Difference between revisions of "Hellas quadrangle"

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48196 1460meteoriteclosest.jpg|Close view of out of place rock, as seen by HiRISE under [[HiWish program]] It may be a meteorite or it may have been tossed here by a nearby impact.
 
48196 1460meteoriteclosest.jpg|Close view of out of place rock, as seen by HiRISE under [[HiWish program]] It may be a meteorite or it may have been tossed here by a nearby impact.
 
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==External links==
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*[https://www.youtube.com/watch?v=Rws1mj1mnIc A trip to Mars with Hubble, Viking, and HiRISE]
  
 
==See also==
 
==See also==

Revision as of 13:13, 24 March 2020

Mars topography (MOLA dataset) HiRes (1).jpg
MC-28 Hellas 30–65° S 60–120° E Quadrangles Atlas

The most famous feature of this area is the Hellas basin, a impact crater 2300 km in diameter.


The Hellas quadrangle covers the area from 30° to 65° south latitude and 240° to 300° west longitude (120-60 E ). When an asteroid slammed into Mars to create a big hole that makes up some of this quadrange, many unbelievable events happened—it was worse than any science fiction movie. Within the Hellas quadrangle lies the classic features Hellas Planitia and Promethei Terra. Many interesting and mysterious features have been discovered in the Hellas quadrangle, including the giant river valleys Dao Vallis, Niger Vallis, Harmakhis, and Reull Vallis—all of which may have contributed water to a lake in the Hellas basin in the distant past.[1] [2] [3] Many places in the Hellas quadrangle show signs that the ground is full of ice, especially with glacier-like flow features.

In this article, some of the best pictures from a number of spacecraft will show what the landscape looks like in this region. The origins and significance of all features will be explained as they are currently understood.

Hellas Basin

The Hellas quadrangle contains part of the Hellas Basin, the largest known impact crater on the surface of Mars and the second largest in the solar system. The depth of the crater is 7152 m[4] (23,000ft) below the standard topographic datum of Mars. As of 2001, based on measurements of the Mars Orbiter Laser zero elevation on Mars (standard topographic dataum) was defined as the average radius of the planet at the equator.[5] This type of agreement was necessary since Mars does not have a sea level like the Earth. Before this data came in from the Mars Orbiter, zero altitude was defined as a specific atmospheric pressure of 610.5 Pascals, about six millibars--it is a special number, it's where water can exist as gas, liquid or solid, called "the triple point of water."[6]

The basin is located in the southern highlands of Mars and is thought to have been formed about 3.9 billion years ago, during a period that geologists call the Late Heavy Bombardment. This was a period of much greater asteroid impacts.

Results of asteroid collision

The physics of this great event boggles the mind. Studies suggest that when an impact created the Hellas Basin, the entire surface of Mars was heated hundreds of degrees, 70 meters of molted rock fell on the planet, and an atmosphere of gaseous rock was formed. Think about hot, molten rock falling to a depth of a 21 story building. On Earth that would cover all homes and most buildings. This rock atmosphere was 10 times as thick as the Earth's atmosphere. In a few days, the rock would have condensed out and covered the whole planet with an additional 10 m of molten rock.[7] When all this rock cooled all the planet would be covered with rock that was as deep as a 24 story building is tall. And this is not made up folks—the proof is the big hole called the Hellas Basin. Imagine if such a thing happened on the Earth.

Strange surfaces—Origin Unknown

In the Northwest portion of Hellas Planitia is a strange type of surface called complex banded terrain or taffy-pull terrain. Its process of formation is still largely unknown, although it appears to be due to erosion of hard and soft sediment along with ductile deformation. Ductile deformation results from layers undergoing strain.[8]

Giant Lake

Early in the planet's history, it is believed that a giant lake existed in the Hellas Basin.[9] Parts of the Hellas Basin are in three different quadrangles: Hellas, Noachis, and Iapygia. Possible shorelines have been discovered. These are evident in alternating benches and scarps visible in Mars orbiting camera narrow-angle images. In addition, Mars orbiting laser altimeter (MOLA) data show that the contacts of these sedimentary units mark contours of constant elevation for thousands of km, and in one case all around the basin. Channels, believed to be formed by water, enter into the basin. The Hellas drainage basin may be almost one-fifth that of the entire northern plains. A lake in Hellas in today's Martian climate would form a thick ice at the top that would eventually sublimate away. That is the ice would turn directly from a solid to a gas. This is similar to how dry ice (solid carbon dioxide) behaves on Earth.[10] Glacial features ( moraines, drumlins, and eskers) have been found that may have been formed when the water froze.[11] [12]

Twisted Ground in Hellas This is a good example of how difficult it would be to walk on Mars.

