Evolution of Water Reservoirs on Mars: Constraints from Hydrogen Isotopes in Martian Meteorites
Hiroyuki Kurokawa, Masahiko Sato, Masashi Ushioda, Takeshi Matsuyama, Ryota Moriwaki, James M. Dohm, Tomohiro Usui
aa r X i v : . [ a s t r o - ph . E P ] M a r Evolution of Water Reservoirs on Mars:Constraints from Hydrogen Isotopes in MartianMeteorites
H. Kurokawa a,b , M. Sato c,b , M. Ushioda b , T. Matsuyama b , R. Moriwaki b , J.M. Dohm d , T. Usui b a Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi464-8602, Japan b Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1Ookayama, Meguro, Tokyo 152-8551, Japan c Department of Environmental Changes, Kyushu University, 744 Motooka, Nishi-ku,Fukuoka 819-0395, Japan d Earth-Life-Science Institute, Tokyo Institute of Technology, 2-12-1-1E-1 Ookayama,Meguro-ku, Tokyo, 152-8550, Japan
Abstract
Martian surface morphology implies that Mars was once warm enough tomaintain persistent liquid water on its surface. While the high D/H ra-tios ( ∼ > −
99 mglobal equivalent layers, GEL) was more significant than in the rest of mar-tian history ( > −
53 m GEL). Combining our results with geological andgeomorphological evidence for ancient oceans, we propose that undetectedsubsurface water/ice ( ≃ − ≃ −
30 m GEL) on Mars.
Keywords:
Mars, meteorites, water reservoir, isotope, atmospheric escape
Email address: [email protected] (H. Kurokawa)
Preprint submitted to Earth and Planetary Science Letters September 17, 2018 . Introduction
Mars is generally considered to be a cold and dry planet, with relativelysmall amounts of water-ice observed at the polar caps (e.g., Jakosky & Phillips,2001; Christensen, 2006). On the contrary, a number of geological obser-vations, such as dense valley networks (Scott et al., 1995; Carr & Chuang,1997; Hoke et al., 2011) and deltas (Cabrol & Grin, 1999; Ori et al., 2000;Di Achille & Hynek, 2010), provide definitive evidence that large standingbodies of liquid water (i.e., oceans and lakes) existed in the early history,the presence of which would have profound implications for the early climateand habitability of Mars (e.g., Carr, 2007; Head et al., 1999; Dohm et al.,2011). The geological observations further include the detection of water-lainsediments and a variety of hydrous minerals (e.g., clays) (e.g., Fialips et al.,2005; Bibring et al., 2006) and evaporites (e.g. gypsum) (e.g., Osterloo et al.,2008) commonly formed by aqueous processes, implying Earth-like hydrologicactivities, with Noachian lakes and/or oceans. Despite such compelling ev-idence for hydrologic conditions that could support oceans and lakes, thereare, however, major gaps in our understanding of the evolution of surfacewater: e.g., what was the global inventory of martian surficial water/ice, andhow did it change through time.The global inventory of ancient surficial water has been estimated basedon the size of reported paleo-oceans (e.g., Head et al., 1999; Clifford & Parker,2001; Carr & Head, 2003; Orm¨o et al., 2004; Di Achille & Hynek, 2010). To-pographic features of putative paleo-shorelines suggest that large bodies ofstanding water once occupied the northern lowlands (Head et al., 1999).Shoreline-demarcation studies of the northern lowlands point to several con-tacts that yield variable sizes of paleo-oceans estimated to range from ∼ × km to 2 × km (corresponding to global equivalent layers (GEL) of130 m to 1 ,
