 |
Up
to now we have two main sources of information with respect to the geochemistry
of Mars. First, we have the direct observations and the data obtained by space
probes, in particular by the Viking missions, as well as by Mars 5 and Phobos 2
and most recently by Mars Pathfinder. Second, we have information from the
SNC-meteorites (see below), which are generally believed - but not absolutely
proven - to represent Martian surface rocks ejected into space by large impacts
of other bodies on the Martian surface.
The most
direct information about the chemical composition of the Martian surface stems
from the XRF-instruments on board the two Viking Landers 1 and 2, and the Mars
Pathfinder APX-spectrometer (Alpha-Proton-X-ray). Although Viking 1 and Viking
2 landed about 6500 km from each other and the Pathfinder another 1000 km to
the east of Viking 1, the chemical composition of the Martian surface soil was
found to be almost identical. Large areas of the Martian surface, and
especially the lowlands are obviously covered with a fine dust thoroughly mixed
by repeated dust storms. However, both the infrared spectrometer and the
gamma-ray spectrometer of Phobos 2 had found considerable regional variations
in the chemistry of the Martian surface. Using the soil data, it has been
generally assumed that the Martian crust is of mafic nature (Mg- and Fe-rich),
with only a low degree of fractionation. This picture changed completely when
the Pathfinder APXS-data became available at the end of 1997. The Pathfinder
rover Sojourner was able to put the sensor head of the APX-spectrometer not
only down to the ground but also to specific rocks. We will return to the rock
data in the Chapter: The Geochemistry of Martian Surface Layers.
The second source of information about the
chemistry of Mars, the SNC-meteorites, (See Figure 1) yielded more detailed
data about their parent body. The 16 currently identified SNC- meteorites
(Shergottites, Nakhlites and Chassigny) are distinguishable from all the other
thousands of meteorites by their oxygen isotopic composition and various trace
element ratios. Large impacts are obviously able to eject surface rocks into
space with velocities exceeding the escape velocity of the Moon (2.4 km/sec)
and Mars (5.0 km/sec), although there is still no generally accepted model for
the dynamics problems involved. The Lunar meteorites were quickly associated
with their parent body because of their similarity with the rocks collected on
the Moon by space missions. In the case of the SNC-meteorites no such ground
truth exists.
The first indication of a Martian
origin of the SNC-meteorites stems from their young crystallisation ages. All
magmatically differentiated meteorites except the Lunar and the SNC-meteorites
are pieces of asteroids. They all have crystallisation ages close to 4.5
billion years and, except for impact heating, no heat source is known which
could melt and differentiate an asteroid billions of years later.
With one exception, the reported crystallisation
ages of SNC-meteorites range between 160 million and 1.3 billion years. Their
considerable fractionation of Rare Earth Elements (REE) excludes an origin from
impact melts. |
|
Figure 1:Shergotty, the meteorite after which the
SNC-subgroup, the Shergottites were named.
It fell on August 25,
1865.
Total recovered mass about 5 kgH.W. |
 |
There is one SNC-meteorite (ALH
84001) which is distinct from all the others as its crystallisation age is
close to 4.5 billion years. Hence, it represents by far the oldest rock
collected from Mars. In all likelihood it comes from the huge areas of the
southern highlands on Mars which according to crater statistics are older than
3.8 billion years. In this meteorite American scientists recently detected
organic compounds - polycyclic aromatic hydrocarbons (PAHs) - the first
observation in a rock from Mars. However, the question, whether these compounds
have biological origin or not, is not yet clear. The PAHs were found on
interior fracture surfaces of ALH 84001 generally associated with
carbonate-rich globules (Figure 2). These carbonate-rich globules contain
microscopic mineral grains of single-domain magnetite and iron sulphides. Both
these mineral grains and the PAHs can be formed by terrestrial bacteria but can
also be made by non-biological processes.
The most
direct evidence for Mars as the parent body of the SNCmeteorites was the
discovery of a trapped gas component (rare gases, nitrogen and CO2)
in these meteorites, very different from those observed in any other meteorite,
but closely matching the highly characteristic element and isotope ratios of
the Martian atmosphere. |
|
Figure 2: Cleavage surface of Martian meteorite ALH84001.
