Planetary and Space Science xxx (xxxx) xxx

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Planetary and Space Science

journal homepage: www.elsevier.com/locate/pss

Atmospheric injection of sulfur from the Medusae Fossae forming events

Lujendra Ojha a,*, Suniti Karunatillake b, Kayla Iacovino c

a Department of Earth and Planetary Sciences, Rutgers, The State University of New Jersey, Piscataway, NJ, USAb Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA, 708033, USAc Astromaterials Research and Exploration Science, Jacobs, NASA Johnson Space Center, Houston, TX, 77058, USA

A R T I C L E I N F O

Keywords:MarsAtmosphereSulfurPyroclasticGeochemistryDust

* Corresponding author. Department of Earth andE-mail address: Luju.ojha@rutgers.edu (L. Ojha)

https://doi.org/10.1016/j.pss.2019.104734Received 30 October 2018; Received in revised forAvailable online xxxx0032-0633/© 2019 Elsevier Ltd. All rights reserved

Please cite this article as: Ojha, L. et al., Atmhttps://doi.org/10.1016/j.pss.2019.104734

A B S T R A C T

A plethora of in-situ bulk chemical and mineralogical analyses, remote sensing data, and geochemical modelssuggest that the Martian crust is sulfur(S)-rich, exceeding the S content of terrestrial basalts by almost an order ofmagnitude. The main source of crustal S on Mars is the volcanic exhalation of SO2 and H2S gases. Volcanic ash,expelled along with gases, may be a sink for up to 30% of the exhaled S gas species. We analyze the elementalcomposition of the Martian shallow sub-surface using the Mars Odyssey Gamma Ray Spectrometer to find areasthat are simultaneously enriched in constituents of major volcanic gases such as S, Cl, and H2O. A large sedi-mentary deposit of likely pyroclastic origin called the Medusae Fossae Formation (MFF) is located within thisregion of possibly extensive alteration by volcanic gases. Based on reasonable terrestrial analog estimates that theMFF scavenged, at maximum, 30% of the exhaled S gas species, we find that a significant amount of S (>1017 kg)would have been delivered to the atmosphere over the time it took for the MFF to be deposited on Mars. The massof the S emitted from the MFF-forming event(s) is up to 8 orders of magnitude higher than the mass of S emittedfrom the largest Quaternary volcanic eruption on Earth. Thus, volcanic degassing from the MFF forming eventscould have significantly affected surface geological processes, climate evolution, and habitability of Mars.

1. Introduction

Sulfur (S) is the tenth most abundant element in the solar system andits complex heterovalence states (S2- to S6þ) allow it to participate innumerous geochemical and biochemical processes. Sulfur can play amajor role in the planetary differentiation process by allowing earlydifferentiation of Fe-Ni-S core and a largely silicate mantle and crust (e.g.Stewart et al., 2007). Sulfur in the atmosphere can form sulfate aerosolsand play a powerful role in the global climate and the chemistry of theatmosphere and surface environments (Bluth et al., 1993; McCormicket al., 1995; Mitchell and Johns, 1997; de Silva and Zielinski, 1998;Robock, 2000; Halevy et al., 2007; Robock et al., 2009). Sulfur may havealso played a major role in the early oxygenation of the biosphere and thecoevolution of life on Earth (e.g. Lyons and Gill, 2010), and has beenrecently found to be associated with organic compounds on Mars(Eigenbrode et al., 2018). As such, the knowledge of the bulk S contentand its spatial distribution on a planet is important to our understandingof the planetary differentiation processes; sedimentary, geomorphic, andaqueous processes; climate evolution; and current and past habitability.

Multiple lines of observations have shown that the Martian regolith isrich in sulfur. In-situ measurements have revealed high concentrations of

Planetary Sciences, Rutgers, Th.

m 25 June 2019; Accepted 2 Sep

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ospheric injection of sulfur fr

S in the Martian soil and dust (e.g. Berger et al., 2016; Gellert et al., 2006;Yen et al., 2005). Remote sensing observations have found evidence formassive and widespread deposits of sulfates in association with outcropsand the bulk regolith throughout the planet (e.g. Gendrin et al., 2005;Murchie et al., 2009; Wray et al., 2011; Carter et al., 2013; Ackiss andWray, 2014; Karunatillake et al., 2014, 2016), and Martian meteoritesare known to contain sulfides and sulfates (e.g. Righter et al., 2009; Franzet al., 2014; Ding et al., 2015).

