Valles Marineris is on Mars.
 Thermokarst activity may have contributed, but erosion by water is a problematic mechanism because liquid water cannot exist in most current Martian surface conditions, which typically experience about 1% of Earth's atmospheric pressure and a temperature range of 148 K (125 C; 193 F) to 310 K (37 C; 98 F)
 Valles Marineris may have been enlarged by flowing water at that time

