NASA Technical Reports Server (NTRS) 19930005210: Outgassing history of Venus and the absence of water on Venus

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LPI Contribution No. 789 135
clouds seeing in the IR, such as cold collar, hot dipole, and polar degree than the Earth and never had an oceanllke surface water
cap, are not observed in UV. The IR data show H= 3-5 km on the ITIaSS.
average for high latitudes outside the cold collar, and the mixing Lower degree of outgasstug for Venus and Its consequences:
ratio varies from 100-200 ppb in hot dipole to 1000 ppb in 1.4°At in the atmosphere of Venus and Earth. 40At in Venus'
inhomogeneous regions with retrieved high diffuse clouds. We find atmosphere is -1/4 of that in Earth's atmosphere, when normalized
inside the cold collar f~ 1-10 ppb and H - 1kin. The comparison by planetary mass and ignoring 40Ar stored in Earth's continental
ofIR and UV data shows that the vertical pro f'deof SOzmay bemore crust [2,16]. Since KAJ and Th/U ratios and K contents invenusian
complex than our two-parametric model, and H decreases with crustal rocks are similar to those in terrestrial rocks [17,18], less
height ath > 69 kan. In this case the differences in Hare explained 4°Ar in Venus' atmosphere implies alower degree of outgassing for
by viewing angle differences between the observations and the 4°Ar [16]. 4°At in Earth's atmosphere represents 62% degassing of
differential opacity at UV and IR. Temporal variations may also the time-integrated _Ar budget of DM (degassed mange), but4°Ar
contribute.
in Venus' aanosphere represents only 15% degassing of itsDM, if
Latitudinal averaged column density at 62 km (near observed the relative masses of DM in Venus and Earth are similar.
upper boundary of clouds athigh latitudes) obtained from thc IR is 2. Comparison of N2, CO 2, and H20 on Earth and Venus.
about 1019 cm-2 at low latitudes and it increases to 10z0 cm-2 at Previous workers noted that the CO2/N 2ratio of surface reservoirs
high latitudes. on Venus and Earth are nearly identical when CO 2stored inEarth's
References: [I] Moroz V. I. ct at., (1986) Applied Optics, 25, continental crust is included [2,9]. However, such comparisons did
N10. [2] Oertel D. et at. (1987) Adv. Space Res., 5, 25. [3] Moroz not take into account the effect of recycling CO 2back toEarth's DM,
V. I.ctat. (1990)Adv. Space Res., 10, 77. [4] Esposilo L.W. (1980) which may be asignificant part of the Earth's CO, budget [19].The
JGR, 85, 8151-8157. [5]Esposito L.W. etat. (198g)JGR, 93, 5267. present venusian crust is hot (surface temperature 740 K) and the
[6] Na C. Y. et at. (1990)JGR, 95, 7485. [7] Yung Y. L. and Dcmore formation of carbonates requires liquid water, atleast on the Earth,
W. B. (1982) Icarus, 51, 199 .... : hence the venusian crust is probably apoor repository of volatiles.
,'N93-14398 Ji
Most of the outgassed volatiles from Venus' DM are, therefore,
likely to reside in the atmosphere. Hence, subduction on Venus, if
OUTGASSING HISTORY OF VENUS AND THE ABSENCE
it occurs, should have little effect on surface CO2budget, analogous
OF WATER ON VENUS. Youxue Zhangt, 2and Alan Zindler t, to the case for N2 on Earth. In this context, the atmospheric
]Lamont-Doherty Geological Observatory and Department of Gee- composition of Venus can beused to estimate total outgassing from
logical Sciences of Columbia University, Palisades NY 10964, the interior.
USA, 2Department ofGeological Scicnccs, University of Michigan, Table 1 compares the volatile inventory of Earth, which is
Ann Arbor M] 48109-1063, USA. corrected for recycling, with that of Venus. Although the atmo-
sphere of Venus hastwice as much N2as the AC* of Earth, ithas only
Similarities in the size and mean density of Earth and Venus about half as much CO 2,and orders of magnitude less water. This
encourage the use of Earth-analogue models for the evolution of sequence is the inverse of the solubilities of these volatile compo-
Venus. However, the amount of water in the present Venus atmo- nents in basaltic melts (Table 1). In the context of a solubility-
sphere is miniscule compared to Earth's oceans [e.g., 1-3]. The controlled degassing model, the relative difference inNz, CO2, and
"missing" water isthus one of the most significant problems related H20 on Earth and Venus can perhaps be explained by a lower
to the origin and evolution of Venus and has been discussed degree of outgassing of Venus compared to Earth.
extensively [e.g., 2-14]. Lewis [4] proposed that Venus accreted For solubility-controlled equilibrium outgassing we can write
with less water, but this has been challenged [10,13]. The high D/H the following equation [20]
ratio in Venus' atmosphere is consistent with anearlier water mass
more than 100 times higher than atpresent conditions and is often
PiVg 0 cO
cited tosupport a"wet" Venus, but this amounts toonly 0.01 to0.1% ciM+ =elM 0 _ -* =1+ Vg
RTm " ci KiRTmM ' =_
of the water in terrestrial oceans [5,12,15, and Table I]and the high
D/H ratio on Venus could easily reflect cometary injection [14].