How climate change caused ice-rich features

Many features on Mars, including ones in Hellas quadrangle, are believed to contain large amounts of ice. The Hellas region displays many strange and beautiful landscapes. Most do not have their counterparts on the Earth. Researchers have struggled to explain these features and others. Mars holds many mysteries. However, after so much coverage by satellites with increasing better cameras, we have made major strides in understanding the mysteries of the Red Planet. Some aspects of the planet are still debated. Many things we do have some understanding of their natures, but some details have yet to be worked out. Most of the strangeness of the Hellas region relates to climate change. Indeed, most of the whole planet’s surface appearance is driven by drastic and frequent climate changes. These changes are due to basic physics. Seasons on the planets, including the Earth, are caused by the tilt of a planet's rotational axis. Because the Earth has a moon of considerable mass, the Earth’s axis does not change much from its usual 23.5 degrees. However, Mars lacks a large moon; consequently its tilt has even been greater than 80 degrees. Note that its tilt at 25 degrees is almost the same as ours. [15] [16] Studies have shown that when the tilt of Mars reaches 45 degrees from its current 25 degrees, ice is no longer stable at the poles. As a result, it will disappear.[17] Furthermore, at this high tilt, stores of solid carbon dioxide (dry ice) sublimate, thereby increasing the atmospheric pressure. This increased pressure allows more dust to be held in the atmosphere. With more dust, more ice will freeze onto the dust. Eventually, moisture in the atmosphere will fall as snow or as ice frozen onto dust grains. Calculations suggest this material will concentrate in the mid-latitudes. And Hellas is in the mid-latitudes of the southern hemisphere.[18] [19] Using decades of date from orbiting satellites together with general principles about weather and climate, researchers have developed theories or models that explain why Mars looks like it does. They call these models or theories general circulation models. These theories predict accumulations of ice-rich dust (which becomes mantle) in the same areas where ice-rich features are found.[20] When the tilt begins to return to lower values, the ice sublimates (turns directly to a gas) and leaves behind a lag of dust.[21] [22] This lag deposit caps the underlying material so with each cycle of high tilt levels, some ice-rich mantle remains behind.[23] After many, many cycles of mantle accumulation some places, especially the Hellas region, accumulate very thick deposits of mantle, technically called latitude dependent mantle (because its occurrence depends on the latitude). Some parts of the mantle may have changed into solid ice in a manner analogous to how snow turns into ice in our Earth’s glaciers. The following pictures show expressions of this mantle in the Hellas region.

Niger Vallis with features typical of this latitude, as seen by HiRISE. Chevron patterns result from movement of ice-rich material. Click on image to enlarge in order to see chevron pattern and mantle

The image at the right shows a good view of this smooth mantle around Niger Vallis, as observed with HiRISE.


What happens next is that cracks appear in the surface. Stress is suggested to initiate a fracture process that produces cracks. Cracks expose more surfaces, and consequently more ice can escape into the planet's thin atmosphere. Conditions on Mars are such that the process called sublimation dominates. In this process ice will change directly into a gas—rather than melting into a liquid first. Dry ice behaves in a similar fashion on the Earth. On Mars sublimation has been observed when the Phoenix lander uncovered chunks of ice that disappeared in a few days.[24] [25]

