Final answer:
It is difficult to learn about Venus' surface through ground-based observation due to perpetual cloud layers. Radar instruments have been used to study the geology of Venus.
Step-by-step explanation:
The main reason it is difficult to learn about Venus' surface through ground-based observation is because the planet is covered by perpetual cloud layers. These clouds make it impossible for us to see the surface visually. To overcome this challenge, scientists have used radar instruments to penetrate through the obscuring layer and study the geology of Venus.