Common surface features of Mars

The common surface features of Mars include dark slope streaks, dust devil tracks, sand dunes, Medusae Fossae Formation, fretted terrain, layers, gullies, glaciers, scalloped topography, chaos terrain, possible ancient rivers, pedestal craters, brain terrain, and ring mold craters.

Slope streaks

When occurring near the top of a dune, dark sand may cascade down the dune leaving dark surface streaks -- streaks that might appear at first to be trees standing in front of the lighter regions.

A new phenomenon known as slope streaks has been uncovered by the HiRISE camera on the Mars Reconnaissance Orbiter. These features appear on crater walls and other slopes, and they are thin and many hundreds of metres long. The streaks have been observed to grow slowly over the course of a year or so, always beginning at a point source. Newly formed streaks are dark in colour but fade as they age until white. The cause is unknown, but theories range from dry dust avalanches (the favoured theory) to brine seepage.^[1]

Examples of dark slope streaks from various parts of Mars are shown below. Click on image to get a better view.

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  Tikonravev Crater floor in Arabia quadrangle, as seen by Mars Global Surveyor. Click on image to see dark slope streaks and layers.

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  Close-up of some layers under cap rock of a pedestal crater and a dark slope streak, as seen by HiRISE under HiWish program.

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  Dark streaks in Diacria quadrangle, as seen by Mars Global Surveyor.

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  Layers and dark slope streaks, as seen by HiRISE under HiWish program

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  Dark slope streaks on mesa, as seen by HiRISE under HiWish program Location is Amazonis quadrangle.

Dust devil tracks

Many areas on Mars experience the passage of giant dust devils. A thin coating of fine bright dust covers most of the Martian surface. When a dust devil travels by, it blows away the coating and exposes the underlying dark surface. These dust devils have been seen both from the ground and from orbit. They have even blown the dust off the solar panels of the two Rovers on Mars, thereby greatly extending their lives.^[2] The twin Rovers were designed to last for 3 months; instead, they have lasted eleven years and are still going. The pattern of the tracks have been shown to change every few months.^[3]

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  Pattern of large and small tracks made by giant dust devils, as seen by under the MOC Public Targeting Program

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  Dust devil tracks, as seen by HiRISE under HiWish program

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  Dust devil tracks, as seen by HiRISE under HiWish program.

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  Russell Crater Dust Devil Changes in Noachis quadrangle, as seen by HiRISE. Click on image to see changes in dust devil tracks in just 3 months.

Layers

Many places on Mars show rocks arranged in layers. Rock can form layers in a variety of ways. Volcanoes, wind, or water can produce layers.^[4]

A detailed discussion of layering with many Martian examples can be found in Sedimentary Geology of Mars.^[5] Layers can be hardened by the action of groundwater. Martian ground water probably moved hundreds of kilometers, and in the process it dissolved many minerals from the rock it passed through. When ground water surfaces in low areas containing sediments, water evaporates in the thin atmosphere and leaves behind minerals as deposits and/or cementing agents. Consequently, layers of dust could not later easily erode away since they were cemented together.

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  Close up view of southern part of Firsoff Crater showing layers, as seen by CTX camera (on Mars Reconnaissance Orbiter).

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  Layers in Firsoff Crater, as seen by HiRISE under HiWish program Note: this image field can be found in the previous image of the layers in Firsoff Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter).

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  Close-up of layers in Firsoff Crater, as seen by HiRISE Note: this is an enlargement of the previous image of Firsoff Crater.

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  Faults and layers in Firsoff Crater, as seen by HiRISE under HiWish program. Arrows show location of faults.

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  Wide-view of layers, as seen by HiRISE under HiWish program. Box shows location of next image. Dark parts of image are dark, basalt sand sitting on level places.

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  Enlargement of previous image showing a fault and layers. Image taken with HiRISE, under HiWish program.

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  Layers in Firsoff crater with a box showing the size of a football field Picture taken by HiRISE under HiWish program.

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  Layers and faults in Firsoff Crater, as seen by HiRISE under HiWish program. Arrows show one large fault, but there are other smaller ones in the picture.

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  Light toned butte on the floor of a crater, as seen by HiRISE, under HiWish program. Light toned material is probably sulfate-rich and similar to material examined by Spirit Rover, and it once probably covered the whole floor of the crater's floor.