In addition, HiRISE has seen fresh craters with ice at the bottom. HiRISE, a powerful telescope in orbit around the planet, observed these ice deposit disappear over time.[27] Eventually, small cracks become large canyons or troughs in the mantle. Small cracks often contain small pits and chains of pits.[28] [29] When parts of this many meters deep mantle start to have cracks, sublimation takes over and many strange landscapes are created. HiRISE has imaged many of these scenes. Pictures in this article show many of these exotically beautiful forms. Changes in Mars's orbit and tilt cause significant changes in the distribution of water ice from Polar Regions down to latitudes equivalent to Texas. During certain climate period’s, water vapor leaves polar ice and enters the atmosphere. The water returns to the ground at lower latitudes as deposits of frost or snow mixed generously with dust. The atmosphere of Mars contains a great deal of fine dust particles. Water vapor condenses on the particles; consequently, they fall down to the ground due to the additional weight of the water coating. This material that falls, along with snow lands in certain places on Mars. A great deal lands in the Hellas region. It appears as a smooth covering. Due to its great age, the Martian surface is very irregular, but where mantle has accumulated it is smooth. When ice at the top of the mantling layer goes back into the atmosphere, it leaves behind dust, which insulates the remaining ice.[30]

Lobate debris aprons (LDA)

One very important feature common in east Hellas are piles of material surrounding cliffs. This formation is called a lobate debris apron (LDA). Recently, research with the Shallow Radar on the Mars Reconnaissance Orbiter has provided strong evidence that the LDAs are glaciers that are covered with a thin layer of rocks.[31] [32] [33] [34] [35] Large amounts of water ice are believed to be in the LDAs. Available evidence strongly suggests that the eastern part of Hellas accumulated snow in the past. When the tilt (obliquity) of Mars increases the southern ice cap releases large amounts of water vapor. Climate models predict that when this occurs, water vapor condenses and falls where LDAs are located. The tilt of the earth changes little because our relatively large moon keeps it stable. The two tiny Martian moons do not stabilize their planet, so the rotational axis of Mars undergoes large variations.[36] Lobate debris aprons may be a major source of water for future Mars colonists. Their major advantage over other sources of Martian water are that they can easily mapped from orbit and they are closer to the equator, where manned missions are more likely to land.

Lineated Valley Fill (LVF)

Reull Vallis with lineated floor deposits, as seen by THEMIS. Click on image to enlarge to see relationship to other features.

On the floors of some channels are features called lineated floor deposits or lineated valley fill. They are ridged and grooved materials that seem to deflect around obstacles. They are believed to be ice-rich. Some glaciers on the Earth show such features. Lineated floor deposits may be related to lobate debris aprons, which have been proven to contain large amounts of ice. Reull Vallis, as pictured below, displays these deposits.[37]

Lineated valley fill, as seen by HiRISE under HiWish program

Upper Plains Unit

Close view of upper plains unit breaking down into brain terrain, as seen by HiRISE under HiWish program As ice leaves the ground, the ground collapses and winds blow the remaining dust away.

Close view of upper plains unit breaking down into brain terrain As ice leaves the ground, the ground collapses and winds blow the remaining dust away.

Some places the mantle has piled up quite deeply. The remains of a 50-100 meter thick mantling, called the upper plains unit, has been discovered in the mid-latitudes of Mars. It was first investigated in the Deuteronilus Mensae region (in the North), but it occurs in other places as well. The remnants sometimes consist of sets of dipping or tilted layers in craters and along mesas.[38] Sets of dipping layers may be of various sizes and shapes—some look like stepped Aztec pyramids from Central America. The Upper plains unit can have several different appearances.

Some regions of the upper plains unit display large fractures and troughs with raised rims; such regions are called ribbed upper plains. Fractures are believed to have started with small cracks from stresses. Stress is suggested to initiate the fracture process since ribbed upper plains are common when debris aprons come together or near the edge of debris aprons—such sites would generate compressional stresses. Cracks expose more surfaces, and consequently more ice in the material sublimates into the planet's thin atmosphere. Eventually, small cracks become large canyons or troughs.[39] [29] The upper plains unit is probably just a very thick pile of mantle that has dropped from the sky. It drapes various surfaces and as is the case for other mantle deposits, it has layers, is fine-grained, and is ice-rich. It is widespread; it does not seem to have a point source. The surface appearance of some regions of Mars is due to how this unit has degraded. This unit also degrades into a feature named brain terrain; it looks like the human brain. Brain terrain is a region of maze-like ridges 3–5 meters high. Some ridges may consist of an ice core, so they may be sources of water for future colonists.