The brownish circular spots (diameter ca. 0.1 mm) consist of carbonates, which
were possibly formed by primitive life forms on Mars. H.W. |
 |
| As seen from Figure 3, all
SNC-meteorites plot in the oxygen isotope diagram along a single line above the
terrestrial fractionation line, indicating that they all come from one and the
same object in the Solar System. In fact, the term SNC-meteorites should be
abandoned as some of the recently recognized Martian meteorites do not fit into
this nomenclature. In addition to Shergottites, Nakhlites and Chassigny two
more groups have been discovered. As we will see later, new evidence has been
collected for the Martian origin of these meteorites. In the following chapters
we will not use the term SNC-meteorites any more, but just call them Martian
meteorites. |
|
click to enlarge  |
Figure 3 :18O / 16O vs. 1
7O l 16O plot for achondrites. The SNC-meteorites clearly form
a separate line from the eucrites and are separated from the terrestrial
fractionation line, indicating separated reservoirs.
H.W |
 |
The bulk chemistry of Mars as inferred from Martian
meteorites All Martian meteorites are igneous rocks of quite
variable composition. Their high Fe0 content reflects the high FeO content of
the Martian mantle, while their high MnO and Cr203
concentrations indicate that, contrary to the Earth's mantle, the Martian
mantle is not depleted in MnO and Cr2O3. Hence, we can
use as a starting point a strictly C1 abundance of MnO (C 1= carbonaceous
chondrites type 1, the most primitive group of meteorites reflecting the
primordial unfractionated abundances in the Solar nebula).
As the ratio FeO/MnO is extraordinarily constant in
all Shergottites with a MnO abundance equal to 1.00, an abundance of FeO of
0.399, corresponding to 17.9 % Fe0, is derived for the Martian mantle. For the
other major elements (MgO, Al2O3, SiO2, CaO)
strictly C1 abundances are assumed for the Martian mantle. In the case of iron
a C 1 Fe/Si ratio is assumed for the whole planet. Hence, the fraction of iron
present in a form other than FeO in the Martian mantle is assumed to have
segregated to the core in the form of iron metal or FeS. It is further assumed
that all refractory lithophile elements are also present in the Martian mantle
in C1 abundances, as only moderate fractionation of these elements has been
observed for the Earth's mantle and in meteorites. Using element correlations
observed in meteorites, the mantle abundances of many siderophile (W Ni, Co,
etc.) and moderately volatile and volatile (Na, K, Rb, Cs, F, Br, etc.)
elements have been calculated (see Table 1 and and Figures 4 and 5).
The similar abundances of several geochemically
very different elements (W to Rb in Figure 5) in the mantle of Mars is further
strong evidence for the formation of the inner planets from two compositionally
different components.
|
|
Figure 4: Correlation of K vs. La in SNC-meteorites from
Mars (SPB), the eucrite parent body (EPB), the Earth and Moon. Assuming a CI
abundance of the refractory element La (=0.48 ppm) in the Martian mantle, the
abundance of the moderately volatile element K (=315 ppm) can be obtained from
this correlation. H. W. |
|
Figure 5: Estimated elemental abundances in the Martian
mantle as derived from Martian meteorites. The higher abundance of moderately
volatile (Na, Ga, P, K, F, Rb, Zn) and volatile elements (Cl, Bra as compared
with the terrestrial mantle is obvious. Note the depletion of all chalcophale
elements (In, Co, Cu, Ni) in the Martian mantle. H.W. |
These components are:
Component A:
Highly reduced and free of
all elements with volatility equal to or higher than Na, but containing all
other elements in C 1 abundance ratios. Fe and all siderophile elements are
metallic, and even Si might be partly present in metallic form. V, Cr, and Mn
are present as metals or sulphides. It is assumed that the degree of reduction
is within the range of that observed in enstatite chondrites or even
higher.
Component B:
Oxidised and
containing all elements - including the volatiles - in C1 abundances. Fe and
all siderophile (Co, Ni, Cu, Ga, W etc.) and lithophile elements are present,
mainly as oxides.
