Diagram of Lake Stratification on MarsThis diagram presents some of the processes and clues related to a long-ago lake on Mars that became stratified, with the shallow water richer in oxidants than deeper water was.
The sedimentary rocks deposited within a lake in Mars' Gale Crater more than three billion years ago differ from each other in a pattern that matches what is seen in lakes on Earth. As sediment-bearing water flows into a lake, bedding thickness and particle size progressively decrease as sediment is deposited in deeper and deeper water as seen in examples of thick beds from shallowest water, thin beds from deeper water and even thinner beds from deepest water.
At sites on lower Mount Sharp, inside the crater, measurements of chemical and mineral composition by NASA's Curiosity Mars rover reveal a clear correspondence between the physical characteristics of sedimentary rock from different parts of the lake and how strongly oxidized the sediments were. Rocks with textures indicating that the sediments were deposited near the edge of a lake have more strongly oxidized composition than rocks with textures indicating sedimentation in deep water. For example, the iron mineral hematite is more oxidized than the iron mineral magnetite.
An explanation for why such chemical stratification occurs in a lake is that the water closer to the surface is more exposed to oxidizing effects of oxygen in the atmosphere and ultraviolet light.
On Earth, a stratified lake with a distinct boundary between oxidant-rich shallows and oxidant-poor depths provides a diversity of environments suited to different types of microbes. If Mars has ever hosted microbial live, the stratified lake at Gale Crater may have similarly provided a range of different habitats for life.
Image Credit: NASA/JPL-Caltech/Stony Brook University