The prevailing scientific wisdom on Mars, based on information gathered by the Spirit, Opportunity, and Curiosity rovers, as well as via multiple orbiting satellites, is that the planet once had substantial reserves of liquid water on its surface. The question of what happened to those reserves, and how Mars transitioned from a warmer, wet world to the frozen arid desert of today, is still a topic of active study and consideration.
We know that Gale Crater, where Curiosity landed, is virtually guaranteed to have been a lake at one point. There are extensive examples of both clay and sulfate mineral types that form in water, as well as visual evidence for features like fans and deltas — two features that are associated with the flowing of liquid water on Earth. Gale itself is believed to have been a freshwater lake that may have been pure enough for humans to drink from it, at least at one point in time. The video below is a good introduction to Gale Crater and the prevailing conditions that Curiosity has been researching, though it predates this paper:
At the lowest levels of Mount Sharp (a raised peak inside Gale Crater), Curiosity found evidence of a long-existing lake. As the rover picks its way up the slopes, it found evidence of rocks enriched in mineral salts, implying these deposits were laid down in briny ponds that underwent repeated episodes of drying and wetting — exactly what you might expect in an area where the hydrological cycle is failing, but not yet failed completely. Curiosity will head for a rock outcrop known as the “sulfate bearing unit” at some point in the future to test it and determine what its composition can tell us about the beginning of the Red Planet’s long dry spell.
As Curiosity climbs, the rock layers it has access to become younger, meaning that the rover is essentially traveling forward into Mars’ present from a starting point in its most distant past. If the sulfate-bearing unit shows strong and increasing concentrations of sulfates, it will imply that Mars’ aridity was a consistent progression, with no significant reversals over time. If the sediment layers the rover finds are thin, with alternating patterns of aridity and moisture, it will imply that Mars’ drying happened in a stepwise fashion over a long period of time.
A network of mud cracks, nicknamed “Old Soaker.” Mud cracks are evidence of periods of wetness being interrupted by dry intervals. Image credit: NASA/JPL-Caltech/MSSS
There are significant questions about how long water persisted on Mars. One reason we believe Mars was wetter in its most distant past is that craters that formed during the Noachian Era (4.1B – 3.7B years ago) are much more heavily eroded than craters formed in the Hesperian (3.7 – 3.0B years ago, though the end-date of the Hesperian is disputed). Gale Crater is thought to have formed 3.8 – 3.5B years ago, putting it in the late Noachian or mid-Hesperian period. Either way, Gale Crater existed when Mars was undergoing its most important transition and contains evidence of the processes that were shaping the planet at that point in time. By the late Hesperian, Mars’ atmosphere is thought to have been close to its current density.
There are multiple theories for how Mars managed to maintain reserves of liquid water for billions of years, despite the problems inherent with the faint, early Sun. Early Mars would have radiated more heat thanks to the decay of elements within the planet’s core. Mars is much smaller than Earth, however, with fewer heavy elements and less of the short-lived, high-energy elements in particular. The Late Heavy Bombardment is estimated to have delivered a 100-km asteroid to Mars every million years or so, with smaller impact rates estimated to be “exponentially higher,” and overall impact rates 500x higher than we observe today.
The Tharsis Bulge may also have played a part. The Tharsis Bulge is an area of volcanism on Mars so huge that the weight of the spot is thought to have caused ‘true polar wander’ on Mars, changing the physical location of the North and South poles. The shutdown of Mars’ geomagnetic dynamo and the loss of the planetary magnetic field undoubtedly contributed as well. Understanding the timeline of how and when Mars lost its water will help us understand which processes were most likely to shape it.
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