Sulfur on the Martian surface is ultimately derived from the mantle byvarious magmatic and hydrothermal processes (Settle, 1979; Bullock andMoore, 2007; Johnson et al., 2008; Craddock and Greeley, 2009; Gaillardand Scaillet, 2009; Righter et al., 2009; Franz et al., 2018). Magmaticdegassing occurs when volcanic gases are exsolved from melts during thedecompression of magma ascending to the near surface. The abundanceof sulfur soluble in an aluminosilicate melt is directly related to its FeOcontent. As Martian basalts contain twice as much FeO as their terrestrialcounterparts (Haggerty, 1978), the melts can dissolve up to 3–4 timesmore S under similar conditions of pressure and temperature. Indeed,thermodynamic modeling suggests that the S concentration of Martianbasalt melt is in the range of 4000–7000 ppm, which exceeds the Scontent of terrestrial basalt melt by a factor of 4–7 (Gaillard and Scaillet,

e State University of New Jersey, Piscataway, NJ, USA.

tember 2019

om the Medusae Fossae forming events, Planetary and Space Science,

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2009). The volume and the species of the degassed sulfur-bearing gasesstrongly depends on the atmospheric pressure at venting conditions, themagma oxygen fugacity (fO2), and the concentrations of other volatileelements present (e.g., H, C, Cl) (Wallace and Carmichael, 1994; Gaillardand Scaillet, 2009; Iacovino, 2015). The lower gravity and atmosphericdensity of Mars compared to Earth facilitates rapid degassing of volatilesduring magma ascent (Wilson and Head, 1983), and the relatively lowviscosity of basaltic magmas compared to terrestrial rhyolitic composi-tions further amplifies the degassing process. The amount of S degassed isaffected in equal magnitude by the fO2 and magmatic water content, andmaximum sulfur degassing is obtained for hydrated-oxidized melts(Gaillard et al., 2012). Other factors such as temperature, melt compo-sition, and fS2 play a secondary role in controlling the volume of degassedsulfur (Galliard and Scaillet, 2014).

The transition from S2- to S6þ in silicate melts occurs over a narrowfO2 interval between quartz-fayalite-magnetite oxygen fugacity buffer(QFM) and QFM þ2 (Jugo et al., 2010). H2S is the dominant form ofgaseous S in melts with lower fO2, lower H2O, and higher pressure,whereas SO2 is dominant in melts with higher fO2, higher H2O, and lowerpressure. In some cases, S2 may also make up a significant portion ofexhaled S gas. The fO2 of Martian basalts encompasses a wide range ofvalues, as evidenced from Martian meteorites, ranging from QFM-5 toQFMþ1.5 (Bunch and Reid, 1975; Herd et al., 2001, 2002; Wadhwa,2001; Sautter et al., 2002; Goodrich et al., 2003; Herd, 2003; Dyar et al.,2005; Shearer et al., 2006; Righter et al., 2008). H2Smay have dominatedduring the early stages of Martian history when degassing occurred froma reduced magma ocean at higher eruptive pressure (Gaillard et al.,2012). The assimilation of crustal materials in the melt would have led tothe increase in the oxidation state of magmas over time, thus, thedominant sulfur species during the past ~4 Ga on Mars was likely SO2

(Gaillard et al., 2012). Thermodynamic modeling within the aboveconstraints illustrates that Martian volcanism may have expelled gaseswith up to ~60mol% SO2 (Gaillard and Scaillet, 2009). The H2S/SO2ratio in exhaled volcanic gas may be altered during and after eruption viathe removal of SO2 by dissolution in surface water or undergroundaquifer, sulfate aerosol formation in the atmosphere, and chemical re-actions between gas and ash in the plume (see review by King et al., 2018and references therein). Halogens such as chlorine (Cl) and fluorine (F)are also released to the atmosphere in the form of HCl, NaCl, KCl; and HF,SiF by volcanic degassing (Giggenbach, 1996 Oppenheimer, 2003),although in a minor amount.

Degassing is the key mechanism responsible for the transfer of sulfurfrom the mantle to the atmosphere on Mars (Franz et al., 2018). Theamount of volcanically injected atmospheric S on Mars has been previ-ously estimated from theoretical considerations. Based on geochemicalmodeling, it has been suggested that degassing from the Tharsis volcanicregion alone could have made 20–60m thick global-equivalent-layer(GEL) of sulfate minerals (Gaillard and Scaillet, 2009). Similarly, John-son et al. (2008) estimated SO2 emission in the range of1.19–4.76� 1014 kg from the Tharsis igneous province, which couldhave supported greenhouse warming up to 25 K greater than what couldbe caused by CO2 alone. Righter et al. (2009) proposed that degassing of2400 ppm S from Fe-rich Martian melts over Mars’s geological historymight have produced all the observed S on the surface. Craddock andGreeley (2009) estimated that as much as 1 bar of sulfur may have beenreleased via juvenile degassing throughout Martian history. It has beenestimated that ~11% of the terrestrial primitive mantle sulfur may havebeen outgassed over geological time on Earth (Canfield, 2004).McLennan and Grotzinger (2009) assumed a similar rate of degassing onMars and estimated the total mass of surficial sulfur to exceed 2.3 �1022 g, which equates to a global equivalent layer of 2-km thick sedi-mentary layer with S content equal to average soil (~6% SO3) (McLennanand Grotzinger, 2009). Collectively, these theoretical estimates illustratethat a large amount of volcanic S has been injected into the Martian at-mosphere by volcanism (also see Nekvasil et al., 2019).

