Professor Jeffrey Andrew-Hanna spoke on Thursday August 23 about the landscape of the Tharsis region of Mars where many different land masses have formed. There is a crevice that is larger than any canyon on the planet Earth and a large active volcano known as Olympus Mons in the same Tharsis region. Andrew-Hanna opens, “It is interesting to study the geophysics of a planet different from ours.” Mars, unlike Earth, is a single plate planet which is vastly different from Earth’s system of tectonic plates.
Mars is divided into two hemispheres: the Northern lowlands and the Southern highlands. This is known as the Dichotomy of Mars, its oldest and most defining feature. Oddly enough, in the southern highlands of Mars, one of the lowest points on the planet, the Valles Marineris exists. Near the Valles Marineris is the Noctis Labyrinthus, a set of canyons to the east. The Valles Marineris, Olympus Mons, and the Noctis Labyrinthus are all within the Tharsis region on Mars. This area itself is unique because scientists are still trying to discover how it was created and how long ago it was formed. One idea is that Tharsis was formed by the impact of a meteor, which caused an effect known as the Borealis Rim. This hypothesis is based upon the fact that the largest impact surfaces are elliptical in shape. However, the projected crater on Mars is four times larger than any other meteorite impacts from recorded history. Initially, Andrew-Hanna was one of the few scientists who believed in a meteorite impact on Mars; “at first people disagreed with me, but over time they have to changed to agree with me.” Andrew-Hanna predicted that a 2,230 kilograms meteorite made impact with Mars and caused a giant elliptical crater.
Valles Marineris adds many stresses to the Tharsis region. The stresses on Tharsis can be classified into three categories. The first stress is the finite element model, which can best be described by a quote from Andrew-Hanna, “It is like pushing down on a board harder on one end than the other.” Andrew-Hanna explained further, “Eventually the board will bend and break.” This statement describes a thicker mass on one end and a lighter mass at the other end of Tharsis. The second stress is the thin shell. This does not rely on the dichotomy of Mars. This idea is that stresses are compressional within Tharsis. The third stress is the Crustal-fit model, which describes a load thickness designed to reproduce present day crustal thickness. It also defines a narrow band of tensile stress which runs perpendicular to the Valles Marineris. This is how the Noctis Labyrinthus is suspected to have formed.
The fault geometry of Mars is used to determine how much extension took place. Typical faults have between 30-60 kilometers of max extension, but these numbers are nearly impossible for the surface of Mars because faults are typically much smaller than these estimates. Other ideas for where the Mars faults arose from include either melting ice or lava flow. The amounts of these substances needed to fill the Valles Marineris is 200 kilometers across and 10 kilometers thick. This is a nearly impossible amount of lava or ice. Another theory is the Boundary Element model, which includes fault slips and elastic deformation. The problem with this model is that the stresses are too large for the surface of Mars to obtain without breaking up. This theory predicts that the surface of Mars will rise about two kilometers, while in reality, it should only rise about 300 meters. The most accurate model is the Subsurface Collapse Model. This model predicts dips that are larger than eighty-five degrees, stresses that range from 100-300 Megapascals and that the surface uplift should be around 300 meters.
The formation of the Valles Marineris is supported by the elastic lithosphere, which is “super isostatic.” It is an isostatic anomaly that is maintained by the elastic strength of the lithosphere. Tharsis is a flexural support and the volcanic center of Mars. This area has abundant evidence of surface and sub-surface igneous activity. This also causes collapse pits that are parallel to the Valles Marineris. At one time, Andrew-Hanna believed that a sedimentary filling completed the troughs with Interior Layer Deposits. Through erosion, the sediment was eventually washed away. Hydrologic effects also cycled away some of the sediment. Andrew-Hanna believed in a “perfect storm” that consisted of its super isostatic crust working with the tensile stress belt and a sedimentary loading, causing the formation of the Valles Marineris.
The final structure discussed was the Olympus Mons, a massive 20 kilometer high shield volcano. The first question about this structure is, how old is it? The answer can be found from the craters. The more craters there are, the older the surface is. Thus the volcano was estimated to be about 3.5 billion years old. By using paleo-topography, one can also find the age of a structure. Andrew-Hanna stated, “If you can date the age of a trough, you can date the age of a volcano.” The troughs form from the flow of lava. At certain points in history, the lava had not flowed downhill. It has even flowed perpendicular to the downhill motion. Thus lava flow started before the volcano was even present. It turns out that lava flow began 3.7 billion years ago. Olympus Mons still spews lava, about 0.1 kilometers a day, which is comparable to volcanoes in Hawaii.
These three features are what define the Tharsis region of Mars. Beginning with the formation of the structures to learning how those structures form is invaluable to understanding foreign planets.
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