Origin of Dao Vallis

Dao Vallis, as seen by THEMIS. Click on image to see relationship of Dao Vallis to other nearby features

Dao Vallis begins near a large volcano, called Hadriaca Patera, so it is thought to have received water when hot magma melted huge amounts of ice in the frozen ground.[40] The partially circular depressions on the left side of the channel in the adjacent image suggests that groundwater sapping also contributed water.[41]

Dust devil tracks

Secchi Crater Floor, as seen by HiRISE. Click on image to see dust devil tracks and a pedestal crater.

Many areas on Mars, including the Hellas quadrangle, experience the passage of giant dust devils. A thin coating of fine bright dust covers most of the Martian surface. When a dust devil goes by it blows away the coating and exposes the underlying dark surface. Dust devils have been seen from the ground and from orbiting spacecraft. They have even blown the dust off of the solar panels of the two Mars Exploration Rovers (Spirit and Opportunity), thereby greatly extending their lives.[42] The twin Rovers were designed to last for 3 months, instead they have lasted far longer; Opportunity lasted more than 14 years. The pattern of dust devil tracks have been shown to change every few months.[43] A study that combined data from the High Resolution Stereo Camera (HRSC) and the Mars Orbiter Camera (MOC) found that some large dust devils on Mars have a diameter of 700 meters and last at least 26 minutes.[44]


Pedestal Craters

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.

Glacial Features

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.[45] [46] Lobate convex features on the surface known as viscous flow features and lobate debris aprons are now almost unanimously regarded as true glaciers.[47] [48] [49] [50] [51] [52] [53] [54] A climate model, reported in the journal Science in 2006, found that large amounts of ice should accumulate in the Hellas region, in the same places where glaciers are observed. Water is transported from the south polar area to northern Hellas and falls as precipitation.[55] The following pictures show many features that are probably glaciers—that is they are mostly ice and move downhill—like rivers—but much slower. Researchers started to call these things flows, but later came to understand that they are probably glaciers with a covering of debris.


High center polygons are visible. Box shows size of football field.

High center polygons are visible. Box shows size of football field.

Picture below shows material moving through a crater rim Lateral moraines are labeled.

Material Flowing through a crater rim Lateral moraines are labeled.

Channels

Channels,as seen by HiRISE under the HiWish program

Channels,as seen by HiRISE under the HiWish program

Today, it is generally accepted that water once flowed in river valleys on Mars.[56] [57] Images of curved channels have been seen in images from Mars spacecraft dating back to the early seventies with the Mariner 9 orbiter.[58] [59] [60] [61] Indeed, a study published in June 2017, calculated that the volume of water needed to carve all the channels on Mars was even larger than the proposed ocean that the planet may have had. [62] [63]

Channel Arrows indicate evidence of a meander.


                    Channel   Arrows indicate evidence of a meander.

Layers

Close view of layers Some layers are breaking up.

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.[64] A detailed discussion of layering with many Martian examples can be found in Sedimentary Geology of Mars.[65]

Honeycomb terrain

Honeycomb terrain is strangely beautiful. It presents with relatively flat-lying “cells” that appear to have concentric layers or bands, similar to a honeycomb. This "honeycomb" terrain was first discovered in the northwestern part of Hellas.[66] Although several ideas have been put forth, the exact geologic process responsible for creating these features remains unresolved.[67] Some calculations indicate that this formation may have been caused by ice moving up through the ground in this region. The ice layer would have been between 100 m and 1 km thick.[68] [69] [70] When one substance moves up through another denser substance, it is called a “diapir.” In this idea, large masses of ice pushed up layers of rock into domes that were subsequently eroded. After erosion cut off the top of the layered domes, circular features remained. Diapirs are thought to be responsible for features on Neptune's moon Triton, Jupiter's moon Europa, Saturn's moon Enceladus, and Uranus's moon Miranda.[71]

Ridge networks

Networks of ridges sometimes show up in low areas like crater floors. There origin is not completely understood.