At the terminus, eruption plumes are composed of ~1–10wt% of gas

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and ~90–99wt% of ash (Sparks et al., 1997). Volcanic ash is comprisedof a mixture of amorphous aluminosilicate glass and crystalline materials(Heiken and Wohletz, 1992) with its specific mineralogy dictated by thesource magma. A significant amount of exhaled volcanic gases can bescavenged by fine ash particles during heterogenous reactions withinerupting plumes (�Oskarsson, 1980; Varekamp et al., 1984; De Hoog et al.,2001; Delmelle et al., 2007, 2018; Self et al., 2008). This may occur as acombination of: 1) chemisorption reactions at temperatures between 190and 800 �C (Rose, 1977; Witham et al., 2005); 2) direct precipitation ofelemental sulfur at moderate temperatures (400–500 �C) (DiFrancescoet al., 2015, 2016; King et al., 2018; Nekvasil et al., 2019); and 3)chemisorption reactions to form sulfate and precipitation to formhalogen salt coatings on ash grains at high temperatures (600–1200 �C)(Ayris et al., 2013; Henley et al., 2015; King et al., 2018; Palm et al.,2018; Renggli and King, 2018).

Water- and acid-soluble species scavenged by ash may mobilized ifdeposits are infiltrated with groundwater (requiring low pH in the case ofacid-soluble species). Pristine ash samples preserve their scavengedvolatiles, and water and acidmay be used to leach them in the laboratory,thus allowing for direct measurement of the bulk composition and con-centration of volatile-bearing precipitates. Comparison of leachate vol-atile concentrations with the total mass of erupted material gives anestimate of volatile mass returned to the surface as fallout. Such in-vestigations for eruptions where total eruptive volatile flux is indepen-dently known have found that ash can scavenge as much as 30% of thesulfur and 10–30% of the total chlorine emitted (Rose, 1977; Varekampet al., 1984; De Hoog et al., 2001; de Moor et al., 2005; Witham et al.,2005; Delmelle et al., 2007, 2018; Self et al., 2008) thus limiting theamount of SO2 and halogens injected into the atmosphere and makingsurface deposits a potentially important sink for erupted volatiles.

The 1974 Fuego Volcano eruptions in Guatemala was one of the firstvolcanoes recognized as releasing excess sulfur and produced more than0.2 km3 of basaltic ash (Rose, 1977; Rose et al., 2008). During this ex-plosion, as much as 33% of the degassed S fell back to Earth as acidaerosol particles absorbed on the ash (Rose, 1977). Given the estimatedmass of the ash deposit, Rose (1977) estimated that 2.2� 1011 g of S musthave been released to the atmosphere. The 1982 eruptions of the ElChichon volcano in Chiapas, Mexico, produced approximately 0.38 km3

of andesitic ash deposit (Varekamp et al., 1984). The fine ash particlesscavenged around 30% of the sulfur emissions from the eruption andcarried it to the ground (Varekamp et al., 1984), which equates to~4� 1011 g of scavenged S. The total mass of S injected into the atmo-sphere by this event equates to ~1� 1013 g (Krueger, 1983). The1982–1983 eruptions of Galunggung in West Java, Indonesia produced~0.37 km3 of andesitic to high-Mg basalt ash (Gourgaud et al., 1989;Gerbe et al., 1992). The analysis of ash leachate from this event showsthat 35% of the exhaled S was scavenged by volcanic ash from theeruptions. Self et al. (2008) estimate that 75% of the exhaled volatilesmay reach the tropospheric-stratospheric boundary via eruptions, while25%may form a ground-level fog with the lava flow. This is in reasonableagreement with de Hoog et al.’s [2001] estimate for S deposited ontoclasts versus that free to circulate, where Galunggung observations sug-gest 35% of exhaled S phases adsorb on pyroclastic debris depositingquickly, while the rest circulates in the atmosphere. Although the exactmechanics of sulfur uptake and their relative efficiency are not wellunderstood (e.g. Schmauss and Keppler, 2014; Delmelle et al., 2018), ashleachate studies on Earth have clearly demonstrated that volcanic ash canscavenge a significant fraction of exhaled sulfur.