K,/--'-,/_-K/'---_/ ,
Nevertheless, many authors begin with the premise that Venus once
tt-v_ ) k,-vj
had an oceanlike water mass on its surface, and investigate the
many possible mechanisms that might account for its loss [e.g., where c°and ciare the initial and final concentrations of gas species
2,6-12]. In this paper we propose that Venus degassed to alower iin the magma, M0and Mare the intial and final mass of the magma
TABLE 1. Comparison of volatile inventory of EarthandVenus.
H20 CO2 N2
Solubility (inreel g-I bar-l) 1.8x 10-6 1.8 x 10-8 -3.6 x 10-9
AC* of Earth(in moles) 8x 1022 (Z4_:9) xl022 (2.0J:0.2) x 102o
Arm of Venus (inmoles) 1016to 1017 (l .l+0.1) x 1022 (4.3:t:0.5) x 1020
Solubility dataarethose inbasaltic magma atIkbar partial vaporpressure and1200°C. Source
of data:water [21]; COg [22,23]; and N2[24], atm of Venus [1-3]. AC* (area +crust) plus a
correction for recycling (recycling of water isignored since thecomparison isnot affected by
augmenting water onEarth's surface).
136 International Colloquium on Venus
(M=M0), Pi is the partial pressure and Vgis the total volume of the TABLE 2. Calcualted degree ofoutgassing from DM
gas phase, R is the universal gas constant and Tm is the magma of Earth (corrected forrecycling) and Venus.
terrmperature, Ki=c./P iwhere Kiis the solubility of iin the melt. and
a2o co2 N2
Fi(=l--ci/c °) is the degree of degassing.
Ifwe assume that the outgassing of Venus canbe approximately Earth 0.45 0.98 0.997
described by this equilibrium degassing formulation as is the case Venus 0.002 0.15 0.50
for Earth [20], then we can estimate the degree of outgassing for
different volatile components in Venus by reproducing the present
atmospheric COz/N zratio from the initial ratio, assumed to be the often suggested that present heat losses on Venus are mostly via hot
same as the initial ratio of the Earth. We note that the H20/N2 ratio spots [e.g., 25,26], as opposed to spreading ridges. The degree of
cannot be reproduced exactly due to loss of water from Venus' melting at hot spots may be higher on average than that at normal
atmosphere. Uncertainties in the initial CO_'q 2ratio for Earth and ocean ridges, at least for the Earth [27]. The elevated surface
the present CO2/N2ratio inVenus' amaosphere (and in the degassing temperatures on Venus, ifitis more than avery recent phenomenon,
species), combine to permit large ranges in the degrees of outgas- also serves to elevate the gcotherm and hence increase the degrees
sing for each of the components. All possible solutions, however, of melting. If so, the average degree of melting on Venus may be
suggest very low degrees of degassing for water (less than 1%), if higher than on Earth, leading to lower volatile concentrations in the
H20 is the major degassing species for water. A "best" solution is undegassed magma. If the rates of magma production or heat
shown in Table 2 with 0.2% outgassing for H20 from the DM of dissipation on Venus and the Earth are the same, the lower initial
Venus. (The total amount of _Ar in Venus' atmosphere also concentration of volatiles results in adecreased level of degassing
provides, in principle, aconstraint on the degree of degassing, but on average because acolumn of magmas must ascend closer to the
it is difficult toutilize the constraint in aquantitative way due to the surface before it reaches saturation and begins to degas.
contribution of continental degassing to40At in Venus' atmosphere The slow outgassing is probably apositive-feedback process. If
[16] and the fact that the degree of degassing for 4°Ar is not the initial outgassing is slow, very little water is outgassed. The
equivalent to that for S6Ar.) deposition of carbonates requires water as a medium, so CO 2
The firactionation effects on volatile-element ratios during out- quickly accumulates in the atmosphere. The surface temperature
gassing can be likened to that on incompatible and compatible then rises due to the greenhouse effect, which then impedes sub-
elements during the partial melting process, although we hasten to duction by increasing the buoyancy of the lithosphere [25,26,28-30].
emphasize that such acomparison should not be taken to imply that This results in even slower outgassing, and hence higher surface
inferred low degrees of degassing for Venus are equivalent to, or a temperatures.
