http://www.physorg.com/news105965930.html

Hematite? Perhaps NASA should begin a search for life in these South Australian rocks? The billion year old hematite found in this drill core study shows how silly and misguided the NASA Mars Rover team scientists were to consider that hematite, if found on Mars, would be a "beacon" for liquid water and a help in the search for life on Mars. They have continually, and incorrectly, argued that hematite is a mineral which usually forms in liquid water. It does not. Indeed, hematite forms from the dehydration of other water-containing iron oxides. The water in these original minerals is lost leaving a recrystallized product behind - hematite (Fe2O3). And the gray hematite ballyhooed by NASA in their seach for water and life is a metamorphic mineral requiring even higher temperatures to form. Thus, in their search for water and for life, NASA sent their rovers to the wrong places on Mars! Interesting results are being obtained but nothing to do with life.

The goal in geochemical interpretation is not to explain origins with just one mineral, it is instead to evaluate the convergence of multiple lines of evidence such as mineral assemblages, stratigraphy, and the overall geologic context. As such, gray hematite (α-Fe2O3) spherules are but one of many reasons that led the Mars Exploration Rover (MER) team to favor a playa setting at Meridiani [e.g., Squyres et al., 2004; dx.doi.org/10.1126/science.1104559] in the Noachian to Hesperian era.

The remote sensing observations of Sinus Meridiani by Christensen et al. [2001; dx.doi.org/10.1029/2000JE001415] demonstrate that lava flows are absent in the vicinity -- the region containing hematite exceeds 175*10^3 km^2 -- and that the morphology most resembles sedimentary outcrop. Their interpretation, which motivated the selection of Meridiani as a landing site for the MER mission, considered two major categories of geochemical processes: (1) chemical precipitation (2) thermal oxidation of magnetite-rich lavas. They had to disfavor the latter because of the geologic setting (e.g., absence of lava flows). Based on additional comparisons with earth analogs they favored one of two sub-processes of the former: (1a) sedimentary ferric hydroxide deposits – on earth these commonly occur in deep water environments [e.g., dx.doi.org/10.1036/1097-8542.312700] – with subsequent (low temperature) burial metamorphism to yield grey hematite (1b) direct precipitation of gray hematite from an Fe-rich brine. As Christensen et al. [2001] note, process (1a) is consistent with the banded-iron origin of the Peculiar Knob Formation [Schmidt et al., 2007; dx.doi.org/10.1029/2006JB004495], and actually requires large quantities of water to be present at Meridiani during Hesperian or earlier times. The reader’s primary argument, that gray hematite is only a product of high-temperature metamorphism, proves that the reader was unaware of the precursor minerals that require an abundance of water.

Nevertheless, observations by the MER Opportunity are more consist with process (1b), which requires less water than (1a). These observations include [e.g., Squyres et al., 2006; dx.doi.org/10.1029/2006JE002771]: (1) The origin of gray hematite as spherules within the pore spaces of the bedrock (2) stratigraphic evidence of festoons (3) presence of jarosite in the Burns formation of the Endurance crater [e.g., Morris et al., 2006; dx.doi.org/10.1029/2006JE002791] (4)abundance of sulfate minerals in the bedrock [e.g., Clark et al., 2005; dx.doi.org/10.1016/j.epsl.2005.09.040] (5)presence of crystal-shaped vugs [e.g., Herkenhoff et al., 2004; dx.doi.org/10.1126/science.1105286]. Jarosite is a key mineral, as it occurs in low-pH aqueous environments on earth and has ~10% stoichiometric H2O by mass [Klingelhofer et al., 2004; dx.doi.org/10.1126/science.1104653].

Even though exact terrestrial analogs of the Meridiani formation have not been found, several terrestrial geologic settings provide consistent analogs and help refine process (1b). For example, a low water:rock aqueous environment produced the gray hematite spherules in the high-porosity Jurassic Navajo Sandstone of Southern Utah [Chan et al., 2004; dx.doi.org/10.1038/nature02600]. Permian strata derived from low-pH ephemeral lakes (Nippewalla group) in what is now Kansas are terrestrial analogs of observations (2) – (5) [Benison et al., 2006; dx.doi.org/10.1130/G22176.1]. Ongoing and past mineral formations in the Rio Tinto basin of Spain include terrestrial analogs of (1) – (5) [Fernandez-Remolar et al., 2005; dx.doi.org/10.1016/j.epsl.2005.09.043].

