4. Mars Lake Sediments
Even though liquid water cannot exist now on Mars’ surface, there is geological evidence for ancient lakes and possibly even oceans. This picture, taken by the Viking 1 Orbiter, shows possible evidence for such an ancient lake. In the center of the image is a round- ended chasm or canyon, deeper than the Grand Canyon on Earth. The walls of the chasm form steps down to its floor, and each step is a layer of tough, strong rock; there must be weaker rock layers between the steps. These layers extend for hundreds of kilometers, and may have formed originally as sediments (mud and sand) deposited in an ancient lake.
On Earth, life is found in hostile environments--in the barren dry valleys of Antarctica, below the surface of the driest deserts, in rocks deep in the Earth (slide #27), and in boiling hot sulfurous springs (slide #28) — but life today is much more abundant and visible in temperate environments with abundant water. Past life on Earth also needed water, as fossil traces of ancient life are found mostly in sedimentary rocks that were deposited in ancient lakes and oceans. Applying this to Mars, it might be easier to find traces of ancient life on Mars in places where there was once water at the surface, like ancient lakes and oceans (now dried and gone). Fossils, maybe only fossil bacteria or algae, may await us in the hardened muds of Mars' ancient lakes, such as the layers in this picture.
The layers in this region were probably deposited one at a time, layer upon layer, with the youngest layer on top. In sequences of rocks like this on Earth, we can trace the evolution of life through the changing types of fossils in the rocks. Perhaps we could see the same on Mars — fossils in the oldest rocks (the deepest) might be different from fossils in the youngest rocks.
Color mosaic from Viking Orbiter images 126A08, 16, 22, and 24. 44°S 259°W. Scene is approximately 120 kilometers across. Image processing by Peter Leth (Pomona College) and Allan Treiman (Lunar and Planetary Institute).