Topography Analysis of the Martian Hematite Region

The Thermal Emission Spectrometer (TES) on the spacecraft Mars Global Surveyor (MGS) recently identified an unusual surface deposit on Mars as a coarse grained form of the mineral hematite. Hematite is a mineral composed of oxidized iron, Fe₂O₃. Several hypotheses have been suggested to explain hematite’s presence on the Martian surface. Possible causes include precipitation from Fe rich standing water, hydrothermal activity, ground water leaching, surface weathering processes and oxidation of magnetite rich lavas. Here the possibility of precipitation from standing water is examined using data from the Mars Orbiter Laser Altimeter (MOLA). In spite of the hematite’s association with a smooth surface deposit, it is found that the current topography of the region in which the hematite was identified is inconsistent with a large standing body of water. It is therefore considered unlikely that the mineral precipitated from a regional scale lake.

Introduction

Hematite is iron oxide, a-Fe₂O₃, more commonly known as rust. Hematite has a rhombohedral crystal structure and deposits can be found with either crystalline, micaceous or massive morphology. The name hematite, or haematite, comes from the Greek word “haema,” for blood because no matter the colour of the mineral, which can be red or black, it always leaves a distinctive red streak. It shares its chemical formula with the mineral Maghemite, g-Fe₂O₃, which is metastable. The difference is crystal structure. Hematite is the stable phase of iron oxide up to ~1390°C, while magnetite (FeFe₂O₄) is stable at higher temperatures. Maghemite can in form in magnetite deposits altered by water or reduced by a biological agent.

On Earth, hematite can form through hydrothermal or ground water activity, but is formed primarily when water dissolves mafic rocks containing iron-bearing minerals such as Olivine or Pyroxene, as illustrated in figure 1. The ferrous iron (Fe⁺²) is oxidized by oxygen into ferric iron (Fe⁺³). The new ions then combine with water molecules to form an insoluble precipitate, hematite. This leads to the association of the mineral hematite with water. On Earth, where one finds hematite, there must have been water.

Hematite on Mars

Identifying the mineral involved three principle steps, subtracting the atmosphere from the TES spectra, normalizing out the “standard” Martian surface and comparing what remained of the spectrum to spectra of minerals obtained in the laboratory. Please see Christensen et. al.(2000) for details.

Figure 1: Precipitation of hematite from water. Press & Siever, Understanding Earth, fig. 6.8.

Hematite was identified on Mars in the Sinus Meridiani region, at about 2°S, 3°W as can be seen in figure 2. It was found to be consistent with coarse grained, tens to hundreds of microns, crystalline hematite as opposed to red nanophase hematite that may be a component of the ubiquitous Martian dust. It was found to cover between 10 to 15% of the surface area in the region although if the grain size is at the low end of the range it could be significantly more and correlates well with a smooth surface that may be of sedimentary origin. Please see figures 3 and 4 for a narrow angle Mars Orbiter Camera (MOC) image of the contact area between the smooth region and the heavily cratered terrain and a lower resolution Viking image of the area for context. The hematite correlates with the smooth area in the images.

Figure 2: TES Mineral Map of Mars from 45°S to 45°N. The pink area near the centre of the image labeled "H" is the hematite deposit.

Formation Theories

There are several theories of how the hematite may have been deposited on the Martian surface. These include precipitation from standing water, hydrothermal activity, ground water activity, surface weathering processes and oxidation of magnetite rich lavas. The first four theories are based on observation of these processes on Earth.

It has been suggested that the hematite on Mars formed in an analogous process to the way Banded Iron Formations (BIFs) form on Earth. It is thought that an early Martian sea may have contained much dissolved iron and silica in anoxic conditions. Then, when the surface layer came into contact with oxygen through some unspecified process large amounts of hematite precipitated out of solution. Silica saturated surface waters may have been periodically overwhelmed by upwelling of iron rich deep waters caused by an underwater heat source. If the Martian hematite deposit is a BIF then there should be some quartz mixed together with it. The spectral signature of quartz has not been detected. Also, for this theory to be true, a large, standing body of water of at least regional scale would be required to exist for a substantial period of time.

Figure 3: (Previous page) MOC image of the contact between the smooth (hematite) region and the surrounding heavily cratered terrain. The top of the image is north. The smooth terrain type in the north of the image correlates with the hematite. Subframe of MOC image ab107704.

Figure 4: Viking context frame for figure 3. The green outline shows the entire MOC frame, while the red shows the subframe in figure 3. The hematite is the smooth area in the top half of the image.

Hydrothermal activity is another process known to create hematite deposits in Earth. This occurs when hot water moves through iron bearing rocks. The water dissolves some of the iron and carries it away. Changes in pressure and temperature cause changes in solubility that can lead to precipitation of iron oxide, hematite. The trouble with this explanation for the hematite on Mars is twofold. First, the size of the hematite region is much larger than any hematite deposit on Earth known to have originated in this way. Second, there is no evidence for any local volcanism or other heat source.

