Difference between revisions of "Geological processes that have shaped Mars: Why Mars looks like it does"

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*[https://www.youtube.com/watch?v=jcaawA7d0ro Sublimation of Dry Ice]
 
*[https://www.youtube.com/watch?v=jcaawA7d0ro Sublimation of Dry Ice]
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* [https://www.youtube.com/watch?v=NiT02piO40c The Geological History of Water on Mars and Astrobiological Implications (Vic Baker)]
  
 
* [https://www.youtube.com/watch?v=z6B742f8yPs  Mars Bunker: Martian Ice Revealed]
 
* [https://www.youtube.com/watch?v=z6B742f8yPs  Mars Bunker: Martian Ice Revealed]

Revision as of 14:32, 1 July 2020

Article written by Jim Secosky. Jim is a retired science teacher who has used the Hubble Space Telescope, the Mars Global Surveyor, and HiRISE.

Earth and Mars Earth is much bigger, but both have the same land area. Mars has about one third the gravity of the Earth.

Mars looks like it does because of certain geological processes. Some of them are common to both the Earth and Mars. However, others are rare or nonexistent on the Earth. Mars shows an extremely old record of the past that is lacking on the Earth. Plate tectonics and vigorous air and water erosion has wiped out nearly all of the past geology of the Earth. In contrast, much of the Martian surface is billions of years old. Another factor that has affected the appearance of Mars is its extreme cold. The coldness of the planet makes carbon dioxide significant. It has influenced Mars both as a gas and as a solid. As a greenhouse gas, early in the history of the planet, it may have been thick enough in the atmosphere to help raise the temperature enough to permit water to flow, to carve rivers, to form lakes and an ocean. Indeed, it may have been warm enough from carbon dioxide for life to first originate on Mars and then travel to the Earth on meteorites. Today, as a solid, carbon dioxide (dry ice) produces the ubiquitous gullies found in numerous areas of the planet.

Erosion Related

As on the Earth material was laid down and then later eroded. Many spectacular scenes are present with places that were mostly eroded, but with remnants remaining in the form of buttes and mesas. Sometimes, sediments were put down in layers. As a result beautiful places were created. On the Earth we admire such layers in Monument Valley and many beautiful canyons. The same types of landscapes show up on Mars. The top layer of buttes and mesas is hard and resistant to erosion. It protects the lower layers from being eroded away. On Mars that hard, cap rock could be made from a lava flow. Many, large areas of Mars have eroded in such a fashion. The remaining structures are called mesas or buttes—if they are small in area. Some mesas and buttes show layers. Mesas show the kind of material that covered a wide area.

As on the Earth, there are landslides. However, they could be a little different since Mars has only about a third of Earth’s gravity.

Common features in certain areas of the Earth’s surface are “Yardangs.” They are found in desert areas which contain much sand. The wind blows sand and shapes the relatively soft grained deposits into the long, boat shapes of yardangs. On Mars it is thought that these forms are the result of the weathering of huge ash deposits from volcanoes. Mars has the biggest known volcanoes in the solar system. Many probably threw out much fine-grained material which was easily eroded to make vast fields of yardangs. Regions called the “Medusa Fossae Formation and Electris deposits contain thousands of yardangs.

Unlike the Earth, Mars shows landscapes that are billions of years old. In that time material has been deposited and then eroded and/or greatly changed. Some features have been “inverted.” Low areas turned into high areas. Low areas like stream beds were filled with erosion-resistant materials like lava and large rocks. Later, the surrounding, softer ground became eroded. As a result, the old stream bed now appears raised. We can tell it was originally a stream bed since the overall shape from above still looks like a stream with curves and branches.

Inverted streams Here a branched stream became filled with hard material and then the surrounding ground was eroded.

Another structure made with erosion is a “pedestal crater.” They are abundant in regions far from the equator. These craters seem to sit on little circular shelves or pedestals.[1] In the impacting process, ejecta fell about the crater and protected the underlying ground from erosion. These craters occur where we think there was a great deal of ice in the ground. So, much of the material that disappeared was just ice. With that being said, pedestal craters give us an indication of how much ice was in the region. In some places hundreds of meters of ice-rich ground were removed to make pedestal craters.[2] [3] [4]

Pedestal crater Surface close to crater was protected from erosion by the ejecta. In the past all the surrounding ground was at the level of the pedestal. Most of the loss is thought to be from the loss of ice."
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
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.