As reinforced by hydrologic models of pre-Amazonian Mars [Andrews-Hanna and Zuber, 2007; dx.doi.org/10.1038/nature05594], an aqueous origin to the Meridiani bedrock is also consistent with global-scale processes on Mars. In conclusion therefore, we can be confident that the collective evidence favors aqueous alteration in a playa setting, in spite of interpretations to the contrary [e.g., [McCollom and Hynek, 2006; dx.doi.org/10.1038/nature05213] and the reader’s erroneous opinions.

"The reader’s primary argument, that gray hematite is only a product of high-temperature metamorphism, proves that the reader was unaware of the precursor minerals that require an abundance of water."

Not so! I wrote that "hematite forms from the dehydration of other water-containing iron oxides. The water in these original minerals is lost leaving a recrystallized product behind - hematite (Fe2O3). "original" = "precursor"


According to the site selection committee report (JGR), the primary mission of the Mars Rovers was to locate and visit areas where geologically recent evidence for liquid water might lead to evidence for life. The remote sensing evidence gathered from space was interpreted to be a signal for widespread gray hematite (specular hematite). This, again, is a recrystallization product of red hematite, which itself requires a hydrous "precursor" mineral. This finding, alone, should have been a red flag to the committee. The identification of this phase from space was based on the interpretation of the remote sensing signals that the flake-shaped crystals were in a preferred orientation with the flat surfaces parallel to the sedimentary surface (much like micas). On Earth, flake-shaped, coarse-grained gray hematite is NEVER a widespread primary "chemical precipitate" such as the precipitates in our Archean banded iron formations, and supposedly present over widespread areas on Mars. Even Christensen noted that it is not a primary precipitate. Amazingly, the site selection committee basically ignored this admission. I agree that the hematite precursor minerals (e.g., goethite, ferrihydrite, lepidocrocite) are the kinds of sedimentary iron minerals they should have been looking for, not those "baked" oxide minerals found in banded iron formations that are billions of years old. To consider hematite, especially gray hematite, as a mineral that "usually forms in water" is not only mineralogically naive but is also flat out wrong! The "water" the Rovers have "found" is in rocks that represent the early history of Mars. These rocks formed in water billions of years ago, but they have nothing to do with recent water or evidence for life....the rationale for the trip! The spin that has been put on these findings has been laughable.

It is true that the aqueous environments at both Meridiani (Noachian-Hesperian) and the "Columbia Hills" (Noachian) are geologically ancient, not recent. Nevertheless, these two sites potentially reflect processes during what Bibring et al. [2007; doi 10.1126/science.1122659] have tentatively termed the "theiikian" era, and were rich in groundwater. In particular, the Meridiani formation was not caused by metamorphism, not even low-temperature burial metamorphism, and the gray hematite there is not analogous to "baked" oxide minerals found in banded iron formations.

To reiterate -- as I noted in the 12 Aug post -- gray hematite at Meridiani is not an analog of banded iron formations on earth. Direct precipitation of gray hematite from Fe-rich brines is the most likely scenario [e.g., Squyres et al., 2006; doi 10.1029/2006JE002771] for their formation. Key terrestrial analogs of the Meridiani gray hematite are:
(1) The low water:rock aqueous environment that produced the gray hematite spherules in the high-porosity Jurassic Navajo Sandstone of Southern Utah [Chan et al., 2004; doi 10.1038/nature02600]
(2) Permian strata derived from low-pH ephemeral lakes (Nippewalla group) in Kansas [Benison et al., 2006; doi 10.1130/G22176.1]
(3) Ongoing and past low-pH aqueous alteration in the Rio Tinto basin of Spain [Fernandez-Remolar et al., 2005; doi 10.1016/j.epsl.2005.09.043]

Therefore, the banded-iron origin of the Peculiar Knob Formation [Schmidt et al., 2007; doi 10.1029/2006JB004495] is unrelated to the processes that produced the sulfate-dominated sedimentary deposits at Meridiani.

