The chemical evolution of the early Martian hydrosphere is discussed. The early Martian ocean can be modeled as a body of relatively pure water in equilibrium with a dense carbon dioxide atmosphere. The chemical weathering of lavas, pyroclastic deposits, and impact melt sheets would have the effect of neutralizing the acidity of the juvenile water. As calcium and other cations are added to the water by chemical weathering, they are quickly removed by the precipitation of calcium carbonate and other minerals, forming a deposit of limestone beneath the surface of the ocean. As the atmospheric carbon dioxide pressure and the temperature decrease, the Martian ocean would be completely frozen. Given the scenario for the chemical evolution of the northern lowland plains of Mars, it should be possible to draw a few conclusions about the expected mineralogy and geomorphology of this regions

Schaefer, M. W.

English

NASA Technical Reports Server

N91-23001 255
CHEMICAL EVOLUTION OF THE F_ARLYMARTIAN HYDROSPHERE; M. W. Schaefer,
University of Maryland and Goddard Space Flight Center, Greenbelt, MD
Mars today is a cold, dry planet, with a thin atmosphere, largely consisting of carbon
dioxide. It may not always have been that way, however. Values of total outgassed C02 from
several to about 10 bars are consistent with present knowledge (18), and this amount of CO2
implies an amount of water outgassed at least equal to an equivalent depth of 500 to 1000
meters (3). Pollack et al. (18) have made extensive calculations deriving the amount of carbon
dioxide necessary to achieve a strong enough greenhouse effect to raise the surface
temperature of Mars above the freezing point of water, and have determined that, for different
values of surface albedo, latitude, and orbitali_osition, pressures of from 0.75 to 5 bars are
necessary. The geological evidence suggests that any such warm, wet period in Mars' history
must have been over by about 3.5 billion years ago.
One may model the early Martian ocean as a body of relatively pure water in equilibrium
with a dense (several bars) carbon dioxide atmosphere. The Juvenile water outgassed by Mars
should have been extremeIy acidic, based on Earth analogy. In such waters all common
components of ordinary rocks are highly soluble, with the exception of SiC) 2 and AI203 (I0].
The chemical weathering of lavas, pyroclastic deposits, and Impact melt sheets would have
the effect of neutralizing the acidity of the Juvenile water. Equillbrium would be achieved
when the weathering rate of the rocks was equal to the dissolution rate of the carbon dioxide
in the atmosphere (which dissolution would itself require the water to be quite acidic, with a
pH no greater than about 5). There was also an extensive groundwater system at thls time (2).
which acted to weather even those parts of the regolith far distant from the ocean. On the
Earth, about 30% (9) of the outgassed water is thought to be geochemically bound in the crust.
One might take this figure as an upper limit to how much of the Martian water budget might
have become bound in its regollth during the relatively restricted time available for such
processes to occur (no more than about 1 billion years, compared to the 4.5 billion years
available on the Earth). Assuming then a total value for outgassed water on Mars of 500 -
I000 m, averaged over the entire planet, and only a few meters of that lost to space through
thermal escape (12), at the time when enough carbon dioxide was lost from the atmosphere
such that temperatures dropped below the freezing point of water on Mars, there should have
been at least 350 - 700 m of water still free and unbound. The formation of the highland-
lowland dichotomy is believed to have taken place by the time of the decline of the heavy
meteorite bombardment (7), thus providing a natural basin into which this water could
collect. The lowland terrain takes up roughly 30% (_+5%) of the surface of Mars; therefore, if
the available water was concentrated in the lowland tea-ram, it would have a depth of some
1000 - 2800 m. This depth is comparable to the height of the boundary scarp between the
highland and lowland terrains, similarly (though probably comcidentally) to the way in
which water on the Earth Is largely contained by the continents to within the lowlands
formed by the oceanic crust. There may be geological evidence for such an ocean on Mars (3,
13, 11, 15-17).
As calcium and other cations are added to the water by chemical weathering, they are
quickly removed by the precipitation of calcium carbonate and other minerals, forming a
deposit of limestone beneath the surface of the ocean. Even in the deepest parts of this ocean,
pressure effects will not be enough to prevent the precipitation of carbonates, unlike the
present situation on the Earth, where carbonate deposits only form in relatively shallow
water. By this process, carbon dioxide is removed from the atmosphere. The time scale on
which this occurs can be as little as 107 years (3, 6, 18), assuming no reworklng of deposited
