(1997).
13. Madden, A. S. & Hochella, M. F. A test of geochemical reactivity
as a function of mineral size: manganese oxidation promoted by
hematite nanoparticles. Geochim. Cosmochim. Acta 69, 389–398
(2005).
14. Baron, F., Gaudin, A., Lorand, J. P. & Mangold, N. New constraints
on early Mars weathering conditions from an experimental
approach on crust simulants. J. Geophys. Res. Planets 124,
1783–1801 (2019).
15. Kite, E. S. & Daswani, M. M. Geochemistry constrains global
hydrology on early Mars. Earth Planet. Sci. Lett. 524, 115718
(2019).
16. Wang, X. et al. The presence of ferrihydrite promotes abiotic
formation of manganese (oxyhydr)oxides. Soil Sci. Soc. Am. J. 79,
1297–1305 (2015).
17. Lan, S. et al. Mechanisms of Mn(ii) catalytic oxidation on
ferrihydrite surfaces and the formation of manganese (oxyhydr)
oxides. Geochim. Cosmochim. Acta 211, 79–96 (2017).
18. Treiman, A. H. et al. Mineralogy, provenance, and diagenesis of a
potassic basaltic sandstone on Mars: CheMin X‐ray diffraction of
the Windjana sample (Kimberley area, Gale crater). J. Geophys.
Res. Planets 121, 75–106 (2016).
19. Zhao, Y. Y. S., McLennan, S. M., Jackson, W. A. & Karunatillake, S.
Photochemical controls on chlorine and bromine geochemistry
at the Martian surface. Earth Planet. Sci. Lett. 497, 102–112
(2018).
20. Sutter, B. et al. Measurements of oxychlorine species on Mars. Int.
J. Astrobiol. 16, 203–217 (2017).
21. Bellucci, J. et al. Halogen and Cl isotopic systematics in Martian
phosphates: implications for the Cl cycle and surface halogen
reservoirs on Mars. Earth Planet. Sci. Lett. 458, 192–202 (2017).
22. Mitra, K. & Catalano, J. G. Chlorate as a potential oxidant on Mars:
rates and products of dissolved Fe(ii) oxidation. J. Geophys. Res.
Planets 124, 2893-2916 (2019).
23. Mitra, K., Moreland, E. L. & Catalano, J. G. Capacity of chlorate to
oxidize ferrous iron: implications for iron oxide formation on Mars.
Minerals 10, 729 (2020).
24. Mitra, K., Moreland, E. L., Knight, A. L. & Catalano, J. G. Rates and
Products of Iron Oxidation by Chlorate at Low Temperatures
(0 to 25°C) and Implications for Mars Geochemistry. ACS Earth
Space Chem. 6, 250–260 (2022).
25. Zheng, D., Yin, Z., Zhang, W., Tan, X. & Sun, S. Novel branched
γ-MnOOH and β-MnO2 multipod nanostructures. Cryst. Growth
Des. 6, 1733–1735 (2006).
26. Dreibus, G. & Wanke, H. Mars, a volatile-rich planet. Meteoritics
20, 367–381 (1985).

7. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
27. Rampe, E. B., Cartwright, J. A., McCubbin, F. M. & Osterloo, M.
M. in D.E. Harlov, L. Aranovich (eds.) The Role of Halogens in
Terrestrial and Extraterrestrial Geochemical Processes 959–995
(Springer, 2018).
28. VanBommel, S., Gellert, R., Berger, J., Yen, A. & Boyd, N. Mars
science laboratory alpha particle X-ray spectrometer trace
elements: situational sensitivity to Co, Ni, Cu, Zn, Ga, Ge, and Br.
Acta Astronaut. 165, 32–42 (2019).
29. Wang, X. et al. Multiphase volatilization of halogens at the soil–
atmosphere interface on Mars. J. Geophys. Res. Planets 126,
e2021JE006929 (2021).
30. Thompson, R. C. Reduction of bromine(v) by cerium(iii),
manganese(ii), and neptunium(v) in aqueous sulfuric acid. J. Am.
Chem. Soc. 93, 7315–7315 (1971).
31. Birk, J. P. Kinetics and mechanism of the reduction of bromate ion
by hexachloroiridate(iii). Inorg. Chem. 17, 504–506 (1978).
32. Le, N. C. & Van Phuc, D. Sorption of lead(ii), cobalt(ii) and
copper(ii) ions from aqueous solutions by γ-MnO2 nanostructure.
Adv. Nat. Sci. Nanosci. Nanotechnol. 6, 025014 (2015).
33. Post, J. E. Manganese oxide minerals: crystal structures and
economic and environmental significance. Proc. Natl Acad. Sci.
USA 96, 3447–3454 (1999).
34. Noda, N. et al. Highly oxidizing aqueous environments on
early Mars inferred from scavenging pattern of trace metals on
manganese oxides. J. Geophys. Res. Planets 124, 1282–1295 (2019).
35. Luther, G. W. The role of one- and two-electron transfer reactions
in forming thermodynamically unstable intermediates as barriers
in multi-electron redox reactions. Aquat. Geochem. 16, 395–420
(2010).
36. Hem, J. D. Rates of manganese oxidation in aqueous systems.
Geochim. Cosmochim. Acta 45, 1369–1374 (1981).
37. Coughlin, B. R. & Stone, A. T. Nonreversible adsorption of divalent
metal ions (Mnii
, Coii
, Niii
, Cuii
, and Pbii
) onto goethite: effects of
acidification, FeII addition, and picolinic acid addition. Environ.
Sci. Technol. 29, 2445–2455 (1995).
38. Jacobsen, F., Holcman, J. & Sehested, K. Oxidation of
manganese(ii) by ozone and reduction of manganese(iii) by
hydrogen peroxide in acidic solution. Int. J. Chem. Kinet. 30,
207–214 (1998).
39. Yen, A., Kim, S., Hecht, M., Frant, M. & Murray, B. Evidence that the
reactivity of the Martian soil is due to superoxide ions. Science
289, 1909–1912 (2000).
40. Zent, A. P., Ichimura, A. S., Quinn, R. C. & Harding, H. K. The
formation and stability of the superoxide radical (O2
−
) on
rock‐forming minerals: band gaps, hydroxylation state, and
implications for Mars oxidant chemistry. J. Geophys. Res. Planets
113, E09001 (2008).
41. Learman, D. R., Voelker, B. M., Madden, A. S. & Hansel, C. M.
Constraints on superoxide mediated formation of manganese
oxides. Front. Microbiol. 4, 262 (2013).
42. Liu, D. & Kounaves, S. P. The role of titanium dioxide (TiO2) in
the production of perchlorate (ClO4
–
) from chlorite (ClO2
–
) and
chlorate (ClO3
–
) on Earth and Mars. ACS Earth Space Chem. 3,
1678–1684 (2019).
43. Rao, B., Anderson, T. A., Redder, A. & Jackson, W. A. Perchlorate