This respondent continues to miss the point, while at the same time continuing to foster the ridiculous notion that grey hematite can precipitate directly from water. The reason for going to Mars was to look for evidence of prebiotic or biotic processes. Such processes, of course, need water. The remote sensing data that supposedly found coarse-grained, platy hematite was part of the rationale for going to these sites on Mars. Again, the idea or belief that coarse-grained platy hematite can precipitate directly from water over large areal extents is mineralogical and geochemical nonsense! Lane et al. (Jour. Geophys. Res. 2002, v. 107) wrote this: "The [TES] observations are consistent with a formational model where the platy, gray hematite originated as an iron-oxide, chemically precipitated from Fe-rich aqueous and/or hydrothermal solutions on EARLY Mars, that was BURIED, RECRYSTALLIZED TO PLATY HEMATITE, and subsequently reexposed as lenses of schistose hematite..." (emphasis added). Nevertheless, and amazingly, NASA site-selection scientists believed (and apparently still do) that hematite, especially gray (specular) hematite, “typically forms in water” (Google that phrase!). Hematite is described as “usually ...associated with a wet environment”. These are demonstrably false and misleading statements. Hematite (red or gray) is anhydrous iron oxide (Fe2O3). Unlike jarosite, it is a mineral that has no “water” in its crystalline structure. It is not a primary mineral in any sediments that are widespread on the Earth. While it is certainly true that hematite is found commonly in water-laid sediments it is always found as an alteration product. It forms secondarily from a number of other iron oxide minerals that ARE the primary iron precipitates in water (precursors). Mineral names include limonite, goethite, lepidocrocite, ferrihydrite, feroxyhyte, the so-called "ferric hydroxides", and some others depending on the composition and temperature of the water. Hematite, however, forms by a dehydration and recrystallization after the structural water of any or all of these initial precipitates has been removed by heating and drying (diagenesis). This is more than a simple academic distinction because if the NASA team had been seriously interested in selecting a place on Mars to land their Opportunity rover where large amounts of water may have existed in the RECENT past they should have been looking for any one of these other minerals, or even any of the aluminosilicate clay minerals, but certainly not gray hematite. Once more, gray hematite is a product of the still further recrystallization of red hematite with elevated temperatures...hardly the process likely to be involved recently over the surface of a frigid Mars or one to have preserved biochemical fossils had life existed there. The world's major iron ore deposits, the billion year old banded iron formations, were formed in large, deep bodies of water and they contain gray hematite now, but only as a result of major thermal recrystallizations that have taken place in the intervening billions of years. The organic carbon content of these BIF oxides is almost zero. Gray hematite is to an iron-rich watery environment (a lake or sea) as anthracite coal is to a watery plant-rich coal swamp. Both are far removed from the water in which their "ancestors" grew. Following the Viking missions, there were few who doubted that some surface water existed on Mars during its EARLY history and which through the photochemical production of oxygen and weathering caused the formation of a variety of hydrous ferric iron minerals. In 1977 Toulmin et al. suggested the phyllosilicate nontronite was a likely Martian iron mineral (Jour. Geophys. Res. v. 82, p. 4625). Presumably, then, the current NASA missions were intent on searching for more recent evidence of water on the Martian surface? Golombek et al. (Jour. Geophys. Res, 2003 v. 108) wrote: "...the science objectives of the missions...are to determine the aqueous, climatic, and geologic history of sites on Mars where conditions may have been favorable to the preservation of evidence of possible prebiotic or biotic processes." In Table 5 the sites of Meridiani and Gusev were, amazingly, given "No obvious concerns" under the criterion: "May preseve (pre) biotic materials." Therefore, when widespread evidence of gray hematite was identified from remote sensing spectrometers in orbit those places should have been eliminated as potential landing sites simply because the odds of finding any of the primary iron oxide precipitate precursors or prebiotic materials there would be slim. Coarse grained, platy gray hematite is hardly a "beacon for aqueous processes” for extensive recent water where evidence for life might reasonably be found. Focusing on gray hematite may, sadly, have represented an “Opportunity” missed because widespread deposits of this mineral almost certainly imply a very ancient, “cooked” terrain. And, since Mars has been called the Red Planet for centuries, the presence of red hematite was a foregone conclusion, there being no other realistic mineral possibility. Hopefully, future missions to Mars will not use hematite as a signal for liquid water, especially gray hematite!

Additional note: An e-mail note from Dr. Robert Berner (Geochemist, Yale University; member NAS) on the subject of gray hematite on Mars:

Years ago I showed that hematite forms mostly from the
dehydration of "limonite". Also, grey hematite is only the long term
result of constant re-crystallization under increasing temperature.
Young hematites in red soils are reddish-orange. Western USA redbed
hematite is truly red. More deeply buried and more heated (eg Newark
Series) hematite is purplish red. The Paleozoic "redbeds" are
purplish. The BIF's become purplish-gray to gray as they have been
metamorphosed to some extent. I believe that gray hematite means heat
and a lot of time. But the precursor ferric hydroxides supports a
water origin.