Some structures on Mars are being “exhumed.” Craters are observed that are being uncovered. In the past, impacts produced craters. Later, they were buried. Now they are in the process of being uncovered by erosion. When an asteroid strikes the surface it generates a hole and throws out ejecta all around it. A circular hole is the result. If we see a half of a crater, we know that that it is being exposed by erosion. Impacts do not produce half holes!

Craters

Impact craters occur on both the Earth and Mars. However, due to the extreme age of the Martian surface, most of Mars shows a high density of impact craters especially in the southern hemisphere. Craters do not last long on the Earth. Remember, the Earth experiences a great deal more erosion due to its thick atmosphere and abundant water. And, at intervals, the crust is taken into the Earth at plate boundaries. We know a fair amount about impact craters because the Earth has impact craters like Meteor Crater in Arizona that we can study easily.

We know that a new crater will have a rim and ejecta around it. Large ones may have a central uplift and maybe a ring around the middle of the floor. We know that the impact brings up material from deep underground.[5] If we study the rocks in the central mound and in the ejecta, we can learn about what is deep underground.

The heat from an impact into ice-rich ground may produce channels emanating from the edge of the ejecta. These have been seen around a number of craters.


Mars shows some interesting variations to the usual appearance of craters. At times the force of an impact reaches down to a different type of layer. The lower layer may be of a different color; therefore the ejecta that is spread on the landscape may be a different color.


Sometimes it looks as if an impact caused rocks to melt and when the molted rocks landed on the crater floor steam explosions occurred with ice-rich ground. What results is ground with a high density of pits.

On occasion, an impact may go down to ice-rich ground or maybe to a layer of ice. Indeed, a number of craters expose ice on their floors which after a period of time disappears into the thin Martian atmosphere.

Then there is a type of crater which is common in locations we think contain much ice. Called “ring-mold” craters, they may be caused by a rebound of an ice layer. Experiments in labs confirm that this behavior can occur. Ring-mold craters are called that because they resemble ring-molds used in baking.


Ring-mold craters form when an impact goes through to an ice layer. The rebound forms the ring-mold shape, and then dust and debris settle on the top to insulate the ice. Ring-mold craters form when an impact goes through to an ice layer. The rebound forms the ring-mold shape, and then dust and debris settle on the top to insulate the ice.


Now, during the impact process much material is sent flying in the air. Some of it will come down and create new craters. These are called secondary craters. They can be identified by all being of the same age. In addition, sometimes molted rock is produced by the impact. If molten rock lands on ice-rich ground, an area with a high density of pits will form. The hot molten rocks cause ice in the ground to burst into steam and cause pits to form.

Glaciers

Mars may have had much water in past ages. Much of that water is now frozen in the ground and locked up in glacier-like forms. Many features have been found that are like glaciers—in that they are mostly made of ice and flow like glaciers. [7] [8] [9] [10] [11] [12] [13] That means they move slowly and in a downhill direction. For ice to exist under today’s climate conditions, it must be covered with a layer of debris—dust, rocks, etc. A layer several meters or a few tens of meters thick will preserve ice for millions of years. [14] Under today’s conditions any exposed ice would undergo sublimation and disappear into the thin Martian atmosphere. That is, it would go directly from a solid to a gas. But, the isulating effect of surface material prevents loss of ice.[11] [14] [15] [16] Martian glaciers show evidence of movement on their surfaces and in their shapes. The actual existence of water ice in some of them has been proven with radar studies from orbit. [17] Some of them look just like alpine glaciers on the Earth. Most show piles of debris called moraine. This was material that was removed from one place and moved along to another by ice. Also, shapes looking just like eskers of terrestrial glaciers are common in places. Eskers form from streams moving under glaciers. These streams deposit rocks in tunnels in the ice at the bottom of glaciers. When the ice goes away, curved ridges stay behind.


