Fe oxidation a solution to acid Mars on a basaltic planet?

This is a review of
"Origin of acidic surface waters and the evolution of atmospheric chemistry on early Mars"
Joel A. Hurowitz, Woodward W. Fischer, Nicholas J. Tosca & Ralph E. Milliken
Nature Geoscience 3, 323 - 326 (2010) Published online: 4 April 2010 doi:10.1038/ngeo831

I've talked about the apparent paradox between the widely accepted view (it seems) that aqueous environments on Mars were predominately acidic and the fact that Mars is predominately a basaltic (and olivine -rich!) planet. The point being that basalt and olivine will act to neutralize any acidic solution they come into contact with. The other point being that there is a lot more rock on Mars than water or acid, so one would expect the rock to win. However, that is only true if there is a fair amount of water available to bring the acid in contact with said rock. This seems to be the case in the groundwater hypothesis for Meridiani which suggests that water was supplied from a global aquifer recharged thousands of miles away.

Anyhow, this paper is a possible solution to this paradox, and presents an interesting possibility.

My biggest issue is that the paper assumes a "neutral" Fe-, SO4-rich starting solution. Of course by doing this there is an assumption made that the water has already had acid added to it (How else would SO4 get there?). So the problem lies with charge balance. In their assumption, the authors presumably assume that the Fe2+ is charge balanced by SO4 -- although that is never indicated.

They conclude that acidity is produced by rapid oxidation of Fe2+ in the presence of potent atmospheric oxidants. While the system they describe is one possibility for the chemical environment of Meridiani Planum, Fe-oxidation cannot be the underlying source of the acidity because of charge balance in the solution. At equilibrium, the charge of an aqueous solution must be balanced, thus every positively charged cation must be balanced by a negatively charged anion. Therefore, what anions are balancing the Fe(2+) in the aqueous solution before it is oxidized? In order to answer this question, one must look back to how the Fe(2+) originally entered the solution. In this case Hurowitz et al. suggest the Fe(2+) is originated from anoxic basaltic weathering. The primary Fe-rich minerals in basalt are olivine and pyroxene which weather according to the following reactions:

Fe2Si2O6 + 2H2O --> 2Fe(2+) + 2SiO2 + 4 OH- (1)
Fe2SiO4 + 2H2O --> 2Fe(2+) + SiO2 + 4OH- (2)

The important thing to note is that the Fe(2+) is charge balanced by OH-. This is inescapable in this simple system because there are no other anions by which to balance the positive charge of the Fe(2+).

Likewise, simple oxidation of the Fe(2+) consumes H+ and in fact makes the aqueous solution more alkaline rather than more acidic (after eq 2 from Hurowitz et al.):

2Fe(2+) + 0.5O2 + 2H+ --> 2Fe(3+) + H2O (3)

Acidity is actually introduced by the hydrolysis of the aqueous Fe:

Fe(3+) + 3H2O --> Fe(OH)3 + 3H+ (4)
Fe(OH)3 --> FeO(OH) + H2O (5)

Fe(OH)3 is insoluble and precipitates from solution, subsequent dehydration of this species yields common iron phases such as goethite and hematite (eq. 5). Viewing equation 4 here in light of equations 1 and 2, it becomes clear that in precipitation, the iron has simply paired up with the OH- that was its original charge balance in the solution. So writing the complete chemical reaction including weathering, hydrolysis, and oxidation – looking only at iron:

2FeO(pyx, ol) + H2O + 0.5O2 --> 2FeO(OH) (6)

This reaction shows that the overall production of acid by weathering of Fe-rich minerals in basalt is in fact neutral. This is born out in the terrestrial environment where oxic weathering of basalt, which includes rapid Fe-oxidation, yields neutral to basic solutions.

In order to avoid this charge balance problem, Hurowitz et al. must provide some means for removal of OH- from the solution between the weathering of the basalt and the precipitation of the Fe at the surface. Charge balance plays an important role here as well, as one cannot simply add another cation to the solution without a anion to balance its charge. The only means that I can think of for getting rid of the OH- is to add some sort of acid to the solution. For example, this could be accomplished through addition of SO2 aerosols. Of course this means that the underlying source of the acid in the system is due to the sulfur and is not due to the iron as maintained by Hurowitz et al.

Another means for avoiding this problem would be to source the iron from something other than pyroxene and olivine. This has been proposed by others who suggest weathering of iron sulfides is the ultimate source of the acidity at Meridiani Planum. In this case, the underlying cause of the acidity would be weathering of iron sulfides while hydrolysis of iron would contribute to the acidity.

Another possibility maybe be chlorite formation, which is precipitated during seawater alteration of basalt and can produce acidic hydrothermal solutions. While this may not be applicable to Meridiani, chlorite has been identified on Mars and this process may be a means for creating acidic solutions on a basaltic planet.

Review of "Dust and ice deposition in the Martian geologic record"

This is a review and discussion of
Tanaka K. L. (2000) Dust and ice deposition in the Martian geologic record. Icarus 144(2), 254-266.