Bob

Precursors support a watery environment? Yes. But gray hematite? Absurd!

It is wrong to state that gray hematite cannot precipitate directly from Fe-rich brines. The gray hematite at Meridiani occurs as spherular concretions that formed within the pore space of pre-existing sedimentary rocks. This process is completely unrelated to the banded Fe formations -- and the related metamorphic dehydration into gray hematite -- on earth that Tennilleguy discusses. Metamorphic high temperature formation of hematite, albeit very common on earth, is only one of several ways in which hematite may form on other planets. It is certainly not the way hematite formed at Meridiani. As I also noted, the gray hematite concretions are but one of many observations indicating that flowing groundwater existed at Meridiani. With the possible exception of Gusev, Meridiani remains the only locality where compelling chemical, mineral, and textural in situ evidence has been found for a groundwater table at any point in the geologic history (ancient or recent) of Mars.

As I mentioned in the 16 Aug post, (spherular) gray hematite concretions formed as direct precipitates from Fe-rich brines at the following terrestrial sites, and they, not the banded Fe formations so common on earth, remain the best analogs of the hematite concretions at Meridiani:
(1) Jurassic Navajo Sandstone of Southern Utah [Chan et al., 2004; doi 10.1038/nature02600]
(2) Nippewalla strata in Kansas [Benison et al., 2006; doi 10.1130/G22176.1]
(3) Rio Tinto basin of Spain [Fernandez-Remolar et al., 2005; doi 10.1016/j.epsl.2005.09.043]

Since gray hematite also forms directly in low-temperature low-pH aqueous environments on earth, its presence cannot be used to exclude localities on other planets as candidates for aqueous activity. At the time of the MER site selection, hydrous Al-phyllosilicates could not be detected due to instrument limitations [Christensen et al., 2001; doi 10.1029/2000JE001415]. But a few regional hot-spots of gray hematite, including Meridiani, were evident. Meridiani also had favorable engineering constraints [Golombek et al., 2003; doi 10.1029/2003JE002074]. Given the geographically restricted nature of the deposit, its engineering safety as a landing site, and the potential for past water activity, the selection committee probably made the best choice they could among 155 [Golombek et al., 2003] potential sites.

Furthermore, as I noted in my 12 Aug post, the initial remote-sensing infrared spectral interpretation [e.g., Christensen et al. 2001; doi 10.1029/2000JE001415 and Lane et al., 2002; doi 10.1029/2001JE001832] that Meridiani may be contain platy gray hematite (with metamorphic implications) turned out to be wrong on the basis of observations by the Opportunity rover.