For Mars, a number of names have been applied to these glacier-like forms. Some of them are tongue-shaped glaciers, lobate debris aprons (LDA’s), lineated valley fill (LVF), and concentric crater fill (CCF).[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]

Lineated valley fill, as seen by HiRISE under HiWish program

Ice in the ground

Mars has some unique landscapes and features that are common just to it. Since so much water is frozen in the ground and since the thin atmosphere of Mars allows ground ice to disappear when it became exposed, unreal scenes can develop. Under current conditions on Mars, ice sublimates when exposed to the air. In that process, ice goes directly to a gas instead of first melting. It often starts with small, narrow cracks that get larger and larger. Once ice leaves the ground there is not much left except dust. And winds will eventually carry the dust away. The end result is various shaped holes, pits, canyons, and hollows. Some of these forms are called brain terrain, ribbed terrain, hollows, scalloped terrain, and exposed ice sheets. [40] All of these may be of use to future colonists who need to find supplies of water.

Open and closed brain terrain [41]

                                Open and closed brain terrain [42]

Ribbed terrain begins with cracks that eventually widen to produce hollows

                    Ribbed terrain begins with cracks that eventually widen to produce hollow

Close view of hollowed terrain caused by ice leaving the ground Box shows size of football field.


                      Close view of terrain caused by ice leaving the ground   Box shows size of football field.

Other signs of water ice in the ground are: lobed (rampart craters), patterned ground, and possible pingos. Pattered ground or polygonal ground is common in ice-rich areas on Earth.


Pingos are mounds that contain a core of ice.[52] [53] [54] They often have cracks on their surfaces. Cracks form when water freezes and expands. Pingos would be useful as sources of water for future colonies on the planet.

Craters with ejecta that look like they were made by an impact into mud are called lobed or rampart craters. They were discovered by early, orbital missions to Mars. They are most common where we expect ice in the ground.

Channels are sometimes found in a crater's ejecta or along the edges of the ejecta. Heat from the ejecta probably melted ice in the ground. Much heat is produced with an impact.

Liquid water

Mars used to have lots of water and maybe a much thicker atmosphere billions of years ago. With liquid water, life is possible. Indeed, life may have first appeared on Mars before it occurred on Earth. Martian organisms could have been knocked off Mars by low angle asteroid impacts and found their way to Earth. Perhaps, the DNA of all Earthly organisms, included us, still contains genes from early Martian life. When we have samples of Mars brought back to Earth, we may find traces of DNA that are like ours. Data are still being gathered and ideas debated, but scientists think that once Mars cooled down and lost its magnetic field, the solar wind may have carried away much of its atmosphere. In addition, some researchers have suggested that some of the atmosphere was splashed out by impacts. After the planet cooled, water became frozen in the polar ice caps and in the ground. But, for some period there was liquid water.

Artist’s conception of how the solar wind strikes Mars, but does not reach the Earth’s surface because of the Earth’s magnetic field

 Artist’s conception of how the solar wind strikes Mars, but does not reach the Earth’s surface because of the Earth’s magnetic field

This poster made by NASA shows the different ways that Mars lost most of its atmosphere after its magnetic field disappeared.

This poster made by NASA shows the different ways that Mars lost most of its atmosphere after its magnetic field disappeared.


Huge amounts of water had to be present to carve the many outflow channels and produce the valley networks. Many of the outflow channels begin in "Chaos Terrain." Such a landscape often is where the ground seems to have just collapsed into giant blocks.[55] [56] It is believed that a shell of ice was created when the planet's climate cooled. Perhaps, at times the shell was broken by asteroid impacts, movements of magma, or faults. Such events would allow pressurized water to rapidly escape from under the shell of ice (shell has been called a cryosphere). Evidence is accumulating for the existence of an ocean. Lakes existed in low spots, especially craters.


Channels that empty into a low area that could have been a lake Arrows show channels that lead to a low area that could have hosted a lake.


Streamlined forms in wide channel These forms were shaped by running water.


Streamlined forms in wide channel

These forms were shaped by running water.


At present, it is hotly debated just how long water stayed around. The sun was not as strong billions of years ago. Greenhouse gases like carbon dioxide, methane, and hydrogen may have made Mars warm enough for liquid water. Massive volcanoes would have given up many of these gases, along with water vapor.

Height of Olympus Mons compared to tall mountains on Earth


Maybe the water just existed for short periods. Some studies have showed that large impacts into icy ground could release water and change the local climate for thousands of years. Also, impacts may have punctured an ice shell and allowed pressurized water to flow out for a time. Any water moving on the surface would quickly freeze at the top. But, it would continue to flow under the ice for a long time and make many of the channels we see today. Heat to allow water to flow may have been from underground flows of magma. On the other hand, many of the features created by liquid water could have formed under massive ice sheets where water was insulated from the Martian atmosphere.

