Astronomy Practice Exam

What has been discovered on Mars that suggests it once had liquid water?

1. Subsurface lakes

2. Ancient riverbeds

3. Current tidal effects

4. Extensive cloud cover

The correct answer is: Ancient riverbeds

The discovery of ancient riverbeds on Mars is significant because it provides substantial evidence that liquid water once flowed on the planet's surface. These riverbeds, which resemble those found on Earth, indicate a history of sustained water activity, thus suggesting that Mars had a much wetter climate in the past. The presence of ancient riverbeds shows that water not only existed but also moved across the landscape, shaping the surface much like it does on Earth today. Such geological features are formed by the erosive power of moving water over time, supporting the idea that liquid water played a critical role in shaping Mars’ surface. Other choices, while they might seem plausible, are either not established as fact (like subsurface lakes) or do not pertain to the historical presence of liquid water in the same direct way as riverbeds do. Current tidal effects are irrelevant to hinting at past conditions, and extensive cloud cover does not directly imply the existence of liquid water at any point in Mars' history.