Mr. Karunatillake continues to "spin" the mission and the decisions made by the site-selection committee. He discusses what they found (or think they found) rather than what they set out to find based on remote sensing data and the interpretations thereof. Direct precipitation from iron-rich brines was not at the top of the list. They were not looking for obscure, rare or unusual mineralogies (which is what they found). They were looking for large-scale sedimentary deposits that occur over wide areas in hopes of finding RECENT evidence for water. After all, there was (and is) little doubt that water was present on ancient Mars. The problem, as I have already said, was the mineralogical interpretation used to make the site selection. The NASA committee believed (or were led to believe) that coarse-grained gray hematite would be a beacon for water-lain sediments. They were quoted repeatedly (and still are) that this mineral "typically forms in water." During the press conferences that preceded the landing some of the scientists were even displaying samples of gray hematite, such as used in jewelry! What is most surprising and difficult to understand is that the site selection committee ignored one of their own…P.R. Christensen. Christensen (and co-authors) repeatedly wrote on this topic. In 2000 (Jour. Geophys. Res. v. 105, p. 9623) they wrote: "We prefer chemical precipitation models and favor precipitation from Fe-rich water on the basis of the probable association with sedimentary material, large geographic size, distance from regional heat source, lack of evidence for extensive groundwater processes elsewhere on Mars. The Sinus Meridiani region may be an ideal candidate for future landed missions searching for biotic and prebiotic environments..." Having said all that they then wrote: "[Previous researchers] suggested that large amounts of ferrous iron were dissolved into acidic and anoxic ancient Martian SEAS and subsequently precipitated as insoluble hydrous ferric oxides [precursors] when the upper layers of the SEAS became oxygenated. Morris (1993) explained the banded nature of the Hamersley (Australia) BIFs by silica-saturated surface waters which were periodically overwhelmed by deeper, ferrous-rich water (source of hydrous ferric oxides [precursors] during convective upwelling in association with mid-ocean ridges or local hot spots. METAMORPHISM (BY BURIAL) OF THESE CHEMICAL PRECIPITATES [precursors] PRODUCED COARSELY CRYSTALLINE MATERIALS LIKE GRAY HEMATITE..." (emphasis added). Two years later, Christensen et. al, (2002, Jour. Geophys. Res. v. 106, 23,873) also wrote: "Formation modes for gray hematite detected by TES can be grouped into two model classes: (1) chemical precipitation and (2) thermal oxidation of magnetite-rich lavas. Chemical precipitation models include 1a, low temperature precipitation of Fe oxides/oxyhydroxides from standing, oxygenated, Fe-rich waters, followed by subsequent alteration to gray hematite. They went on to warn that "...models 1a and 1b require an oxidative alteration process (e.g., burial metamorphism) to convert Fe oxide/oxide assemblages (e.g., red hematite, goethite, ferrihydrite, goethite [sic], and siderite to coarse-grained (>10 µm) gray hematite." It is clear that the Earth’s banded iron formations were an important impetus at the outset. They may not be now (because what was found has nothing to do with BIFs), but they surely were then! Burial metamorphism is the common mechanism for the formation of gray hematite, but this process is not what one would desire when looking for biotic and prebiotic materials. As indicated earlier, oxide facies of BIFs have extremely low amounts of organic carbon in them. Catling and Moore (Sixth International Conference on Mars (2003) wrote: "Detailed features in the hematite spectral signature of the Sinus Meridiani region show that the spectrum is consistent with emission dominated by crystal c-faces of hematite, implying that the hematite is specular. Gray specular hematite (also known as “specularite”) is a particular gray crystalline form that has intergrown, hexagonal plates with a silvery metallic luster. We believe that the key to the origin of specularite is that it requires crystallization at temperatures in excess of about 100°C. In reviewing the occurrence of gray hematite on Earth, we find no exceptions to this warm temperature requirement."

That's essentially what Bob Berner said. Gray hematite is NOT "typically formed in water". NASA made a mistake! No amount of “spin” can alter this. Neither are gray hematites “typical” of an environment favorable for the preservation of biotic (or prebiotic) materials.

Fe-rich brines? Even if they are gray hematite, which is doubtful, they are not "typical" and even they require the precursor minerals that Mr. Karunatillake critcized me for not being aware of. At least he admits that the remote sensing prediction for the presence of platy, coarsely crystalline gray hematite was wrong. Indeed, the entire mineralogical reason for going where they went was wrong! As Christensen repeatedly noted gray hematite does not form directly from water (lakes, seas) and now, after landing it isn't even present as postulated? Maybe Mr. Karunatillake should pass that along to the NASA folks? In SCIENCE (v. 306, p. 1736), Christensen et al. state: "The presence of coarsely crystalline hematite exposed on the surface has been predicted from orbital TES data and was confirmed in the Mission Success panorama..." How so? The published Mini-TES spectra (Fig. 7a, 9) are cut off near 400 cm-1 so that the critical 390 cm-1 emissivity band is not shown. Thus, it’s not certain that the hematite present there is even gray hematite. After the landing of the Opportunity rover Soderblom et al. (SCIENCE, 2004, v. 306, p. 1723) wrote: "Meridiani Planum is unique in its orbital signature; the MGS Thermal Emission Spectrometer showed the strongest, most extensive signature for coarsely crystalline hematite of any region on the planet. Because most modes of hematite formation involve liquid water, Meridiani Planum is a prime site for exploration." Wrong then, still wrong!

The hematite in the Navajo Sandstone is red, not gray, and did not precipitate directly. It is, like other sedimentary hematites, a diagenetic recrystallization product. Beitler et al's paper is even titled: Diagenetic analogs to hematite regions on Mars: examples from Jurassic sandstones of southern Utah, USA". Chan was quoted: "Pink Navajo Sandstone contains 1 to 2 percent hematite (Fe2O3). When there is a higher concentration of iron oxide cement (5 to 25 percent), the sandstone often looks deep brownish-red," Deep reddish brown is not gray, and it certainly is not "coarsely crystalline" there either. The Rio Tinto is red and is also a diagenetic product from goethite (precursor). This is not direct precipitation, is localized, and is not an example from over the broad geographical extent desired as an appropriate analog for Mars. Such rare, localized occurrences are now being "spun" as evidence for widespread sedimentary deposits.