Layers

Many locations display layered formations. Some are mostly just made of ice and dust. These types of layers are common in the polar ice caps, especially the northern ice cap.[58] Other, rockier layers, are visible in the walls of impact craters and canyon walls.


And then there are layers that may be more recent, they may be connected to repeated climate changes. Some have regularity to them. The climate of Mars changes drastically due to changes in the tilt of its rotational axis. At times, like now, it is close to the Earth’s 23.5 degrees. But, at times it may be as much as 70 degrees.[59] [60] [61] Tilt governs the seasons and where ice is distributed. Currently, the largest deposit of ice is at the poles. At other times could have been at mid-latitudes. Imagine how it would be to have Pittsburgh under an ice cap. Mars may have had ice caps at the latitude of Pittsburgh.

There is an ice-rich material that falls from the sky. It is called latitude dependent mantle.[62] It is thought to come from snow and ice-coated dust. At times, there is a lot of dust in the air. When that happens, moisture will freeze onto dust grains. When the ice-coated dust particle gets heavy enough, it will fall. Recent accumulations of this mantle look smooth. In some places the mantle is layered. Some formations, particularly in protected spots in craters and against mounds, suggest that these layered formations had many more layers. The wind sometimes shapes them into layered mounds.

Mantle in a crater The mantle here has made everything look smooth on one side of the crater.


               Mantle in a crater  The mantle here has made everything look smooth on one side of the crater.

Mesa in crater with layers


                         Layers in crater  They were protected from erosion by being in the crater.

The older layers visible on crater and canyon walls may have different sources. Some are from lava flows or ash from volcanoes. Some may have formed under water like most layered sedimentary rocks on the Earth.[63] Curiosity, our robotic explorer, has found that layers in Gale Crater were made from sediments at the bottom of a lake. Some may be just from dust and debris settling in low areas and then being cemented by rising groundwater carrying minerals like sulfates and silica.[64] [65] [66] Sometimes a crater may have been filled up with layered rocks and then the rocks may have been eroded by the wind in such a way to just leave a layered mound in the center of the crater. Gale crater, where Curiosity is exploring, was like that.

Rock layers in the Murray Buttes area in lower Mount Sharp They look like rocks formed at the bottom of lakes and their chemistry proves it.


                                            Rock layers in the Murray Buttes area in lower Mount Sharp

Igneous effects

Volcanic vent with lava channel

                      Volcanic vent with lava channel

Igneous refers to rock that is heated to a molten condition. On Mars, this is a major shaper of landscapes. Lava comes out of the ground at holes called vents. Flows of lava can be about as fluid as water and move long distances. Sometimes the top cools to a solid, but the liquid rock continues to flow underneath a hard crust. Giant pieces of this stiff crust can move around as “lava rafts.”


In other places, lava travels in channels. When they make a hard crust, lave tunnels are created. A picture below shows lava tunnels.[67] After the liquid lava moves away, an empty tunnel can be formed. These are significant for future colonists as they may be where our first colonies will be built. There people would be protected from surface radiation. We have already found spots that might be openings to these tunnels in HiRISE images.


Lava tubes and lava tunnels Future colonists may live in lava tunnels.


Possible cave entrance to a lava tunnel Future colonies may live in caves for protection from weather and radiation.
Some of the Martian volcanoes, as seen by Viking 1

There are huge volcanoes that were noticed by our first spacecraft to orbit the planet. The first satellite to orbit the planet was only able to see a few volcanoes peeking above a massive global dust storm. Since Mars has not had plate tectonics for nearly all of its history, volcanoes can grow very large.[68] Lava and ash can erupt from the same spot for long periods of time. On the Earth, the plates move so volcanoes can only grow so big.

Volcanoes are only the surface manifestations of liquid rock. There is more molted rock moving under the surface than what we see above ground in volcanoes. Molted rock is called magma when underground. Stretching out around volcanoes underground are various structures. Vast linear walls, called dikes radiate out from volcanoes. On Mars they can be many miles in length. Many form by moving along cracks or weak parts of rocks. Some scientists have suggested that they from long troughs when they melt ground ice. Troughs are some of the longest features on Mars.



