Okay, so after mapping the entire planet at 230 meters per pixel -- what next? First, you take your map of the networks and see if there are any correlations with other globally mapped data. For instance, you can ask: what's the age of the terrain on which these valleys formed? The answer is, unsurprisingly, that it's mostly pretty old terrain, from what's called the Noachian era. We knew that already. The Noachian era was the time in Mars' history that was dominated by asteroid impacts, before impact activity waned. You have to be careful, though, to avoid confusing correlation with causation. The age of the terrain on which a valley is superposed only gives you a maximum age for the valleys. Suppose, for the sake of argument, that valleys formed at the same rate everywhere on Mars at all times, just like impact craters. You'd see more of them on the oldest terrain, because it's been exposed for the longest time to accumulate the most.

So if you actually want to know when the valleys formed -- if you want to do better to know they can be no older than the end of the Noachian -- you have to find a way to figure out how old the valleys themselves are. You do that by finding something that has happened to the valleys that you can age-date. Hoke and Hynek selected ten very large, well-preserved valley networks and counted the craters that overlap them to age-date them, the same way you age-date a lava flow. This is a little tricky because of the more linear than areal nature of the valley networks, but geographic information systems let you do this.

And here's the surprise. All ten of the big networks that they looked at had a very narrow span of ages, from the end of the Noachian to the beginning of the Hesperian, or roughly 3.8 to 3.6 billion years ago. That's a very narrow span, at a very ancient time. And yet it's also a long span, too long to be considered geologically instantaneous. Several of the networks have distinctly different ages, though others appear to have formed at the same time. Hoke and Hynek say:

"This research suggests the late Noachian and early Hesperian were characterized by roaming zones of precipitation that occurred during either continuously warmer and thicker atmospheric conditions or intermittently clement conditions, with precipitation occasionally returning to previously rainy regions and overall continuing near Meridiani Planum longer than in Terra Sabaea."

The narrowness of the time frame is interesting. It is also interesting that it happens after Mars' major impacts. Why is that important? Because some people have suggested that Mars could've been ordinarily dry, cold, and desert-like, much like today, and that valley networks only formed when major impacts briefly enriched the atmosphere with volatile gases. But if rain only happened mostly after all of Mars' major impacts, not during the impact era, then the impact-generated atmosphere explanation for valley networks just doesn't work.

There are valley networks that are younger than this brief period. They are isolated in a few locations that tend to be on the flanks of volcanoes. When Hynek, Beach, and Hoke looked at the younger volcanoes closely, they found that the younger volcanoes (like Arsia Mons, for instance) have networks that look quite different from the ancient ones. They're much more primitive (less branched) and are often associated with volcanic pit crater chains. They look more like the kinds of channels that form from groundwater springs in hydrothermal environments, not from rain.

So what does this all mean? Hynek, Beach, and Hoke tell the following story: Mars started out with a thin atmosphere and cold, dry conditions, somewhat like today's. The early Sun's energetic ultraviolet radiation would've stripped away Mars' atmosphere as it does today. But toward the end of the Noachian, the massive volcanic eruptions that began to build Tharsis had thickened the atmosphere, at the same time that the Sun's ultraviolet radiation was declining. Mars spent roughly 200 million years with a thicker atmosphere and more rainfall before volcanic activity declined enough that it could no longer resupply Mars' atmosphere with gases being stripped away by the Sun.

This is not inconsistent with the paper I wrote about earlier, where Edwin Kite and coauthors explained the distribution of sedimentary rocks on Mars with very infrequent episodes of snowmelt punctuating ordinarily dry and cold conditions. The briefly wet Mars that Hynek, Beach, and Hoke talk about could predate Kite's dry, cold Mars.

Is a 200-million-year-long Paradise sufficient for life to originate on Mars, and flourish? We don't know, because we don't know how life got started on Earth. What we can say is that if we want to look at Mars rocks that may contain evidence for life, we need to be looking at rocks from that span of time -- from the very end of the Noachian to the very beginning of the Hesperian.

Hmm, let's see, how old is Gale crater? Let's go look up Brad Thomson's paper, "Constraints on the origin and evolution of the layered mound in Gale Crater, Mars using Mars Reconnaissance Orbiter data." From the abstract: " The formation age of the layered mound, derived from crater counts and superposition relationships, is ∼3.6–3.8 [billion years old] and straddles the Noachian–Hesperian time-stratigraphic boundary." Hot diggity dog. Curiosity appears to be in the right place!

That's no accident, of course. The scientists involved in landing site selection were keenly interested in exploring this particular era of Mars' history, the time straddling the boundary between the Noachian and Hesperian eras. Curiosity may possibly already have seen rocks of this vintage; we don't know yet the age of the rocks at Yellowknife Bay. They could be Noachian, though. But there's more of a story to tell in that thick stack of rocks mounded in the center of the crater.

Let's rove to those old rocks and see if we can tell if Hynek, Beach, and Hoke were right about them forming at a time when Mars was wetter -- and if the rocks above them record the loss of Mars' Paradise. Curiosity is now making landfall at her first waypoint on the trek from Yellowknife Bay to Gale's central mountain. This is just the first of several science stops that will provide clues as to how the Yellowknife Bay sediments relate to the history of Gale -- and, by extension, to the history of Mars.