On Earth, movement of groundwater can aide in hematite formation, most often between the annual highest and lowest levels of the local water table or in regions of high rainfall and alternating wet and dry seasons. The chemical process is essentially the same, only the source of the water and the mechanism for moving it changes. The primary objection to this theory is the requirement for a large amount of surface water from which the groundwater springs. There is little or no evidence of surface features carved by rainfall on Mars.

Another proposed mechanism is that of surface weathering processes. It has been argued that water in the atmosphere could have reacted with the rocks in the same manner and produced a hematite coating. There are two problems with this idea. First, if this were the case one would expect to find hematite to be widespread on the surface. Second, the kind of hematite produced by surface weathering processes is nanophase, red hematite. TES has identified coarse grained, black, crystalline hematite.

Finally, oxidation of magnetite rich lavas can produce hematite. Magnetite oxidizes to hematite at temperatures as low as a few hundred degrees Celsius. However, there is no evidence for lava flows in Sinus Meridiani. The main advantage of this model is that it does not require large amounts of liquid water.

Christensen et. al. examined these theories, and based on the association of hematite with an apparently sedimentary layer, the deposit’s large size, its distance from a near surface heat source and the lack of evidence for extensive groundwater activity anywhere else on Mars, they prefer the precipitation from standing water explanation.

Topography

The question that this paper is attempting to answer is whether or not the topography of the region can help to decide between these theories. Topography is especially important to the standing water formation theory. Elevation maps of the region where constructed using data from MOLA. The smooth layered deposit was identified and the elevation maps were examined with an eye to implications for large, regional scale standing bodies of water about the size of the hematite deposit. No closed contours were found. The present topography is not consistent with a large, regional scale lake as can be seen in figures 5 and 6.

The lack of closed contours in this region throws a great deal of doubt on the postulated existence of a large, regional scale lake that seems to be required for Christensen’s favoured theory to be correct. The topography data show no obvious contradictions with the other theories.

There is some morphological evidence of a lakebed, but it is purely circumstantial. In figure 5 one can see what appears to be channels running into the hematite region. On inspection of MOC images it remains uncertain whether or not the channels in fact end when they meet the smooth layered unit that corresponds to the hematite deposit or if they run underneath it and the hematite was laid down after. One can also imagine a situation where an ice dam might have prevented water from flowing out of the region, holding it long enough for hematite to precipitate. This seems plausible since the average slope in the region is of order ~10^-2. However, on first glance this seems unlikely due to the lack of obvious erosional features that would have resulted when the dam gave way.

Figure 5: Elevation map of the Sinus Meridiani region. The hematite region is roughly enclosed by the ellipse. The (0,0) point corresponds with 0°N, 0°W.

Figure 6: Elevation map of the Sinus Meridiani region. The hematite region is, roughly, enclosed by the ellipse. One can see there are still no closed contours on a larger scale. The (0,0) point corresponds with 0°N, 0°W.

Conclusions

Given present topography, there cannot have been a large, regional scale lake in the Sinus Meridiani region. This calls into question the validity of the theory of precipitation from standing water as an explanation for the presence of hematite. The lack of a large, regional scale lake is not, so far as the author is aware, inconsistent with any of the other five theories.

The author would like to speculate on the process of formation of the hematite. The large outflow channels on Mars originate somewhere on the dichotomy and run northward toward lower elevation. It has been suggested that the cause of the catastrophic floods could be sudden breaches in the permafrost layer on the slope leading down to Vastitas Borealis. Therefore, if the hematite region proved to be slightly higher in elevation than the beginning of the outflow channels, it might be possible for a break in the permafrost to cause slow release of water driven by the lower hydrostatic pressure at greater altitude instead of the catastrophic release that carved the channels. This could be a source of water for the groundwater theory. It bears investigation.

References

Byrne, Personal Communication, 2001.

Christensen et. al., ‘Detection of crystallin hematite mineralization on Mars by the Thermal Emission Spectrometer : Evidence for near surface water.’, JGR 105, p.9623, 2000.

Christensen et. al., ‘Identification of a basltic component on the Martian surface from thermal emission spectroscopy’, JGR 105, p.9609, 2000.

Hamilton, ‘Thermal infrared emission spectroscopy of the pyroxene mineral series’, JGR 105, p.9701, 2000.

Hamilton & Christensen, ‘Determining the modal mineralogy of mafic and ultramafic igneous rocks using thermal emission spectroscopy’, JGR 105, p.9717, 2000.

Hurlbut & Sharp, ‘Dana’s Minerals and how to study them’, 4^th Ed., John Wiley & Sons Inc., 1998.

Naboko, ‘Hydrothermal mineral-forming solutions in the areas of active volcanism’, Oxonian Press, New Delhi, 1982.

Press & Siever, ‘Understanding Earth’, 3^rd Ed., W. H. Freeman and company, 2001.

Smith et. al., ‘Separation of atmospheric and surface spectral features in Mars Global Surveyor Thermal Emission Spectrometer (TES) spectra’, JGR 105, p.9589, 2000.

Thompson & Thompson, ‘Atlas of alteration: a field and petrographic guide to hydrothermal alteration minerals’, Geological Association of Canada mineral deposits division, 1996.