Posted by DR Bethany Ehlmann, Brown University.
The past decade of high resolution orbital imaging of Mars has revealed gullies, dune forms, fresh impact craters, polar layered deposits, and sedimentary stratigraphic sections through the use of the Mars Orbiter Camera (MOC; 1.5 m/pixel), the High Resolution Stereo Camera (HRSC; 2.3 m/pixel), the Context Imager (CTX, 5m/pixel), and the High-Resolution Imaging Science Experiment (HiRISE; 25cm/pixel). These have permitted detailed studies of aeolian, glacial/periglacial, and past fluvial processes that have shaped the development of Mars’ landscapes. Equally, the past five years of Mars exploration with orbital visible/near-infrared spectroscopy have led to the discovery of numerous classes of alteration minerals including clays, sulfates, and carbonates that provide information on the duration and chemical conditions of aqueous alteration, using the Observatoire pour la Minéralogie l’Eau les Glaces et l’Activité (OMEGA; 300m/pixel) and the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM; 18m/pixel).
Now, these data sets are being merged. By utilizing morphology with infrared color and spectra for mineral identification, more information can be obtained about the changing nature of aqueous processes using compositional stratigraphy, and sands and sediments can be better traced back to sources. Below are examples showing of the power of combining morphology and mineralogy and the diverse ways this has been utilized to find out more about Mars.
Stratigraphy and Mars’ Changing Aqueous Processes
In many studies of fluvial systems, a key question is duration. How long did a given fluvial system persist? Was it persistent or episodic? Does this have implications for the duration of liquid water on Mars, locally or globally? In and around the Nili Fossae of Mars, there has been widespread gradation: eroded channels and valleys, filled craters and fan/deltaic deposits (see October 2008 image). These events took place sometime after the formation of the Isidis basin but largely before emplacement of the early Hesperian Syrtis Major lava flows (Mangold et al., 2007).
Image 1 shows a 40km crater in the region, with over a kilometer of fill. Examination of the center of the crater with CTX images shows that sedimentary deposits within the crater have been eroded by a channel system that drained through a breach in the crater wall to the northwest. The morphology further shows the possibility of distinctive sedimentary units indicated by variations in albedo. With the addition of infrared spectral data, the bright-dark relationship is shown to be accompanied by distinctive mineralogic stratigraphy: a thin, bright capping layer of kaolinite- (Al2Si2O5(OH)4) bearing materials overlies a massive sedimentary unit(s) of Fe/Mg smectite clay-bearing materials ((Fe, Mg)3(Si,Al)4O10(OH)2.
The smectites have the same composition as those found regionally in in-situ bedrock, and have probably been locally sourced during an initial period of gradation. The kaolinite has no local bedrock exposure. Rather than being transported to the crater, it likely formed in-situ by leaching of the underlying smectite (loss of Mg, Ca, Fe) in a period when there was little erosion of the sediments in the crater. Finally, a more intense period of fluvial activity led to erosion of the deposits and breaching of the crater (Ehlmann et al., 2009). The identification of these three separate episodes of aqueous activity using morphology alone, would have been challenging, without the additional information provided by mineralogy.
A second example of the power of combined mineralogic/morphologic analysis for stratigraphy is shown in Image 2. Here, the Heseperian lavas flows of the Syrtis Major formation cover older bright, fractured Noachian deposits. High resolution grayscale images show few differences in the underlying units, save for that the uppermost units appear to be more regularly layered. However, with the addition of infrared color, two distinct units of altered minerals can be discerned, and using spectroscopic information, these have been identified. Here at NE Syrtis, there is a unique stratigraphy of iron sulfate overlying carbonate, which is being exposed by the erosion of overlying lavas (Mustard and Ehlmann, 2010). This suggests a transition in the aqueous alteration environment from neutral-to-alkaline to acidic that is preserved in the rock record.
Traverse Planning for Landed Operations
Finally, combined mineralogic/morphologic information is of high value in planning the traverses of rovers on Mars surface. For example, in the sulfate plains of Meridiani Planum, the Opportunity Rover is presently heading toward the rim of Endeavor crater in part because of the mineralogic signature of Fe/Mg smectites in the ejecta and rim rocks and the desire to investigate these clays in-situ [Wray et al., 2009]. The high resolution imagery has helped the rover avoid hazards and plan the most scientifically interesting and efficient route to these deposits. For the landing site selection process for the 2011 Mars Science Laboratory (MSL), high-resolution morphology with mineralogy is being utilized extensively to understand and discriminate the habitability potential, biologic preservation potential, and safety of candidate landing sites (e.g. image 2 is near one such site under consideration).
The CTX and HiRISE high resolution cameras and the CRISM hyperspectral infrared imager on the Mars Reconnaissance Orbiter will continue to acquire additional high resolution data. Keys for the future will be obtaining topographic information, e.g. digital elevation models to permit improvements in stratigraphy and additional coverage of the planet to further understand the distribution and transport of minerals. We are only beginning synergistic use of orbital mineralogic and morphologic data at outcrop scales, but it promises to be a fruitful area for future Mars science.
Ehlmann, BL, et al. (2009) Identification of hydrated silicate minerals on Mars using MRO-CRISM: geologic context near Nili Fossae and implications for aqueous alteration, J. Geophys. Res, doi:10.1029/2009JE003339. [Abstract]
Mangold, N., et al. (2007) Mineralogy of the Nili Fossae region with OMEGA/Mars Express data: 2. Aqueous alteration of the crust, J Geophys Res, 112, E08S04, doi:10.1029/2006JE002835. [Abstract]
Mustard, JF, et al (2009). Composition, Morphology, and Stratigraphy of Noachian/Phyllosian Crust around the Isidis basin. J. Geophys. Res,114, E00D12, doi:10.1029/2009JE003349. [Abstract]
Mustard, JF and Ehlmann, BL (2010). Intact stratigraphy traversing the phyllosilicate to sulfate eras at the Syrtis-Isidis contact, Mars. Lunar & Plan. Sci. Conf. 41, abstract #2070. [Abstract]
Wray, J. J., et al. (2009), Phyllosilicates and sulfates at Endeavour Crater, Meridiani Planum, Mars, Geophys. Res. Lett., 36, L21201, doi:10.1029/2009GL040734 [Abstract]