500 m, respectively) (Carr & Head, 2003, and references therein).Though the shoreline demarcations (Parker et al., 1989, 1993), supported byMars Orbiter Laser Altimeter (MOLA) topography (Head et al., 1998, 1999),could not be confirmed using Mars Orbiter Camera (MOC) and ThermalEmission Imaging System (THEMIS) image data (Malin & Edgett, 1999,2001; Ghatan & Zimbelman, 2006), this variation has been interpreted toreflect the historical change in the ocean volume. For example, two majorcontacts (contact-1: Arabia shoreline and contact-2: Deuteronilus shoreline)2ndividually represent the larger Noachian and smaller Hesperian oceans, re-spectively (Parker et al., 1993; Clifford & Parker, 2001; Carr & Head, 2003).Although these geomorphologic studies have provided significant constraintson the history of martian paleo-oceans, they lack information about pre-Noachian (Frey, 2006) oceans because no geologic records are available. Fur-thermore, the shoreline-demarcation approaches would not be applicable tothe youngest Amazonian (3.1 Ga to present) era, during which the surface wa-ter would have occurred mostly as ice (Clifford & Parker, 2001; Carr & Head,2010).This study endeavors to trace the global inventory of surficial waterthrough time beginning with the embryonic stages of development of Mars(i.e., 4.5 Ga) to present day based on a geochemical approach of hydrogenisotopes (D/H: deuterium/hydrogen). Hydrogen is a major component ofwater (H O) and its isotopes fractionate significantly during hydrological cy-cling between the atmosphere, surface water, and ground and polar cap ices.Telescopic studies have reported that the hemispheric mean of the martianatmosphere has a D/H ratio of 6 times ( δ D ≃ δ D = [(D / H) sample / (D / H) reference − × < . . . . . . ≃ −
2. Calculation
The amount of water loss due to the atmospheric escape between time t and t can be calculated from an assumed amount of present water reservoirusing the following equations: L t − t = R t − R t = R t × (cid:20)(cid:18) I t I t (cid:19) − f − (cid:21) , (1)and f = d[D] / [D]d[H] / [H] . (2)Here L t − t is the amount of water loss due to the atmospheric escape duringthe time from t to t , R and I are an amount of water reservoir and a D/Hratio at each time, respectively, f is the fractionation factor, and [H] and [D]are the abundances of H and D in the combined reservoirs in atoms cm − (Lammer et al., 2003). Both the volumes of water reservoir and water lossare expressed in ocean depth [m] as a global equivalent layer (GEL). Usingthe density of water of 10 kg m − and the surface area of 1 . × m , 1 mGEL corresponds to 1 . × kg of water. Eq. 1 can be rewritten as: R t R t = (cid:18) I t I t (cid:19) − f , (3)which gives a ratio of the amount of water for t and t . Eq. 3 is used inSections 3.2, 3.3, and 3.4 to discuss the evolution of the amount of waterthrough time. 4e employ the fractionation factor f of 0 . f = 0 .
9, Pineau et al., 1998), we consider the D/H frac-tionation to be solely due to atmospheric escape. We calculate surface waterloss in two stages: Stage-1 (4 . − . . . ≃ . I at the boundary con-ditions (4 . . δ D value of 275 for the4 . ∼ . δ D range (1200 − . δ D value (5000) of the present martian water reservoir is obtained fromD/H analyses of geochemically enriched shergottites (Shergotty and LAR06319) that crystallized near the surface in the recent past (0 . − .
18 Ga,Greenwood et al., 2008; Usui et al., 2012). This high δ D value is consistentwith those of water in the present martian atmosphere ( ≃ Curiosity roverobservations (Webster et al., 2013).D/H ratios of martian meteorites reflect complex geologic histories in-cluding the terrestrial weathering after the meteorite falls. This study em-ploys D/H datasets obtained only from recent in situ ion microprobe mea-surements to minimize the effect of terrestrial contamination. The ALH84001 carbonates formed by ancient aqueous activity possess surficial wa-ter components, whereas the D/H ratios of magmatic phosphates (apatites)are interpreted as representing mixing of “magmatic” and “surficial” wa-ter components (Boctor et al., 2003). However, as D/H ratios of apatitesin geochemically enriched shergottites are indistinguishable from that of the5urrent martian atmosphere, they are interpreted as representing a D/H ra-tio of near-surface water that is exchanged with the atmosphere (previouslycalled the exchangeable reservoir) (Greenwood et al., 2008). The D/H ratiosof apatites employed in this study approximate the surficial water D/H ratioswhen the host meteorites crystallized near the surface.
