[9][10][11][12] For decades many features on Mars, including ones in Nilosyrtis Mensae, were believed to contain large amounts of ice.
[13][14] Ice was found in many locations in the northern hemisphere, including Nilosyrtis Mensae.
[18] Furthermore, at this high tilt, stores of solid carbon dioxide (dry ice) sublimate, thereby increasing the atmospheric pressure.
[21] When the tilt begins to return to lower values, the ice sublimates (turns directly to a gas) and leaves behind a lag of dust.
[22][23] The lag deposit caps the underlying material so with each cycle of high tilt levels, some ice-rich mantle remains behind.
Inverted crater mesa, Nilosyrtis Mensae. This is thought to be an old
impact crater
that was eroded, filled in and then eroded again, so that now it is a low
mesa
surrounded by a bouldery slope. Image is about 900 m wide.
Bedrock in Nilosyrtis Mensae. Image is about 1.5 km wide. In this enhanced-color image, the blue and green colors are generally due to
mafic
(magnesium and iron rich) minerals that are not altered by water, while the warmer colors are due to altered minerals like clays. The structure in this scene is complex, from impact and perhaps fluvial and volcanic processes, tectonic faulting, and erosion. This is an old terrain with a complex geologic history.
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