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  Layered terrain in Aeolis quadrangle, as seen by HiRISE under HiWish program.

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  Layers exposed at the base of a group of buttes in Mangala Valles in Memnonia quadrangle, as seen by HiRISE under HiWish program. Arrows point to boulders sitting in pits. The pits may have formed by winds, heat from the boulders melting ground ice, or some other process.

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  Layers under cap rock of a pedestal crater, as seen by HiRISE under HiWish program. Pedestal crater is within the much larger Tikhonravov Crater.

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  Close-up of some layers under cap rock of a pedestal crater, as seen by HiRISE under HiWish program.

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  Layers in a butte in Arabia, as seen by HiRISE under HiWish program.

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  Layers in Arabia, as seen by HiRISE under HiWish program.

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  Butte in Crommelin Crater, as seen by HiRISE under HiWish program. Location is Oxia Palus quadrangle.

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  Layers in Crommelin Crater, as seen by HiRISE under HiWish program. Location is Oxia Palus quadrangle.

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  Layers in Crommelin Crater, as seen by HiRISE under HiWish program. Arrow indicates fault. Location is Oxia Palus quadrangle.

Sand dunes

Many locations on Mars have sand dunes. An erg (or sand sea), made up of aeolian dune fields referred to as the Circumpolar Dune Field^[6] surrounds most of the north polar cap.^[7] The dunes are covered by a seasonal carbon dioxide frost that forms in early autumn and remains until late spring.^[7] Many martian dunes strongly resemble terrestrial dunes but images acquired by the High-Resolution Imaging Science Experiment on the Mars Reconnaissance Orbiter have shown that martian dunes in the north polar region are subject to modification via grainflow triggered by seasonal CO2 sublimation, a process not seen on Earth.^[8] Many dunes are black because they are derived from the dark volcanic rock basalt. Extraterrestrial sand seas such as those found on Mars are referred to as "undae" from the Latin for waves.

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  Dark dunes (probably basalt) which form a dark spot in Noachis. Picture from Mars Global Surveyor.

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  Wide view of dunes in Noachis, as seen by HiRISE.

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  Close-up View of dunes in previous image, as seen by HiRISE. Note how sand barely covers some boulders.

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  Proctor Crater Ripples and Dunes, as seen by HiRISE.

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  Dunes among craters, as seen by HiRISE under HiWish program. Some of these are barchans.

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  Dunes in two craters, as seen by HiRISE under the HiWish program.

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  Dunes and craters, as seen by HiRISE under HiWish program

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  Dunes on a crater floor, as seen by HiRISE under HiWish program. Most of these are barchans. Box shows location of next image.

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  Dunes on a crater floor, as seen by HiRISE under HiWish program. Most of these are barchans. Note: this is an enlargement of the center of the previous image.

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  Dunes, as seen by HiRISE under HiWish program. Location is Eridania quadrangle.

Gullies

Martian gullies are small, incised networks of narrow channels and their associated downslope sediment deposits, found on the planet of Mars. They are named for their resemblance to terrestrial gullies. First discovered on images from Mars Global Surveyor, they occur on steep slopes, especially on the walls of craters. Usually, each gully has a dendritic alcove at its head, a fan-shaped apron at its base, and a single thread of incised channel linking the two, giving the whole gully an hourglass shape.^[9] They are believed to be relatively young because they have few, if any craters.

On the basis of their form, aspects, positions, and location amongst and apparent interaction with features thought to be rich in water ice, many researchers believed that the processes carving the gullies involve liquid water. However, this remains a topic of active research.

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  Gullies with alcove, channel, apron—the parts of a complete gully. Picture taken with HiRISE under HiWish program.

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  Gullies with remains of a former glacier in crater in Terra Sirenum, as seen by HiRISE under HiWish program. Location is the Phaethontis quadrangle.

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  Gullies near Newton Crater, as seen by HiRISE under the HiWish Program. Location is the Phaethontis quadrangle.

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  Gullies in a crater in Terra Sirenum, as seen by HiRISE under the HiWish Program. Location is the Phaethontis quadrangle.

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  Close-up of gully showing multiple channels and patterned ground, as seen by HiRISE under the HiWish program. Location is the Phaethontis quadrangle.