Gullies

Close view of gullies, as seen by HiRISE under HiWish program Curves in channels are evidence that these gullies were not created by landslides.

Gullies occur on steep slopes, especially on the walls of craters. Gullies are believed to be relatively young because they have few, if any craters. Moreover, they lie on top of sand dunes which themselves are considered to be quite young. Usually, each gully has an alcove, channel, and apron.[72] [73] [74] For years, many believed that gullies were formed by running water, but further observations demonstrate that they may be formed by dry ice. Recent studies, using the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter (MRO), examined gullies at 356 sites, starting in 2006. Thirty-eight of the sites showed active gully formation. Before-and-after images demonstrated the timing of this activity coincided with seasonal carbon dioxide frost and temperatures that would not have allowed for liquid water. When dry ice frost changes to a gas, it may lubricate dry material to flow especially on steep slopes.[75] [76][77] In some years frost, perhaps as thick as 1 meter, triggers avalanches. This frost contains mostly dry ice, but also has tiny amounts of water ice.[78]


Gullies in crater

Close view of gullies in crater Polygons are visible in this close view.


Close view of gullies in crater Polygons are visible in this close view.

Polygons

Some surfaces on Mars display polygons. These may be of different sizes. Polygons are an example of patterned ground. Polygonal, patterned ground is quite common in some regions of Mars.[79] [80] [81] [82] [83] [84]

Exposed ice sheets

Thick deposits of ice were found by a team of researchers using instruments on board the Mars Reconnaissance Orbiter (MRO).[85] The team of scientists found eight eroding slopes that showed exposed water ice sheets as thick as 100 meters. Seven of the locations were in the southern hemisphere. Much evidence of buried ice under the ground on vast regions of Mars has already been found by past studies, but this study found that the ice was only covered by a layer of about 1 or 2 meters thick of Martian soil.[86] [87] [88] Shane Byrne of the University of Arizona Lunar and Planetary Laboratory, Tucson, one of the co-authors remarked that future colonists of the Red Planet would be able to gather up ice with just a bucket and shovel.[89] The layered ice is exposed in triangular shaped depressions. They are unique in that one wall is very steep and faces the pole. Confirmation that water-ice makes up the layers came from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on board the Mars Reconnaissance Orbiter (MRO). Spectra gathered by CRISM showed strong signals of water.[90] These layers are especially prominent in depressions in Hellas quadrangle as shown in the enlarged views below.

Besides being of great value to future explorers, these ice layers could help us better understand the climate history of Mars. They provide a record of the past. The large variations in the tilt of the planet cause dramatic climate variations. Mars does not possess a large moon to keep its tilt stable. Today, ice is concentrated at the poles, with a greater tilt, more ice will exist at mid-latitudes. These climate changes may be able to be measured with study of these layers.

These triangular depressions are similar to those in scalloped terrain. However scalloped terrain, displays a gentle equator-facing slope and is rounded. Scalloped terrain lacks the sharp vertical wall of these depressions.

Scalloped topography

Scalloped topography is common in the mid-latitudes of Mars, between 45° and 60° north and south. In the region around Hellas it is found in locations called Peneus Patera and Amphitrites Paterae[93] [94] in the southern hemisphere. It 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.[95] Scalloped depressions are believed to form from the removal of subsurface material, possibly interstitial ice, by sublimation (direct transition of a material from the solid to the gas phase with no intermediate liquid stage). This process may still be happening at present.[96] This topography may be of great importance for future colonization of Mars because it may point to deposits of pure ice.[97]

Additional Images in Hellas quadrangle

Possible dike and troughs The arrows point to the possible dike along the left edge of picture. Straight features are rare in nature; they are often due to dikes and joints.


Possible dike and troughs The arrows point to the possible dike along the left edge of picture. Straight features are rare in nature; they are often due to dikes and joints.


Ridges forming from cracks Box in upper left shows size of football field. Just imagine trying to hike across such a landscape.

Ridges forming from cracks Box in upper left shows size of football field. Just imagine trying to hike across such a landscape.

External links

See also

References:

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