As seen from Table 1 and Figure 5,
the Martian-meteorites indicate abundances of moderately volatile (Na, K, Rb,
Zn) and volatile elements (Cl, Br, I) in the Martian mantle, in excess of those
in the terrestrial mantle, except for chalcophile elements (TI, In). A mixing
ratio of component group A/component group B of 60:40 is obtained for Mars,
compared with a ratio of 85:15 for the Earth. There are, however, a number of
elements supposedly derived from component group B that in the Earth's mantle
have abundances similar to that of Fe, Na, Ga, K, F and Rb, but in the Martian
mantle have considerably lower abundances (Co, Ni, Cu, In). These elements all
have a strong chalcophile character. Their low abundances are taken to be an
indication of an homogeneous accretion on Mars. Hence, contrary to an
inhomogeneous accretion of the Earth as frequently favoured, on Mars the two
components had the chance to equilibrate with each other and were probably
supplied almost simultaneously to the growing planet. The high abundance of
component group B, which supplied large amounts of sulphur, was obviously
responsible for FeS becoming a major phase and at its segregation extracted all
chalcophile elements according to their sulphide-silicate partition
coefficients.
The sulphide-silicate equilibrium in
the Martian mantle indicates its saturation with FeS. The FeO content of the
Martian mantle is about a factor of 2 higher than that of the terrestrial
mantle. As a consequence, the sulphur abundance in the Martian mantle is
expected to be substantially above the S abundance in the Earth's mantle as the
solubility of FeS in silicates increases with the FeO content. Hence, the
observed high concentrations of sulphur in mantle derived magmas as represented
by the Shergottites (sulphur content between 600 and 2800 ppm) is not
surprising. If one assumes that sulphur is present in the whole planet with an
abundance similar to that of elements with similar volatility, one finds a C1
and Si normalised sulphur abundance of 0.35, while for Ni and Co an abundance
of 1.00 was taken for the estimation of the bulk composition of Mars listed in
Table 1.
The size and the composition of the
Martian core, with over 14% S as given in Table 1, fits well not only with the
geophysical data, i. e. the Martian moment of inertia factor and the planet's
density, but also fits within the estimated uncertainty, the concentration
required for a completely fluid core that is only weakly convecting, which in
turn could explain the very weak magnetic field of Mars.
The water abundance given in Table 1 is derived
from element correlations observed in Shergottites and cosmochemical
constraints. Two quite different approaches yielded an almost identical value;
it might however be an upper limit only for the water in the Martian mantle.
(For further discussion, see the next chapter.)
|
Table 1.
Bulk Composition of Mars as
derived from Martian meteorites
(Dreibus and Wänke,
1984).
| MgO |
30 |
.2 |
Cl
ppm |
38 |
|
Fe |
77 |
.8 |
| Al2O3 |
3 |
.02 |
Br
ppb |
145 |
|
Ni |
7 |
.6 |
| SiO2 |
44 |
.4 |
I |
32 |
|
Co |
0 |
.36 |
| CaO |
2 |
.45 |
Co
ppm |
68 |
|
S |
14 |
.24 |
| TiO2 |
0 |
.14 |
Ni |
400 |
|
|
|
|
| FeO |
17 |
.9 |
Cu |
5 |
.5 |
|
|
|
| Na2O |
0 |
.50 |
Zn |
62 |
|
|
|
|
| P2O5 |
0 |
.16 |
Ga |
6 |
.6 |
|
|
|
| Cr2O3 |
0 |
.76 |
Mo
ppb |
118 |
|
|
|
|
| MnO |
0 |
.46 |
In |
14 |
|
|
|
|
| K
ppm |
305 |
|
Tl |
3 |
.6 |
|
|
|
| Rb |
1 |
.06 |
W |
105 |
|
|
|
|
| Cs |
0 |
.07 |
Th |
56 |
|
|
|
|
| F |
32 |
|
U |
16 |
|
|
|
|
H2O added during
accretion = 3.4 %
H2O retained in the mantle = 36 ppm, equivalent
to a surface layer of 130 m on 100 % degassing. |
|
|
The Geochemistry of the Martian Surface
Layers Two of the eleven known Shergottites,
i.e. namely Shergotty and Zagami, have compositions very similar to the Viking
soil. (Table 2.) The other Martian meteorites differ considerably in their
chemical composition but generally have a mafic to ultaramafic character. As
evident from Figure 6, all Martian meteorites, except the pyroxene cumulates,
plot in the Mg/Si vs. Al/Si diagram along one fraction line (red line) as do
the Viking soils and Pathfinder soils. Terrestrial samples (blue dots) plot
along a very different line.
