Eos Chasma is a chasma in the southern part of the Valles Marineris canyon system of the Margaritifer Sinus quadrangle region of the planet Mars. Eos Chasma’s western floor is mainly composed of an etched massive material composed of either volcanic or eolian deposits later eroded by the Martian wind. The eastern end of the Eos chasma has a large area of streamlined bars and longitudinal striations. This is interpreted to be stream-carved plateau deposits and material transported and deposited by flowing fluid. (wikipedia entry)
A chasma is a deep, elongated, steep-sided depression. On Mars they are named after the nearest named albedo feature on the Schiaparelli or Antoniadi maps.