result of, lower degrees of melting. At low degrees of partial In summary, there are two major lines of evidence tosupport our
melting, large proportions of incompatible elements (such as K), contention that Venus outgassed to a lower degree than did the
and only very small proportions of the compatible elements (such as Earth. (1) There is less 4°Ar in Venus' atmosphere than in the
Ni), partition into the melt. At high degrees of melting, large terrestrial atmosphere. This implies that the time-integrated degree
proportions of both compatible and incompatible elements go into of degassing for Venus is lower than that for Earth. (2) When
the melt. Similar effects are produced during different degrees of recycling effects are corrected for, the major volafiles of both
degassing. The degree of outgassing for Earth is very high, so that planets show a relationship between solubility and mass on the
even water has been outgassed to -50%. The degree of outgassing surface. This relationship is consistent with outgassing on both
for Venus is low, hence only incompatible volatiles have outgassed planets being controlled by melt-vapor partitioning, provided that
significantly, while H20, avery compatible volatile in silicate melts Venus outgassed to alower degree. In the context of this scenario,
(100 times more soluble than CO 2and -500 times more soluble than the absence of water on Venus' surface isjust the most conspicuous
N2[21-24]), remains predominantly within the mantle. expression of alower degree of outgassing than Earth. We note that
To illustrate the implication of the solubility-controlled outgas- our model can be tested in the future for other volatile components
sing model for Venus, we first assume that Venus accreted the same that have solubilities between those of CO 2and water, or higher than
amount of water as the Earth although lower or higher amounts of that of water. Venus probably never had much water on its surface,
initial water are possible and consistent with the model. Taking the even if Venus and Earth accreted the same amount of water.
"best" solution shown in Table 2,only --0.2% of this water has been Therefore, there isno compelling need toexplain the loss of massive
outgassed from the Venus' mantle to its atmosphere. The total water quantities from Venus' atmosphere.
amount of outgassed water is ~3 × 10z_mol or -0.4% of the water References: [1] Hoffman J. H. et al. (1980) JGR, 85,
mass now present in terrestrial oceans, enough to generate l3m of 7871-7881. [2] Donahue T. M. and Pollack J. B. (1983) In Venus
water on the surface of Venus ifit were all present atthe same time. (D. M. Hunten et al., eds.), 1036. [3] yon Zahn U. et al. (1983) In
This estimate is similar to an independent estimate of 8m of water Venus (D. M. Hunten et al., eds.), 299-430. [4] Lewis J. S. (1972)
[15]. There is now 1016to 10t7tool of water in Venus' atmosphere, EPSL, 15, 286-290. [5] Donahue T. M. et al. (1982) Science, 216,
which requires atime-integrated escaping rate of 7 x 1010mol/yr, 630-633. [6] Kasting J. F. and Pollack J. B. (1983) Icarus, 53,
-30 times the present loss rate [7,14]. Such losses are capable of 479-508. [7] Kumar S. et al.(1983)Icarus, 55, 369-389. [8] Kasting
generating a 100-fold enrichment in the D/H ratio over Venus' J. F. et al. (1984) Icarus, 57, 335-355. [9] Krasnopolsky V. A.
history [5]. The important point is that Venus never had much water (1985) Icarus, 62,221-229. [10] Kasting J. F. (1988) Icarus, 74,
at its surface, and the absence of aglobe-encircling ocean on Venus 472--494. [11] Zahnle K. J. and Kasting J. F. (1986) Icarus, 68,
must have had profound effects on its geological evolution, making 462--480. [12] Hunten (1990) Icarus, 85, 1-20. [13] Wetherill G. W.
it very different from that of Earth. (1985) Science, 228, 877-879. [14] Grinspoon D. H. and Lewis J.S.
Discussion: It is difficult to explain why Venus outgassed to (1988) Icarus, 74, 21-35. [15] McElroy M. B. etal. (1982) Science,
alower degree than the Earth. One possibility is that alower degree 215, 1614-1615. [16] Turcotte D. L. and Schubert G. (1988) Icarus,
of degassing reflects the lack of plate tectonics on Venus [16]. Itis 74, 36--46. [17] Surkov Yu. A. (1977) Proc. LPSC8th, 2665-2689.
LPI Contribution No. 789 137
[18] Surkov Yu. A. (1983) In Venus (D. M. Hunten et al., eds.), personalcommunication. [25]McGill O. E.etal.(1983)InVenus
154-158. [19] 2_ang Y. andZindler A. (1988) Chem. Geol., 70, 43; (D.M. Hunt_n ¢tal.,eds.).69-130. [26]PhillipsR.I.(1983)In
Zhang Y. and Zindler A., in preparation. [20] Zhang Y. and Zindler Venus (D.M. Hunten etal.,e_.).159-214, [27]KleinE.M. and
A. (1989) JGR, 94, 13719-13737. [21] Hamilton D. L.et al. (1964) Langmuir C.H. (1987)JGR, 92,8089---8115.[28]Anderson D.L.
J. Petrol., 5, 21-39. [22] Stolper E. M. and HoUoway J.R. (1988) (1981)GRL, 7,101-102. [29]PhillipsR.J.etal.(1981)Science,
EPSL, 87, 397--408. [23] Pan V. et ai. (1991) C-CA, 55, 1587-1595. 212,879-887. [30]Tuz_otteD. L,(1989)JGR, 94,2779--2785.
[24] Javoy M, et al. (1986) Chem. Geol., 57, 41-62; Javoy M.,