Free Astronomy Magazine March-April 2021

39 MARCH-APRIL 2021 MARS ROVERS E levation map of the alluvial delta, fed through Peace Vallis, which originated the dry streams discovered by Curiosity. The cross inside the ellipse indicates the rover’s landing site, very close to the sites of Link and Hottah. [NASA/JPL- Caltech/Univ. Of Arizona, ASU] A second important question con- cerns where these clays formed. They may have formed outside what could be the crater’s bottom, such as on the crater ring, by the ac- tion of simple temporary streams. Alternatively, they might have formed on the bottom of the crater due to the action of lake waters, starting from rocky fragments rol- led along the slopes and from dust carried by the winds. Since the raw material is substantially formed by volcanic minerals in both cases and since their transformation into clay where and how those clay minerals formed within Gale Crater could provide the chronology of the lake it hosted. The longer this lake lasted before evaporating, the more likely it is that it may have harbored ele- mental life forms. A key question was whether Yellowknife Bay’s clay minerals formed earlier than the lake’s supposed epoch. mentary life forms. This discovery confirmed the dominant view of the history of Mars, which indicates that there were cool, calm waters where clay minerals could form about 4 bil- lion years ago. That environment, which was favorable to life, then ex- perienced a growing drought that made those waters increasingly acidic and brackish. Understanding