3. Results and Discussions
Using Eq. 1, the estimated water loss in Stage-1 ( L . − . ) and Stage-2 ( L . − ) is obtained as a function of the amount of the present waterreservoir R present (Fig. 2). The ranges of L . − . and L . − at a given R present (i.e., width of the stripes in Fig. 2) reflect the range of the D/Hratios at 4 . − L . − . and L . − are positively correlated with R present .Our model further indicates that a ratio of water loss between the stages( L . − . /L . − ) is independent of R present and that L . − . is alwaysgreater than L . − at any R present . Dividing Eq. 1 for Stage-2 by that forStage-1, the ratio can be written as, L . − . L . − = (cid:16) I I . (cid:17) − f h(cid:16) I . I . (cid:17) − f − i(cid:16) I I . (cid:17) − f − , (4)where f and f are the fractionation factors in both Stage-1 and -2. As-suming the same fractionation factor f for both Stage-1 and -2, the ratio ofwater loss is given by L . − . L . − = I − − f . − I − − f . I − − f . − I − − f . (5)This equation shows that the ratio of water loss is determined only from theD/H ratios and the fractionation factor in Stage-1 and -2. Because the δ Dvalue of 1200 − . L . − . /L . − given by Eq. 5 is ≃ . − .
5. Thisindicates that the water loss is more significant in Stage-1 when comparedto Stage-2. As the period of Stage-1 (0 . ≃
10 times shorter thanStage-2 (4 . .2. Minimum estimate of water loss The amount of the “observable” current surface water reservoir is dom-inated by the polar layered deposits (PLD). Assuming that the PLDs aremainly composed of water-ice, they are expected to contain H O of 1 . − . × km in the North polar region (Zuber et al., 1998) and 1 . × km in the South polar region (Plaut et al., 2007), respectively; their total sum(2 . − . × km ) corresponds to 20 −
30 m GEL. We employ this value(20 −
30 m) as the minimum estimate for the amount of present waterreservoir R present , because the existence of “missing” water-ice reservoirs hasbeen proposed (e.g., Carr & Head, 2003). For example, ice-rich mantles andcovering sediments in the mid-latitude possibly contain a large amount ofice (Murray et al., 2005; Page, 2007; Christensen, 2006; Page et al., 2009).Furthermore, there is increasing evidence that vast reservoirs of water-icepotentially exist in parts of the high latitudes, as indicated by geomor-phology (Baker, 2001; Kargel, 2004; Soare et al., 2007, 2011, 2012, 2013a,b;Smith et al., 2009; Lefort et al., 2009; Levy et al., 2009a,b, 2011), in situ analysis through the Phoenix Lander (Smith et al., 2009), and the MarsOdyssey Gamma Ray Spectrometer (Boynton et al., 2002, 2007).As there is a positive correlation between the amount of present waterreservoir and the water loss (Fig. 2), the minimum R present of 20 −
30 m GELyields the minimum estimate of the water loss in each stage (Fig. 2). Theminimum water loss in Stage-1 ( L . − . ) and -2 ( L . − ) are calculatedto be 41 −
99 m GEL and 10 −
53 m GEL, respectively (Table 2). The sumof these values ( R present , L . − , L . − . ) yields the minimum estimatesfor the amounts of martian water reservoirs of 30 −
83 m GEL at 4 . −
150 m at 4 . −
30 m) and the uncertainty of the D/H ratio at 4 . − . − . × molecules s − in Stage-1 and0 . − . × molecules s − in Stage-2, respectively.We employ the fractionation factor f of 0 . f of 0 .
