And there are indications that groundwater persisted for far longer.
Ars Technica said:Gale Crater, the site being explored by the Curiosity rover, was chosen as a landing site because its structure and composition suggested that it might preserve information about Mars' past. As Curiosity climbed the slopes of the crater's central peak, various discoveries have clearly indicated that Mars had a watery past.
Now, scientists have put all these individual discoveries into a big-picture view of the history of Gale Crater. The picture shows that the crater was water-filled for hundreds of millions of years—and warm for much of that time. A separate paper indicates that long after the crater filled up with wind-blown sand, groundwater still percolated through the area.
The new study is built on lots of individual analyses of rock samples done by Curiosity as it headed up the slopes. Various instruments revealed the types of rocks and their chemical composition at specific locations up the slopes, building a picture of the different layers of deposits.
Individually, these results are rather dry, to put it mildly. "The Murray formation can be subdivided into two groups (facies): one that is recognized by abundant ferric iron–bearing minerals (e.g., hematite) accompanied by phyllosilicates and another that is recognized by high concentrations of silica minerals accompanied by magnetite," the paper says.
But collectively, they tell an important story. Different minerals are more or less common under acidic or basic conditions; something called jarosite found in some deposits forms under fairly acidic conditions, for example. The presence of oxygen can be determined based on how common some forms of iron are. Sedimentary rocks composed of large grains would have formed near the shore of the lake in Gale Crater, while smaller particles travel further and contribute to rocks formed in deeper waters.
The researchers also measured what's termed a "chemical index of alteration," which tracks how much rocks from elsewhere on Mars underwent chemical reactions before getting incorporated into sedimentary rocks. This provides some measure of the temperatures.
Putting this all together, the authors suggest that Gale Crater's lake went through several different periods. While the first material to arrive as sediment was transported in a cold climate, things soon warmed up. During this warm period, the lake ended up stratified, meaning it had distinct layers. Near the surface, UV light and atmospheric oxygen created an oxidizing environment. This oxidized some of the sulfur in the rocks, ultimately creating enough sulfuric acid to lower the pH there. Deeper in the lake, there was little oxygen, and the pH stayed closer to neutral.
Later still, there was some deposition of salt-rich deposits. This may represent the period when Mars was losing much of its atmosphere and its waters were evaporating away.
The key thing is that the period when the conditions were warm and watery lasted a long time. "Only a small component of the observed stratigraphy express[es] geochemical properties consistent with a cold climate," the authors conclude. They estimate that environment fit the definition of habitable for a period of about 700 million years, ending at 3.1 billion years ago.
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