Anyone know much about mars?

Okay I was just searching for flyby pics of mars, and I came across this Rosetta one:



Now to me, that just looks like some big ass battle went on. How come only the bottom half is covered in black dust and craters? Just that part of the planets atmosphere is different or something ? Would that make sense?

Can someone explain or shed some light?

Land of Rust

Its dirty red color must have dislodged memories of blood-soaked battlefields, inspiring early observers to name the planet for the Roman god of war. Although we now know Mars’ surface to be entirely blood-free, the color of both substances can be explained by the same chemical reaction – the oxidation of iron, better known as rust. The reddish-brown dust that veils much of Mars’ rocky surface is rich in iron oxide. Areas that are comparatively free of this dust appear darker, exposing the underlying rocky terrain and creating a pattern of contrasts that changes as colossal global windstorms redistribute the dust. Particles of dust suspended in the planet’s thin atmosphere give the sky a pinkish orange glow.

Some great stuffs here... check it out!
www.christa.org...
mars.jpl.nasa.gov...