Concentric crater fill
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A concentric crater fill is a landform where the floor of a crater is mostly covered with a large number of parallel ridges. It is common in the mid-latitudes of Mars, and is widely believed to be caused by glacial movement. Areas on Mars called Deuteronilus Mensae and Protonilus Mensae contain many examples of concentric crater fill.
Concentric crater fill, like lobate debris aprons and lineated valley fill, is believed to be ice-rich. Based on accurate topography measures of height at different points in these craters and calculations of how deep the craters should be based on their diameters, it is thought that the craters are 80% filled with mostly ice. That is, they hold hundreds of meters of material that probably consists of ice with a few tens of meters of surface debris. The ice accumulated in the crater from snowfall in previous climates. Recent modeling suggests that concentric crater fill develops over many cycles in which snow is deposited, then moves into the crater. Once inside the crater shade and dust preserve the snow. The snow changes to ice. The many concentric lines are created by the many cycles of snow accumulation. Generally snow accumulates whenever the axial tilt reaches 35 degrees.
Sometimes boulders are found on concentric crater fill; it is believed they fell off the crater wall, then were transported away from the wall with the movement of the glacier. Erratics on Earth were carried by similar means.
High resolution pictures taken with HiRISE reveal that some of the surfaces of concentric crater fill are covered with strange patterns called closed-cell and open-cell brain terrain. The terrain resembles a human brain. It is believed to be caused by cracks in the surface accumulating dust and other debris, together with ice sublimating from some of the surfaces. The cracks are the result stress from gravity and seasonal heating and cooling.
Wide-view of concentric crater fill, as seen by HiRISE. Location is the Casius quadrangle.
Well-developed hollows, as seen by HiRISE under the HiWish program. Location is the Casius quadrangle.
This series of drawings illustrates why researchers believe many craters are full of ice-rich material. The depth of craters can be predicted based upon the observed diameter. Many craters are almost full, instead of having bowl shape; hence it is believed that they have gained much material since they were formed by impact. Much of the extra material is believed to be ice that fell from the sky as snow or ice-coated dust.
- Casius quadrangle
- Climate of Mars
- Deuteronilus Mensae
- Fretted terrain
- Impact crater
- Glaciers on Mars
- Protonilus Mensae
- Water on Mars
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