While several works in the past have estimated the amount of sulfurthat could have been injected into the Martian atmosphere from theo-retical considerations (Johnson et al., 2008; Craddock and Greeley, 2009;Gaillard and Scaillet, 2009; Righter et al., 2009), constraints from thecompositional analyses of available Martian geochemical data has so farbeen lacking. In this work, we rely on the elemental geochemistry of thenear-surface regolith of Mars provided by The Mars Odyssey Gamma RaySpectrometer Suite (GRS) to provide an estimate of the volume of S gases

L. Ojha et al. Planetary and Space Science xxx (xxxx) xxx

that would have been delivered to the Martian atmosphere during largepyroclastic eruptions. In section 2, we describe the GRS instrument suiteand our methodology. In section 3, we present the S, Cl, and H2O massfractions (as percentage, wt%) from a region on Mars that is significantlyenriched in these elements. We describe a large sedimentary unit of likelypyroclastic origin on Mars called the Medusae Fossae Formation (MFF)and provide reasonable lower bounds on the amount of sulfur that wouldhave been delivered to the Martian atmosphere when this unit wasdeposited on Mars. We then discuss the significance of our results.

2. Methods

The shallow subsurface composition of the Martian crust to decimeterdepths has been derived by GRS. GRS measures the spectrum of gammaphotons emitted from the Martian surface; characteristic spectral peaksfrom specific nuclear reactions allow the quantification of most majorrock-forming elements, along with select minor and trace elements (Al,Ca, Cl, Fe, H, K, S, Si, Th) (Boynton et al., 2007). Peak area above thecontinuum can be used to infer the wt% of each element over an area ofthe planet’s surface, leading to chemical abundance maps excludingapproximately the � 50� latitudes where H increases rapidly (Fig. 1).Recent synthesis of Mars Reconnaissance Orbiter (MRO) CompactReconnaissance Imaging Spectrometer for Mars (CRISM) derivedmineralogy and GRS derived chemistry also suggests that GRS-chemistrymay represent fine sediments – such as of eruptive or eolian provenance –

Fig. 1. GRS derived abundance maps for S, Cl, and H2O. GRS derived elementalabundance maps for S (a), Cl (b), and H2O (c) in wt%, overlain on a Mars OrbiterLaser Altimeter (MOLA) shaded relief (Smith et al., 1999). The outline of theMedusae Fossae Formation (MFF) is shown in black.

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more than underlying primary igneous rocks at key volcanic provincesmantled by dust (Viviano et al., 2019). Thus, the near global coverage,sensitivity to decimeter depth, and association with fine sediments makesGRS chemical maps an ideal dataset for a study of this nature.

Previously, GRS maps have been used to study chlorine enrichment insedimentary deposits (e.g. Keller et al., 2006; Diez et al., 2009), globalbulk soil hydration by sulfates (e.g. Karunatillake et al., 2014, 2016),bulk crustal compositions (e.g. Boynton et al., 2007; King and McLennan,2010), and igneous processes (e.g. Baratoux et al., 2014; Hood et al.,2016; Susko et al., 2017). Several chemical maps were unavailable infinal form to prior workers due in part to overlapping spectral peaks atcontrasting intensity corresponding to different elements, particularlyaffecting S (cf., Evans et al., 2007). Continued refinements to GRSspectral analyses and forward modeling from spectra to concentration(Boynton et al., 2007; Evans et al., 2007; Karunatillake et al., 2007)allowed Al, Ca, and S abundance trends to be analyzed in a series ofsubsequent works (Baratoux et al., 2014; Karunatillake et al., 2014, 2016;Hood et al., 2016; Susko et al., 2017; Ojha et al., 2018) as also used in thiswork.

We further resolve the discrepancy between the mapping resolutionand the analytical resolution in prior works (e.g., Karunatillake et al.,2014) by rendering chemical maps at the same coarse resolution as usedfor analyses. As in our prior work (Ojha et al., 2018), that helps preventapparent variations in the maps that lack spatial precision to supportinterpretations. Specifically, spatial smoothing needed for numericalprecision causes spatial autocorrelation (Karunatillake et al., 2011, 2012)and limits the spatial resolution of the S wt% map. Therefore, we use5� � 5� bins (referred to as “pixels” in the typical parlance of otherremote sensing datasets) for the Cl and H2O wt% data set, and re-binnedS wt% from the original 10� � 10� resolution to 5� � 5�. Fig. 1 shows theresulting wt% maps for Cl, S, and H2O.