Besides the direct action of lava and magma, volcanoes affect Mars with just their weight. The mass of a volcano stretches the crust and makes cracks form. The large canyon system of Valles Marineris may have been started with some sort of stretching of the crust. But, its stretching may have been caused by rising mantle plumes or maybe the rise of Tharsis where so many volcanoes are located.[69] Cracks in the crust are called faults. Faults on Mars are often double faults. A center section is lower than the sides. This arrangement is called a graben. On the Earth they can turn into lakes like Lake George in New York State. Graben on Mars can be thousands of miles long.


Sometimes lava moves over frozen ground. That results in steam explosions. Large fields of small cones can be produced when this happens. Those cones are called “rootless cones” since they do not go down very far.

Volcanoes sometimes explode with great amounts of ash that travels long distances, covering everything. Some of the layers seen on Mars are probably from these ash deposits. These deposits do not contain boulders and are easily eroded by just the wind. Two areas on Mars have widespread and thick deposits made in this way; they are called the Medusae Fossae Formation and the Electris deposits. These relatively soft deposits often form shapes called yardangs. They are sort of boat shaped and show the direction of the prevailing wind when they were created.

Much of the atmosphere of Mars came from volcanoes. Volcanoes give off large amounts of carbon dioxide and water, along with other chemicals. Some of these chemical compounds are “greenhouse gases” that served to heat up early Mars. A few places are thought to be where volcanoes erupted under ice. The shapes that resulted look like those made on Earth when a volcano erupted under the ice.

Large group of concentric cracks Location is Ismenius Lacus quadrangle. Cracks were formed by a volcano under ice.

Large group of concentric cracks Location is Ismenius Lacus quadrangle. Cracks were formed by a volcano under ice.

Bright dust

A thin coating of bright-toned dust covers almost all parts of Mars. It has a rust brown color. It is not too noticeable until it is not here. Some things remove the dust and then reveal the dark underlying surface. The contrast between this thin coating and the underlying dark rock is striking. Much of the difference derives from how NASA pictures are processed.[70] To bring out more detail, the brightest tone is considered white, while the darkest black. It only takes a very thin layer of dust to make a difference in the over-all appearance of a picture. Experiments on Earth found that the layer may be only as thick as the diameter of a human hair.[71] Incidentally, the dust has the color of rust because it is rust—it is oxidized iron.

Dark slope streaks occur when bright dust avalanches down steep slopes like crater walls. They can be very long and elaborate. These movements are affected by obstacles like boulders. A streak may split into two when encountering a boulder. They may be initiated when an impact happens nearby.[72][73] [74]

Dark slope streaks on layered mesa, as seen by HiRISE under HiWish program

                                Dark slope streaks on layered mesa
Dark slope streaks As these streaks moved down, boulders changed their appearance.

Another thing that causes light and dark patterns is a dust devil. These miniature tornadoes remove the bright dust and make straight and/or curved tracks. They are common especially in areas with much dust cover and at certain times of the day. They have been observed both from orbit and from the ground. We even have movies of them in action. They can form beautiful scenes. And, the arrangement of the tracks can be different in just a few months.[75]

Dust devil tracks near crater

The atmosphere of Mars contains a great deal of fine dust. Large dust storms happen just about every Martian year. A year on Mars is about 23 of our months. Dust storms typically occur when it is spring or summer in the southern hemisphere. At that time, Mars is at its closest to the sun. Unlike the Earth, Mars has a very elliptical orbit which brings it much closer to the sun than at other times. This makes for differences in season both in intensity and length. For example the southern summer is much shorter than that of the north. However, the summer season in the southern hemisphere is much more intense.

Comparrsion of the orbits of Earth and Mars. The Earth’s orbit is almost a perfect circle.[76] [77]

Comparison of the orbits of Earth and Mars. The Earth’s orbit is almost a perfect circle. Mars changes its distances to sun a great deal--this changes makes drastic seasonal changes.

Dry Ice

Some of the strangest things on Mars involve dry ice—solid carbon dioxide. The cold conditions on Mars cause much of the carbon dioxide to freeze out of the atmosphere. Both ice caps contain some dry ice. Each year about 25% of the atmosphere freezes out onto the poles. This is so much that the gravity of the planet shifts. [78] [79] Winds and weather systems that almost look like the Earth’s are produced by so much dry ice changing to a gas at these times.