formation by ozone oxidation of aqueous chlorine/oxy-chlorine
species: role of ClxOy radicals. Environ. Sci. Technol. 44,
2961–2967 (2010).
44. Hamilton, G., Chiswell, B., Terry, J., Dixon, D. & Sly, L. Filtration
and manganese removal. J. Water Supply Res. Technol. Aqua 62,
417–425 (2013).
45. Chen, L., Zhang, J. J. & Zheng, X. L. Coupling technique for deep
removal of manganese and iron from potable water. Environ. Eng.
Sci. 33, 261–269 (2016).
46. Adam, L. C., Fabian, I., Suzuki, K. & Gordon, G. Hypochlorous acid
decomposition in the pH5–8 region. Inorg. Chem. 31, 3534–3541
(1992).
47. Farley, K. A. et al. Light and variable 37
Cl/35
Cl ratios in rocks from
Gale crater, Mars: possible signature of perchlorate. Earth Planet.
Sci. Lett. 438, 14–24 (2016).
48. Brundrett, M., Yan, W., Velazquez, M. C., Rao, B. & Jackson, W. A.
Abiotic reduction of chlorate by Fe(ii) minerals: implications for
occurrence and transformation of oxy-chlorine species on Earth
and Mars. ACS Earth Space Chem. 3, 700–710 (2019).
49. Miller, L. G., Baesman, S. M., Carlström, C. I., Coates, J. D. &
Oremland, R. S. Methane oxidation linked to chlorite dismutation.
Front. Microbiol. 5, 275 (2014).
50. Zhang, X. et al. Reaction of methane and UV-activated
perchlorate: relevance to heterogeneous loss of methane in the
atmosphere of Mars. Icarus 376, 114832 (2022).
51. Smith, M. L., Claire, M. W., Catling, D. C. & Zahnle, K. J. The
formation of sulfate, nitrate and perchlorate salts in the Martian
atmosphere. Icarus 231, 51–64 (2014).
52. Catling, D. C. et al. Atmospheric origins of perchlorate on Mars
and in the Atacama. J. Geophys. Res. Planets 115, E00E11 (2010).
53. Turner, A. M., Abplanalp, M. J. & Kaiser, R. I. Mechanistic studies
on the radiolytic decomposition of perchlorates on the Martian
surface. Astrophys. J. 820, 127 (2016).
54. Wu, Z. C. et al. Forming perchlorates on Mars through plasma
chemistry during dust events. Earth Planet. Sci. Lett. 504, 94–105
(2018).
55. Carrier, B. L. & Kounaves, S. P. The origins of perchlorate in the
Martian soil. Geophys. Res. Lett. 42, 3739–3745 (2015).
56. Melwani Daswani, M. & Kite, E. Paleohydrology on Mars
constrained by mass balance and mineralogy of pre‐Amazonian
sodium chloride lakes. J. Geophys. Res. Planets 122, 1802–1823
(2017).
57. Kim, Y. S., Wo, K. P., Maity, S., Atreya, S. K. & Kaiser, R. I.
Radiation-induced formation of chlorine oxides and their
potential role in the origin of Martian perchlorates. J. Am. Chem.
Soc. 135, 4910–4913 (2013).
58. Kang, N., Anderson, T. A., Rao, B. & Jackson, W. A. Characteristics
of perchlorate formation via photodissociation of aqueous
chlorite. Environ. Chem. 6, 53–59 (2009).
Publisher’s note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional affiliations.
Springer Nature or its licensor (e.g. a society or other partner) holds
exclusive rights to this article under a publishing agreement with
the author(s) or other rightsholder(s); author self-archiving of the
accepted manuscript version of this article is solely governed by the
terms of such publishing agreement and applicable law.
© The Author(s), under exclusive licence to Springer Nature Limited
2022

8. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
Methods
Geochemical modelling of Mn(ii) oxidation by O2
TherateofoxidationofdissolvedMn(ii)bymolecularoxygenat25 °C
wasimplementedinTheGeochemist’sWorkbenchmoduleReact59
using
amodifiedLawrenceLivermoreNationalLaboratorydatabase60,61
.The
hydrolysisandcomplexationconstantsofMnwereadjustedtomatch
thosereportedbyMorganetal.11
.ThegassolubilityofCO2 andtheacid
dissociationconstantsinthedatabasewereverifiedtobeequaltothose
reportedbyref.11
.Weimplementedtheratelawfromref.11
−d [Mn (II)] /dt =4 [O2] { k1 [MnOH
+
] + k2 [Mn (OH)2 (aq)] + k3 [Mn (CO3)
2−
2 ]
(1)
wherek1,k2,andk3 aretherateconstantsforeachkinetictermwithlog
k1 = −1.78,logk2 = +1.32andlogk3 = −1.09.
Theoxidationof1 mmol l−1
Mn(ii)intherangeofexpectedO2 con-
centrations(0.03,0.01and10−5
bar)(ref.6
)wasmodelledinabackground
fluidof100 mmol l−1
MgCl2 thatisconstantlybufferedbyO2 and0.5 bar
CO2 asafunctionofpH(7–4).TwotypesofpHsystemswerealsostudied,
fixed pH systems and free-drift pH systems. The fixed pH systems are
idealscenariosinwhichthereisperfectpHbufferingandthepHnever
undergoesanychangeduringreaction.Thefree-driftpHsystemsdisplay
pHbufferingonlyfromthedissolvedspeciespresent.Allnaturalsystems
fallinbetweenthesetwoendmemberscenarios.Purelyaqueoussystems
behaveasfree-driftsystemswhilereactionsinfracturesorporespaces
behaveasanintermediatebetweenfree-driftandpHbufferedsystems
owingtothepHbufferingarisingfromwater–rockinteraction.
Rhodochrosite(MnCO3)wassuppressedinthesecalculationsbecause
itsformationabovepH5.56lowersthedissolvedMn(ii)concentrationin
ourmodelandthusslowstherateofoxidation62
.Simulationsabovethis
pH value should thus be considered lower bounds for the timescale of
Mn(ii)oxidationbyO2.NotethatthesolublerangeofMn(ii)extendsto
pH6ifdissolvedMn(ii)concentrationsarelimitedto~150μmoll−1
andto
pH7iflimitedto~4μmoll−1
.Inaddition,bixbyite(Mn2O3)wassuppressed
becauseitisnotreportedinstudiesofMnoxidation11
.
Mn(ii)oxidationbyoxyhalogenspecies
Laboratoryexperimentswereconductedtoinvestigatewhetherchlo-
rate and/or bromate are capable of oxidizing Mn(ii) in Mars-relevant
fluids at ambient conditions (24 ± 1 °C, 1 atm). To isolate the effects
of chlorate or bromate on dissolved Mn(ii), all reactions were con-
ducted inside a Coy Laboratory Products vinyl anaerobic chamber
(N2 = 97%, H2 = 3%), with <1 ppmv O2 concentration, maintained using
palladium catalysts, to inhibit O2 interference in Mn(ii) oxidation.
All reactors were wrapped with Al foil to inhibit any probable Mn
photo-oxidation. Stock solutions of manganese(ii) chloride tetrahy-
drate[MnCl2·4H2O],manganese(ii)sulfatemonohydrate[MnSO4·H2O],
magnesium chloride hexahydrate [MgCl2·6H2O], magnesium sulfate
hexahydrate[MgSO4·6H2O],sodiumchlorate[NaClO3]andsodiumbro-
mate[NaBrO3]werepreparedusingAmericanChemicalSociety-grade
FisherScientificreagentsindeionized,deoxygenatedwaterandkept
insidetheanaerobicchamberindarkreagentbottles.Thestocksolu-
tionsweredilutedinappropriateamountstopreparetheexperimental
solutionsandachievethedesiredstoichiometry.
Twotypesofexperimentswereconductedforstudyingtheeffect
ofchlorateand/orbromateondissolvedMn(ii):(1)kineticexperiments
and(2)batchexperiments.Whilethekineticexperimentsweresetup
to track the concentration of Mn(ii) and pH as a function of time, the
batchexperimentsweresetuptostudythemineralproductsofMn(ii)
oxidation by chlorate and/or bromate. The batch experiments were
greater volume (150 ml) replicates of the kinetic experiments (50 ml)
to extract a substantial amount of mineral product for identification
and characterization. Reactors of batch experiments were sealed in
air-tightserumbottlesusingbluebutylrubberstoppersandbrought
outside the anaerobic chamber to be placed on a shaker table set to
~170 rotations per minute to promote mixing and avoid gravimetric
settling of any mineral produced. The kinetic experiment reactors