We sought to find regions on Mars that are statistically enriched inelements such as S, Cl, and H2O which are major constituents of volcanicgases (Oppenheimer, 2003). Fluorine, despite its significance tomagmatic conditions and surficial weathering as discussed in Gale craterworks (e.g., Cousin et al., 2017; Forni et al., 2015) is unavailable inGRS-derived chemical maps. To that end, we used an enhanced Student’st-test parameter, ti, that measures the error-weighted deviation for eachelement from its bulk-average on Mars at each 5� � 5� GRS grid (Kar-unatillake et al., 2009):

ti ¼ ci � mffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffis2m;i þ s2

q ; (1)

where ci is the wt% of an element, m is the global arithmetic mean wt%,sm,i is the numerical uncertainty of ci, and s is the standard deviation ofthe data. Here we define areas of significant enrichment as those with tivalues greater than 1.5 (i.e. statistical confidence in directional deviation> 94%).

The correlation coefficient ( \Þ between two elements (a, b) wascomputed as follows:

\ða;bÞ ¼ 1N � 1

XNi¼1

�ai � μaσa

��bi � μbσb

�; (2)

where N is the number of observations, μa and σa are the mean andstandard deviation respectively of a, and μb and σb are the mean andstandard deviation of b. To assess the significance of the apparent cor-relation, we resampled the original data set a thousand times andcalculated the slope, y-intercept, and the correlation coefficient for eachnew subsample. This process of ‘bootstrap’ provides better information onthe characteristics of the dataset than statistical parameters (mean,standard deviation, correlation coefficients) computed from the full dataset (Efron, 1979).

We estimate the mass of S release to the atmosphere by a simple massbalance relation. Given the mass of a volcanic deposit (Mdeposit) and the

L. Ojha et al. Planetary and Space Science xxx (xxxx) xxx

concentration of S in the deposit (Cs), the mass of the S present in thatdeposit (Smass) is given by:

Smass ¼Mdeposit � Cs: (3)

Assuming that the volcanic deposit scavenged X% of the exhaled S,the S released to the atmosphere (SatmÞ is given by the following relation:

Satm ¼ SmassðX=100Þ : (4)

3. Results and discussion

The only area of Mars that is simultaneously enriched in S, Cl, andH2O is the equatorial region among the large volcanic provinces ofTharsis, Elysium, and Apollinaris Patera (hereafter referred to as S-Cl-Henriched region) (Fig. 2), likely associated with the siliciclastic fines thatmantle these areas (cf., Viviano et al., 2019). The correlation coefficientbetween S and Cl within the Cl-enriched region (i.e. area with ti >¼ 1.5for Cl in Fig. 2) is 0.76, with S modeling ~60% of the Cl variance (Fig. 3).In comparison, the correlation coefficient between S and Cl for theMartian mid-latitude is 0.62, with S modeling only ~38% of Cl variance(Ojha et al., 2018). The results from the bootstrap method (correlation0.76� 0.04) clearly highlight that the high correlation between S and Clis not due to statistical outliers (Fig. 3). Given our prior observations of Hvs S trends at regional scales, the positive S intercept to regression(Fig. 3a) would be consistent with the availability of S to associate withnon-Cl phases. That also converges with the positive S intercept observedin H-S bivariate space, likewise attributed to excess S in a reduced state,such as elemental deposits (Karunatillake et al., 2014).

The S-Cl-H enriched region of Mars has a relatively low thermalinertia (Fig. 2 (d)). Thermal inertia is a key property of a regolith thatdepends on particle size, the degree of induration, rock abundance, andexposure of bedrock within the top few centimeters of the subsurface.Fine particles such as volcanic ash and dust have low thermal inertia;bright unconsolidated fines have thermal inertia between 28 and135 Jm�2 K�1 s�1/2 (‘thermal inertia unit’ or TIU) (e.g. Putzig et al.,2005). The mean thermal inertia of the S-Cl-H enriched region is< 100

Fig. 2. Contour map showing regions on Mars with significant enrichment of S (a)contour labels correspond to a modified ‘t’ parameter (Equation (1)) that show regionMars. The background is the shaded relief map of Mars from Mars Orbiter Laser Altimtop of the daytime thermal inertia map from Thermal Emission Spectrometer. The y

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TIU (Fig. 2), suggesting that the source of the high concentration of S, Cl,and H2O in the S-Cl-H enriched region can be associated with fine par-ticles, at least in the top few centimeters of the regolith.

