page_title: Composition of Mars
section_title: Evidence of water
hierarchical_section_title: Composition of Mars / Opportunity rover discoveries in the Margaritifer Sinus quadrangle / Evidence of water
caption_attribution_description: English: Voids on bedrock on Mars. Picture taken by Opportunity (JPL/NASA/Cornell/USGS)
context_page_description: The composition of Mars covers the branch of the geology of Mars that describes the make-up of the planet Mars.
context_section_description: Examination in 2004 of Meridiani rocks, showed the first strong in situ evidence for past water by detecting the mineral jarosite, which  only forms in water.  This discovery proved that water once existed in Meridiani Planum.  In addition, some rocks showed small laminations (layers) with shapes that are only made by gently flowing water.  The first such laminations were found in a rock called "The Dells."  Geologists would say that the cross-stratification showed festoon geometry from transport in subaqueous ripples.  A picture of cross-stratification, also called cross-bedding, is shown on the left.
  Box-shaped holes in some rocks were caused by sulfates forming large crystals, and then when the crystals later dissolved, holes, called vugs, were left behind.  The concentration of the element bromine in rocks was highly variable probably because it is very soluble.  Water may have concentrated it in places before it evaporated.  Another mechanism for concentrating highly soluble bromine compounds is frost deposition at night that would form very thin films of water that would concentrate bromine in certain spots.
caption: Voids or "vugs" inside the rock
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