Final answer:
Darker areas on Mars are attributed to iron oxides in the soil, changing seasonal surface markings, and the effects of wind redistribution creating contrasts in terrain. The dark sand dunes and terraced layers suggest periodic climate changes influenced by Mars' orbit and tilt.
Step-by-step explanation:
The reason some areas of Mars are darker than others can be attributed to several factors. One explanation is the presence of iron oxides in the soil, which give Mars its reddish color and cause darker surface markings. These areas sometimes change in outline and intensity with the martian seasons, suggesting seasonal processes like wind redistribution or the flow of water or dark sediment.
Furthermore, dark sand dunes are found at higher latitudes on Mars, indicating that wind plays a significant role in moving material around the planet's surface. This wind action creates contrasts between darker areas of sand and the lighter underlying material. Additionally, the cyclic deposition of dust and ice contributes to the appearance of terraced layers, inferring changes in the martian climate over thousands of years similar to Earth's ice ages, affected by Mars' orbit and tilt variations.