Small data set, high impact

[Musings by Talitha Santini]

The first article for the revived Journal Club was ‘Jarosite as an indicator of water-limited chemical weathering on Mars' (Elwood Madden et al. [2004] Nature, 431:821-823), chosen by Talitha because it was a short and reasonably simple article about soils on another planet. The authors used observations of jarosite and gypsum as alteration products of the basaltic parent rock at the Meridiani Planum landing site on Mars to argue a case for chemical weathering on Mars being water-limited. Using geochemical modelling software, a basaltic mineral assemblage (using data from Rosenbauer et al. [1983]) was titrated into a fluid containing SO₄^2-, Na⁺, K⁺, Ca²⁺, Fe²⁺, Mg²⁺, Al³⁺, and dissolved SiO₂, under current Martian atmospheric O₂ and CO₂ fugacities, at 298 K and 10⁴Pa total atmospheric pressure. The authors modelled both mineral assemblages as basalt weathering progressed, and the final mineral assemblages at different water:rock ratios. Modelling indicated that jarosite could only be present as a result of basalt weathering if (a) a large quantity of water reacted completely with a small amount of rock (for example, water creating an alteration rind on rock surfaces); or (b) a small amount of water reacted only partially with a large amount of rock. It was concluded that once jarosite formed, water must have been removed quickly in order to halt chemical weathering before pH increased and jarosite was converted to an iron (oxy)hydroxide - hence the article's title.

Once we started examining the article, we realized how little data were required to have an article published in Nature. The authors used only five pieces of information about Mars (the presence of a basaltic parent rock, the presence of jarosite and gypsum, and Martian atmospheric O₂ and CO₂ fugacities and atmospheric pressure) and a geochemical model designed for Earth surface conditions. This got us thinking about having a look at some Martian soil data from NASA (presented in Rieder et al. [1997]) and publishing our own articles. It's a good example of researchers considering data critically and extracting as much information as possible with the tools they have available to them. The tools we have available aren't perfect though, and one of the criticisms of this article was the ‘Earth-centric' modelling - that is, the use of Earth surface conditions to simulate Martian weathering processes. Where possible, the authors used (current) Martian conditions; however, in the case of temperature and pressure, current Earth conditions were used.

The ‘suppression of mineral phases...at the discretion of the operator' also raised debate about using geochemical models to simulate weathering. The problem is that models generally predict phase assemblages at thermodynamic equilibrium, which may not actually occur in the field. The authors cite the example of slow goethite formation hindering its occurrence in acid mine drainage environments despite being the thermodynamically stable iron phase. Kinetics were only included in this model ‘through the suppression of mineral phases unlikely to form in a geologically relevant time period.'

So why should we care about soils on Mars, anyway? We came up with a few answers to this: for the pure basic science objectives of understanding the properties and history of Martian surface materials and understanding how soils develop under different environmental conditions; because soil on another planet is inherently interesting; and because we may even want to annex Mars one day as a replacement Earth, in which case an understanding of its soils would be pretty important.

The next article will be chosen by Bree, with the next meeting scheduled for July 1.

Image from www.andersonfreepress.net