The discovery that Mars' Gale Crater was once Gale Lake adds a powerful piece of evidence for an ancient wet and warm climate that lasted much longer than previous predictions. Now, if only the computer models would agree.
To account for a lake that lasted for millions or even tens of millions of years means the Martian atmosphere would have had to be not only far thicker than the puny envelope of gases that surrounds it today, but also loaded with water, said Ashwin Vasavada, deputy project scientist for NASA's Curiosity Mars rover.
The Curiosity science team announced Monday that the 96-mile-wide crater where the rover landed in August 2012 was once a lake.
"The landscapes of Mount Sharp indicate that rivers, lakes and groundwater were present over millions of years, something that would be impossible on Mars today," Vasavada said.
Today, water on Mars is frozen around the planet's poles. Even if the atmosphere were thicker (generating pressure that would permit water to exist as a liquid, rather than just as solid or gas) water would still preferentially gather in the polar regions, leaving the atmosphere dry. Gale Lake would have evaporated quickly.
"To get a long-lived lake in Gale Crater there must have been so much water in the climate system that the frozen latitudes were essentially filled up, that water was forced to warmer latitudes where it would exist as liquid," Vasavada said.
To humidify the atmosphere, Mars would need a vigorous hydrological cycle fed by either warmer ice at lower latitudes or a large expanse of liquid water, like an ocean.
"A humid atmosphere would slow the evaporation of Gale Lake and also resupply water to precipitation," Vasavada said.
Since the 19070s-era Viking days, scientists have been looking for remnants of a Martian ocean. They've found tantalizing hints, such as a network of valleys and channels cut into the highlands that lead downward toward a large basin. However, concrete evidence, like a shoreline, may have been obliterated by erosion.
"There is no smoking gun for an ocean in the northern hemisphere," Vasavada said.
Even accounting for greenhouse gases, computer models currently fall short in explaining how Mars could have stayed warm enough to sustain a lake like Gale for millions of years.
"Constructing a model of Mars ancient climate that was thick, warm and humid for millions of years has proven pretty challenging," Vasavada said.
One option is that perhaps Mars didn't warm up and stay that way, but heated up periodically from volcanism, changes in its orbit and/or large asteroid strikes.
"Each event may have created warm and wet conditions for hundreds or maybe thousands of years, perhaps enough to fill Gale Crater with one more layer of sediment," Vasavada said.
The question is crucial to understanding how long Mars may have had conditions suitable for life to evolve.