Arabia quadrangle
Map of Arabia quadrangle from Mars Orbiter Laser Altimeter (MOLA) data. The highest elevations are red and the lowest are blue.
15°00′N 337°30′W / 15°N 337.5°W / 15; -337.5Coordinates: 15°00′N 337°30′W / 15°N 337.5°W / 15; -337.5
Image of the Arabia Quadrangle (MC-12). The region is dominated with heavily cratered highlands; the northeast part contains Cassini Crater.
The Arabia quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The Arabia quadrangle is also referred to as MC-12 (Mars Chart-12).[1]
The quadrangle contains part of the classic area of Mars known as Arabia. It also contains a part of Terra Sabaea and a small part of Meridiani Planum.It lies on the boundary between the young northern plains and the old southern highlands. The quadrangle covers the area from 315° to 360° west longitude and 0° to 30° north latitude.
2 Layers
3 Craters
4 Possible methane
5 Deformation bands
6 Geological history
7 Dark slope streaks
9 Other Mars quadrangles
The surface of the Arabia quadrangle appears to be very old because it has a high density of craters, but it is not near as high in elevation as typical old surfaces. On Mars the oldest areas contain the most craters; the oldest period is called Noachian after the quadrangle Noachis.[2] The Arabia area contains many buttes and ridges. Some believe that during certain climate changes an ice-dust layer was deposited; later, parts were eroded to form buttes.[3] Some outflow channels are found in Arabia, namely Naktong Vallis, Locras Valles, Indus Vallis, Scamander Vallis, and Cusus Valles.[4]
Map of Arabia quadrangle with major craters.
Naktong Vallis, as seen by HiRISE.
Indus Vallis, as seen by HiRISE.
Layers[edit]
Many places in Arabia are shaped into layers.[5] The layers can be a few meters thick or tens of meters thick. Recent research on these layers by scientists at California Institute of Technology (Caltech) suggest that ancient climate change on Mars caused by regular variation in the planet's tilt, or obliquity may have caused the patterns in the layers. On Earth, similar changes (astronomical forcing) of climate results in ice-age cycles.
A recent study of layers in craters in western Arabia revealed much about the history of the layers. Although the craters in this study are just outside the boundary for the Arabia quadrangle the findings would probably apply to the Arabia quadrangle as well. The thickness of each layer may average less than 4 meters in one crater, but 20 meters in another. The pattern of layers measured in Becquerel crater, suggests that each layer was formed over a period of about 100,000 years. Moreover, every 10 layers were bundled together into larger units. The 10-layer pattern is repeated at least 10 times. So every 10-layer pattern took one-million years to form.
The tilt of the Earth's axis changes by only a little more than 2 degrees; it is stabilized by the relatively large mass of our moon. In contrast Mars's tilt varies by tens of degrees. When the tilt (or obliquity) is low, the poles are the coldest places on the planet, while the equator is the warmest - as on Earth. This causes gases in the atmosphere, like water and carbon dioxide, to migrate pole ward, where they freeze. When the obliquity is higher, the poles receive more sunlight, causing those materials to migrate away. When carbon dioxide moves from the poles, the atmospheric pressure increases, maybe causing a difference in the ability of winds to transport and deposit sand. Also, with more water in the atmosphere sand grains may stick and cement together to form layers. This study of the thickness of layers was done using stereo topographic maps obtained by processing data from the high-resolution camera onboard NASA's Mars Reconnaissance Orbiter.[6]
Recent research leads scientists to believe that some of the craters in Arabia may have held huge lakes. Cassini Crater and Tikonravov Crater probably once were full of water since their rims seem to have been breached by water. Both inflow and outflow channels have been observed on their rims. Each of these lakes would have contained more water than Earth's Lake Baikal, our largest freshwater lake by volume. The watersheds for lakes in Arabia seem to be too small to gather enough water by precipitation alone; therefore it is thought that much of their water came from groundwater.[7]
Another group of researchers proposed groundwater with dissolved minerals came to the surface, in and later around craters, and helped to form layers by adding minerals (especially sulfate) and cementing sediments. Upon close examination, Arabia layers appear to have a slight tilt. This tilt supports formation with the action of a rising water table. A water table generally follows the topography. Since the layers slope slightly down toward the northwest, the layers may have been created by groundwater, rather than a single large sea that has been suggested.
This hypothesis is supported by a groundwater model and by sulfates discovered in a wide area.[8][9] At first, by examining surface materials with Opportunity Rover, scientists discovered that groundwater had repeatedly risen and deposited sulfates.[10][11][12][13][14] Later studies with instruments on board the Mars Reconnaissance Orbiter showed that the same kinds of materials exist in a large area that included Arabia.[15]
Layers in Gill crater (Martian crater), as seen by HiRISE under HiWish program.
Close-up of layers in Gill crater, as seen by HiRISE under HiWish program.
Close-up of layers on edge of a mesa in Gill crater, as seen by HiRISE under HiWish program.
Layers under cap rock of a pedestal crater, as seen by HiRISE under HiWish program. Pedestal crater is within the much larger Tikhonravov Crater.
Close-up of some layers under cap rock of a pedestal crater, as seen by HiRISE under HiWish program.
Close-up of some layers under cap rock of a pedestal crater and a dark slope streak, as seen by HiRISE under HiWish program.
Layers in a butte in Arabia, as seen by HiRISE under HiWish program.
Layers in Arabia, as seen by HiRISE under HiWish program.
West side of Danielson Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Danielson has a great deal of regular layering.
Some of the layers of Danielson Crater is just visible in this CTX image. Dunes are also visible.
Many layers are visible in this HiRISE image. Dark dust serves to highlight some of the layers.
Craters[edit]
Impact craters generally have a rim with ejecta around them, in contrast volcanic craters usually do not have a rim or ejecta deposits. As craters get larger (greater than 10 km in diameter) they usually have a central peak.[16] The peak is caused by a rebound of the crater floor following the impact.[17] Sometimes craters display layers. Since the collision that produces a crater is like a powerful explosion, rocks from deep underground are tossed unto the surface. Hence, craters can show us what lies deep under the surface.
Some craters in Arabia are classified as pedestal craters. A pedestal crater is a crater with its ejecta sitting above the surrounding terrain and thereby forming a raised platform. They form when an impact crater ejects material which forms an erosion resistant layer, thus protecting the immediate area from erosion. As a result of this hard covering, the crater and its ejecta become elevated, as erosion removes the softer material beyond the ejecta.[18] Some pedestals have been accurately measured to be hundreds of meters above the surrounding area. This means that hundreds of meters of material were eroded away. Pedestal craters were first observed during the Mariner missions.[18][19][20]
Researchers believe over 200 new craters are formed each year on Mars, based on study of years of HiRISE images.[21][22]
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.
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.
Pasteur Crater Floor, as seen by HiRISE. The scale bar is 1,000 meters long
Henry Crater Mound, as seen by HiRISE. The scale bar is 500 meters long
Mounds in craters like Henry are formed by the erosion of layers that were deposited after the impact.
Crater in the middle of Cassini, as seen by HiRISE. Layers may have been deposited under water since it is believed that Cassini once held a giant lake.