Much of the Martian surface is covered with a thick ice-rich, mantle layer that has fallen from the sky a number of times in the past.^{[1] [2] [3]} In some places a number of layers are visible in the mantle.^[4]

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  Layers in mantle deposit, as seen by HiRISE, under the HiWish program. Mantle was probably formed from snow and dust falling during a different climate. Location is Thaumasia quadrangle
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  HiRISE image showing smooth mantle covering parts of a crater in the Phaethontis quadrangle. Along the outer rim of the crater, the mantle is displayed as layers. This suggests that the mantle was deposited multiple times in the past. Picture was taken with HiRISE under HiWish program. The layers are enlarged in the next image.
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  Enlargement of previous image of mantle layers. Four to five layers are visible. Location is the Phaethontis quadrangle.
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  Surface showing appearance with and without mantle covering, as seen by HiRISE, under the HiWish program. Location is Terra Sirenum in Phaethontis quadrangle.
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  Mantle layers, as seen by HiRISE under HiWish program. Location is Eridania quadrangle
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  Close view of places covered and not covered by mantle layer which falls from the sky when climate changes. Location is Eridania quadrangle. Picture taken with HiRISE under HiWish program.
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  Close up view of mantle, as seen by HiRISE under the HiWish program. Mantle may be composed of ice and dust that fell from the sky during past climatic conditions. Location is Cebrenia quadrangle.
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  Smooth mantle with layers in Hellas quadrangle, as seen by HiRISE under HiWish program
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  Close view of mantle, as seen by HiRISE under HiWish program Arrows show craters along edge which highlight the thickness of mantle. Location is Ismenius Lacus quadrangle.
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  Close view that displays the thickness of the mantle, as seen by HiRISE under HiWish program Location is Ismenius Lacus quadrangle.
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  Close view of mantle, as seen by HiRISE under HiWish program Location is Hellas quadrangle.
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  Close view of the edge of mantle, as seen by HiRISE under the HiWish program Location is Hellas quadrangle.

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  Wide view of surface with spots displaying mantle, as seen by HiRISE under HiWish program Location is the Arcadia quadrangle.
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  Close view of mantle, as seen by HiRISE under HiWish program
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  Close view of mantle, as seen by HiRISE under HiWish program

It fell as snow and ice-coated dust. There is good evidence that this mantle is ice-rich. The shapes of the polygons common on many surfaces suggest ice-rich soil. High levels of hydrogen (probably from water) have been found with Mars Odyssey.^{[5][6] [7] [8] [9]} Thermal measurements from orbit suggest ice. ^{[10] [11]} The Phoenix (spacecraft) discovered water ice with made direct observations since it landed in a field of polygons. ^{[12] [13]} In fact, its landing rockets exposed pure ice. Theory had predicted that ice would be found under a few cm of soil. This mantle layer is called "latitude dependent mantle" because its occurrence is related to the latitude. It is this mantle that cracks and then forms polygonal ground. This cracking of ice-rich ground is predicted based on physical processes.^{[14][15] [16] [17] [18] [19] [20]} Another type of surface is called "brain terrain" as it looks like the surface of a human brain. Brain terrain lies under polygonal ground when the two are both visible in a region.

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  Context picture showing origin of next picture. The location is a region of lineated valley fill. Image from HiRISE under HiWish program.
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  Open and closed-cell brain terrain, as seen by HiRISE, under HiWish program.
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  Brain terrain being formed from a thicker layer, as seen by HiRISE under HiWish program. Arrows show the thicker unit breaking up into small cells.

Since the top, polygon layer is fairly smooth although the underlying brain terrain is irregular; it is believed that the mantle layer that contains the polygons needs to be 10–20 meters thick to smooth out the irregularities. The mantle layer lasts for a very long time before all the ice is gone because a protective lag deposit forms on the top.^{[21] [22] [23]} The mantle contains ice and dust. After a certain amount of ice disappears from sublimation the dust stays on the top, forming the lag deposit. ^{[24] [25] [26] [27]}

The total amount of water locked up in the mantle has been calculated based on the total area of polygonal ground and an estimated depth of 10 meters. This volume is equal to a layer 2.5 meters deep spread over the entire planet. This compares to a 30-meter depth over the whole planet for the water locked up in the north and south polar caps.^[28]

Mantle forms when the Martian climate is different than the present climate.^{[29] [30] [31]} The tilt or obliquity of the axis of the planet changes a great deal.^{[32] [33] [34]} The Earth’s tilt changes little because our rather large moon stabilizes the Earth. Mars only has two very small moons that do not possess enough gravity to stabilize its tilt. When the tilt of Mars exceeds around 40 degrees (from today's 25 degrees), ice is deposited in certain latitude bands where much mantle exists today.^{[35] [36]}

See also

• Climate of Mars

References