It is reported that the SNC meteorites are a factor of 2 richer in moderately volatile elements (Na, K, P, etc.) and some volatile elements (Cl, Br, I, S) compared to the Earth and Mars. It is proposed that Mars accreted homogeneously, with the result that all the water except trace amounts (on reaction with metallic Fe) was converted to H(sub 2) and escaped. This model predicts a total Mars H(sub 2)O inventory of 18 ppm (equivalent to an ocean 80 m deep on the planet), and a value for Earth of 300 ppm, close to the actual abundance

Dreibus, G.

Waenke, H.

English

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WATER ON MARS H. W_nke and G. Dreibus, Max-Planck-lnstitut fiir
Chemie, Saarstrasse 23, D-6500 Mainz, F.R.Germany.
With the assumption that the SNC-meteorites are indeed Martian rocks,
these meteorites allow very definite conclusions about the bulk composition of
Mars. Figs. 1 and 2 show correlations of the moderately volatile K and of the
volatile Br with the refractory element La. As to a first approxi_nation one
can assume C 1-abundances for the refractory elements for all terrestrial
planets, these correlations clearly indicate higher abundances of moderately
volatile and volatile elements on Mars as compared to the Earth. In the case
of Br, the difference _nay actually be smaller as on the Earth the oceans re-
present such an ideal reservoir for Cl and Br that today the major portions of
the whole terrestrial inventory of Cl and Br reside in the oceans.
According to the two component model for the formation of terrestrial
planets as introduced by Ringwood (1,2) and slightly modified by ',V_nke(3),
the composition of these planets can successfully be described as mixtures of:
Component A: Highly reduced and free of all elements with equal or higher vol-
atility than Na, but containing all other elements in C 1-abundance ratios.Fe
and all siderophile elements and even Si partly in metallic form. And
Component B: Oxidized and containing all elements - including the volatiles -
in C l-abundances. Fe, all siderophile and lithophile elements as oxides.
From the analytical data of SNC-_eteorites (4,5), Dreibus and W_nke (6,7)
concluded that the mixing ratio of component A/component B for Mars is 60:40,
compared to a ratio of 85:15 for the Earth. They further concluded that Mars
accreted almost homogeneously contrary to the inhomogeneous accretion of the
Earth, as proposed by Jagoutz et al. (8) and in more detail by W_nke (3).
On Mars because of the homogeneous accretion almost all H20 added (from
component B) reacted with meta]lic Fe (and, of course, with other metallic
phases like Si of component A) and was reduced to H2 which escaped. The huge
quantities of H2 will not only have greatly accelerated the extraction of gas-
eous species from the interior of the planet but also have furthered the re-
moval of these species from the gravity field of the planet due to hydrodynam-
ic escape by keeping the rqean ,nolecular weight of the atmosphere low. Hence,
in their esti_qate of the amount of H20 left on Mars after accretion, Dreibus
10000
1000
100
I0
ppm K e_" _//
Shergotty parent body //IFm/_- •
ct •
I _ _ o Moon/ Y./ Y_
- -- t// /'-'-Moon "sPE_;tgo!
_o.°__// - .oon t,:._k_,..
chassignites)/ _Eucrffe parent body cnass,gn,tes/
/J 'EPB' ppm Lo
, , ,i .... i I J , , ,I
01 1 10 100
Fig. 1 _ig. 2
Morgan and Anders, 1979 (19); Morgan
and Anders, !980 (20); '_nke, 1981 (3)
1000
100
10
I , ' ' ' I ' ,/ ' I
ppb Br /
/ o
Shergotty ,/0 z_Earth's
crust * ocean/
EETA 79001B / 0
EETA 79001A /10
• MARS as SPB \_ °'_
7AfHoA5/_QO MEoAK;TU and QZ)O_ C)_q_
/ Anders 1980
/
r-I EARTH
Wanke 1981
I
0.1
MARS • Morgan and
Anders 1979 ppm Lo
, , , , i , , , , I
1.0 I0
https://ntrs.nasa.gov/search.jsp?R=19890001451 2020-03-20T06:07:06+00:00Z