The S-Cl-H enriched region includes a vast sedimentary deposit calledthe Medusae Fossae Formation (MFF) (Tanaka, 2000; Bradley et al.,2002; Hynek, 2003; Kerber et al., 2011; Ojha et al., 2018; Ojha andLewis, 2018), that was likely emplaced during the Hesperian era (Kerberand Head, 2010) [3.7–3.0 Ga] (Fig. 1; Fig. 4). The lack of boulders alongerosional contacts (Malin and Edgett, 2000; Hynek, 2003), differentcrater depth-diameter ratios than areas known to be caprock (Barlow,1993), erosional morphology (Schultz and Lutz, 1988; Malin and Edgett,2000; Hynek, 2003), low density (Ojha and Lewis, 2018), and lowthermal inertia and radar signals (Hynek, 2003) are highly suggestivethat the MFF is composed of fine-grained materials. At present, the arealextent of the MFF exceeds 2� 106 km2 (20% of the continental UnitedStates), but the original extent of the MFF could have exceeded5� 106 km2 (50% of the continental United States) (Bradley et al., 2002;Dunning et al., 2018; Dunning, 2019). The current volume of the MFFexceeds 1.4� 106 km3, equivalent to ~10-m global-equivalent-layer(GEL), making it one of the largest sedimentary deposits on theMartian surface (Bradley et al., 2002).

A variety of formation mechanisms have been proposed to explain thedepositional history of the MFF including glacial activity emplacingpaleo-polar deposits which incorporated eolian sediments (Schultz andLutz, 1988), direct atmospheric deposition of suspended eolian sediment(Tanaka, 2000), coarse-grained ignimbrite deposits from pyroclasticdensity currents (Scott and Tanaka, 1982; Mandt et al., 2008),fine-grained distal ashfall from volcanic eruptions (Tanaka, 2000; Brad-ley et al., 2002; Hynek, 2003; Kerber et al., 2011; Ojha and Lewis, 2018;Ojha et al., 2018), and water-ice (Watters et al., 2007; Wilson et al.,2018). Several aspects of these proposed hypotheses have been criticallyassessed to arrive at the conclusion that the formation is most likelycomposed of volcanic ash, ignimbrites, or loess (see Mandt et al., 2008and reference therein for a review).

The MFF has a mean S and Cl wt% of 2.62� 0.24 and 0.65� 0.04respectively, which compared to the mid-latitudinal average is higher bya factor of 1.2x and 1.4x respectively. While S, Cl, and H2O show higher

, Cl (b), H2O (c), and the overlap of the contours for all the elements (d). Thes with significant enrichment of S, Cl, and H2O compared to the bulk-average ofeter (MOLA). (d) The overlap of the 1.5-ti contour for S, Cl, and H2O overlaid onellow ‘X’ marks the location of Apollinaris Patera.

Fig. 3. Correlation between GRS derived S and Cl for the Cl-enriched region. (a) The black dots show the real data from the GRS-derived data and the black line showsthe mean regression line for S and Cl in the Cl-enriched region. The slope of the line is a ratio that relates the wt% of sulfur to the wt% of chlorine within the Cl-enriched region. (b)–(d) Results from the bootstrap method showing the histogram for the slope, Y-intercept, and the correlation coefficient from 1000 differentresampled dataset.

Fig. 4. The overlap of the 1.5-ti contour for S (green), Cl (red), and H2O (blue)overlaid on top of the MOLA shaded relief. The black lines show the geologicalboundary of the MFF. Symbols are the same as Fig. 2(d).

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enrichment separately (Fig. 1), the spatially colocated enrichment isdistinct for the MFF (Fig. 4). If the MFF was only enriched in S, then avariety of processes could have been responsible for such enrichmentincluding oxidative weathering of S-bearing minerals, interaction withmagmatic or hydrothermal S-bearing gas, immiscible sulfide fluids, andsulfide minerals (King and McSween, 2005). Similarly, if the MFF wasonly enriched in Cl, then processes including chemical alteration throughhydrothermal activity and leaching of Cl by liquid water could have beenresponsible for the enrichment. The collective enrichment of major vol-canic gases (S, Cl, and H2O) in the MFF and plethora of previous worksthat have identified the MFF as a likely pyroclastic deposit (Tanaka,

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2000; Bradley et al., 2002; Hynek, 2003; Mandt et al., 2008; Kerber et al.,2011; Ojha and Lewis, 2018; Ojha et al., 2018) however suggest thesource of enrichment is most likely due to interaction with volcanicgases. We disfavor an aeolian origin for the bulk of the MFF because,despite its huge volume, the MFF is pervasively indurated, welded, orjointed on a regional scale. Ancient loess deposits on Earth seldomdisplay these properties regionally (Mandt et al., 2008). Water-ice isunstable at any depth near the equator of Mars (Schorghofer and Ahar-onson, 2005; Schorghofer, 2007), so while minor amount of ice may betrapped within the MFF (Watters et al., 2007; Wilson et al., 2018), thebulk of the MFF volume cannot be explained by the deposition ofwater-ice.

