The great drying: Mars as it may have appeared billions of years ago on the left, and Mars as it appears today on the far right. Source: Wikipedia
NASA's Mars Curiosity Rover was sent off to Mars in fall of 2011, and deposited on the surface using a nail-biting Rube Goldberg scheme in August 2012. Its primary mission is to look for the ingredients of life. Liquid water is considered by most astrobiologists to be the most important ingredient for life in all the universe. So, when NASA announced last week that Curiosity, supported by other missions, had uncovered compelling evidence for recent, free-flowing liquid water on or very near the Martian surface, that was a huge deal in the search for extraterrestrial life.
Not only does liquid water offer some hope that a few hardy microbes might still be clinging to life here and there under the red, alien Marscape, but surface water is consistent with a much warmer, much wetter Mars. It is consistent with an ancient biosphere that might have supported a rich ecosystem on par with those rich biomes blooming on Earth at the same time.
The evidence that Mars was far more habitable by terrestrial standards has been growing for decades. So much so that it's worth speculating about how and why Mars dried up, cooled down, and turned into the arid, frozen husk of a world we see today. Journey below and join in that converstion, fellow Martian wannabes.
Most "Marsologists" think the planet was once a very different place than it is today. Both Earth and Mars had mostly taken shape by 4.5 billion years ago. Both had accreted out of the solar nebula swirling around our nascent sun. The sun was smaller and dimmer then, and Earth and Mars were probably covered in thick clouds belched up by the prodigious residual heat of planetary formation. It likely took tens of millions of years before a stable, solid surface congealed on either world. But cool they did, and then something unexpected happened: The inner solar system was bombarded by icy comets and hunks of frozen rock from the edge of the solar system.
That's when paleobiologists believe the first simple replicating organisms developed on Earth—which implies it had to happen pretty quickly. The earliest marine sediments we have found on earth, dating to about 3.8 billion years ago, already show geochemical signatures consistent with widespread colonies of chemosynthetic bacteria. If the conditions on Mars were the same, it’s reasonable to infer that there might have been life in the oceans of that planet, too. That life could have even come from Earth, chipped off by impacts and deposited on Mars, or vice-versa for that matter! But there were big changes in the air for Mars.
Planetary scientists believe the molten iron core of Mars cooled many times faster than ours, which makes sense; Mars is a much smaller planet, which means a greater amount of surface area to volume, and less insulation between the hot core and the cooling surface. Mars would radiate internal heat much faster into the infinite cold sink of space than a larger planet. Once that core began setting like cement, the Martian oceans were doomed. The great internal dynamo deep inside seized up. Mars lost its protective magnetic field, and nothing stood between the harsh solar wind and the Martian atmosphere. Or so the theory goes ...
Models suggest that the atmosphere would be stripped away quickly, perhaps in only a few hundred million years. As the ambient pressure dropped and the sun beat down, the oceans would first evaporate and then sublimate. Water vapor rose high into the Martian sky where it was broken down into constituent oxygen and hydrogen by unfiltered UV. The lighter hydrogen quickly escaped, and the reactive oxygen left behind combined with any elements at hand. Carbon became CO2 and iron and other minerals in the drying valleys oxidized. In as little as a billion years after its birth as a warm, wet world, Mars was transformed into the bone dry, frozen, rusty planet we see today.
There is no chance our own world will follow suit. We only have to look at our nearest planetary neighbor to prove this. Venus also lost its magnetic field long ago, perhaps starting with an ancient cataclysm that flipped the planet on its head, where the sun wore away whatever residual rotation it had over millions of years. When Venus basically stopped rotating, its internal dynamo was also shut down, any magnetic fields it had waned, and the full force of a nearby sun blasted into its atmosphere. But Venus didn't end up in a near vacuum like Mars—quite the opposite! It has the thickest atmosphere of any terrestrial planet or Earth-like moon in our solar system. Venus is almost exactly the same size and mass as Earth, bigger than Mars, and plenty big enough to hold onto gasses like carbon dioxide or water vapor.
The ambient pressure and average temperature on Mars are both very low by terrestrial standards—so low that liquid water on the surface should simultaneously freeze and boil away. But there are periods of relative warmth on the planet. This water is probably saturated with salt and other natural antifreeze agents, and water might feel just enough pressure if barely under the surface, trickling between grains of sand, to keep from boiling away into the rarefied Martian atmosphere.
However liquid water may wrap around the rules of ideal gasses and planetary physics, its presence signals a big jump in the odds that Mars was once home to life of some sort and may still be today. And if microbes did exist on Mars, or still do in small pockets under the surface, it will mean that life is common in the universe. What that says about the odds of complex life or intelligent life evolving from humble beginnings remains unknown.