July 21, 2012
Credits: NASA/JPL/MSSS - Mars Global Surveyor Mission - Credits for the additional process.: Dr Paolo C. Fienga/Lunar Explorer Italia/IPF
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The (main) factors influencing the Initial Formation of the Water-Ice Patch that is so incredibly bright and well visible (even in this frame, taken by the NASA - Mars Reconnaissance Orbiter from over 400 Km of altitude) inside Louth Crater, are likely bound to remain a mystery); however, Its location and relative proximity to the Radial Water-Ice Deposits that lie around the North Polar Cap of Mars (between Longitudes 90 to 250 and Latitudes 75 to 78 (unofficially named "Mrs. Chippy's Ring") suggests linkages to these off-cap Water-Ice Deposits. On the other hand, it is highly unlikely that a Direct Transfer of Icy Material might have occurred by way of Aeolian Activity (alone) from "Mrs. Chippy's Ring" to Louth Crater (i.e.: by way of so-called Saltation of Ice Grains), given (among others) the distance of more than 100 Km intervening between the two locations. There is also very little evidence to suggest that Louth is a Remaining Outlier from a much larger Polar Cap. Therefore, the suggested (and, in a way, preferable) mode of deposition of the Ice at Lauth was via Atmospheric Deposition of Water Vapor, where Water Particles are preferentially trapped in the center of the Crater. In constant competition with this "Depositional Process", though, a strong Aeolian Activity and Sublimation also appear to be shaping and stripping back the Water-Ice Patch, and it is possible, given its offset position, that it formerly occupied a larger area within Louth and that it has retreated Poleward in (relatively) recent times. As far as the Stability of the Water-Ice Patch is concerned, if we consider the Icy Material to be a section of a sphere that is approx. 12 Km across and 200 mt deep, the Water-Ice Deposit should roughly be 11 Km3 in volume. Its location within the Crater, however, is offset to the Poleward Side. Vasavada et al. used Thermal Modeling to suggest Craters on the Moon and Mercury at similar latitudes will experience coldest average temperatures on that portion of their Floors which is adjacent to the Equatorward Rim. However the Depth-to-Diameter Ratio of Louth Crater is only approx. 0.42, so the Walls appear to be less than 5° in height when standing at the Craters' deepest point. Furthermore, Mars also has a much larger Obliquity than either the Moon or Mercury, so that the Sun may appear up to 40° above the Horizon at this location. These two properties mean that a Direct Shadowing of the Crater Floor that is put into effect by the Crater Walls is not effective at Louth. The higher Albedo (and today's APOD proves this fact beyond any reasonable doubt) and, in particular, the Thermal Inertia of the Ice, works as a great help to stabilize it against ablation (---> removal), but this circumstance does not explain, anyway, the reason why the Icy Material began forming there in the first place. All that said, the factors controlling the size and shape of the Water-Ice Patch could be narrowed to: (A) the assumption that the shape of Louth Crater exerts (some) control over the Local Meteorology and therefore on the location of the Water-Ice Patch, or (B) the Thermodynamic Properties of the Material underlying the Ice Deposit can actually fully determine its exact location. Longitude of image center: 257,06° West This frame has been colorized in Absolute Natural Colors by (such as the colors that a human eye would actually perceive if someone were onboard the NASA - Mars Global Surveyor Orbiter and then looked down, towards the Surface of Mars), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. |
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