Mars: internal composition. On the hydrogen in the Martian core Schmidt Institute of Physics of the Earth Russian Academy of Sciences N.Zharkov V.N.Zharkov

1. Cosmochemical model of Mars 2. Cosmogonical aspects of the problem 3. Dynamical high pressure data 4. Experimental data on the system Fe-S-H 5. An estimate for the hydrogen content in the Martian core 6. Conclusion

Cosmochemical model of Mars The idea of the origin of the terrestrial planets from planets from planetesimals oxidized to varying degrees was discussed by Dreibus and Wänke (1989). The composition of the Earth and Mars is considered to be a certain mixture of component A and B. Component A. This substance is highly reduced. Protobodies consisting of the A component filled the feeding zone of the Earth. Component B. This substance is highly oxidized and contains all elements, including volatiles, with abundances like those in meteorites of class C1. Component B constituted the protobodies in the zone of the contemporary asteroidal belt. Dreibus and Wänke concluded that components A and B are mixed in Mars in the ratio 60:40, whereas the ratio 85:15 was reported for the Earth. It was inferred that the accretion of Mars was almost homogeneous in contrast to the chemically inhomogeneous accretion of the Earth.

Cosmogonical aspects of the problem The growing Proto Mars, in the stage of accretion, is hited by protobodies with parabolic velocities protobody G is the gravitational constant M PM is the mass of the growing Proto Mars r PM is the radius of growing Proto Mars M PM r PM vpvp

Dynamical high pressure data T.J. Ahrens, J.D. O’Keefe and M.A. Lange (1989) studied in laboratory conditions the question as to what are the shock pressures at which the minerals containing volatiles (CO 2, H 2 O and SO 2 ) begin to loose them. They found that consolidated minerals start losing volatiles at shock pressures within the range GPa Such shock pressures are achievable when the impactor has a velocity of ~2-3 km/s, colliding with a target of the same material. Consequently, the dehydratation of planetesimals that belong to component B(C1) begins when the radius of growing proto-Mars is r~0.4R and attains 75% at r = R (R being the radius of Mars) ‏

Experimental data on the system Fe-S-H We assume that in the Martian core: 180 ≤ p ≤ 450 kbar, 1500 ≤ T ≤ 3000K The phase FeS IV corresponds to the conditions of the Martian core. The equation of state in the Birch-Murnaghan form, which allows one to calculate the density of the Martian core composed of pure FeSIV at T=2000K (Fei et al., 1995) ‏ p(ρ,T)=p(ρ,800K)+αK T (T-800), f=1/2[(ρ/ ρ 0 ) 2/3 -1] p(ρ,800K)=3f(1+2f) 5/2 K T [1+3/2(K’ T -4)f], (1) ‏ where ρ 0 =4.94±0.05 g/cm 3 - the density at zero pressure, K T =54±6GPa, K’ T =( ∂ K T / ∂ p)=4, αK T =3.7x10 -3 GPa/K, α - the coefficient of thermal expansion. Fig.1 The phase diagram of FeS (Fei et al., 1995) ‏

The melting points are shown for iron and FeS. Figures present the liquidus and solidus curve for the model of the Martian core with a sulfur content of 14.5wt% Fig. 2. (a) Measured melting temperatures of Fe and FeS up to 50 GPa (Williams and Jeanloz, 1990). Vertical bar indicates experimental errors. (b) Calculated liquidus and solidus curves for the Martian-core composition in model DW (Longhi et al., 1992)

Fig. 3 The solubility of hydrogen in γ-Fe as a function of the temperature and of the hydrogen pressure (Sugimoto and Fukai, 1992). The solubility is seen to grow rapidly with the pressure of hydrogen. At pressures of ~7GPa and temperatures distinctly smoler than those in the Martian core, the concentration of hydrogen in γ-Fe may be as high as unity.

Fig. 4. The p-T phase diagram for iron (Akimoto et al., 1987) and the p H2 -T phase diagram for the system Fe-H (Ponyatovsky et al., 1984; Antonov et al., 1982; Suzuki et al., 1984). One’s attention is attracted by the fact the dissolution of hydrogen in iron considerably reduces the melting point of pure iron and causes the contraction of the stability field of γ-Fe and, therefore, the expansion of the stability field of the ε-Fe phase.

