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.

Reading the layers of history

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.

Going underground

History didn't end when Gale Crater dried up. The layers upon layers of sediment that built up on the crater floor during this period were eventually eroded by the wind, exposing them on Mount Sharp for Curiosity to sample. This period has been long enough that the winds blew sandy deposits into place that gradually solidified to form rocks.

One of these deposits, visited by Curiosity, is the subject of some independent work that shares features with that described above. Both of the papers describe how, in the later periods of Gale Crater's evolution, some of the rocks formed from lakebed sediments had groundwater seeping through their cracks. This groundwater left behind some tell-tale salts and minerals within these cracks, a process we're familiar with here on Earth.

The key finding in the second paper is that the groundwater mineral deposition didn't only occur in rocks made from lakebed sediments. Instead, it continued into the rocks made from windblown sand and dust as well.

Why is this important? The lake bed was apparently gone over three billion years ago. But it took a long time after that for the winds to carve away most of that sediment, and longer still for it to deposit enough sand and dust to form an additional layer of rock made of wind-blown material. The new results suggest that all of this had happened before groundwater seeped through the rocks and deposited additional minerals. That means that groundwater was present here long after the lake was gone.

"Consequently," the researchers conclude, "the timescale for potential habitability, at least in the subsurface of Gale, must be substantially extended."

We don't know what that timescale might be because we don't have a date on the formation of the wind-blown rocks in Gale Crater. But the two sets of results indicate that Mars had the potential to host life for over a billion years. And here on Earth, it took far less than that for life to begin.

Science, 2017. DOI: 10.1126/science.aah6849 (About DOIs).

Geophysical Research Letters, 2017. DOI: 10.1002/2017GL073323 (About DOIs).