This elongated depression is about 78 km in length, opens from just under 10 km wide at one end to 25 km wide at the other, and reaches a depth of 2 km. It is located at about 21°S / 55°E, and was probably caused by the impact of a train of projectiles. The data were acquired during orbit 8433 on 4 August 2010 using the High Resolution Stereo Camera on Mars Express. (Credit: ESA/DLR/FU Berlin (G. Neukum))
The Scars Of Impacts On Mars -- Science Daily
ScienceDaily (Mar. 7, 2011) — ESA's Mars Express has returned new images of an elongated impact crater in the southern hemisphere of Mars. Located just south of the Huygens basin, it could have been carved out by a train of projectiles striking the planet at a shallow angle.
The large Huygens basin (not visible in the main image but seen in the wider contextual image) is about 450 km in diameter and lies in the heavily cratered southern highlands. In this area there are many impact scars but none perhaps are more intriguing than the 'elongated craters'.
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My Comment: That is one hell of an impact.