2011 Annual Science Report
Rensselaer Polytechnic Institute Reporting | SEP 2010 – AUG 2011
Project 5: Vistas of Early Mars: In Preparation for Sample Return
To understand the history of life in the solar system requires knowledge of how hydrous minerals form on planetary surfaces, and the role these minerals play in the development of potential life forms. One hydrous mineral found on Earth and inferred from in situ measurements on Mars, is the mineral Jarosite, KFe3(SO4)2(OH)6. We are investigating whether radiometric ages, specifically 40Ar/39Ar ages on jarosite can be interpreted to accurately record climate change events on Mars. This project not only requires understanding the conditions required for jarosite formation and preservation on planetary surfaces, but also assessing under what conditions its “radiometric clock” can be reset (e.g., during changes in environmental conditions such as temperature). By studying jarosites formed by a variety of processes on Earth, we will be prepared to analyze and properly interpret ages measured from jarosite obtained from future Mars sample return missions.
The formation of supergene jarosite at planetary surfaces (i.e. Earth and Mars) requires the presence of water, however preservation of the mineral requires sustained aridity. As a potassium-bearing mineral, jarosite can be dated by the 40Ar/39Ar method. Therefore, jarosite 40Ar/39Ar ages can be used to date surface processes such as weathering and environmental transitions (i.e. aridification) on Earth and Mars. In preparation for Mars sample return missions and to better interpret jarosite ages from a thermochronological perspective, the diffusion kinetics of argon in jarosite were determined. Incremental fractional loss measurements indicate an activation energy (E) of 37.8 ± 1.5 kcal/mol and a log Do/a2 of 5.68 ± 0.63 s-1 corresponding to a closure temperature of 143 ± 28 °C, assuming a cooling rate 100°C/Ma (Figure 1).
Downward extrapolation of these parameters to Martian surface temperatures (≤ 22°C) predicts <1% fractional loss of Ar over 4.0 Ga. Forward modeling of 40Ar/39Ar age spectra using the least retentive E, Do/a2 pairs predict that if held at 22°C or less for 4.0 Ga, supergene jarosite would preserve original growth ages manifest as plateau ages consisting of >95% of the gas release (Figure 2).
Because of its susceptibility to mineralogical breakdown, 40Ar/39Ar ages on preserved Martian jarosite will reflect the time since water was present at a location that has since undergone aridification and remained hydrologically inactive and thermally quiescent.
As a proof of concept for applying jarosite 40Ar/39Ar chronometry to constrain the timing and rate of aridification for once-aqueous environments, a vertical profile of jarosite-bearing samples was collected from the top of the Paleocene Fort Union Formation in the Bighorn Basin of Wyoming. The Fort Union Formation exposed at the McDermotts Butte area (Figure 3) consists of interbedded fluvial sheet sandstones and floodplain mudstones (15, 16). Paleosols developed on the fine-grained floodplain deposits contain rhizoliths, relative abundances of goethite and hematite, and preserve jarosite indicating a general history of deposition, water saturation and soil formation, burial with sustained saturation, and finally excavation and drainage. This hydrologic sequence of saturation followed by drainage and sustained aridity (Figure 3) is analogous to that interpreted for the stratigraphy exposed at Burns Cliff at Meridiani Planum (Grotzinger et al., 2005).
Jarosite precipitation during drainage of the Fort Union Formation is a time-transgressive process where the oldest jarosite (first-formed) should be at the top of the section and progressively younger jarosite should be present downsection. The ages measured from 5 jarosite samples collected along a 25 cm vertical profile are in agreement with this prediction. Jarosite ages systematically decrease downsection (Figure 4) indicating a jarosite precipitation front migrated downward through the
section at an average rate of 3.6 cm/Ma. The precipitation front likely reflects slow drainage through this portion of the sedimentary column. This result indicates that jarosite dissolution rates may be slower than those derived from laboratory experiments and that jarosite ages can record geologically slow environmental transitions over very small spatial distributions. Additionally, jarosite-bearing samples retrieved by future Mars sample return missions will afford determining when and how long water existed at the Mars surface.
PROJECT INVESTIGATORS:Suzanne Baldwin
PROJECT MEMBERS:John Delano
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Biosignatures to be sought in Solar System materials