The large
concentrations of sulphur (3.5 %) and chlorine (0.8 %) in the Viking soils and
very similar concentration in all Pathfinder soils are not noticeably
accompanied by large concentrations of their respective cations. The most
likely cations for the sulphates, Mg and Ca, have even higher concentrations in
Shergotty and Zagami than compared with the Viking soils. This observation
speaks for a direct introduction of SO2 and probably also HCl to the Martian
regolith via gas-solid reactions. |
|
click to enlarge |
Figure 6: Mg/SI vs. AI/Si diagram. Samples from Mars plot
along a very different fractionation line (red) as terrestrial samples (blue).
The fact that the data points for the Martian soils and rocks (red and open
diamonds) fall close to the fraction line defined by the Martian meteorites
(red triangles) is an additional proof of their Martian origin. The open red
triangles represent cumulate rocks among the Martian meteorites. Like in the
case of terrestrial rocks, these cumulates (mainly pyroxene cumulates) fall off
the fractionation line. H. W. |
 |
Table 2.
Comparison Shergotty -
Mars
| |
% |
|
|
% |
|
| SiO2 |
51 |
.4 |
|
43 |
.0 |
| FeO |
19 |
.4 |
|
16 |
.2 |
| CaO |
10 |
.0 |
|
5 |
.8 |
| MgO |
9 |
.28 |
|
6 |
.0 |
| Al2O3 |
7 |
.06 |
|
7 |
.02 |
| TiO2 |
0 |
.87 |
|
0 |
.6 |
| Na2O |
1 |
.29 |
|
-- |
|
| S |
0 |
.13 |
|
3 |
.5 |
| Cl |
0 |
.01 |
|
0 |
.8 |
| CO3 |
< 0 |
.2 |
|
< 2 |
|
| H2O |
< 0 |
.02 |
|
< 2 |
|
| Sr ppm |
51 |
|
|
58 |
|
|
|
|
Over the years, the dry Martian
mantle as given in Table 1 has been questioned since water-rich inclusions are
observed in Martian meteorites. However, it was not known if the host phases of
these inclusions have crystallised from mantle-derived magmas or represent
material from a water-rich Martian crust taken up by intrusions and overplating
of mantle-derived magmas. Concerning the contradictory evidence of a dry
Martian mantle, as indicated by the low water content of Martian meteorites,
and the erosion-like Martian surface features, which seem to require large
amounts of water, measurements of the oxygen isotopes and the H/D ratio of
water extracted from Martian meteorites is of great interest.
It was observed that for all the Martian meteorites
analysed, apart from the presence of terrestrial contamination, a large
fraction of the water, although Martian, is not derived from the Martian
mantle, but obviously represents Martian surface water with an oxygen isotope
composition up to three times further away from the terrestrial isotope
fractionation line than the oxygen in the silicates of Martian meteorites. At
the high temperatures during magma generation in the Martian mantle, isotopic
equilibration between oxygen of the silicates and of water would certainly have
been established. Hence, only a fraction of the water found in Martian
meteorites can be mantle derived and the other non-terrestrial part must come
from the Martian surface. The oxygen isotopes of the surface component might
have been created by non-linear isotope fractionation by non-thermal escape of
oxygen to space.
Of course, it could also be that
the water added to Mars during accretion had oxygen isotopes much further away
from the terrestrial isotope fractionation line than the oxygen of Martian
mantle silicates and that isotopic equilibration of these two oxygen reservoirs
never took place. Such a scenario has severe consequences for the accretion
processes. Even in the case of Earth, if it has received most of its present
water by a late veneer, it might be that this water had oxygen isotope ratios
very different from those of the oxides in the mantle. Later on, subduction and
recycling of the oceanic crust might have continuously brought water from the
surface into the originally dry mantle and isotopic equilibration with the
oxygen of the silicates took place there.