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  Group of gullies in Thaumasia quadrangle, as seen by HiRISE under the HiWish program.

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  Enlargement of part of previous image showing smaller gullies inside larger ones. Water probably flowed in these gullies more than once.

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  Gullies and massive flow of material, as seen by HiRISE under HiWish program. Gullies are enlarged in next image. Location is Bamberg crater

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  Close up view of some gullies, as seen by HiRISE under the HiWish program.

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  Gullies in a crater, as seen by HiRISE under HiWish program. Location is in the Mare Acidalium quadrangle.

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  Close-up of gullies in a crater from previous image. Image taken by HiRISE under HiWish program.

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  Wide view of group of gullies, as seen by HiRISE under HiWish program. Note that part of this image is enlarged in the following image. Location is Diacria quadrangle.

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  Close-up of gullies, as seen by HiRISE under HiWish program. Streamlined features in the channels are suggestive of a formation by running water. Location is Diacria quadrangle.

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  Gullies in crater in Phaethontis quadrangle, as seen by HiRISE under HiWish program

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  Floor of crater from previous image in Phaethontis quadrangle, as seen by HiRISE under HiWish program Ring mold crater is caused by asteroid reaching an ice layer. Pits on floor may be from ice escaping the ground.

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  Gullies in crater, as seen by HiRISE under HiWish program. Location is Eridania quadrangle.

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  Close up of gullies in crater showing channels within larger valleys and curves in channels. These characteristics suggest they were made by flowing water. Note: this is an enlargement of the previous image by HiRISE under HiWish program. Location is Eridania quadrangle.

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  Close up of gully network showing branched channels and curves; these characteristics suggest creation by a fluid. Note: this is an enlargement of a previous wide view of gullies in a crater, as seen by HiRISE under HiWish program. Location is Eridania quadrangle.

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  Gullies in two levels of a crater wall, as seen by HiRISE under HiWish program. Gullies at two levels suggests they were not made with an aquifer, as was first suggested. Location is Phaethontis quadrangle.

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  Image of gullies with main parts labeled. The main parts of a Martian gully are alcove, channel, and apron. Since there are no craters on this gully, it is thought to be rather young. Picture was taken by HiRISE under HiWish program. Location is Phaethontis quadrangle.

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  Close-up of gully aprons showing they are free of craters; hence very young. Location is Phaethontis quadrangle. Picture was taken by HiRISE under HiWish program.

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  Gullies on wall of crater, as seen by HiRISE under HiWish program Location is the Mare Acidalium quadrangle.

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  Close-up of gully channels, as seen by HiRISE under HiWish program. This image shows many streamlined forms and some benches along a channel. These features suggest formation by running water. Benches are usually formed when the water level goes down a bit and stays at that level for a time. Picture was taken with HiRISE under HiWish program. Location is the Mare Acidalium quadrangle. Note this is an enlargement of a previous image.

Medusae Fossae Formation

The Medusae Fossae Formation is a soft, easily eroded deposit that extends for nearly 1,000 km along the equator of Mars. Sometimes the formation appears as a smooth and gently undulating surface; however, in places it is wind-sculpted into ridges and grooves.^[10] Radar imaging has suggested that the region may contain either extremely porous rock (for example volcanic ash) or deep layers of glacier-like ice deposits amounting to about the same quantity as is stored in Mars' south polar cap.^[11][12]

The lower portion (member) of Medusae Fossae Formation contains many patterns and shapes that are thought to be the remains of streams. It is believed that streams formed valleys that were filled and became resistant to erosion by cementaion of minerals or by the gathering of a coarse covering layer. These inverted stream beds are sometimes called sinuous ridges or raised curvilinear features. They may be a kilometer or so in length. Their height ranges from a meter to greater than 10 meters, while the width of the narrow ones is less than 10 meters.^[13]

The wind has eroded the surface of the formation into a series of linear ridges called yardangs. These ridges generally point in the direction of the prevailing winds that carved them and demonstrate the erosive power of martian winds. The easily eroded nature of the Medusae Fossae Formation suggests that it is composed of weakly cemented particles, and was most likely formed by the deposition of wind-blown dust or volcanic ash. Layers are seen in parts of the formation. A resistant caprock on the top of yardangs has been observed in Viking,^[14] Mars Global Surveyor,^[15] and HiRISE photos.^[16] Very few impact craters are visible throughout the area so the surface is relatively young.^[17]

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  Medusae Fossae Formation as seen with Mars Odyssey's THEMIS. Notice elongated formations called yardangs.