Quoted by Robert Roy Britt (SPACE.com, 2/3/04) “The Growing Case for Water on Mars” (http://www.space.com/scienceastronomy/mars_hematite_040203.html) Bob Craddock said it best: “It won’t be easy to put the hematite into context. What’s found might have formed early in Mars’ history, merely adding to a practically closed case for a wet young planet.” And…that's exactly what happened! Interesting results, fabulous technology, but no recent water, no evidence for life. A bummer?

As noted by Tennilleguy on 17 Aug, the science objectives were stated explicitly as: "...determine the aqueous, climatic, and geologic history of sites on Mars where conditions may have been favorable to the preservation of evidence of possible prebiotic or biotic processes," with emphasis on "...past aqueous activity..." throughout [Golombek et al., 2003 doi 10.1029/2003JE002074]. Limiting the focus to a particular geologic period (recent or ancient), does not seem to have been a key criterion of site selection. Besides, it is well known that potentially recent aqueous activity, such as gullies [Malin et al., 2006 doi 10.1126/science.1135156], were inaccessible under MER engineering constraints. Such exploration may remain beyond our reach until technologies -- such as those of the Arctic Mars Analog Svalbard Expedition (AMASE) -- mature.

A key purpose of the rovers was to establish the geochemical context of previous aqueous activity and any site where that may have occurred was of great interest. Even though there is increasing evidence for a significant amount of flowing water on ancient Mars, the particular geochemical settings of ancient water activity are very poorly constrained. Therefore, the in situ investigation of ancient aqueous environments is an essential part of understanding the evolution of Mars and the possibility of life on that planet.

It is important to keep in mind that geomorphic evidence, as tantalizing as it may be, had failed to locate sites with in situ evidence of recent or ancient groundwater for Pathfinder and Viking missions, which may have also become Spirit's fate had it not ascended the "Columbia Hills." In light of all this and keeping in mind the clearly stated science goal of searching for past water activity, Meridiani, with its prominent hematite deposits, probably seemed a promising choice.

The site selection discussion clearly states that TES-spectral analysis could only interpret the hematite to be in any one of the following forms:
(1)discrete grains [Christensen et al., 2001 doi 10.1029/2000JE001415]
(2) crystals embedded in glassy coatings [Minitti et al., 2002 doi 10.1029/2001JE001518]
(3)a platy outcrop [Lane et al., 2002 doi 10.1029/2001JE001832]
Opportunity's discovery of spherular hematite concretions -- in the setting I tabulated on 12 Aug -- is consistent with (1). On 17 Aug, I merely highlighted that (3) is ruled out by the rover observations.

Golombek et al. [2003] clearly state that identifying the processes that led to the hematite deposit would be part of the science tasks at Meridiani; they do not emphasize one process (such as burial metamorphism or high-temperature oxidation of magnetite rich lava) over another (such as low temperature dissolution).

I agree that I failed to clarify the diagenetic origin of hematite in examples (1)--(3) in my 17 Aug post. The diagenetic processes in question involve low temperatures and significant quantities of oxidizing water. These are consistent with the interpretation that hematite concretions at Meridiani formed during a groundwater recharge event, most likely via replacive processes [McLennan et al., 2005 doi 10.1016/j.epsl.2005.09.041; Fig 4 and Section 5.2]. Experiments with synthetic Martian basalts are consistent with this scenario [Tosca et al., 2005 doi 10.1016/j.epsl.2005.09.042].

In summary then, the selection committee was aware of and clearly stated all forms the hematite at Meridiani could take and all possible chemical processes that may have produced it [Golombek et al., 2003]. The site selection process seems to have been well-informed and carefully considered, not misguided.

A layperson's questions:

Can you elaborate on the spherule "blueberries" and how they might've formed? Can water so thoroughly penetrate and flow through so much rock as to completely fill voids in the rock with the obviously harder materials? Why didn't the softer base rock wash away in the process? It seems to me that for the mineral to collect in such quantity, the water must have actually flowed through the rock continually - regularly depositing new material.

On the other hand, it seems to me that a static soak should have left a very hard and thick surface layer of the same material above the layer which includes the blueberries. Is there evidence of such a layer?

