John Bridges: Mars Science Laboratory Blog
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We are continuing our detailed traverse around the Pahrump area. I will be Geo ScienceTheme Lead tomorrow and we aim to do more contact science. The MastCam image is of Book Cliffs. You can see the Dust Removal Tool area where we now have APXS compositional data. The ridges in this sediment are clear signs of the passage of water from through cracks after the sediment was buried. One of the significant points about this for understanding the history of water in Gale Crater is that there must have been a large amount of water passing through the sediment over a protracted period of time to create a relatively large mass of secondary minerals.
This MAHLI image shows the importance of the DRT Dust Removal Tool brushes. A section of Pahrump has been exposed by the DRT to reveal striking relict crystal structures in the fine grained sediment. These will have formed after the rock lithified, perhaps by evaporation at the same time as the gypsum veins formed in Yellowknife Bay. When we see textures like this we always think about terrestrial analogues, the flagstones which pave some of the streets of Scotland, taken from the 'Old Red Sandstone' ~350 Myr shallow lake beds of Orkney and Caithness (see picture of these in outcrop) are one type of terrestrial rock that contains such structures.
Mars Science Laboratory has changed our view of Mars: following the 2 Viking landers of 1976 and the Pathfinder Lander in 1997 we had an idea that Mars was predominantly made of basaltic igneous rocks. However, on the basis of what we now know from our 10 km of travels in Gale Crater we are now a lot more receptive to the idea that much of Mars is made of sedimentary layers deposited by water. So looking back at this first colour image from the Viking 2 lander (which landed near Mie Crater in Utopia Planitia in the northern plains in 1976) some of the samples - like the one in the white inset lines - may be layered. Accurately determining the origins of rocks without getting right up close is very difficult, MSL will stimulate us to look again at what we think we know about Mars.
This NavCam image shows one of the outcrops we have been analysing in detail at the Chinle outcrop in Pahrump. This could be one of the classic MSL outcrops which helps unlock how Mt Sharp formed and what sort of lakes and deltas were present on Mars. You can see fine scaled layering and cross-bedding, and we are taking lots of ChemCam analyses to check the variation in composition. We know from the orbital near Infrared analyses by CRISM that we are moving into an area with more Fe oxide, and this will lead up to the 'Hematite Ridge' which is one of our main mission targets. Piecing together why there were was a change from the Yellowknife Bay type of conditions with deposition of fine grained sediment into a lake, to these Fe oxide rich lacustrine sediments will help unravel early Mars.