As
subduction of crust material seems not to occur on Mars, material from a late
veneer could have been added to the upper crust only and would remain
unrecognized in mantle-derived magmas.
The
depletion of chalcophile elements in the Martian crust has been used to infer a
homogeneous accretion scenario for Mars. In the same way as discussed above for
sulphur, homogeneous accretion had the consequence that water added to the
growing planet by component group B reacted with the metallic Fe of the
component group A, oxidising it to FeO, while the thereby generated hydrogen
escaped. Hence, we should expect a very dry Martian mantle, because water,
although added in large quantities to the planet during accretion, was reduced
to H2, except for trace amounts. The huge amount of hydrogen created not only
greatly enhanced degassing of the planet during its accretion stage, but
probably also led to an hydrodynamic escape process which took heavier species
with it.
In this way, one could explain the low
abundance of primordial rare gases in the Martian atmosphere, while the low
absolute abundance of Argon-40, the decay product of Potassium-40 in the
Martian atmosphere, might be explained by a combination of atmospheric loss
over geologic time and a low degassing rate of Mars due to the absence of plate
tectonics. The preliminary results of the soils and rocks from APXS analyses of
the Pathfinder mission are summarized in Table 3. As mentioned above, the
Pathfinder soils had a very similar composition as the Viking soils. Figure 7
illustrates the homogeneous composition of the dust component of the Martian
surface. However, the preliminary data of the rocks at the Pathfinder landing
site show that their composition is very different from the soil composition.
In fact, the in situ analyses of these rocks yielded a big surprise to most
scientists in the field. |
|
click to enlarge |
Figure 7: Comparison of the chemical composition of five
measured Pathfinder soils A-2, A-4, A-10, and A-15 and the range of the Viking
soil data. All element concentrations were normalized to a SiO2 content of 44 %
by weight. H. W. |
 |
Table 3.
Mars Pathfinder Alpha Proton
X-Ray Spectrometer
Prelimerary Results from X-Ray
Mode*
| Oxides |
Mean Soil
A-4,
A-5
A-10, A-15 |
A-8
Scooby
Doo |
A-3
Rock
Barnacle
Bill |
A-7
Rock
Yogi |
A-17
Rock
Shark |
"soil-free
rock" |
| |
Weight
% |
Weight
% |
Weight
% |
Weight
% |
Weight
% |
Weight
% |
|
| Na2O |
2.4 |
2.0 |
3.2 |
1.7 |
2.0 |
2.6 |
|
| MgO |
7.8 |
7.1 |
3.0 |
5.9 |
3.0 |
2.0 |
|
| Al2O3 |
8.6 |
9.1 |
10.8 |
9.1 |
9.9 |
10.6 |
|
| SiO2 |
48.6 |
51.6 |
58.6 |
55.5 |
61.2 |
62.0 |
|
| SO3 |
5.9 |
5.3 |
2.2 |
3.9 |
0.7 |
0.0 |
|
| Cl |
0.6 |
0.7 |
0.5 |
0.6 |
0.3 |
0.2 |
|
| K2O |
0.3 |
0.5 |
0.7 |
0.5 |
0.5 |
0.7 |
|
| CaO |
6.1 |
7.3 |
5.3 |
6.6 |
7.8 |
7.3 |
|
| TiO2 |
1.2 |
1.1 |
0.8 |
0.9 |
0.7 |
0.7 |
|
| FeO |
16.5 |
13.4 |
12.9 |
13.1 |
11.9 |
12.0 |
|
|
|
* Data from R. Rieder et al.: "The
chemical Composition of Martian soil and Rocks Returned by the Mobile
Alpha-Proton-X-Ray Spectrometer: Preliminary Results from the X-Ray
Mode."
Science 278, 1997; pp. 1771-1774. |
|
|
Contrary to the Martian soil
and the Martian meteorites which point to a mafic Martian crust, the Pathfinder
rocks turned out to have low magnesium but high SiO2 contents. That means they
are felsic rocks with a high degree of fractionation which is also reflected by
their rather high potassium content.