This paper's abstract opens with the statement:

The polar layered deposits of Mars demonstrate that thick accumulations of dust and ice deposits can develop on the planet if environmental conditions are favorable. These deposits appear to be hundreds of millions of years old, and other deposits of similar size but of greater age in nonpolar regions may have formed by similar processes. Possible relict dust deposits include, from oldest to youngest: Noachian intercrater materials, including Arabia mantle deposits, Noachian to Early Hesperian south polar pitted deposits, Early Hesperian Hellas and Argyre basin deposits, Late Hesperian Electris deposits, and the Amazonian Medusae Fossae Formation.

I think Tanaka lays out a convincing argument throughout this paper tying together age relationships, photo-geologic observations, and a big picture view of Mars as possibly only he can. He lays out a potential history of Mars with the Noachian being marked by heavy deposition of dust/ice material created by pyroclastic eruptions, fluvial activity, impacts, and weathering. This is followed by a cooling of the climate where only episodic deposition occurs in only favorable places at the end of the Noachian/Hesperian. This period and subsequent periods are then marked primarily by deep erosion and redeposition of this material at the poles and in the Argyre and Hellas basins.

There is also an extensive discussion critical of the polar wander idea of Shultz and Lutz (1988). He cites problems with age-dating potential paleo-polar materials, and inconsistencies of the story with regard to the timing and growth of Tharsis. The conclusion being that any significant polar wander occurred very early in martian history and it's record has been destroyed.

It is amazing to note that this paper was written at the cusp of the revolution of scientific data from Mars, with only the preliminary returns from the MOC and MOLA to inspire it. I do not possess enough familiarity with the data to know how much things have changed, but to my eye many of the observations used to back up the arguments in this paper are still valid today. In fact much of the data returned has strengthened this argument.

One topic that I think is not treated well in this paper is the role of obliquity variations in driving dust/ice deposition and erosion processes. Given this driver, it may have been possible to have dust/ice deposition throughout martian history -- with gradually depleted ice reservoirs as increasing amounts of ice finds stable locations within the crust which are immune to climate changes caused by obliquity variations. This water could also be lost to space.

Finally in the conclusions Tanaka provides a means for discriminating between ice/dust deposits and fluvial deposits:

Such deposits [dust/ice deposits] should be fine grained and thus have low thermal inertia. They may be layered, reflecting climate-induced cyclic variations in composition. Airfall deposits may be draped over preexisting topography and may erode by mass wasting and (or) eolian erosion. In contrast, fine-grained fluvial deposits should have flat to gently sloping surfaces, be restricted to depressions, and have feeder channels and a local source region.

Of course the main idea behind the source of these deposits today is that they are the result of groundwater upwelling and evaporation similar to what has been proposed for Meridiani. I'm not clear how this changes these criteria or not. I think that the draping should be a defining characteristic distinguishing between these two depositional mechanisms, but aeolian reworking can always be invoked to explain draping no matter what mechanism is proposed.

Mars...The Big Picture

People who know me, know that I like to talk about the "Big Picture" a lot. So perhaps the best place to go with this is to outline Mars surface science from the Biggest picture I can imagine (that still retains some sort of usefulness). I want to preface this by saying that I'm not attempting to be comprehensive here, I'm just trying to lay out things from my perspective. So sorry geophysicists, there isn't going to be alot about differentiation and planetary evolution.

I think the biggest picture question in my view is the question that most papers are ultimately addressing, and that is the "Warm, wet early Mars" idea. This idea is not well defined and as such discussions of this idea without clear delineations of what exactly is being discussed can be confusing. In my mind the question is:

Was early Mars warm enough, and wet enough, for a long enough period to allow for life to evolve?

This of course leaves a lot to be desired since we don't know how long it takes for life to evolve, or how warm or wet it needs to be. So many people have simply focused on a simpler version:

How warm, how wet, and for how long?

I think most Mars scientists would say that early Mars was warm and wet for at least very short periods - either in short impact induced periods, or in longer ocean forming epochs with rain and an active hydrological cycle.

I think many people would point to the results from the Opportunity rover, as well as the photos of outflow channels and valley networks, to say that Mars had an extended period where it resembled a terrestrial desert. Playa lakes formed inside craters, rainfall was limited but present, hydrology was mainly in the subsurface, and oceans may or may not have been present.

There are others, including myself, who tend towards the idea that Mars has been primarily cold and dry throughout its history, never resembling terrestrial environments, and rarely if ever being warm enough to allow for rain.

So in my mind, the most interesting martian science is that which ultimately addresses these questions through testing our various hypotheses about features on Mars.

Anyways, that is the short version, comments are of course encouraged.

Blog v 2.0

Ok, after a long period of innactivity, and some discussions. I've decided to refocus this blog on Mars science. The goal is to make it a science blog for Mars. Hopefully this will encourage others to join in and post their thoughts as well.

I deleted the older posts, and kept the Mars focused stuff. I'm going to start by reviewing older papers that I think have particular bearing on the issues I think are important. I'd like to start with Ken Tanaka's paper on ice rich sediments from 2000.

Any other requests for discussion topics? I'll open the floor -- What do you want to talk about?