The compressibility of FeH is described by the Vinet equation P=3K T0 (ρ/ρ 0 ) 2/3 [1-(ρ 0 /ρ) 1/3 ] exp{3/2(K’ T0 -1) [1-(ρ 0 /ρ) 1/3 ]} (2) In which the zero- isotherm values of the parameters are ρ 0 = 6.7 g/cm 3, K T0 =121±19 GPa, K’ T0 =5.31±0.9 To assess the isotherm for FeH at T=2000K, we added the value of the thermal pressure, taken for estimates to be equal to 10GPa, to the zero isotherm, as it was done for the pure- iron isotherms calculated from equation (1). For example, adding (FeH) 0.2 to (ε- Fe 0.8 ) reduced the density by about 0.32 g/cm 3. The hydrogen is brought into the iron component of the forming planet through the reactions Fe + H 2 O → FeO + H 2 Fe + ( x /2)H 2 → FeH x Fig.5 Isotherms T=2000K of(1)  -Fe, (2) FeH And their mixtures with molecular concentrations x FeH =0.1, 0.2, 0.5, 0.7 (dashed lines).

AN ESTIMATE FOR THE HYDROGEN CONTENT IN THE MARTIAN CORE The oxidized component B is assumed to have a composition equivalent to that of carbonaceous chondrites of class C1. The water content attains in such bodies ~ 18 – 22 wt% (Loders, Fegley, 1998). To estimate the mass of H 2 that can be buried in the interior of growing proto- Mars, we put r≈0.5R and ρ≈3.5 g/cm 3. Then one easily obtains ~ (6-7.2)∙10 23 g of hydrogen. The mass of the iron-nickel (Fe 0.9 N 0.1 ) core comprises in model DW ~ 1.2x10 26 g. Therefore, if all the buried hydrogen ultimately becomes a constituent of the Martian core, we obtain the composition (Fe 0.9 Ni 0.1 ) H It is the lover bound for the hydrogen content in the Martian core. The maximum amount of hydrogen that could be produced in the process of the accumulation of Mars is ~ (5 – 6)10 24 g of H 2 Thus, the upper bound, not achievable in practice, is N Fe /N H =1.2x10 26 /5.6x(5-6)x10 24 ~ => FeH The isotherms T = 2000K of ε - Fe, FeH and their mixtures containing the molecular concentrations of FeH х=0.1, 0.2, 0.5 and 0.7 are shown in Fig 5. It can be seen from this figure that addition of the molecular concentration х= 0.1 of FeH to the iron core of Mars reduces its density by about 0.16g/cm 3.

Composition : Fe with 14.2 wt % S, 7.6 wt % Ni (Dreibus and Wänke, 1985). In this study : Fe-Ni, S, hydrogen. new high P-T measurements of the density of Fe (  -Fe) and FeS (Kavner et al., 2001) CORE 10 mol % of hydrogen →  = g/cm 3 (Zharkov, 1996) ‏ If the core is liquid →   =  0.3 g/cm 3

If there is no hydrogen in the core, the Fe/Si ratio ranges from 1.35 to 1.38, and Fe# ranges from 0.24 to 0.20, respectively. The presence of hydrogen leads to the increase of the Fe/Si ratio and the decrease of Fe# in the mantle due to the increase of the core radius. The core radius as a function of the Martian mantle Fe#: 14 wt % S (according to the DW model) ‏ (according to the DW model) ‏ hydrogen mol % hydrogen mol % 50-km-thick crust. To satisfy the bulk chondritic ratio, more than 50 mol % of hydrogen must be incorporated into the core.

Interior structure model of Mars (in collaboration with T.Gudkova)‏ The varied parameters: ferric number of the mantle (Fe#) ‏ sulfur content in the core hydrogen content in the core Distributions of density , pressure P, temperature T, compressional and shear velocities as a function of radius Fe/Si  1.7