]]8 WATER ON MARS ORIG'NAL PAGE 1.3
OF POOR QUALITY
W_nke, H. and Dreibus, G.
and W_nke (7) assumed that at the end of accretion the concentration of H20
and other volatiles at the surface of the planet was zero. From the Br/La con-
centration (Fig. 2), they could calculate the absolute amounts of CI, 3r and I
on Mars using C i-abundance ratios of CI/Br and I/Br. In this way, they found
for Mars an absolute C1 concentration of 34 ppm. Assuming the c_,Iposition of
component B to be identical to that of C l-meteorites, 320 ppm C1 were actual-
ly added to the planet, of which all except the 34 ppm was lost during the
accretion process together with H2 and other gases, yielding a depletion fac-
tor for C1 of 0. II.
During accretion even the relatively small Mars was heated to tempera-
tures which probably led to partial melting of the silicates (9). Nevertheless
we have to expect that the solid or liquid phases of the planets' interior
were in solubility equilibrium with the t_nporary atmosphere during the accre-
tion process. The solubility of HCI in basaltic melts exceeds that of H20 by
about a f_ctor of 200 (I0), yielding for H20 a depletion factor of 0.11:200 =
5.5 x 10-q. C l-chondrites contain about 20% H2O, however, the preterrestrial
!120 content is only 4.5% (II), the rest is mainly terrestrial contamination.
Hence, in order to find the composition of component B which we assume to be
identical to C l-chondrites, we devide the C i-values by 0.85. During accre-
tion the hydrogen of the hydrocarbons of the C l-material will at an interme-
diate stage also be present in form of H20. For the following, we will there-
fore use a C l-concentration of 7.1% H20 as deduced from the H abundance of
7900 ppm (12). With this value we find that 3.3 % H20 was added to Hars during
accretion. Together With the depletion factor of 5.5 x 10-4 , we find that a
total of 18 ppm !12,3 was retained in the interior of the planet corresponding
to 23 ppm H20 in the mantle. Making the unrealistic assumption of a 100% re-
lease, this would yield a water layer of 80 fn covering the whole planet.
From SNC-meteorites a value of 2_7 ppm K for bulk Mars is obtained (5,7),
leading to a total of 1840 x lO-dcm3/g 4OAr produced in 4.5 x I09 years The
observed amount of 4OAr in the Martian abnosphere corresponds to 48x]O-dcm3/g.
Hence, if we neglect a possible loss of _UAr from the atmosnhere into space,
we find a release factor for 4OAr of 0.026. Assuming the release factor of H20
to be identical to that of 4OAr, we derive at a water layer of ?.i m or a
total of 0.3 x 106 km3. The low 4OAr concentration in the Hartian atmosphere
leading to the very low release factor may be in part due to rewaoval of CUAr
from the atmosphere by energetic impacts in geologic time (13). Hence the
release factor would in fact be higher than 0.026 and consequently the amount
of water at the surface of Mars would become higher, too.
The assumptions s,lade above may not be as unrealistic as they ,night sound
at first sight. Because of the even smaller C1 abundance on Earth compared to
liars only a small fraction of tile total H20 of the Earth could be explained in
this way. However, the Earth probably accreted inhomogeneously. According to
the _qodel of W_nke (3), the accretion of the Earth started with the highly
reduced and volatile-free material of component A. After the Earth had reached
about 60% of its present mass more and _._ore of the oxidized c_nponent B was
added. However small amounts of metal still present were responsible for the
extraction of highly siderophile elements (Ir, etc.).Towards the very end of
accretion metal became unstable, hence, even the highly siderophiles reiaai=led
in the mantle in their C i-abundance ratios. The concentration of __.8 ppb Ir
