Determining the mineral content of Mars using reflectance spectroscopy
from Earth based telescopes is complicated by interference from
the atmospheres of both Earth and Mars.
For instance, detecting water molecularly bound into clay on the
martian surface is impeded by the water in Earth's atmosphere.
Both Earth's atmospheric H2O
and this clay-bound water tend to absorb at 1.4, 1.9 and 3.0 µm.
Furthermore, the carbon dioxide(CO2) absorbs at 1.4 µm
and in the range of 1.9 to 2.1 µm, and has absorption bands at
many infrared wavelength ranges that are important for determining
mineral contents. Scientists must attempt to compensate for these
obstacles when making interpretations of Mars reflectance spectra.
The surface of Mars has generally
been divided into two types -
high and low albedo (having a high or low level of reflectance
or more simply bright and dark regions).
The high - albedo areas have a similar reflectance to that of
martian dust storms, indicating that those regions may be covered
with a wind blown dust that is sometimes redistributed by these
storms. The regions with low albedo have characteristic 1 µm
absorption bands which seem to indicate the presence of pyroxenes
and perhaps olivine. In addition, the overall trend of the
reflectance spectrum for the low-albedo regions is downward,
because the surface is coated with brighter, oxidized (rusted) dust,
but has underlying darker mafic rocks. Mars is the “red planet”
because of this thin layer of rust-like dust particles,
which complicates attempts to identifiy the rock types beneath.
The oxidized iron which lends its color to Mars was determined
by the Viking mission to make up about 18 to 20% of the soil
tested. To replicate the Fe3+ absorption indicators
observed in Mars reflectance spectra, it is proposed that
a poorly crystalline or nanophase form of hematite
(-Fe2O3) is present. A mineral is nanophase if it
has crystalline structure, but is in very small particles
(in this case, less than 10 nanometers across). A poorly crystalline
material has little internal structure beyond the molecular
level(e.g., window glass, which has the same chemical foundation
as quartz but is much less structured). In fact, glass can be thought
of as a very thick liquid.
One earth analogue to the characteristic dust and soi
l observed on Mars is altered volcanic material.
Palagonitization is a process where poorly chrystalline
materials formed through weathering of volcanic ash.
This product can be called palagonitic material (or soil).
Other possibilities for the iron-bearing materials on Mars
include clays such as smectites and serpentines.
As noted,scientists have also determined that molecular water
exists in the structure of one or more minerals on Mars,
based on the absorption bands of water that are present
at 1.4 and 1.9 µm. Many Earth clays match these bands,
but they all have an additional absorption band at 2.2-2.3 µm
which is not present in Mars absorption spectra. A band in the range
2.2-2.3 µm is observed for OH bound to Al, Fe, Mg or
Si present in clays or altered glass. Mixtures of minerals
result in weakening and masking of spectral bands. For example
10-20 percent montmorillonite clay, which has a band at 2.2 µm, could
be present in a mixture without observing this band.
It has been suggested that a mixture with palagonites could be up
to 15 percent montmorillonite clay before that 2.2 µm
band would become visible.
Several factors of Martian surface composition remain unexplained.
Mars exhibits a weak absorption band at 2.36 m which is not
replicated by any of the minerals suggested above. Such bands are
present in minerals with Mg-OH chemical bonds, such as talc and
serpentine, but the bonds on Mars are weaker than would be
expected from those minerals. It is possible that these clays
are only a small part of the mix of Martian minerals, that the
wind deposited soils have a structure of medium rigidity between the
palagonites and a highly formed crystal, or that Mars has a
great deal of a particular type of mineral called scapolite -
which can contain other ions in the gaps between its molecules.
The martiam soil may also contain carbonates, sulfates, and nitrates.
(sulfer was detected by the Viking landers).
Reflectance spectra of Mars contain no evidence of the 2.55 µm band
characteristic of carbonates. Most of the strong absorption bands
for nitrates are between 3 and 4 µm, a range for which less
data are present. The main absorption bands for sulfates are
interfered with by the strong absorption by atmospheric CO2.
Mars Atmosphere x-section
Surface Oxidation
The complications of atmospheric absorption and surface oxidation
have made determining the surface composition of Mars from
Earth based spectroscopy difficult. Many uncertainties regarding the
causes of certain spectral features still remain. Current missions (MER,
Beagle) may elucidate the chemsitry of martian soils aiding in the
interpretation of remotely sensed spectra such as the data to be
acquired by CRISM.
Click here to view a table of likelihood
of finding various minerals on the Moon, Mercury, and Mars.
References
Englert, Peter A. and Pieters, Carle M. (1993)
Remote Geochemical Analysis: Elemental and Mineralogical Composition
New York: Cambridge University Press, 312-317.
Rencz, Andrew N.(ed.) (1999) Remote Sensing for the Earth Sciences.
New York: John Wiley and Sons, Incorporated.
Viking Orbiter and Lander Images courtesy of NASA.
