Final answer:
Evidence for past liquid water on Mars, and thus potential life, includes the presence of recurring slope lineae, spectral analysis suggesting liquid water flow, mudstone indicative of wet environments, and discoveries of water and organic materials in Martian samples.
Step-by-step explanation:
The evidence presented in the paragraph from "Scratching the Surface Of Mars" supports the idea that life may have existed on Mars through indications of water. The key evidence includes the presence of recurring slope lineae, which are dark streaks in Mars' craters like Horowitz crater, suggesting the flow of salty liquid water. These lineae move downslope for about 100 meters, providing strong evidence that liquid water flows on or just below the surface of the planet. Spectral analysis corroborates this theory.
Additionally, the Curiosity and Perseverance rovers have taken further steps by finding mudstone, which implies the existence of wet environments necessary for life. Moreover, the discovery of both water and organic compounds in Martian samples hints at the possibility of ancient oceans and potentially life. The crux of the search for Martian life rests on the requirement of liquid water, as this is essential for life as we know it.
Lastly, mudcracks found in Gale crater, believed to be filled with a form of calcium sulfate, serve as another indicator that liquid water was present on Mars' surface in the past. These geological features are typically associated with drying and cracking of wet soil or sediment, adding to the body of evidence for past water activity on Mars.