carbonates, or when such reworking is postulated, a time scale of up to about a billion years is
reasonable (18). As the carbon dioxide is removed from the atmosphere, the heat trapped due
to the greenhouse effect becomes unimportant, the planet's surface cools to below the freezing
point of water, and the surface of the ocean freezes. When the surface of the ocean freezes, the
removal rate of carbon dioxide from the atmosphere is decreased, due to the decreased rate of
regolith weathering caused by the lesser mobility of water through the atmosphere-regolith
system, but it is not halted. The ice layer will act to trap beneath it an artificially high
concentration of dissolved carbon-bearlng species, brought in by the percolation of
groundwater. The freezing of the water will have the additional effect of further increasing
https://ntrs.nasa.gov/search.jsp?R=19910013688 2020-03-19T17:41:23+00:00Z
256 EARLY MARTIAN HYDROSPHERE: Schaefer M. W.
the cation concentration in the remaining liquid, due to the exclusion of cations from the
crystal structure of the ice. This behavior is observed in frozen lakes in the terrestrial Arctic
(8] and Antarctic (14], and there results in the precipitation of carbonates on the lake beds.
As the atmospheric carbon dioxide pressure and the temperature continue to decrease.
there should come a tlme when the Martian ocean is almost completely frozen, perhaps
overlying a layer of salts or concentrated brines above the carbonate layer. Due to the high
expected ratio of calcium (or other cation) concentration relative to HCO3- concentration in
the water, calcium carbonates are preferentially deposited, and the remaining fluid gradually
evolves to form a Ca-Na sulfate-chloride brine [5). If the entire Martian CO2 budget of, say 1 -
10 bars, were to be used to form this carbonate deposit, it would imply an average thickness
for the deposit of I00 to 1000 meters.
Once the ocean is completely frozen, the sublimation and ablation of ice from the
uppermost surface of the ocean is no longer compensated for by the freezing of the water
below; the ice itself gradually disappears, starting at the warmest areas near the highland-
lowland boundary, migrating into the regollth and leaving only a residual cap at the north
pole. Or it could be that the ocean would be so reduced in volume by the time it froze
completely that it would be not significantly larger than the present polar cap. Also at this
time, carbon dioxide from the atmosphere may be adsorbed onto the cold regolith.
Eventually, through the action of freezing and the removal of water from the atmosphere-
regollth system by irreversible geochemical weathering, the entire water budget of Mars will
be tied up in its present reservoirs in the polar caps and regolith.
Given the preceding scenario for the geochemical evolution of the northern lowland
plains of Mars, it should be possible to draw a few conclusions about the expected mineralogy
and geomorphology of this region. The basement material should be a hlghly-altered
regolith, though it is impossible to say with any certainty whether the original material was
mostly basalt flows, ash deposits, or some other volcanic product. One would expect,
however, that it should have been highly brecciated by the early, heavy meteorite
bombardment of the planet. It should contain a significant proportion of clays and other
hydrous mlneral_ Overlying this basement, and covering most of the northern plains, are
carbonate deposits, several hundred meters thick or more. Likely to be primarily calcium
carbonate, these deposits would be progressively more sulfate- and salt-rich in their upper
layers. The topmost layers of these deposits would be primarily gypsum (CaSO4-2H20] or
mirabflite (Na2SO4-10H20}, and assorted salts (5). Continued meteoritic impacts, and dust
storms, have acted to mix these sedimentary materials with the volcanic materials common
over most of the rest of the planet, to form a surflcial mantling of dust of uniform
composition. Viking chemical analyses indicate that the surficial deposits in Chryse
Planttla and Utopia Planitia may contain as much as 8 - 15% sulfates, and up to 10%
carbonates (4), consistent with such a volumetric mixing model. It has also been suggested by
several researchers that brines may be at least metastable at present in the regolith of Mars
(1), particularly in the region of Solis Lacus, where anomalous radar reflectivity has been
observed (19,20).
[1) Brass, G. W., Icarus 42,20, 1980; (2) Cart, M. H., JGR82, 4039, 1979; (3) Carr, M. H., Icarus 68.
187, 1986; [4) Clark, B. C., Van Hart, D. C., Icarus 45, 370, 1981; [5) Eugster, H. P., Hardie, L. A.,
Ch. 8 in Lakes: Chemistry, Geology, Physics, Springer-Verlag, 1978; (6) Fanale, F. P., Salvafl,
J. R., Banerdt, W. B., Saunders, I_ S., Icarus 50, 381, 1982; (7) Greeley, 1%, Pfmwtanj
_, Allen and Unwln, 1985; (8) Hail, D. IC, Arct/c 33, 343. 1980; (9) Holland, H. D., The
ChemlcaIEvolution of the Atmosphere and Oceans, Princeton, 1984; (10) Loughnan, F. C..
Chemlral Weathering of the Sll/cate Mtnera/s, Elsevier, 1969; (I 1) Lucchitta, B. IC, Ferguson"