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  Yardangs of various sizes, as seen by HiRISE under HiWish program.

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  Yardangs of various sizes, as seen by HiRISE under HiWish program.

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  Yardangs formed in light-toned material and surrounded by dark, volcanic basalt sand, as seen by HiRISE under HiWish program.

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  Close-up image of yardangs, as seen by HiRISE under HiWish program. Arrows point to transverse aeolian ridges, TAR's, a type of dune. Note this is an enlargement of the previous image from HiRISE.

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  Layers in lower member of Medusae Fossae Formation, as seen by HiRISE. Location is Aeolis quadrangle.

Fretted terrain

Fretted terrain is a type of surface feature common to certain areas of Mars and discovered in Mariner 9 images. It lies between two different surfaces. The surface of Mars can be divided into two parts: low, young, uncratered plains that cover most of the northern hemisphere, and high-standing, old, heavily cratered areas that cover the southern hemisphere and a small part of the northern hemisphere. Between these two zones is the fretted terrain, containing a complicated mix of cliffs, mesas, buttes, and straight-walled and sinuous canyons. Fretted terrain contains smooth, flat lowlands along with steep cliffs. The scarps or cliffs are usually 1 to 2 km high. Channels in the area have wide, flat floors and steep walls.^[18] Fretted terrain is most common in northern Arabia, between latitudes 30°N and 50°N and longitudes 270°W and 360°W.^[19] Parts of the fretted terrain are called Deuteronilus Mensae and Protonilus Mensae.

In fretted terrain, the land seems to transition from narrow straight valleys to isolated mesas. Most of the mesas are surrounded by forms that have been called a variety of names (circum-mesa aprons, debris aprons, rock glaciers, and Lobate Debris Aprons).^[20] At first they appeared to resemble rock glaciers on Earth, but scientists could not be sure. Eventually, proof of their true nature was discovered by radar studies with the Mars Reconnaissance Orbiter and showed that they contain pure water ice covered with a thin layer of rocks that insulated the ice.^{[21][22][23][24][25][26]}

In addition to rock covered glaciers around mesas, the region has many steep-walled valleys with lineations—ridges and grooves—on their floors. The material comprising these valley floors is called lineated valley fill. In some of the best images taken by the Viking Orbiters, some of the valley fill appeared to resemble alpine glaciers on Earth. Given this similarity, some scientists assumed that the lineations on these valley floors might have formed by flow of ice in (and perhaps through) these canyons and valleys. Today, it is generally agreed that glacial flow caused the lineations.

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  Fretted terrain of Ismenius Lacus showing flat floored valleys and cliffs. Photo taken with Mars Orbiter Camera (MOC)on the Mars Global Surveyor.

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  Enlargement of the photo on the left showing cliff. Photo taken with high resolution camera of Mars Global Surveyor (MGS).

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  Wide view of mesa with CTX showing Cliff face and location of Lobate Debris Apron (LDA) Location is Ismenius Lacus quadrangle.

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  Enlargement of previous CTX image of mesa This image shows the cliff face and detail in the LDA. Image taken with HiRISE under HiWish program. Location is Ismenius Lacus quadrangle.

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  Lobate Debris Apron in Phlegra Montes, as seen by HiRISE. The debris apron is probably mostly ice with a thin covering of rock debris, so it could be a source of water for future Martian colonists. Image from the Cebrenia quadrangle. Scale bar is 500 meters long.

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  Reull Vallis with lineated floor deposits, as seen by THEMIS. Image located in Hellas quadrangle. Click on image to see relationship to other features.

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  Wide CTX view showing mesa and buttes with Lobate debris aprons and lineated valley fill around them. Location is Ismenius Lacus quadrangle.

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  Close-up of Lineated Valley Fill (LVF), as seen by HiRISE under HiWish program Note: this is an enlargement of the previous CTX image.

Glaciers

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.^[27][28]

Martian glacier moving down a valley, as seen by HiRISE under HiWish program.