016 employedin this study is likely to be minimum (i.e., largest fractionation), our modelyields the minimum estimate of global water loss. Even if it is granted, ourmain conclusion, more water loss in Stage-1 than Stage-2, would not change,because f is thought to be greater in the older Stage-1 than in the youngerStage-2 due to the hotter exobase condition in Stage-1 than in Stage-2.Our model does not take into account the effects of water supply bymagmatic outgassing and by late accretion of water-bearing bodies such asasteroids and comets. Prolonged igneous activities would have delivered mag-matic water from the martian interior to the surface. Because such magmaticwater is assumed to have unfractionated primordial hydrogen isotopic compo-sitions (e.g., ≃ . . −
230 to +340, Alexander et al., 2012) and Oort-cloud comets ( ∼ kg comets (corresponding to ∼
100 m GEL) with a D/H ra-tio of 1000 increases the D/H ratio of surface water reservoir by ∼ kg comets with a probable Xe / H O ratio of ∼ − (Swindle, 2012) results in a supply of 10 kg Xe, which is 10 times largerthan the martian atmospheric Xe. Since it is almost impossible for such alarge amount of Xe to escape during 4 Gyrs, such a significant supply ofcomets is unlikely. 8 .3. Comparison with geological records Our model requires an amount of surface water at a specific age to calcu-late amounts of surface water at other ages for which the meteorite data areavailable. In the previous section, we have reported the minimum volumesfor the surface waters at 4 . . R present ) of 20 −
30 m GEL. However, as noted ear-lier, PLD represents the minimum estimate for the present water inventoryand the existence of “undetected” current water-ice reservoirs has been pro-posed. Thus, through utilizing geological estimates for the volume of ancientoceans, this section examines the transition of volumes of Mars ocean from4 . ≃ . . ≃ . . − . ≃ . ≃ − −
30 m GEL) (Fig. 4). Such missing water-ice reservoirs might be mid-latitudeice mantles and ice-rich sediments whose amounts are not well constrained, orunder-ground ice implied by
Mars Express ’s dielectric mapping observations(Mouginot et al., 2012). Note that an inefficient fractionation (larger f ) canalso explain the great water loss without a large D/H fractionation, thoughit requires an unrealistic atmospheric condition or other escape mechanismssuch as impact erosion.The comparison also suggests that the initial water amount at 4 . ∼ − m GEL, which corresponds to ∼ − − − wt % of total Mars mass. Formation theories of terrestrial planetssuggest a wide range of the water mass fraction of bulk Mars because theinitial condition is unknown and the formation process involves stochasticproperties. Lunine et al. (2003) calculated the accretion of Mars, assumingthat planetary embryos initially range in a wide position of orbital radii.They obtained the water mass fraction of Mars to be 0 . − .
063 wt %.More recently, in the framework of the “Grand Tack” scenario (Walsh et al.,2011) in which planetary embryos range in a more compact position, Brasser(2013) estimated the water mass fraction of Mars to be 0 . − . ∼ − − − wt % of the Marsmass) is consistent with these theoretical predictions.10 .4. Constraints for oxygen sinks The “bottleneckh to restrict the water loss is remaining oxygen as a resultof the hydrogen escape. As oxygen escapes mainly due to the interaction ofthe uppermost atmosphere with solar wind, the amount of oxygen escape isinfluenced by the presence and intensity of martian magnetic field. Fig. 3and Table 2 compare our minimum estimates for water loss in Stages-1 and-2 with that estimated based on oxygen escape models. An oxygen escapemodel by Lammer et al. (2003) proposes the water loss of 24 −
58 m GELduring the recent 3 . ≃ −
133 m GEL withoutthe magnetic protection in the pre-Noachian period. Our minimum estimatesof water escape ( L . − . = 41 −
99 m GEL and L . − . = 10 −
53 m GEL)are consistent with these oxygen escape models. This consistency indicatesthat no- or weak- dynamo would have contributed to the significant waterloss during pre-Noachian.Calculated water loss based on any geological estimates of paleo-oceansis distinctly greater than our estimate based on the PLD. In one instance,this study employs the Late Hesperian VBF ocean (Carr & Head, 2003) tocalculate the amount of water loss. Because it should have existed in a pe-riod within Stage-2 (4 . δ D value between 1200 (the lowest end at 4 . and S − in the fresh basalts are oxidized to Fe and S and that the density ratio of rock to water as 3 : 1, 14 . and MgSO , 0 .
99 m of the fresh basaltsis required to consume oxygen in 1 m GEL of water. Such mass balancecalculations suggest that the surface oxidation process appears insufficientto account for the greater water loss (up to 520 m GEL, Table 2) estimatedbased on the VBF ocean, because at least the equivalent volume of basalticcrust (i.e. ≃
500 m GEL) should be required. This implies the existence ofunknown oxygen escape mechanisms or unrevealed oxygen sinks.
