Trying to understand the ancient climate of our own planet is hard enough, but to understand Mars' climatic history, planetary scientists have had to turn to a rather inventive method of climate forensics.
In case you didn't get the memo, Mars used to be a lot wetter than it is now; water flowed across its surface and vast lakes - or even seas - used to cover huge swathes of land. But as the red planet's atmosphere was stripped away by the solar wind, global air pressure plummeted, leaving Mars to freeze-dry. The liquid water froze into the crust and sublimated while any atmospheric moisture was lost to space.
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However, the biggest puzzle for scientists isn't necessarily why Mars is now so dry now, but how it was able to sustain liquid water on its surface at all.
In a new study published in the journal Nature Geoscience, Edwin Kite, a planetary geologist of the California Institute of Technology (Caltech), tackled the problem by first devising a novel means of measuring the thickness of the Martian atmosphere in the planet's past.
By measuring impact craters on the Martian surface, Kite was able to gauge how thick the atmosphere was in Mars' ancient past. Kite's team focused on the 3.6-billion-year-old Aeolis Dorsa region, measuring 319 craters.
As a meteorite blasts through a planetary atmosphere, the thicker the atmosphere, the greater the drag. Therefore, the impact energy of a falling space rock should relate to the thickness of the atmosphere - and therefore its atmospheric pressure.
Fascinatingly, the team found that when the impact craters were excavated, the Martian atmosphere must have had a pressure of 0.9 bar - 150 times higher that Mars' current atmospheric pressure and approximately equivalent to Earth's current sea level pressure of 1 bar. With an atmospheric pressure so high, suddenly it doesn't seem like too much of a stretch to think liquid water could have existed for extended periods of time on the surface.
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But there's a problem. Mars is located 50 percent further away from the sun than Earth is, so the amount of solar energy it receives is far too low to keep any water on its surface in a liquid state. To add to the puzzling nature of Mars' wet past, the young sun was radiating even less energy in the past.
As a consequence, according to Kite, Mars would have needed to have far higher atmospheric pressures to make liquid water exist on the surface - a pressure of around 5 bar, or 5 times the Earth's atmospheric pressure at sea level.
"If Mars did not have a stable multi-bar atmosphere at the time that the rivers were flowing - as suggested by our results-then a warm and wet CO2/H2O greenhouse is ruled out, and long-term average temperatures were most likely below freezing," writes Kite and co. in their study.
If Mars was so cold and atmospheric pressures had to have been so high to keep water in a liquid state, how could Mars have accommodated liquid water at all?
In a separate paper published in the same journal, Sanjoy Som of NASA Ames Research Center outlined some possible mechanisms that may have allowed Mars to maintain its liquid reservoir of water.
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Perhaps the Mars water is heavily laced in salts that lower the freezing point of water, allowing water to flow at temperatures that would have otherwise caused it to freeze. This theory has been bandied around as a possible explanation for pools of water that may be accumulating near the Martian surface. The Martian regolith is packed with perchlorates, a highly toxic oxidizing agent that could create briny pockets of liquid water.
Alternatively, periods of intense volcanic activity may have released vast quantities of greenhouse gases, incubating any surface water in a liquid state.
Som also points to "transient intervals" where cyclical changes in Mars' tilt created atmospheric conditions favorable for a thicker atmosphere. Every 120,000 years, the red planet's tilt undergoes precession, which would have influenced the quantity of sunlight hitting the poles. This cycle may have caused episodic freezing and thawing of the Martian surface water.
Although this is a puzzle, the facts are laid out in front of the Mars rovers working on the surface and orbiters that survey the planet from hundreds of miles overhead: Mars used to be a lot wetter than it is now. But how could the small world have sustained liquid water for any period of time? That's for planetary scientists to try to work out.