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This scar, known as Aganippe Fossa, is a patchy, roughly 600-km-long feature known as a ‘graben’: a ditch-like groove with steep walls on either side.
Aganippe Fossa cuts across the lower flank of one of Mars’s largest volcanoes, Arsia Mons. Mars Express regularly observes Arsia Mons and its nearby companions in the region of Tharsis, where several of Mars’s behemoth volcanoes are found. This includes Olympus Mons, the tallest volcano in the Solar System (visible in the context map associated with this new image, as is Arsia Mons).
Arsia Mons itself measures 435 km in diameter and rises more than 9 km above the surrounding plains. For context, the highest dormant volcano on Earth, Ojos del Salado on the Argentina-Chile border, tops out at under 7 km.
Seeping lava
We’re still unsure of how and when Aganippe Fossa came to be, but it seems likely that it was formed as magma rising underneath the colossal mass of the Tharsis volcanoes caused Mars’s crust to stretch and crack.