Lake, rain or ash?
Opportunity’s mini-TES results leave three explanations for how the haematite may have formed in the crater’s soil:
• In sediment from an iron-rich lake or ocean. This theory is supported by the apparently uniform, thin layering of the bedrock. Evidence of cross-bedding – a slight offset in the tilt of some of the layers – would be strong evidence for this scenario, revealing the action of waves as the layers were deposited. If the rover’s microscope also shows images of rounded grains in the rock, that would be typical of sedimentary formations.
• Chemical weathering of a surface layer…