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  Mesa in Ismenius Lacus quadrangle, as seen by CTX. Mesa has several glaciers eroding it. One of the glaciers is seen in greater detail in the next two images from HiRISE. Image from Ismenius Lacus quadrangle.

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  Glacier as seen by HiRISE under the HiWish program. Area in rectangle is enlarged in the next photo. Zone of accumulation of snow at the top. Glacier is moving down valley, then spreading out on plain. Evidence for flow comes from the many lines on surface. Location is in Protonilus Mensae in Ismenius Lacus quadrangle.

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  Enlargement of area in rectangle of the previous image. On Earth the ridge would be called the terminal moraine of an alpine glacier. Picture taken with HiRISE under the HiWish program. Image from Ismenius Lacus quadrangle.

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  Probable glacier as seen by HiRISE under HiWish program. Radar studies have found that it is made up of almost totally of pure ice. It appears to be moving from the high ground (a mesa) on the right. Location is Ismenius Lacus quadrangle.

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  Lobate Debris Aprons (LDA's) around a mesa, as seen by CTX Mesa and LDA's are labeled so one can see their relationship. Radar studies have determined that LDA's contain ice; therefore these can be important for future colonists of Mars. Location is Ismenius Lacus quadrangle.

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  Close up of Lobate debris apron (LDA), as seen by HiRISE under HiWish program

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  Wide CTX view of mesa showing Lobate Debris Apron (LDA) and Lineated Valley Fill. Both are believed to be debris covered glaciers. Location is Ismenius Lacus quadrangle.

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  Close-up of Lobate Debris Apron from the previous CTX image of a mesa. Image shows open-cell brain terrain and closed-cell brain terrain which is more common. Open-cell brain terrain is thought to hold a core of ice. Image is from HiRISE under HiWish program.

Concentric crater fill

Concentric crater fill, like lobate debris aprons and lineated valley fill, is believed to be ice-rich.^[29] 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.^{[30][31][32][33]} That is, they hold hundreds of meters of material that probably consists of ice with a few tens of meters of surface debris.^[34][35] The ice accumulated in the crater from snowfall in previous climates.^[36][37][38] 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.^[39]

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  Crater showing concentric crater fill, as seen by CTX (on Mars Reconnaissance Orbiter). Location is Phaethontis quadrangle.

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  Close-up view of concentric crater fill, as seen by HiRISE under HiWish program Note: this is an enlargement of previous image of a concentric crater. Location is Phaethontis quadrangle.

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  Crater with concentric crater fill, as seen by CTX (on Mars Reconnaissance Orbiter). Location is Casius quadrangle.

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  Well-developed hollows, as seen by HiRISE under the HiWish program. Location is the Casius quadrangle. Note: this is an enlargement of the previous image that was taken by CTX.

Chaos terrain

Chaos terrain is believed to be associated with the release of huge amounts of water. The chaotic features may have collapsed when water came out of the surface. Martian outflow channels commonly begin with a Chaos region. A chaotic region can be recognized by a tangle of mesas, buttes, and hills, all chopped through with valleys which in places look almost patterned. Some parts of this chaotic area have not collapsed completely—they are still formed into large mesas, so they may still contain water ice.^[40] Chaotic terrain occurs in numerous locations on Mars, and always gives the strong impression that something abruptly disturbed the ground. Chaos regions formed long ago. By counting craters (more craters in any given area means an older surface) and by studying the valleys' relations with other geological features, scientists have concluded the channels formed 2.0 to 3.8 billion years ago.^[41]

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  Huge canyons in Aureum Chaos, as seen by THEMIS. Gullies are rare at this latitude. Image from Margaritifer Sinus quadrangle.

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  Iani Chaos, as seen by THEMIS. Sand from eroding mesas is covering brighter floor material. Click on image to see relationship of Iani Chaos to other local features. Image from Margaritifer Sinus quadrangle.

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  Erosion in Aram Chaos, as seen by THEMIS. Image in Oxia Palus quadrangle.

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  Blocks in Aram showing possible source of water, as seen by THEMIS. Image in Oxia Palus quadrangle.

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  Light colored layers in Eos Chaos, as seen by HiRISE.

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  Ister Chaos, as seen by HiRISE.