The
compositional differences between the rocks and soil in which they are embedded
become even larger when it is noticed that the APXSdata of the rocks are
falsified by dust adhering to the surface of the rocks in varying amounts. The
Martian soil contains sulphur and chlorine in concentrations far above those to
be excepted in igneous rocks. If one plots the sulphur concentrations against
the measured concentrations of the other elements (Figure 8), linear
regressions are obtained. The regression lines in Figure 8 were calculated
using data of rocks only. As it can be seen the data points of the soil
measurements fall to sulphur-rich side of the correlation lines. Extrapolation
to zero sulphur yields the composition of a soil-free rock. The good
correlation observed for all elements indicates that the compositions of all
rocks are very similar and close to the hypothetical soil-free rock. Figure 6
shows that in the Mg/Si versus Al/Si diagram the Viking soils, Pathfinder
soils, Pathfinder rocks as well as the calculated soil-free rock plot close to
the Mars fractionation line, derived from Martian meteorites. The location of
the Pathfinder rocks is that to be expected for highly fractionated Martian
crustal rocks. Terrestrial analogues would be andesites or in a broader sense
granites but without any genetic link. |
|
click to enlarge |
Figure 8: Linear regression lines for Si02, U02,
MgO, and Al203 versus S of Pathfinder rocks (blue circles
with error bars). Soil data points were not included in the regression, but
plot generally close to the regression lines defined by the rocks. The zero S
values represent a "soil-free" rock composition.
H. W. |
 |
|
click to enlarge |
Figure 9: Histogram of some element concentration in the
Pathfinder rock Barnacle Bill (hatched bar), Pathfinder soil (filled bar), and
Martian meteorites. The bars for the Martian meteorites represent the mean
value of 11 meteorites of higly different compositions, with the lowest and
highest value given by the arrows. For all elements the Martian soil can be
interpreted as a mixture of Pathfinder rocks and a more mafic component similar
to that represented by the Martian meteorites. H.W |
 |
The composition of Barnacle
Bill or that of the soil-free rock in Figure 6 represents the Al-rich end-point
of the Martian mantle-crust frationation line, with the Martian lherzolites
ALHA 77005 and LEW 88516 and the dunite Chassigny on the Al-poor side. The
basaltic Shergottites (QUE 94201, Shergotty, and Zagami), which form a second
fractionation line (dashed line in Figure 6), could be derived from younger
intrusions into the older Martian crust. As these meteorites are assumed to
have been ejected from Mars in one event about 2.8 million years ago, they must
come from one location and might represent related flows devired from a common
source, containing increasing portions of cumulus pyroxenes, inversely
correlated with their Al content.
It is generally
assumed that the soil is derived from the rocks by diminution due to impacts
and weathering as well by introduction of volcanic gases mainly SO2
and HCI. Figure 9 demonstrates the compositional differences between the
Martian soil and the rocks at the Pathfinder landing site. Obviously, the soil
composition cannot be explained by weathering and diminution of the rocks
analysed at the Pathfinder landing site. Addition of matter richer in Fe and
Mg, but poorer in potassium seems an unavoidable necessity. This matter must be
derived from more distant geologic provinces being compositionally very
different to the Pathfinder rocks.
As evident from
Figure 9, Martian meteorites have the required composition. Keeping in mind
that all but one of these meteorites have an age of 1.3 billion years or
younger, one might speculate that the rocks with high MgO and Fe0 contents
originate from younger geologic provinces. For example, from the Tharsis region
with its huge shield volcanoes like Olympus Mons, the highest mountain in the
Solar System (base diameter 500 km, caldera diameter 72 km, height 25 km). The
Pathfinder rocks are thought to be much older (> 3 billion years).
On the Expected Dominance of SO2 in Martian Volcanic
Gases As discussed above, Shergottites
contain mantle-derived concentrations of about 200 ppm H2O, about
100 ppm CO2 but between 1200 and 5600 ppm SO2, while
terrestrial basalts contain about 2000 ppm H2O and similar
concentrations of CO2 and SO2.