Region of South Pole with ice cap Southern ice cap is much smaller than the North’s.
Spiral troughs in the northern ice cap

In the winter dry ice accumulates. So, large areas appear white. When things warm up in the spring, the landscape gets many dark spots and areas. [80][81] [82] In the past, observers thought that Mars was full of life. They saw the northern ice cap get smaller and smaller. At the same time, they watched the area get darker. They concluded that the darkening was vegetation growing from the water coming out of the ice caps. What was happening was the dry ice was disappearing. Today, we can watch this darkening occur in great detail. [83]

In some places, there are many geyser-like eruptions of gas and dark dust.[84] High pressure gas and dust explode out of the ground. Winds often blow these eruptions into dark plumes. After many observations, scientists concluded that what happens is that a transparent-translucent dry ice slab forms in the winter. With increased sun in the spring, pressure builds up under this slab as light heats cavities under the slab and causes dry ice to turn into a gas. At weak areas in the slab, the gas comes out along with dark dust.[85] [86] The channels may get dark from the dust and make a pattern that looks like a spider. These patterns are called “spiders.” [87] [88] [89] [90] [91] [92] The official name for spiders is "araneiforms."[93]

Close view of spiders


                                              Close view of spiders

Around the southern cap, dry ice makes round, low areas that look like Swiss cheese. [94] [95] [96] [97] So, it is called “Swiss cheese terrain.” The roundness of the pits is believed to be related to the low angle of the sun.[98]

HiRISE view of South Pole Terrain.

                                     HiRISE view of South Pole Terrain.

The ice caps contain a great deal of water ice. The northern cap has a covering of dry ice only 1 meter thick in the winter, but the southern cap always has a coating of dry ice up to 8 meters thick. Large deposits of dry ice are also buried in the water ice of the cap at some locations.

Gullies

Since 2000, researchers have been studying gullies that are common in the mid-latitudes on steep slopes. They look like they were carved by liquid water. After many years of observations, it has been concluded that today they are being made by chunks of dry ice sliding down slopes.[99] [100] [101] However, some scientists concede that water may have been involved in their formation in the past.[102] [103] [104]

Gullies near Newton Crater

                 Gullies near Newton Crater

Other features

The surface of Mars is very old—billions of years. This is plenty of time for rocks to have broken down into sand. In low places, like crater floors, sand accumulates and makes dunes. Some are quite pretty. And the colors used by NASA make them even more pretty—they can appear blue, purple, green, or turquoise.

Colorful dunes in the Mare Tyrrhenum quadrangle[106]

                 Colorful dunes in the Mare Tyrrhenum quadrangle[107]


Dunes

Related to dunes are something called transverse aeolian ridges (TAR’s). They look like small dunes. They are often parallel to each other. They generally are in low areas and one of the most common landforms on Mars.[108] They are mid-way in height between dunes and ripples; they are not well understood.[109] [110]

Some landscape expressions are mysteries. There are different ideas for what caused them. In rocks of certain ages, often at the bottom of low spots are complex arrangements of ridges. These are walls of rock.[111]

Linear ridge networks

                                            Linear ridge networks

Of eerie beauty are odd arrangements visible on the bottom of the Hellas Impact basin. We are not sure exactly what caused them. They have been called honeycomb terrain or banded terrain.

Wide view of features on floor of Hellas impact basin. The exact origin of these shapes is unknown at present.


Wide view of features on floor of Hellas impact basin.

Close view of center of a Hellas floor feature

Close view of center of a Hellas floor feature

Close-up of banded terrain on the floor of the Hellas basin, as seen by HiRISE

            Close-up of banded terrain on the floor of the Hellas basin, as seen by HiRISE

Floor features in Hellas Planitia

                      Honeycomb terrain on floor of Hellas Basin  The exact origin of these shapes is unknown at present.

Mars is one planet that we can see the surface clearly. Its super thin atmosphere (about 1% of the Earth’s) makes it easy to observe. Early telescopes revealed many markings and patterns. As we sent better and better cameras to examine it, more mysteries and more beautiful scenes emerged. We were able to answer many questions, but always more questions arose concerning what we were seeing.

References

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External links

See Also

Recommended reading

  • Grotzinger, J., R. Milliken (eds.). 2012. Sedimentary Geology of Mars. Tulsa: Society for Sedimentary Geology.
  • Kieffer, H., et al. (eds) 1992. Mars. The University of Arizona Press. Tucson
  • history.nasa.gov/SP-4212/ch11
  • 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.