In my own limited experience, I've observed that softer materials tend to form around and cement between harder materials. One of my favorite items in my collection is a piece of natural concrete, in which mostly spherule pebbles can be observed to be thusly preserved.

Thanks.

More spin! The evidence (much of it already presented) reveals clearly that the Mars scientists, for the most part, actually believed that gray hematite "typically forms in water" or "usually forms in water". Google those phrases!

Here is what one reporter (Jim Erickson, Arizona Daily Star, March 6, 2000) wrote:

"Global Surveyor researchers are searching those sites for water-formed minerals, using a heat-sensitive device designed at Arizona State University. The thermal emission spectrometer, known as TES, was designed by Philip R. Christensen's research team. It identifies minerals by measuring their color in the infrared portion of the spectrum. The spectrometer has been mapping Mars since Global Surveyor's main mission began in 1998. So far, TES has seen no sign of salts or carbonates like limestone - minerals considered the best indicators of ancient lakes or oceans. And TES has seen no trace of quartz or calcite, two minerals commonly formed in hot springs. But the infrared instrument did find an outcropping of the mineral hematite about half the size of Arizona, Christensen said. Hematite is an iron-rich mineral that usually forms in water. On Earth, vast deposits of hematite around the Great Lakes were formed in an ocean several billion years ago, creating the primary source of iron in the United States. The Martian hematite deposit found by TES is in a flat region of layered deposits that could be sediments from a lake or small sea, Christensen said. The hematite region is NASA's primary landing site for a Mars probe scheduled for launch next year, he said."

Banded iron-formations! ocean! gray hematite!...usually forms in water! Seems clear that this is where they went wrong using gray hematite. It occurs in the banded iron formations...but this is metamorphic...no life!

There is no evidence that the widespread (half the size of Arizona?) formation of gray hematite (from its precursors!) takes place anywhere under conditions that might be suitable for the preservation of life. In spite of this, Table 5 of the site selection paper found the sites of Meridiani and Gusev to have "No obvious concerns" under the criterion: "May preserve (pre) biotic materials. The formation of gray hematite should have been, in fact, a big concern!

Statements such as: "Low temperature dissolution" or "The diagenetic processes in question involve low temperatures and significant quantities of oxidizing water" apply only to the hematite PRECURSORS. Yes, this is what they should have been looking for, not hematite. With respect to hematite, it is quite wrong to say low-temperature dissolution! Dissolution? This doesn't even make any sense. Hematite (Fe2O3) is anhydrous...it contains no water of crystallization. It is the precursor minerals that do. Gray hematite does not form from oxidizing waters at low temperature. Again, as Berner said, gray hematite is evidence of higher temperatures and lots of time. A continued process of recrystallization from colloidal-sized, initial water-containing yellow to red or reddish-brown precursor minerals (e.g. "rust") takes place to cause the large increase in crystal size...a primary characteristic of gray hematite. Grind up gray hematite and it turns red...mineral streak.

What are the diagnostic, distinguishing characteristics of gray hematite that the rovers found? What are the diagnostic, distinguishing characteristics of gray hematite in the concretions of the Navajo Sandstone? Isn't the identification of gray hematite in the "blueberries" a presumption...an after-the-fact interpretation? Without the much-ballyhooed remote sensing TES identification of platy, coarse-grained gray hematite prior to the mission there likely would be few, if any, who would have labeled the "blueberries" gray hematite. How is gray hematite distinguished from well-crystallized red or purple hematite...TES spectra? Or is it black-and-white, color "enhanced" , or false color photos combined with wishful thinking? Where IS the coarse-grained, platy gray hematite that was seen from space?


A layperson's question:

"Can you elaborate on the spherule "blueberries" and how they might've formed? Can water so thoroughly penetrate and flow through so much rock as to completely fill voids in the rock with the obviously harder materials? Why didn't the softer base rock wash away in the process? It seems to me that for the mineral to collect in such quantity, the water must have actually flowed through the rock continually - regularly depositing new material."