The volcanic origin of the S and Cl enrichment at the MFF is furthersupported by the observed S/Cl molar ratio at the MFF. The experi-mentally determined pressure dependent solubility of S and Cl in melts(e.g., Carroll and Webster, 1994; Webster et al., 1999) indicate that bothspecies should be present in low abundances in vapors produced duringearly degassing of ascending mafic magma (Aiuppa et al., 2004). How-ever, both may play a major role in shallow-depth degassing of basalticmelts (Gerlach and Graeber, 1985; Metrich et al., 1993), and significantsolubility contrast between the species makes S/Cl molar ratios a usefultracer in understanding the degassing processes and eruptive phenom-ena. For example, S is inferred to degas at low pressure from thesulfide-saturated basaltic magma of the mid-oceanic ridge basalt(MORB), whereas in oxidized, water-richmagmas sulfur exsolves into thevapor phase at high pressure (Spilliaert et al., 2006; Burton et al., 2007).Chlorine remains dissolved in the melt up to very shallow depths; hence,

Fig. 5. S/Cl molar ratio for the GRS mapped regions of Mars.

Fig. 6. Minimum and maximum estimates of sulfur that would have beendegassed into the Martian atmosphere as a function of how much S was retainedby the fine particles of the MFF. If the MFF retained 100% of the total exhaledgas then the mass of sulfur degassed into the atmosphere would equate to zero.

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gases emitted by explosive volcanic activity have higher S/Cl molar ra-tios than those emitted by persistent gas emission (e.g. Allard et al., 2005;Oppenheimer et al., 2006; Burton et al., 2007). Halogens only prevailover S (e.g. S/Cl and S/F gas ratios of <1) in the late stages of (residual)degassing, such as at lateral (secondary) vents and in the final phases ofeffusive eruptions (Aiuppa, 2002, 2009; Oppenheimer et al., 2011;Mather et al., 2012). Combining the range of S and Cl estimate of theMFF, we calculate an average molar S/Cl ratio of 4.1� 0.4 for the MFF(Fig. 5) which is consistent with the > 1 S/Cl molar ratios expected ofexplosive volcanic exhalation. It should be noted however that no regionon Mars has a molar S/Cl ratio of less than 3.5 suggesting that volcanicexhalation may have interacted with much of the Martian regolith(Fig. 5). While higher S/Cl ratios occur elsewhere, MFF remains signifi-cant for the underlying enrichment of both S and Cl. The high S/Clelsewhere may also reflect leaching of the more aqueously mobile Clphases or volatilization compared to S (Zhao et al., 2018). Our MFF ob-servations compare favorably with studies targeted at examining thevariability of the S/Cl ratio across the in-situ soil profile. For example, atGusev Crater and Meridiani Planum, S/Cl mass ratio can range from 1.8to 3.6 (i.e., 2–4M ratio) as often noted for surface observations to as highas 4–5 (4–6M ratio) even at ~15 cm shallow depths (Karunatillake et al.,2013).

We estimate the current mass of the MFF to be 2.5� 1018 kg based onthe gravity-derived mean density of 1765 kgm�3 (Ojha and Lewis,2018), and an estimated volume of 1.4� 106 km3 (Bradley et al., 2002).Given the mean S content of 2.62wt % for the MFF, the mass of S withinthe current deposit of the MFF is 6.5� 1016 kg, assuming compositionalhomogeneity at depth. If the S content of the MFF is approximately 30%of the total mass of the exhaled S over the time of the deposition of theMFF, then the mean amount of S injected into the atmosphere exceeds2.2� 1017 kg, equivalent to 4.4� 1017 kg of SO2. For reference, the meantotal mass of the present-day Martian atmosphere is ~2.5� 1016 kg. Thepercentage of S gas scavenged by ash may vary widely depending uponash composition, mineralogy, and average particle surface area, but 30%is among the highest values reported from leachate studies on terrestrialeruptions and so represents the most conservative estimate for total Syield from the MFF. If the fine particles of the MFF scavenged less than30% of the S, then this estimate could be higher by up to a factor of 5(Fig. 6). Further, the MFF has undergone significant erosion since itsemplacement (e.g. Dunning, 2019); similar calculation for the unknownoriginal volume of the MFF would yield an even higher amount of SO2injected into the atmosphere. Alternatively, if the enrichment of S islimited to the first few meters of the MFF (~1% of the volume of theMFF), then the mean mass of S within the current deposit of the MFFequates to ~7� 1014 kg. Assuming that this mass is approximately 30%

6

of the total mass of the exhaled S over the time of the deposition of theMFF, the mean amount of S injected into the atmosphere exceeds2� 1015 kg. Similarly, given the mean Cl content of 0.65wt % for theMFF and assuming that approximately 10% of the total mass of theexhaled Cl was scavanged by the MFF the mean amount of Cl injectedinto the atmosphere exceeds 1.63� 1017 kg.

