rstevenson wrote:Here's a 3D image of part of Arabia Terra, as referenced on this page, about 1/3 of the way down.
As a geologist, I find this image very intriguing. Flat lying strata comprising the Mesas shows no evidence of deformation, indicating that the deposits that formed the Mesas were laid down during a period of no tectonic activity (at least in the Arabia Terra region), with folding and faulting absent. Much the same geological conditions were prevalent when the strata of Mesas in Arizona were deposited.
Secondly, the Mesas are predominantly S – N aligned teardrop shapes in the form of giant ‘ventifacts’ carved by southerly winds, with the elongated ends of the teardrops pointing downwind and the median spines parallel to the prevailing wind direction. It is possible that the Mesas have been eroded into these shapes over time by katabatic winds blowing towards the polar ice caps due to seasonal heating of the equatorial regions causing a wide band of elevated pressure encircling the planet. Note that the younger parabolic dunes have the same S – N alignment, but the material comprising them has a different provenance, possibly extensive areas of dark Iron and Magnesium rich bedrock exposed by wind ablation in equatorial and temperate regions.
<<The surface of Mars has a very low thermal inertia, which means it heats quickly when the sun shines on it. Typical daily temperature swings, away from the polar regions, are around 100 K. On Earth, winds often develop in areas where thermal inertia changes suddenly, such as from sea to land. There are no seas on Mars, but there are areas where the thermal inertia of the soil changes, leading to morning and evening winds akin to the sea breezes on Earth.
At low latitudes the Hadley circulation dominates, and is essentially the same as the process which on Earth generates the trade winds. One of the major differences between Mars' and Earth's Hadley circulations is their speed which is measured on an overturning timescale. The overturning timescale on Mars is about 100 Martian days while on Earth, it is over a year.
At higher latitudes a series of high and low pressure areas, called baroclinic pressure waves, dominate the weather. Mars is dryer and colder than Earth, and in consequence dust raised by these winds tends to remain in the atmosphere longer than on Earth as there is no precipitation to wash it out (excepting CO2 snowfall).>>
<<Arabia Terra is a large upland region in the north of Mars. It is densely cratered and heavily eroded. This battered topography indicates great age, and Arabia Terra is presumed to be one of the oldest terrains on the planet. It covers as much as 4500 kilometers at its longest extent, centered roughly at 19.79°N 30°ECoordinates: 19.79°N 30°E with its eastern and southern regions rising 4 kilometers above the north-west. Alongside its many craters, canyons wind through the Arabia Terra, many emptying into the large Northern lowlands of the planet, which borders Arabia Terra to the north.
Arabia contains many interesting features. There are some good examples of pedestal craters in the area. A pedestal crater has its ejecta above the surrounding terrain, often forming a steep cliff. The ejecta forms a resistant layer that protects the underlying material from erosion. Mounds and buttes on the floor of some craters display many layers. The layers may have formed by volcanic processes, by wind, or by underwater deposition. Dark slope streaks have been observed in Tikhonravov Basin, a large eroded crater. The streaks appear on steep slopes and change over time. At first they are dark, then turn a lighter color, probably by the deposition of fine, light colored dust from the atmosphere. These streaks are thought to form by dust moving downslope in a way similar to snow avalanches on Earth.>>
<<During past ages, there was rain and/or snow on Mars; especially in the Noachian and early Hesperian epochs.
Some locations on the Red Planet show groups of layered rocks. Rock layers are present under the resistant caps of pedestal craters, on the floors of many large impact craters, and in the area called Arabia. In some places the layers are arranged into regular patterns. It has been suggested that the layers were put into place by volcanoes, the wind, or by being at the bottom of a lake or sea. Calculations and simulations show that groundwater carrying dissolved minerals would surface in the same locations that have abundant rock layers. According to these ideas, deep canyons and large craters would receive water coming from the ground. Many craters in the Arabia area of Mars contain groups of layers. Some of these layers may have resulted from climate changes. The tilt of the rotational axis of Mars has repeatedly changed in the past. Some changes are large. Because of these variations of climate, at times the atmosphere of Mars will be much thicker and contain more moisture. The amount of atmospheric dust also has also increased and decreased. It is believed that these frequent changes helped to deposit material in craters and other low places. The rising of mineral-rich ground water cemented these materials. The model also predicts that after a crater is full of layered rocks; additional layers will be laid down in the area around the crater. So, the model predicts that layers may also have formed in intercrater regions; layers in these regions have been observed. 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. On Earth, mineral-rich waters often evaporate forming large deposits of various types of salts and other minerals. Sometimes water flows through Earth’s aquifers, and then evaporates at the surface just as is hypothesed for Mars. One location this occurs on Earth is the Great Artesian Basin of Australia. On Earth the hardness of many sedimentary rocks, like sandstone, is largely due to the cement that was put in place as water passed through.
Many areas on Mars show inverted relief. In those places, former stream channels are displayed as raised beds, instead of stream valleys. Raised beds form when old stream channels become filled with material that is resistant to erosion. After later erosion removes surrounding soft materials, more resistant materials that were deposited in the stream bed are left behind. Lava is one substance that can flow down valleys and produce such inverted terrain. However, fairly loose materials can get quite hard and erosion resistant when cemented by minerals. These minerals can come from groundwater. It is thought that a low point, like a valley focuses groundflow, so more water and cements move into it, and this results in a greater degree of cementation.
Orbiting probes showed that the type of rock around Opportunity was present in a very large area that included Arabia, which is about as large as Europe. A spectroscope, called CRISM, on the Mars Reconnaissance Orbiter found sulfates in many of the same places that the upwelling water model had predicted, including some areas of Arabia. The model predicted deposits in Valles Marineris canyons; these deposits have been observed and found to contain sulfates. Other locations predicted to have upwelling water, for example chaos regions and canyons associated with large outflows, have also been found to contain sulfates. Layers occur in the types of locations predicted by this model of groundwater evaporating at the surface. Layers have been observed around the site that Opportunity landed and in nearby Arabia. The ground under the cap of pedestal craters sometimes displays numerous layers. The cap of a pedestal crater protects material under it from eroding away. It is accepted that the material that now is only found under the pedestal crater’s cap formerly covered the whole region. Some craters contain mounds of layered material that reach above the crater’s rim. Gale Crater and Crommelin Crater are two craters that hold large mounds. Such tall mounds were formed, according to this model, by layers that first filled the crater, and then continued to build up around the surrounding region. Later erosion removed material around the crater, but left a mound in the crater that was higher than its rim.>>
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