Twenty years ago, the question of whether or not Mars had ever been home to significant oceans or bodies of water was a matter of considerable debate. Thanks to the sustained efforts of probes like Spirit, Opportunity, and Curiosity, we now have evidence that water did flow on Mars, in significant quantities and amounts.
But simply knowing that Mars had water doesn’t tell us much about how the water got there in the first place or what happened to it. Research published in Nature today suggests that the Martian landscape was badly scarred by two massive impacts, each creating craters up to 30km in diameter, and possibly occurring a few million years apart. For comparison’s sake, a 30km meteor impact on Earth would be a large-scale disruption, capable of creating a nuclear winter, but probably not large enough to trigger a mass extinction (the Chicxulub impact believed to have killed off (or at least substantially accelerated) the extinction of the dinosaurs left a 110km crater.
According to the researchers, there’s substantial evidence of tsunami activity on Mars in the form of erosion channels, large boulders deposited miles away from ancient shorelines, and general sediment deposits that largely correspond to what we see on Earth in the aftermath of a tsunami. According to computer models, the best explanation for how these structures are arranged on Mars is a series of significant meteor strikes separated by approximately three million years.
These strikes would have generated tsunamis of enormous power — and thanks to the specific qualities of the Martian terrain, they would’ve flowed over a much larger area than tsunamis typically do on Earth. There are significant low-elevation plains in this area of Mars, which would’ve given the water a fairly gently slope to cover.
These strikes are thought to have occurred in the late Hesperian period in Mars’ history, from 3.61 to 3.37 billion years ago. This was the period of time when Mars was transforming from a warmer, wetter environment into a cold, dusty one. Massive impact events or volcanic activity from beneath the planet’s crust is thought to have periodically punctured the ice-covered ocean or breached the permafrost layer we alluded to earlier, causing massive amounts of water to flood the surface in violent bursts before refreezing again in the thin atmosphere and increasingly frigid environment. While this time frame is after the Late Heavy Bombardment period (when the chance of a significant asteroid impact was some 500x higher than today), it could still have been 80x higher than the present rate — more than long enough to produce these massive impacts.
One of the longstanding questions about Mars is whether or not life could have or did evolve in its oceans before they dried up. Research has indicated that much of the debris field created by these impacts is composed of ice — ice that may have lain undisturbed for billions of years, and might still contain evidence of life. Just as amber can preserve the bodies of insects for millions of years, permanent ice could have done the same for microscopic single-celled life that evolved on Mars.
For those of you wondering why Mars became colder and drier as it aged, the current answer is thought to be linked to Martian volcanic activity. The Tharsis region of Mars (sometimes called the Tharsis Bulge) is thought to have once been one of the most volcanically active areas in the entire solar system. Researchers have estimated that the total amount of CO2 produced by Tharsis could have shrouded all of Mars in a 1.5 bar atmosphere (our own atmosphere is roughly 0.99 bar). The size of the bulge is roughly equivalent to the dwarf planet, Ceres, and its formation is thought to have played a profound role in shaping the Martian climate.
One reason why Mars’ cooled is because the late heavy bombardment and volcanism both began to ebb while the sun may have played a substantial role in stripping Mars’ atmosphere away from the planet. All of these factors may well have played a part — so long as the Tharsis Bulge was pumping huge amounts of CO2 into the atmosphere, Mars might have held out against the sun’s high-energy bombardment. Once less energy was being injected into the system, however, the planet began to cool. The liquid oceans turned to ice and eventually sublimated into space. The Gaian hypothesis for the Great Filter, which we covered earlier this year, posits that planets only retain water for significant periods of time if biological life modifies conditions on the planet to make such retention more likely.
Alternately, the entire mystery of what happened to Mars’ water could be explained as the last-ditch attempt of Martian Ice Warriors to protect their civilization and the rest of the galaxy from an alien lifeform known as the Flood that’s capable of surviving in liquid water. But probably not.