Featured images for October 2008:
Deltaic sediments on Mars
Images and caption contributed by Bethany Ehlmann
![[Higher resolution version]](jezerodelta.jpg){kind=link}
The Nili Fossae region of Mars has a diversity of minerals that include mafics and phyllosilicates. The mineral assemblage suggests widespread liquid water activity and a variety of alteration processes from surface weathering to hydrothermal processes (Mangold et al., 2007). This region also boasts sapping channels and well-developed valley networks. Studies of these landforms suggest regional fluvial activity extending from the Noachian to the early Hesperian epochs (i.e. 3.96 to 3 Ga). The most well-developed valley system in this region is a network of tributaries that feed into and breach the Jezero crater rim. They create spectacular bedded sediments on the crater floor, most likely laid down in a crater lake (Fassett and Head, 2005). The western delta is particularly well preserved and is shown in Image 1 using data from the Mars Reconnaisance Orbiter .
The color data of the Compact Reconnaissance Imaging Spectrometer (CRISM) highlights the diverse mineralogy within the crater (Image 1). The crater walls (blue) are pyroxene-bearing while the uppermost surface of the delta and crater fill (purple) are mafic in composition. Olivine sands (yellow) fill in depressions in the crater and are blown in from the regional olivine deposits which are the largest on Mars (Hoefen et al., 2003; Hamilton and Christensen, 2005). While the crater and uppermost surfaces are mostly composed of primary igneous minerals, the bulk of the delta deposit and underlying crater fill is light-toned sediments, which are rich in iron-magnesium smectite clay (green) (Ehlmann et al., 2008). This is one of the few instances on Mars where geomorphic evidence of water and evidence for aqueous chemical alteration are found in the same place. It is likely that these clays were transported into Jezero crater as suspended sediments or bedload, sourced from the clay-rich rocks in the Nili Fossae region (Image 2; Mangold et al., 2007; Ehlmann et al., 2008).
High resolution data from the HiRISE camera (0.25 m/pixel) contain evidence of migrating distributary channels, which indicate that a significant portion of the delta plain sediment was deposited via lateral accretion (Ehlmann et al., 2008). Scroll bars in meander loops and epsilon cross-bedding provide evidence for the migration of distributory channels across the fan surface (Image 3). Discontinuous mesas with the same stratigraphic sequence as on the delta are found near the delta, and indicate that the delta was once more extensive and has subsequently eroded back, forming the over-steepened delta front that is observed in high resolution images.
Further Reading:
Ehlmann, B.L. et al. 2008. Clay minerals in delta deposits and organic preservation potential on Mars. Nature Geoscience 1, 355-358. [Abstract]
Fassett, C.I. and Head, J.W. 2005. Fluvial sedimentary deposits on Mars: Ancient deltas in a crater lake in the Nili Fossae region. Geophysical Research Letters 32, L14201, doi:10.1029/2005GL023456. [Abstract]
Hamilton, V.E, and Christensen, P. R. 2005. Evidence for extensive, olivine-rich bedrock on Mars. Geology 33, 433-436. [Abstract]
Hoefen, T.M., et al. 2003. Discovery of Olivine in the Nili Fossae Region of Mars. Science 302, 627-630. [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]
Poulet, F. et al. 2005. Phyllosilicates on Mars and implications for early martian climate. Nature 438, 623-627. [Abstract]