Final answer:
Gullies, both on Mars and Earth, are formed through erosional processes often related to the action of water. Images from Mars orbiters show gullies that suggest episodic water flows potentially from underground sources. On Earth, river erosion creates similar landscapes in a continuous process.
Step-by-step explanation:
Gullies Formed by Erosion
Gullies observed on the steep walls of valleys and craters, particularly on Mars, are indicative of erosional processes, often caused by the temporary flow of surface water. These erosional features appear to be very young, with a lack of superimposed impact craters and some crossing over recent wind-deposited sands. This suggests that there might be liquid water underground intermittently breaking through to the surface to cause these formations before it freezes or evaporates.
The Mars Global Surveyor and the Mars Reconnaissance Orbiter have provided high-resolution images showing these gullies, and some even display seasonal changes. Dark streaks within these gullies elongate over a few days with the Martian seasons, indicating downhill flow which could either be water or dark sediment. If it is water, this hints at a potential underground source, such as aquifers, contributing to the seasonal water flow that shapes these gullies.
On Earth, river erosion similarly affects landscapes, carving river valleys in the mountains and creating cliffs and caves along coasts. These natural activities demonstrate how erosion and weathering shape different planetary surfaces, including Mars, through the action of water.