Northwest of Hellas
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The Mare Serpentis region (Latin for Sea of the Snakes) is located northwest of the giant Hellas impact basin and was named after the constellation Serpens in the northern celestial hemisphere. The observation shows impact craters of different ages, randomly distributed on the surface. The younger craters in the image show more distinct crater rims, the older ones appear much more smoothed and eroded. Especially peculiar are the different interior fillings of the craters. The surface patterns in these craters reflect the climatic conditions in which the fillings were formed, providing an insight into the history of the Martian climate.
The upper left flat-floored crater shows a distinct radial ejecta layer with many stripes and measures roughly 22 km in diameter. Compared to the other large craters, it appears to be the youngest. The smaller crater to its left still hosts some of the ejecta of the larger crater. The 22 km crater shows a so-called concentric crater fill. This is a common landform in the Martian mid latitudes (30°-60° N and S) and describes a crater filling deposit with an approximately concentric pattern. It develops when debris masses very slowly flow down along parts or all parts of the crater wall, often converging at or near the center of the crater floor. The debris is mixed with ice that condensed from the atmosphere. The smaller craters on the right side of the HRSC observation also show a ice-rich crater fill, however not concentric, but lobate. The distinct rims also indicate a younger age of these craters.
The largest impact crater in the image shows a width of 45 km, strongly eroded crater rims, a flat floor with some interesting erosional features and a very eye-catching dark color. Interestingly, all craters on the right side of the image show this dark color, presumably created by dark sands, transported by wind.
In the lower left part of the image, some small valleys can be spotted, leading into an ancient, nearly completely vanished impact crater (see annotated image), which indicate a past era of liquid water at the surface. In the upper central part of the image a strongly eroded impact crater also shows some little valleys leading into what remained from the crater. Below it, a crater approximately 18 km in diameter displays distinct alcoves and channels in the crater wall. The crater floor also shows a lobate filling, here in an apparently earlier stage.
In the lower right portion of the image, a 35 km impact crater gives interesting insights into the subsurface. Here, an approximately 20 km long, 9 km wide and several hundreds of meters deep pit opens and exposes different alternating layers in the subsurface. Some of the layers appear to be composed of brecciated materials. The curvilinear form of a prominent fault inside the pit suggests a formation by collapse. This kind of collapses are known to happen in volcanically active regions where ancient lava tubes or magma chambers emptied and as a consequence created voids in the underground which later collapsed. The region in and around Mare Serpentis is known to have been volcanically active in the past.