Because NASA asserts that it "typically forms in water" I think we are expected to believe that low-temperature, oxidizing Fe-rich waters actually precipitated gray hematite. That's nonsensical! If there was Fe-rich water passing through the sediment the minerals that might have precipitated would have been the precursor phases...goethite, ferrihydrite etc., not hematite, and certainly not coarse-grained gray hematite! For the precursors to have been diagenetically altered to hematite, esp. gray hematite, the water of crystallization (not adsorbed water) would have to be removed. When hematite is formed from them the water is now gone...the rock is dry. Life is expected there? That's like looking for plants or plant fossils in anthracite coal because peat and coal swamps are wet environments. Using the NASA mantra for gray hematite one might say that anthracite "typically forms in water". Or what about limestone? Does marble "typically form in water" too? Marbles are coarsely-crystalline calcite.

ubavontuba: Can you elaborate on the spherule "blueberries" and how they might've formed? Can water so thoroughly penetrate and flow through so much rock as to completely fill voids in the rock with the obviously harder materials? Why didn't the softer base rock wash away in the process? It seems to me that for the mineral to collect in such quantity, the water must have actually flowed through the rock continually - regularly depositing new material.


McLennan et al. [2005; link at dx.doi.org/10.1016/j.epsl.2005.09.041] provide a very clear and descriptive answer to your question. I'll try to summarize it here.

Summary observations that help elaborate the origin of hematite spherules are: evaporite (sulfates including jarosite, some of it with chemically bound H2O)-rich (sandstone) outcrop, crystal molds, elongated vugs, festoons, and cross-bedding.

The spherules do not disrupt the laminations in the outcrop, sometimes have banding that follows the laminations in the rock, are not more abundant at bedding planes than elsewhere, are sometimes conjoined as doublets or triplets, do not have resolvable internal structure at 100 microns or coarser resolution, and have distinct mossbauer, infrared, and chemical signatures (hematite evident and Fe dominates composition). Collectively, these suggest that the spherules formed at the site (as opposed to being carried over by water currents, volcanic activity, or impact ejecta) as concretions. The first two observations also indicate that the concretions formed after evaporite sediments were deposited. The absence of lamina disruption favors the formation of concretions by replacing soluble minerals and utilizing pre-existing pore space. One additional observation, the very nearly spherical shape of the concretions with only about a 6% elongation in one direction, shows that associated fluids were flowing very slowly relative to the rate of concretion (4mm size concretions could form in less than 10^3 mars years).

The summary observations and spherule properties, taken together, is consistent with a playa environment with a fluctuating groundwater table in the form of a highly concentrated brine. Previously deposited sediments would not dissolve completely and wash away in such a setting. The presence of jarosite indicates that the fluid pH was less than 5. Note that the low-pH and the basaltic source material distinguish the Meridiani setting from playas common on earth.

Goethite can form via the dissolution of jarosite when groundwater interacting with the evaporite sediments is altered by a higher pH solution for a geologically short period (e.g., groundwater recharge event). Under such conditions, goethite is thermodynamically unstable at the atmosphere-fluid interface, coagulating and altering to hematite at temperatures less than 100 C. Another possibility is the oxidation of a soluble Fe-sulfate (such as melanterite) to produce goethite followed by the same alteration to hematite. Tosca et al. [2005 link at dx/doi.org/10.1016/j.epsl.2005.09.042] discuss the thermodynamics of the concretion-forming chemistry in detail.

Tennilleguy: What are the diagnostic, distinguishing characteristics of gray hematite that the rovers found?
A high mass fraction of Fe (observed via the Alpha Proton X-ray Spectrometer), evidence for hematite from mossbauer spectroscopy, IR spectral evidence for hematite (Mini TES), and visible-to-near IR evidence for hematite (Pancam) are the diagnostic signatures of the spherules. [e.g., Suqyres et al., 2006 doi 10.1029/2006JE002771; paragraph 34].

Since spherules eroded from the outcrop form a lag deposit on the surface, they appeared as coarsely crystalline hematite to the TES (i.e, option 1 that I listed on 21 Aug).

Tennilleguy: Where IS the coarse-grained, platy gray hematite that was seen from space?
The IR spectral signature of Meridiani hematite spherules is consistent with that observed by TES, including the absence of a 390 cm^-1 absorption feature [Glotch et al., 2006 doi 10.1016/j.icarus.2005.11.020]. This enabled Glotch et al. [2006] to suggest two possible structural scenarios for the spherules:
(1) randomly oriented platy crystals may be present within the spherules
(2) spherules may have grown concentrically as an anhedral mass with [001] oriented radially

It is not possible to distinguish between the two by means of the IR spectrum. However, neither form of spherules can occur in the high-temperature regime (excess of 500 C), which is certainly consistent with the low-temperature aqueous processes postulated with the remaining in situ geochemical observations at Meridiani.

Thanks for the responses. It still doesn't make complete sense to me though.