were kept inside the anaerobic chamber on end-over-end rotators.
The kinetic experiments were sampled by extracting a small aliquot
fromthesereactorstodeterminethe[Mn(ii)]andpHinregularweekly
timeintervals.Thesolutionsandthemineralprecipitatesofthebatch
reactorswereanalysedonlyattheendoftheexperiments.
Theexperimentalsolutions(bothkineticandbatch)weresetupwith
threetypesofoxidants:(A)onlybromate,(B)onlychlorateand(C)both
bromate and chlorate. All experiment types contained Mars-relevant
backgroundsaltsmagnesiumchlorideormagnesiumsulfate63,64
toserve
asionicstrengthbuffers(~100 mmol l−1
).Thebackgroundsaltsalsopro-
videanionswithdifferentabilitiestocomplexdissolvedMn(ii),thereby
having the capacity to affect reaction rates and mineral products. The
initialpHoftheexperimentalsolutionswassetusing1 mol l−1
solutions
ofhydrogenchlorideandsodiumhydroxideto7,5and3usingaThermo
ScientificOrionStarpHmeterwithanAgClelectrode.Theexperimental
pH was allowed to drift freely as a response to Mn(ii) oxidation and to
determine the effect of Mn(ii) oxidation on the acidity of the solution.
Analogousoxidant-freecontrolexperimentswerealsopreparedatpH 7
and 3 to verify the absence of inadvertent oxidation by stray oxidants
(forexample,O2)inthereactors(SupplementaryFig.5).
Oxidation experiments using only bromate sought to determine
whetherthisoxyhalogenspeciescanoxidizeMn(ii)andproduceMn(iii/
iv)mineralsinMars-relevantfluids.Solutionscontaining~100 mmol l−1
(or10mmoll−1
)Mn(ii),aseitherMnCl2 orMnSO4,werepreparedinback-
groundsaltmixturesof0.1moll−1
Mg-chlorideorMg-sulfate,respectively.
Stocksolutionofsodiumbromate(2moll−1
)wasaddedtotheexperimen-
talsolutionstobringthebromateconcentrationapproximatelyequalto
Mn(ii)(~100or10 mmol l−1
),asperrequirement.
Similarly, oxidation experiments using only chlorate sought to
determine whether this oxyhalogen species can oxidize Mn(ii) and
produce Mn(iii/iv) minerals in Mars-relevant fluids. Oxidant type A.
Solutions containing ~100 mmol l−1
Mn(II), as either MnCl2 or MnSO4,
werepreparedinbackgroundsaltmixturesof0.1 mol l−1
Mg-chlorideor
Mg-sulfate,respectively.Stocksolutionofsodiumchlorate(2 mol l−1
)
wasaddedtotheexperimentalsolutionstobringthechlorateconcen-
trationapproximatelyequaltoMn(ii)(~100 mmol l−1
).
A third set of experiments evaluated Mn(ii) oxidation in mixed
chlorate/bromatesystems.Experimentalsolutionswith100 mmol l−1
chlorateand10 mmol l−1
bromatewasmixedwith100 mmol l−1
Mn(ii)
in both Mg-chloride and Mg-sulfate fluids starting at initial pH 7, 5
and 3. All other experimental conditions remain identical except the
inclusion of both chlorate and bromate together to replicate Martian
fluidconditions.Theconcentrationofbromatecouldrangeashighas
10%ofthechlorateinMartianfluids27,65,66
,andthereforeexperimental
solutionsinvestigatedMn(ii)oxidationwith100 mmol l−1
chlorateand
10 mmol l−1
bromate. Control experiments investigated Mn(ii) oxida-
tionin100 mMchlorateand10 mMbromateseparatelyforcomparison.
Analyticalmethods
Thebromateconcentrationsintheexperimentalsolutionsweremeas-
ured at the start of the reaction and at the end of the experiment with
Thermo Fisher Scientific Dionex Integrion high-pressure ion chro-
matograph using an IonPac AS11 analytical column equipped with an
AG11 guard column, ADRS 600 suppressor, a conductivity detector
and an AS-DV automated sampler. Analytical conditions consisted of
12 mMKOHeluentataflowrateof1 ml min–1
.Bromateconcentrations
atthestartoftheexperimentweredeterminedbeforeadditionofdis-
solved Mn(ii) and were corrected for the dilution resulting from this
final solution component. The Mn(ii) concentration was measured
periodicallyusingiCAP7400Duoinductivelycoupledplasmaoptical
emission spectroscopy. The first data point was taken immediately
afterthestartoftheexperiments,anditrepresentstheinitialamount
ofdissolvedMn(ii)presentinthesolutions.

9. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
The minerals precipitated in the reactors were collected at the
end of the experiment by filtration using a 0.22 µm pore size mixed
cellulose ester membrane. The filtered solid products were dried in a
vacuum desiccator for 3–4 days before characterization using XRD,
XPS and VNIR spectroscopy. XRD was performed using a Bruker d8
Advance diffractometer configured with a Cu source operating at
40 kVand40 mAandaposition-sensitive,energy-dispersiveLynxEye
XEdetector.Thedriedmineralsamplesweregroundusingamortarand
pestle before analysis, and a zero-background silicon sample holder
was used because the amount of mineral precipitated in these batch
reactionswaslimitedinsomesystems.TheXRDscanswereperformed
usingcontinuousscanningwitha0.05°stepsizeand1 scounttimeper
step. The Bruker Diffrac.Eva programme was used for processing the
rawdataandidentificationofmineral(s)basedonpatternmatching.
TheXPSanalyseswereconductedusingaPhysicalElectronics5000
Versa Probe II Scanning ESCA microprobe equipped with a monochro-
matic Al Kα X-ray source. The binding energy was calibrated using the
C1s248.8eVadventitiouscarbonpeak.TheMn3ppeakhasbeenshownto
providerobustquantificationofMnoxidationstate67
;thusthispeakwas
usedtoquantifytheratioofMnoxidationstatesinsamples.Peakswere
fittedinCasaXPSsoftwarewithanasymmetricLorentzian(LA)function
andaniteratedShirleybackground.Otherpeakshapefunctions,suchas
SumGaussian–Lorentzianwerealsoexplored,butthesefunctionsyielded
lower quality fits with larger errors. The proportion of Mn(ii), Mn(iii)
andMn(iv)ineachsamplewasdeterminedbyfittingacomponentpeak
for each of these oxidation states and determining its relative area, an
approachthathasbeenemployedtoquantifyMnoxidationstatesinfiltra-
tionmediasamplesfromwatertreatmentplantsandMn(hydr)oxides67,68
.
Eachofthecomponentpeakswasfixedattheprimarypeakposition(>80%
of total peak area) identified for Mn(ii), Mn(iii) and Mn(iv) standards.
Thesethreestandards,manganese(ii)chloride[MnCl2],manganese(iii)
oxyhydroxide[MnOOH]andmanganese(iv)oxide[MnO2],wereselected
tobestrepresentpossibleformationproductsfromoxidationbychlorate.
MonteCarloanalysiswasusedtoassessthestabilityofpeakfitsandobtain
associatederrorsforeachcomponent.AVoigtlineshapewasusedforall
fits(LA(1.53,243)).SincesamplepeakscouldbefitwithonlyMn(iv)and
Mn(iii)components,thestandarddeviationwasthesameforbothofthese
components;thatis,anincreaseintheproportionoffittedMn(iii)by4%
wouldresultina4%decreaseintheproportionoffittedMn(iv).
TheVNIRreflectancespectraofthemineralprecipitatesproduced
were collected using an Analytical Spectral Devices (ASD) portable
VNIR spectrometer. The powdered samples of the mineral precipi-
tates were measured using an ASD Muglight, a bench-top probe from
0.35–2.50 μm spectral range with a fixed illumination angle of 30°.
The spectra were normalized using a Spectralon diffuse reflectance
standardtoradiancecoefficients.
Dataavailability
The data associated with the manuscript are available at: https://fig-
share.com/s/e12d62da416302225cf3.
Codeavailability
The code to model Mn(ii) oxidation by O2 in the Geochemist’s Work-
bench is available at https://figshare.com/articles/online_resource/
MnII_oxidation_species_Morgan_V7_rea/21066232.
References
59. Bethke, C. M. Geochemical and biogeochemical reaction
modeling. (Cambridge University Press, 2007).
60. Catalano, J. G. Thermodynamic and mass balance constraints on
iron-bearing phyllosilicate formation and alteration pathways on
early Mars. J. Geophys. Res. Planets 118, 2124–2136 (2013).
61. Delany, J. & Lundeen, S. R. The LLNL Thermochemical Database
Report UCRL-21658 (Lawrence Livermore National Laboratory,
1990).
62. Sternbeck, J. Kinetics of rhodochrosite crystal growth at 25°C:
the role of surface speciation. Geochim. Cosmochim. Acta 61,
785–793 (1997).
63. Vaniman, D. T. et al. Magnesium sulphate salts and the history of
water on Mars. Nature 431, 663–665 (2004).
64. Fox-Powell, M. G., Hallsworth, J. E., Cousins, C. R. & Cockell, C. S.
Ionic strength is a barrier to the habitability of Mars. Astrobiology
16, 427–442 (2016).
65. Gellert, R. et al. Alpha Particle X‐ray Spectrometer (APXS): results
from Gusev crater and calibration report. J. Geophys. Res. Planets
111, E02S05 (2006).
66. Marion, G., Catling, D. & Kargel, J. Br/Cl partitioning in chloride
minerals in the Burns formation on Mars. Icarus 200, 436–445
(2009).
67. Ilton, E. S., Post, J. E., Heaney, P. J., Ling, F. T. & Kerisit, S. N. XPS
determination of Mn oxidation states in Mn (hydr) oxides. Appl.
Surf. Sci. 366, 475–485 (2016).
68. Cerrato, J. M., Hochella Jr, M. F., Knocke, W. R., Dietrich, A. M. &
Cromer, T. F. Use of XPS to identify the oxidation state of Mn in
solid surfaces of filtration media oxide samples from drinking
water treatment plants. Environ. Sci. Technol. 44, 5881–5886
(2010).
Acknowledgements
This research was funded by NASA Science Mission Directorate Future
Investigators in NASA Earth and Space Science and Technology
(FINESST) programme through award no. 80NSSC19K1521. J.G.C.
was supported by the NASA Exobiology programme through award
no. 80NSSC18K1292. G.J.L. was supported by the National Science
Foundation Graduate Research Fellowship Program under grant no. DGE-
1745038 and DGE-2139839. Discussions with B. Jolliff, R. Arvidson and J.
Hurowitz improved this manuscript. P. Carpenter is thanked for assistance
with XRD data collection and Rietveld refinements. R. Arvidson and A.
Knight are thanked for assistance in VNIR data collection.
Authorcontributions
K.M. and J.G.C. designed the study. K.M. conducted the experiments
with assistance from E.L.M. G.L. contributed X-ray photoelectron
spectroscopy analyses. J.G.C. and K.M. performed the thermodynamic
and kinetic modeling. K.M. analyzed the results and wrote the original
manuscript, with additional text provided by J.G. and further editing
by G.L. and E.L.M. All authors have read and agreed to the published
version of the manuscript.
Competinginterests
The authors declare no competing interests.
Additionalinformation
Extended data is available for this paper at
https://doi.org/10.1038/s41561-022-01094-y.
Supplementary information The online version
contains supplementary material available at
https://doi.org/10.1038/s41561-022-01094-y.
Correspondence and requests for materialsshould be addressed to
Jeffrey G. Catalano.
Peer review information Nature Geoscience thanks Yasuhito Sekine
and the other, anonymous, reviewer(s) for their contribution to the
peer review of this work. Primary Handling Editor: Tamara Goldin, in
collaboration with the Nature Geoscience team.
Reprints and permissions informationis available at
www.nature.com/reprints.

10. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
ExtendedDataFig.1|ReactionofMartianbasaltwithwaterequilibratedwith0.5 barCO2 and0.03 barO2.(a)RelationshipofdissolvedCO2 andO2
concentrationstopH.(b)Massfractionofmineralalterationproductions.

11. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
ExtendedDataFig.2|Mineralogyofthesolidsproducedbyoxidationof
dissolvedMn(II)by10 mmol L−1
bromate.XRDpatternsofthesolidsproduced
byreactionof10 mmol L−1
Mn(II)with10 mmol L−1
bromatein100 mmol L−1
magnesiumchloridefluids.Patternsarevisuallyoffsetforclarity.Diagnostic
peaksarelabeledandindicatedwithdashedlines.SeeExtendedDataTable4for
completesampledetails.

12. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
ExtendedDataFig.3|ImagesofthesolidsproducedbyoxidationofdissolvedMn(II)bybromate.Mineralsprecipitatedfollowingreactioninmagnesiumchloride
(a-d)andmagnesiumsulfate(eandf)fluids.

13. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
ExtendedDataFig.4|ComparisonofVNIRspectraofthemanganeseoxidemineraldetectedatEndeavorcrater,Mars,andproducedinMn(II)oxidation
experiments.SolidsproducedfromoxidationofdissolvedMn(II)bybromatein(a)magnesiumchlorideand(b)magnesiumsulfatefluids.Thelabelsindicatethe
initialpHofthesamples.SeeExtendedDataTable5forcompletesampledetails.

14. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
Extended Data Table 1 | Homogeneous and heterogeneous Mn(II) oxidation rates by oxygen

15. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
Extended Data Table 2 | Fluid composition of the kinetic experiments shown in Fig. S1 with [Mn(II)]≈[ClO3
−
] ≈ 100mmolL−1

16. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
Extended Data Table 3 | Fluid composition of the kinetic experiments shown in Fig. 2 with [Mn(II)]≈[BrO3
−
] ≈ 100mmolL−1

17. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
Extended Data Table 4 | Fluid composition of the mineral precipitation experiments shown in Extended Data Fig. 2 with
approximately 100mmolL−1
[Mn(II)], and 10mmolL−1
[BrO3
−
]

18. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
Extended Data Table 5 | Fluid composition of the mineral precipitation experiments shown in Fig. 3 with [Mn(II)]≈[BrO3
−
] ≈
100mmolL−1

19. Nature Geoscience
Article https://doi.org/10.1038/s41561-022-01094-y
Extended Data Table 6 | Percentage of Mn(II) oxidation in open systems buffered by 0.21bar O2 in presence of 4 ×10−4
bar CO2
under identical conditions as the experiments with oxyhalogens