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  Close-up of Ister Chaos, as seen by HiRISE.

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  THEMIS image of wide view of following HiRISE images. Black box shows approximate location of HiRISE images. This image is just a part of the vast area known as Aureum Chaos. Click on image to see more details.

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  Aureum Chaos, as seen by HiRISE, under the HiWish program.

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  Close up view of previous image, as seen by HiRISE under HiWish program. Small round dots are boulders.

Mantle

Much of the surface of Mars is covered by a thick smooth mantle that is thought to be a mixture of ice and dust.^[42] This ice-rich mantle, a few yards thick, smoothes the land. But in places it displays a bumpy texture, resembling the surface of a basketball. Because there are few craters on this mantle, the mantle is relatively young.

It’s generally accepted that mantle is ice-rich dust that fell from the sky as snow and ice-coated dust grains during a different climate ^[43] One evidence of its ice-rich nature is the presence of gullies which form when some of the ice melts.^[44][45][46]

The images below, all taken with HiRISE, show a variety of views of this smooth mantle.

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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.

Scalloped topography

Scalloped topography is common in the mid-latitudes of Mars, between 45° and 60° north and south. It is particularly prominent in the region of Utopia Planitia^[47][48] in the northern hemisphere and in the region of Peneus and Amphitrites Patera^[49][50] in the southern hemisphere. Such topography consists of shallow, rimless depressions with scalloped edges, commonly referred to as "scalloped depressions" or simply "scallops". Scalloped depressions can be isolated or clustered and sometimes seem to coalesce. A typical scalloped depression displays a gentle equator-facing slope and a steeper pole-facing scarp. This topographic asymmetry is probably due to differences in insolation. Scalloped depressions are believed to form from the removal of subsurface material, possibly interstitial ice, by sublimation. This process may still be happening at present.^[51]

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  Scalloped ground, as seen by HiRISE under HiWish program.

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  Close up of scalloped ground, as seen by HiRISE under HiWish program. Surface is divided into polygons; these forms are common where ground freezes and thaws. Note: this is an enlargement of a previous image.

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  Scalloped ground, as seen by HiRISE under HiWish program.

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  Close-up of scalloped ground, as seen by HiRISE under HiWish program. Surface is divided into polygons; these forms are common where ground freezes and thaws. Note: this is an enlargement of a previous image.

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  Stages in scallop formation, as seen by HiRISE. Location is Hellas quadrangle.

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  Periglacial Scallops and polygons, as seen by HiRISE under HiWish program.

Ancient rivers?

There is great deal of evidence that water once flowed in river valleys on Mars. Pictures from orbit show winding valleys, branched valleys, and even meanders with oxbow lakes.^[52] Some are visible in the pictures below.

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  Channel within a larger channel, as seen by HiRISE under HiWish program

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  Channels in Arabia, as seen by CTX This channel winds along for a good distance and has branches. It ends in a depression that may have been a lake at one time.

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  Channel in Arabia, as seen by HiRISE under HiWish program. This is an enlargement of the previous image that was taken with CTX to give a wide view.

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  Channels in Sklodowska Crater, as seen by HiRISE under the HiWish program.

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  Stream meander and cutoff, as seen by HiRISE under HiWish program.

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  Oxbow lake, as seen by HiRISE under HiWish program.

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  Valleys as seen by HiRISE under HiWish program

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  Channel on floor of Newton Crater, as seen by HiRISE under HiWish program.

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  Branched channel, as seen by HiRISE under HiWish program.

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  Branched channel, as seen by HiRISE under HiWish program.

Pedestal crater

Pedestal craters are believed to be caused by a crater's ejecta protecting the material beneath it from eroding. The underlying material is probably ice-rich; hence these craters indicate where and how much ice was present in the ground.^{[53][54][55][56]}

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  Pedestal crater, as seen by HiRISE under HiWish program. Top layer has protected the lower material from being eroded.

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  Pedestal craters and layers in Tikonravev Crater in Arabia, as seen by Mars Global Surveyor (MGS), under the MOC Public Targeting Program. Layers may form from volcanoes, the wind, or by deposition under water. Some researchers believe this crater once held a massive lake.

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  Pedestal craters form when the ejecta from impacts protect the underlying material from erosion. As a result of this process, craters appear perched above their surroundings.