H. M., Cummers, C., JGR91, E166, 1986; (12) McEhoy, M. B., Scfence 175, 443, 1972; (13)
McGilI, G. E., in/JJnar and P/anetanj Sclmu_ XV/, Lunar and Planetary Institute, 534, 1985;
(14) McKay, C. P., Nedell, S. S., Icarus 73, 142, 1988; (15} Parker, T. J., Schneeberger, D. M.,
Pleri, D. C., Satmders, R. S., NASA TM-89810, 319, 1987; (16) Parker, T. J., Schneeberger, D.
M., Pieri, D. C., Satmders, I% S., NASA TM-89810, 322, 1987; (17) Parker, T. J., Saunders, R. S.,
LPITech. RepL 88-05, 100, 1988; (18) Pollack, J. B., Kasting, J. F., Richardson, S. M., Poliakof,
I¢., lore'us 71, 203,1987; (19] Zent ,A. P., Fanale, F. P.,JGR91, D439, 1986; (20) ZLsk, S. H.,
Mouglnis-Mark, P. J., Nature 44, 735, 1980.
257
GLOBAL RELATIONSHIPS BETWEEN VOLCANIC VENTS
AND FRACTURES RADIAL TO LARGE IMPACT BASINS ON MARS.
Byron D. Schneid and Ronald Greeley, Department of Geology,
Arizona State University, Tempe, AZ 85287.
The relation of volcanic vents on Mars to impact basins has
been studied previously (1,2,3,4,5,6). It has been asserted
that the concentric fractures around impact basins extend into
the crust and might localize some features, including volcanoes
(e.g. 4). In this study, we assess the possibility of radial
fractures inferred to be associated with impact basins as an
additional control on the location of volcanoes on Mars.
Geologic mapping at 1:2 million scale enabled 250 central vents
and fissure vents to be identified. Patterns of vent
distribution (fig. I) superimposed on a globe show that most are
located on three distinct circles. The first is a great circle
which passes through Arsia Mons, Pavonus Mons , Ascreaus Mons,
and Tempe Fossae, along Protonilus Mensae (an area of fractured
terrain), through Syrtis Major, Hadriaca Patera, and a series of
fissure vents southwest of Tharsis. A similar great circle
trends SW to NE from the Hellas basin, through Hadriaca Patera,
Tyrhenna Patera, Elysium, Alba Patera (which is approximately
antipodal to the Hellas basin), southern Tempe Fossae, the
eastern Valles Marineris chaotic region, and the Amphitrites
Patera vents on the southwest rim of the Hellas basin. The
third series of vents is on a small circle -4800 km in diameter
centered at ~ 104°W, 2°N. This site is near the center of the
Tharsis gravity anomaly (7) and the loci of associated tensile
stresses (8). Most fissure vents not located on the Tharsis
trend of volcanics are on this small circle, as are Alba Patera
and other central vents.
There are two more possible great circles which may be
superimposed onto the martian globe. The first can be traced
along the escarpment dividing the northern lowlands from the
southern highlands, across Isidis Planitia (the site of a
possible impact basin at ~ 273°W, 13°N), fractured terrain in
Solus Planum (a possible fissure vent source area), and through
Juventae Chasma. This circle may reflect the role of inferred
radial fractures in modifying the surface without associated
volcanism. The second possible great circle passes through the
Hellas impact basin, some large unnamed central vent volcanoes
(at ~ 205°W, 48°S) , Apollonaris Patera, the escarpment north
of Alba Patera and the Tempe Fossae region, and into Acidalia
Planitia. Acidalia Planitia is also along the trend of the
Tharsis chain of volcanoes and may indicate an impact site
centered near 30°W, 60°N.
Although concentric fractures of smaller impact basins may
influence local vent sites, the global setting appears to be
governed by radial fractures associated with major impact
basins. This is supported by the association of one or perhaps
two great circles with the Hellas impact basin, and possible
great circles associated with the Isidis basin and Acidalia
Planitia. The distribution also suggests that larger impacts
produce larger fractures and can, therefore, accommodate more
258
VOLCANIC VENTS AND FRACTURES RADIAL TO IMPACT BASINS
Schneid, B.D. and Greeley,R.
volcanic vents. Isidis, Argyre, Procellarum, and Hellas basins
in that order, have an increasing number of vents inferred to be
associated with them.
REFERENCES: (I) Albin, E.F. and Greeley, R., Proc. LPSC XVII, pp. 7-8,
1986. (2) Albin, E., Masters Thesis, Arizona State University Press, 1986.
(3) Craddock, R.A., Greeley, R., and Christensen, P.R.,
Geophysical Research, in press. (4) Schultz, P.H., Schultz, R.A., and
Rogers, J., Journal of Geophysical Research, 82, BI2, pp.9803-9820, 1982.
(5) Schultz, P.H., Proc. LPSC XV, pp.728-729, 1984. (6) Wichman, R. and
Schultz, P.H., NASA TM-89810,pp.474-475, 1987. (7) Phillips, R.J. and
Lambeck, K., Reviews of Geophysical Space Research, 18, pp. 27-76, 1980.
(8) Phillips, R.J. and Ivens, E.R., Physics of Earth and Planetary
/_, 19, pp. 107-148, 1979.
0o
. KEY
O IMPACT BASINS
A LARGE FISSURE
VENTS
• LARGE CENTRAL
VEN_ <25,000 km2
f300" 12Q<
180 ° 120" 60" 0° 300 °
65"
30"
O"
.30"
• ._>%_ 240°
180"
.>40" 180;5 *
.65 •
180" 120" 60" 0° 300 ° 240"
Figure 1. Vent locations, impact basin_, and _reat and s=all circles.
O"
_0"
.... 65"
180"


Chemical evolution of the early Martian hydrosphere

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