Sulfur yields for ancient or unmonitored eruptions on Earth arecommonly estimated to a first order via a petrological approach.Assuming all erupted S gas was dissolved in basaltic melt prior to theeruption, one can use measured pre-eruptive S concentrations to calcu-late the total S yield, so long as the volume, density, and the crystalcontent of the erupted material are known.We also computed the mass ofthe exhaled S from the MFF following a similar approach. Given thepublished estimate for the range of S concentrations expected in pre-erupted Martian basalts (4000–7000 ppm; Gaillard and Scaillet, 2009),and the known volume of the MFF, we estimate that 2.8� 1013 to1.8� 1016 kg of S would have been delivered to the Martian atmosphereover the depositional timescale. This is an extreme minimum value for

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the S degassing from the MFF, since the petrologic method is known tocommonly underestimate total S yields, particularly for large explosiveeruptions, by a factor of 10–100 (Shinohara, 2008; Iacovino et al., 2016).This excess S is typically thought to exist as a pre-eruptive fluid phase(exsolved from magma) and so is not accounted for in dissolved S con-centrations. Including a pre-eruptive fluid phase for the MFF, ourpetrologic S yield estimate increases to 2.8� 1014 to 1.8� 1018 kg S. Thisrange is in a good agreement with our S yield estimate from ash scav-enged using S concentrations measured in the MFF.

Based on petrologic and geochemical modeling, we estimate that asignificant amount of S (>1017 kg) could have been delivered to the at-mosphere over the time it took for the MFF to be deposited on Mars. Wecan compare the mass of the S emitted from the MFF forming events tothe largest quaternary volcanic eruption on Earth, which was the rhyo-litic Toba Tuff (Mason et al., 2004). Toba erupted about 1016 kg of rockand emitted on the order of 1010–1012 kg of S (Oppenheimer, 2002;Chesner and Luhr, 2010; Costa et al., 2014). The S emitted from the MFFis up to 8 orders of magnitude higher by mass than the Toba Tuff. Theoutgassing of S from the Martian volcanoes could have played animportant atmospheric role on early Mars and could have affected thesurface chemistry. The timescale of the MFF’s eruption is not known. Theresidence time of SO2 in the Martian atmosphere heavily depends onatmospheric composition and redox, and estimates range from ~1 to100s of years (Settle, 1979; Levine and Summers, 2008; Johnson et al.,2009). Therefore, it is not clear what and how big the immediate climaticimpacts would have been from the volcanic exhalation of the MFF. If thebulk of the S was released in short bursts, then climatic conditionsenabling the existence of liquid water may have been maintained (e.g.Halevy et al., 2007). High sulfate contents can dramatically lower the pHof surface waters suppressing carbonate saturation in favor of sulfiteminerals, thereby explaining the lack of widespread carbonates on theMartian surface (e.g., Halevy and Schrag, 2009). However, it has alsobeen shown that the warming associated with atmospheric sulfur couldhave been offset by cooling from sulfate and sulfur aerosols in earlyMartian atmosphere (Tian et al., 2010).

4. Conclusion

We analyze the elemental composition of a large sedimentary depositof likely pyroclastic origin onMars called the Medusae Fossae Formation.Using GRS-derived chemical maps, we find sections of the MFF to besignificantly enriched in S, Cl, and H2O. We constrain the mass of theatmospherically injected S over the course of the MFF deposition usingreasonable terrestrial estimates. Assuming that the MFF scavenged, atmaximum, 30% of the exhaled S gas species, we find that a significantamount of S (>1017 kg) would have been delivered to the atmosphereover the time it took for the MFF to be deposited on Mars. If the MFF wasdeposited on a short timescale, then climatic conditions enabling theexistence of liquid water may have been maintained by this amount ofexhaled S gases.

Acknowledgments

LO developed the hypotheses, analyzed the data, and wrote the coreof the manuscript. SK contributed with chemical constraints, contextfrom GRS-derived data, statistical analysis methods, and geochemicalliterature review. KI performed petrologic calculations, terrestrial com-parison, and co-wrote sections of the manuscript regarding volcanicdegassing, leachate studies, and petrology. All three participated inwriting the manuscript and revisions. SK’s work is supported by NASA-MDAP Grant 80NSSC18K1375, research enhancement award (REA) bythe Louisiana Space Consortium via NNX15AH82H, and research awardprogram (RAP) grant LEQSF-EPS(2017)-RAP-22 from the LouisianaBoard of Regents. All data used here are freely available via the NASAPlanetary Data System (PDS) and cited papers. Reviews from PenelopeKing and an anonymous reviewer significant helped improve the paper.

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