Saturday, September 16, 2017

Muddy Impact on Mars

Water-rich impact crater on Mars. Credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO

Over billions of years, meteorite impacts have altered the surface of Mars. Current images from the High Resolution Stereo Camera (HRSC) operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) on board the ESA Mars Express spacecraft show an impact crater over 30 kilometers in size with a prominent ejecta blanket. It is located to the north of the largest impact basin on Mars, Hellas Planitia, where some scientists believe there was once a large lake.

When a meteorite impacts, material from below the surface is uncovered. The ejected rock masses settle around the crater mostly radially in what is known as an ejecta blanket. Depending on the size and speed of the impact, this material can originate from different depths and settle kilometers away from the crater. The properties of these deposits depend on the size of the crater, the composition of the substrate and the presence of an atmosphere. The deposits may consist of finely dispersed, disconnected loose material, for example, or a coherent mass, which then produces a more compact ejecta blanket. Scientists can therefore learn about the composition of the subsurface of Mars through analysis of the different ejecta blankets.

Many impact craters on Mars exhibit such lobe-shaped ejecta blankets as can be seen in this crater in the center of images 1, 2, 3 and 5. They have clear, wall-shaped rims called ramparts, meaning steep embankment or wall. In this rampart crater 32 kilometers in size, up to three layers of lobe-shaped ejecta blanket can be seen. They run radially from the crater and have tongue-shaped offshoots, some of which end in a steep wall. If a crater has such a lobe-shaped ejecta blanket, it indicates the presence of water or ice in the subsurface at the time of impact. This melting subsurface ice or water is mixed with the released material to form a free-flowing mass that is ejected from the point of impact, thus forming the typical lobe shape. The ejecta blanket obtains its flow structure from this mixture of water and rock. By determining the age of the ejecta blanket, the time of the meteorite impact can be calculated, which gives an indication of when water or ice was present in the Martian soil.

The impact crater with the prominent ejecta blanket is located north of the extensive impact basin Hellas Planitia, in a region that may have once belonged to the hollow of a lake within the basin in Mars' early history. Some scientists believe old coastlines can be retraced that appear to show the former bank line of the lake. However, conclusive evidence has yet to be found. Nonetheless, a number of indications of glacial processes can be seen in and around Hellas, regarding ice in the ground and glacial deposits. Old dried up river valleys can also be seen to the south of the ejecta blanket on closer inspection (in the images to the left and further) and have been cut off and covered by the blanket. All of these surface features substantiate the assumption that at the time of impact of the small crater described here, ice or groundwater was present near the surface, which led to the ‘molten’ appearance of its ejecta blanket.


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