in the Earth's mantle equal to 1.9 ppb Ir for tile whole Earth, corresponds to
0.39% C 1-material. With a C I H20 content of 7.1%, we find that 280 ppm !120
were added to the Earth after metallic iron respectively !Ji became unstable.
These 280 ppm remained in form of water while oF the much larger amounts of
the water (,_ I%) added before metal became unstable almost all was converted
WATER ON MARS .... '.i: _.._ _ ,;,.'-T_ _,_, 119
Of POOR QUALFrf
W_nke, H. and Dreibus, G.
to H2. Hence, we would expect a total H20 content of the Earth of about 300
ppm. The contrioution of the crust (mainly the oceans) to the bulk Earth H20
content is already close to 300 ppm so that very little would remain for the
contribution of the mantle. However it seems likely that this contribution is
indeed much smaller as generally assumed. Of the total halogen inventory of
the Earth more than 90% were found to be concentrated in the crust. A similar
distribution is expected for water.
The large amounts of H2 generated during accretion may have removed most
of the prihlordial rare gases delivered by c_ponent B so that even these gases
were contributed predominantly by the 0.39% C 1-material added at the very end
of accretion. The 0.39% C 1-material would bring 0.29 x lo-lOcm3/g 132Xe to
the bulk Earth compared to 0.16 x lo-lOcm3/g 132Xe present in the atmosphere.
Most of the Earth's 132Xe inventory is certainly in the atmosphere, neverthe-
less a surprisingly good agreement, considering the large uncertainties in our
exercise. In the case of 36Ar only 16% of the 210 x lO-lOcm3/g 36Ar in the
Earth atmosphere could be derived by 0.39% C l-material. However we have the
strong suspicion that in respect to rare gases the C 1-chondrites of today do
not represent the material actually added at the accretion stage of the ter-
restrial planets. As about 90% of the radiogenic 4OAr produced during the 4.5
x 109 yrs was obviously lost from C 1-chondrites by diffusion a similar loss
of 36Ar may be inferred. Preferential diffusion loss of Ne, Ar, and Kr rela-
tive to Xe would also explain the striking difference of Kr/Xe-ratios in Earth
and liars as conpared to that in C 1-chondrites (Fig. 3). In this way, the "Xe
problem" might be solved (14-18).
Ref.: (1) Ringwood A.E. (1977) Geochem. J. 11, 111. (2) Ringwood A.E. (1979)
On the origin of the Earth and t,loon,Springer, N.Y. (3) W_nke H. (1981) Phil.
Trans.R.Soc. Lond. A303, 287. (4) Burghele A. et al. (1983) LPS-XIV, 80. (5)
Laul J.C. et al. (T9-_-_)LPS-XVI, Supp. A, 17. (6) Dreibus G. and W_nke H.
(1984) 27th In. Geol. Con. Vol.l__l,VNU Sci. Press, 1. (7) Dreibus G. and W_nke
H. (1985) Meteoritics 20, 367.(8) Jagoutz
lO-S
lO
10-'/
O)
U
m-,
r"
:3
J: _-m,<
10 "12
solar solar values normalized
to CI values at _Ar
erlus
%%
_'_)- ..... -'0
Earth _,,%_
%
""'0"
I I I I
ZONe_Ar e_Kr 132Xe
E. et al. (1979) LP_--1Oth, 2031. (g)
Kaula W.M. (1979) JGR 84, 999. (10)
Anderson A.T. (1975) Rev. Geophys. Space
Phys. 13, 37. (11) Kaplan I.R. (1971) in
Handboo-k- of Elemental Abundances in
Meteorites. B. Mason (ed), 21, Gordon.
(12) Anders E. and Owen T. (1977) Science
198, 453. (13) Watkins H. and Lewis J.S.
_B4) Workshop on Water on Hars, LPl
Tech, Rep. 85-03, p. 93. (14) Canalas R.
A. et al. (1968) JGR 73, 3331. (15)
Vanale F.P. and Cannon W.--A. (1971) EPSL
I I, 362. (15) Bernatowicz T.J. and Podo-
sek F.A. (1978) in Terr. Rare Gases, E.C.
Alexander _ ft. Ozima (eds) 99. (17) Podo-
sek F.A. et al. (1980) GCA 44, 1875. (18)
Wacker J.F. and Anders E. _i-994) GCA 43,
2373. (19) Morgan J.W. and Anders E.
(1979) GCA 43, 1601. (20) Horgan J.W. and
Anders E (_80) Proc. _atl. Acad. Sci.,
USA, Vol. _7, Klo. I?, Geophys. 6973.
I0"" (Fig. 3)


Water on Mars

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