Considering the
similar abundances of H2O, CO2 and SO2 in
terrestrial magmas, these three compounds are also found in about equal
abundances in the gases of terrestrial volcanoes. On a planet with a mantle
considerably poorer in water than Earth, but similar to or richer in
SO2, it is to be expected that SO2 will dominate the
exhalation gases, although part of the sulphur might degas in the form of
H2S and elemental S.
In order to explain
the run-off channels and valley networks present on the ancient, heavily
cratered Martian terrain, it has been suggested that Mars was warmed by the
greenhouse effect of a dense CO2 atmosphere. However, it has been
shown recently that CO2 alone does not give enough greenhouse
warming in the early Solar System history when the luminosity of the Sun was
about 25 to 30% lower than today. In addition, a dense CO2
atmosphere leads to the formation of CO2 clouds, which reflect
Sunlight. SO2 is a very efficient greenhouse gas and its importance
for heating of the Martian atmosphere has been discussed. However, the lifetime
of SO2 in the Martian atmosphere is the order of months and was
considered to be too short for a noticeable effect.
At present, the mean surface temperature of Mars at
low latitudes is 218 K or -55°C, while at the poles the temperature drops
to less than -140°C. Considering the lower Solar luminosity 3.5 billion
years ago, the equatorial mean temperature would have dropped to about 200 K or
-73°C, or close to the freezing point of SO2. Thus, without an
appreciable greenhouse effect, H2O should have been a solid at all
latitudes, CO2 a solid or gas depending on the latitude, and
SO2 a liquid or solid, depending on latitude. On Earth, the
CO2 from erupting lavas amounts to less than 10% of the amount of
CO2 emitted to the atmosphere from fracture zones and diffusive loss
through volcano flanks. Under similar conditions on Mars SO2,
CO2 and H2O would have migrated through the
(mega)regolith towards the surface. Most of the CO2 would have been
quickly transferred to the atmosphere, while SO2 gas would have fed
solid respectively liquid SO2 tables at low depths. Water vapour
will be trapped at even greater depths and correspondingly higher
temperatures.
The degassing rate of SO2
from erupting magmas is too small for a substantial contribution of
SO2 to greenhouse warming on a global scale. However, local warming
by a volcanic intrusion or large impact could liquefy stored SO2 and
drive it to the surface as a liquid at temperatures close to the SO2
triple point (16 mbar and -73°C). Evaporation of stored liquid or solid
SO2 could lead to a sudden release of enough SO2 into the atmosphere
for a global temperature rise. In this way evaporation of SO2 on a
global scale might be triggered. Global warming would lead to melting of water
ice stored at greater depths. Global warming would also explain the break-out
of water in areas not directly connected with volcanic activity, if a late
veneer has supplied enough water stored in near surface layers in the form of
ice. Water in the atmosphere would further add to the greenhouse warming.
The warm climate would last until the
SO2 released into the atmosphere is all oxidized to SO3
and bound in sulphate in the Martian surface layers. As the temperature drops
the original values of volcanic SO2 and H2O would again
be maintained in the megaregolith until the next large eruption leading to
global warming. The whole cycle may have been repeated many times over a long
period of time until the volcanic activity ceased. Instead of one warm period
lasting for several 100 million years one might think of many shorter periods
in the same time interval.
Mars contains
considerable amount of FeS. Although most of the FeS today resides in the
Martian core a small fraction has remained in the mantle, from which
sulphur-rich magmas were formed. The oxygen required to transform FeS to
SO2, or to SO3 (sulphate), may have had an important
influence on the oxygen fugacity of the Martian surface and might well be the
limiting factor for the existence of water. |
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The picture was taken by the Imager for Mars Pathfinder
(IMP) on Sol 4. The Sojourner rover has travelled to area of soil and several
rocks. It tracks are clearly visible in the soft soil seen in the foreground,
and were made in part by the rover's material abrasion experiment. Scientists
were able to control the force of the rover's cleated wheels to help determine
the physical properties of the soil. Rover Sojourner is just using its
|
Alpha Proton X-Ray Spectrometer (APXS) instrument - which
results have been given wide coverage in the above article - to study an area
of soil. Sunlight is striking the area from the left, creating shadows under
Sojourner and at the right of local rocks. The large rock Yogi can be seen at
upper right. NASA /JPL |
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click to enlarge