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  Drawing shows a later idea of how some pedestal craters form. In this way of thinking, an impacting projectile goes into an ice-rich layer—but no further. Heat and wind from the impact hardens the surface against erosion. This hardening can be accomplished by the melting of ice which produces a salt/mineral solution thereby cementing the surface.

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  Dark slope streaks near the top of a pedestal crater, as seen by HiRISE under the HiWish program. Note in the middle of the picture, one can see the track of a boulder that rolled down a slope.

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  Pedestal crater, as seen by HiRISE under HiWish program The ejecta is not symmetrical around crater because the asteroid came at a low angle out of the North East. The ejecta protected the underlying material from erosion; hence the crater looks elevated. The location is Casius quadrangle.

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  Close-up of East side (right side) of previous image of pedestal crater showing polygons on lobe. Since the margin of the crater has lobes and polygons, it is believed there is ice under the protective top. Picture taken with HiRISE under HiWish program. Note: this is an enlargement of the previous image.

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  Dark slope streaks and layers near a pedestal crater, as seen by HiRISE under the HiWish program.

Brain terrain

Brain terrain is a feature of the Martian surface, consisting of complex ridges found on lobate debris aprons, lineated valley fill and concentric crater fill. It is so named because it suggests the ridges on the surface of the human brain. Wide ridges are called closed-cell brain terrain, and the less common narrow ridges are called open-cell brain terrain.^[57] It is thought that the wide closed-cell terrain contains a core of ice, and when the ice disappears the center of the wide ridge collapses to produce the narrow ridges of the open-cell brain terrain.

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  Closed-cell brain terrain, as seen by HiRISE under the HiWish program. This type of surface is common on lobate debris aprons, concentric crater fill, and lineated valley fill.

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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.

Ring mold craters

Ring mold craters are believed to be formed from asteroid impacts into ground that has an underlying layer of ice. The impact produces an rebound of the ice layer to form a "ring-mold" shape.

,

• 

  Ring mold craters on floor of a crater, as seen by HiRISE under HiWish program Location is Ismenius Lacus quadrangle.

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  Ring mold craters of various sizes on floor of a crater, as seen by HiRISE under HiWish program Location is Ismenius Lacus quadrangle.

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  Wide view of a field of ring mold craters, as seen by HiRISE under HiWish program

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  Close view of ring mold crater, as seen by HiRISE under HiWish program Note: this is an enlargement of the previous image of a field of ring mold craters.

Defrosting

In the spring, various shapes appear because frost is disappearing from the surface, exposing the underling dark soil. Also, in some places dust is blown out of in geyser-like eruptions that are sometimes called "spiders." If a wind is blowing, the material creates a long, dark streak or fan.

• 

  Spiders and frost in polygons during northern spring, as seen by HiRISE under HiWish program.

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  Close-up view of spider among polygons or patterned ground, as seen by HiRISE under HiWish program.

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  Spiders shaped by the wind into streak or fans, as seen by HiRISE under HiWish program. Polygon surface has frost in the troughs along the edges.

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  Group of dunes with most of the frost gone, as seen by HiRISE under HiWish program. Some ripples are visible.

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  Close-up of defrosting dunes, as seen by HiRISE under HiWish program. Some ripples and a small channel are also visible.

See also

• Barchan
• Chaos terrain
• Climate of Mars
• Dark slope streaks
• Dust devil tracks
• Fretted terrain
• Geology of Mars
• Geyser (Mars)
• Glacier
• Glaciers on Mars
• Groundwater on Mars
• Ismenius Lacus quadrangle
• Lineated valley fill
• List of areas of chaos terrain on Mars
• Martian chaos terrain
• Martian gullies
• Water on Mars
• Yardangs on Mars

References

Recommended reading

• Lorenz, R. 2014. The Dune Whisperers. The Planetary Report: 34, 1, 8-14
• Lorenz, R., J. Zimbelman. 2014. Dune Worlds: How Windblown Sand Shapes Planetary Landscapes. Springer Praxis Books / Geophysical Sciences.
• Grotzinger, J. and R. Milliken (eds.). 2012. Sedimentary Geology of Mars. SEPM.

External links

• Martian Ice - Jim Secosky - 16th Annual International Mars Society Convention

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