4. Conclusion
Geological and geomorphological studies have revealed that Mars oncecontained large amounts of liquid water on its surface. We estimate theamount of water loss due to atmospheric escape in two stages based on theD/H data of martian meteorites. We demonstrate that the amount of waterloss is positively correlated with the present water inventory and that thewater loss during 4 . . cknowledgments We acknowledge T. Ikoma and H. Genda for fruitful discussions. Thiswork was supported by the program for the “Global Center of Excellence forthe 21st Century in Japan” to Department of Earth and Planetary Sciences,Tokyo Institute of technology, and by “2013 Tokyo Institute of TechnologyChallenge Research Award” and by a NASA Mars Fundamental ResearchProgram grant to TU. HK is supported by Grants-in-Aid from the Ministryof Education, Culture, Sports, Science and Technology (MEXT) of Japan(23244027). 13 tage Period Estimated absolute age range [Ga]21 AmazonianHesperianNoachianPre-Noachian 3.3-2.9 to present3.7-3.5 to 3.3-2.94.2 to 3.7-3.54.5 to 4.2 boundary at 4.1 Ga
Table 1: Geologic-mapping-based, time-stratigraphic information, including theNoachian, Hesperian, and Amazonian Periods (Scott & Carr , 1978). The geologic peri-ods have been given estimated absolute age ranges based on impact crater models (Table1 is modified from Hartmann & Neukum (2001)). Note that we performed comparativeanalyses among the estimated water amounts of the pre-Noachian, approximately referredto here as Stage-1, and Noachian-Amazonian, approximately referred to as Stage-2, basedon recent D/H dataset from martian meteorites. ime4.5 Ga 4.1 Ga Present δD 275‰ δD 1200-3000‰ δD 5000‰ H, H , D, HD H, H , D, HD f f L L R present R present R present L L L escape escapeStage-1 Stage-2 water reservoir water reservoir water reservoir Figure 1: Schematic illustration of the two-stage model for the evolution of the global sur-face water reservoir on Mars. R present is the size of the present water reservoir, L . − . and L . − are the water loss during Stage-1 and -2, and f is the fractionation factor(see text). W a t e r l o ss [ m ] Present water reservoir [m]20 30 L L Minimum surface water inventory
Figure 2: Water loss during Stage-1 ( L . − . , blue) and Stage-2 ( L . − , red) asa function of present water reservoir R present . The width of the blue and red stripes isderived from the δ D range (1200 − a t e r r e s e r v o i r [ m G E L ] O xy gen e sc ape [ m G E L ] Time [Ga]
Terada et al. (2009) Lammer et al. (2003)
Time [Ga] B k m [ n T ] Figure 3: Upper diagram: evolution of martian water reservoir estimated from theminimum amount of the present water reservoir in PLD (blue). The error bars are derivedfrom the uncertainty of the amount of present water reservoir R present (20 −
30 m) andthe δ D value (1200 − −
34 mGEL) and Terada et al. (2009) (18 −
78 m GEL). Lower diagram: magnetic field observedover large basins are plotted as function of N(300) crater age of Frey (2008) (modifiedafter Lillis et al. (2008)). B is magnetic field magnitude at 185 km altitude abovethe martian datum.
10 100 1000 100004.5 4 3.5 3 2.5 2 1.5 1 0.5 0
Head et al. (1999)Carr & Head (2003)Di Achille & Hynek (2010)Carr & Head (2003) Carr & Head (2003)Head et al. (1999) W a t e r r e s e r v o i r [ m G E L ] Time [Ga]
Minimum estimate PLDTotal present water~ 10 - 10 m GEL ?
500 m200 m100 m50 m30 m20 m
Figure 4: Evolution of water reservoirs for different amounts of present water reservoirs(black lines; 20 , , , , , and 500 m GEL) and geological estimates of wateramount: Contact-1 and Arabia shoreline (dark blue), Noachian ocean based on deltaand valleys (light blue), Late-Hesperian ocean based on VBF (orange), and Contact-2and Deuteronilus shoreline (red). The gray area indicates the evolution of surface waterreservoir calculated based on the minimum present water reservoir (20 −
30 m GEL)estimated from PLD. We assume δ D = 3000 at 4 . able 2: Estimated water loss during Stage-1 and -2. Estimates based on oxygen escapecalculations are from 1: Terada et al. (2009) (18 −
78 m GEL in the original paper withdifferent conversion) and 2: Lammer et al. (2003) (14 −
34 m GEL in the original paperwith different conversion). See text for details.
Method Stage-1 Stage-2Based on PLD 41 −
99 m GEL 10 −
53 m GELBased on Carr & Head (2003) 53 −
280 m GEL 120 −
520 m GELOxygen escape 31 −
133 m GEL −
58 m GEL Terada et al. (2009). Originally 18 −
78 m GEL with a different conversion in whicha 1 m GEL ocean contains 8 × water molecules (= 2 . × kg). Lammer et al. (2003). Originally 14 −
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