Valles Marineris
This geological feature is 4,000 km long

Now this is a rather odd formation, rather unlike the previous picture. But how was this Mars feature formed? Wiki says there are a number of explanations, but presently it is believed that Valles Marineris is thought to be a large rift valley, that is it would be due to the movement of tectonic plates, but rifts on Earth do not tend to stay straight over such long distances.
But what if Valles Marineris canyon was not due to tectonic plates but was actually an impact site?
Not an impact site like the previous meteor impact pic, but a comet or other massive icy object coming at Mars on a grazing trajectory (basically coming right across the horizon, rather than straight down). And if this object was about 80% or more H2O, then it would have time, coming through Mars’ thin atmosphere, to partially liquify/vaporize before impact, though perhaps 40-50% of the object would still be solid on impact. So such an object hitting Mars would explain the sort of “splat” features of Valles Marineris (as opposed to an impact caused by a soley stoney object). That the deepest and widest cut appears in the center of the Valles Marineris feature also fits a grazing trajectory scenario.

Click on image to go to the next page