APOD: Perseverance 360: Unusual Rocks and... (2021 Mar 09)

[Chris Peterson]

Thank you, Chris and Johnny. My geology understanding is weak, but still wondering why there are these sharply angular rocks right next to very smooth, rounded ones. If they've both endured eons of sand blasting, and if they are apparently from the same rock formation, wouldn't they erode similarly? Are they ejecta from relatively recent meteorite impacts?

It's a complex environment. Rocks themselves alter the path of wind and therefore of wind-driven ablation. Rocks will erode differently because of different compositions. If they're sedimentary, the orientation of the rock is important. Some many have been more recently exposed by loss of regolith around them. Look at any rock field on Earth that contains ventifacts, and you'll see the same kind of variation.

[Roger Thomas Tobari]

In the center of the panorama is a rock that seems different than the others. It seems to have had its top sheared off. I've seen rocks in areas where glacial activity has grinded the tops of rocks smooth to a polished surface. What could have produced this shearing and why did it not happen to the surrounding rocks?

[neufer]

In the center of the panorama is a rock that seems different than the others. It seems to have had its top sheared off. I've seen rocks in areas where glacial activity has grinded the tops of rocks smooth to a polished surface.

What could have produced this shearing and why did it not happen to the surrounding rocks?

• Past glaciers sheared/polished the rocks underneath
  and later dropped 'a thin layer of rocks' on top

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<<The polar ice caps of Mars show geologic evidence of glacial deposits. The south polar cap is especially comparable to glaciers on Earth. Topographical features and computer models indicate the existence of more glaciers in Mars' past. At mid-latitudes, between 35° and 65° north or south, Martian glaciers are affected by the thin Martian atmosphere. Because of the low atmospheric pressure, ablation near the surface is solely caused by sublimation, not melting. As on Earth, many glaciers are covered with a layer of rocks which insulates the ice. A radar instrument on board the Mars Reconnaissance Orbiter found ice under a thin layer of rocks in formations called lobate debris aprons (LDAs).

Martian glacier as seen by HiRISE. Glacier is moving down valley, then spreading out on plain. Evidence for flow comes from the many lines on surface. The rimming ridges at the end of the glacier are probably moraines. Location is in Protonilus Mensae in Ismenius Lacus quadrangle.

Glaciers, loosely defined as patches of currently or recently flowing ice, are thought to be present across large but restricted areas of the modern Martian surface, and are inferred to have been more widely distributed at times in the past. Lobate convex features on the surface known as viscous flow features and lobate debris aprons, which show the characteristics of non-Newtonian flow, are now almost unanimously regarded as true glaciers.>>