Curiosity Reveals Curious Results

The Curiosity mission has brought back some… curious results about ancient Mars’s climate! As Josh Roberts wrote in December, Curiosity has been studying Gale Crater to learn about the evolution of habitability on the Red Planet. Curiosity already found evidence of a long period where a liquid water lake was present on ancient Mars. In a paper released this week in the Proceedings of the National Academy of Sciences, mission scientists discussed results showing that the presence of this ancient water is rather surprising.

The goal of this particular study was for Curiosity to dissect layers of rock in Gale Crater that date back to around 3.5 billion years ago, during the Noachian and Early Hesperian periods (yeah, Mars has different names for its geological periods than Earth does). The authors were specifically looking to determine the levels of atmospheric carbon dioxide, or CO₂, during that time. They created a model that showed how certain minerals—such as iron, magnetite, and clay—would react given different levels of CO₂, and they then compared the model to Curiosity’s samples.

The model had to make certain assumptions about how much CO₂ is required for liquid water to exist on the planet. If levels of CO₂ are too low there wouldn’t be enough atmospheric pressure to keep the water from turning gaseous and floating away. On the other hand, if CO₂ levels are too high, then too much heat could be trapped and that would also cause liquid water to turn into a gas (just look at Venus, a very extreme example). But just the right amount of CO₂ is important for the existence of liquid water since it is able to trap heat, allowing temperatures to be high enough so that the water doesn’t simply turn into ice (like most of the water on Mars today).

With all of the above considered, previous atmospheric models have shown that levels of CO₂ ought to be in the range of hundreds of millibars in order to have temperatures that would allow liquid water to be stable on Mars. However, the results of this study showed that CO₂ levels would have only been in the tens of millibars. This low level of gas would imply that the temperatures 3.5 billion years ago would have been so low that any water on the surface of Mars should have been frozen. Since we have a mountain of data showing that Gale Crater had one of the youngest known habitable conditions, this must mean there was something else causing a temperature spike that allowed for liquid water to exist on Mars’s surface.

“Curiosity’s traverse through streambeds, deltas, and hundreds of vertical feet of mud deposited in ancient lakes calls out for a vigorous hydrological system supplying the water and sediment to create the rocks we’re finding,” said Curiosity Project Scientist Ashwin Vasavada of NASA’s Jet Propulsion Laboratory. “Carbon dioxide, mixed with other gases like hydrogen, has been the leading candidate for the warming influence needed for such a system. This surprising result would seem to take it out of the running.”

So what could have warmed this part of ancient Mars? The Curiosity scientists propose that either the Gale Crater lake may have formed in colder conditions than previously thought, or perhaps our current climate models are missing a critical component. Either way, it looks like there’s certainly more work ahead for Curiosity!

Image: NASA/JPL-Caltech/MSSS